Brain, Vol. 123, No. 3, 620-640,
March 2000
© 2000 Oxford University Press
Invited review |
Functional MRI evidence for a role of frontal and inferior temporal cortex in amodal components of priming
1 Department of Radiology, Massachusetts General Hospital, Nuclear Magnetic Resonance Center, Charlestown and Department of Radiology, Harvard Medical School, 2 Department of Psychology, Harvard University, Cambridge and 3 Departments of Psychology, Radiology, and Anatomy and Neurobiology, Washington University, St Louis, Missouri, USA
Correspondence to:
Dr Randy L. Buckner, Department of Psychology, Washington University, Campus Box 1125, One Brookings Drive, St Louis, MO 63130, USA E-mail: rbuckner{at}artsci.wustl.edu
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Abstract |
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Changes in human brain activity associated with repetition priming during word generation were characterized across a series of neuroimaging and behavioural studies. Repetition priming was consistently observed behaviourally as a decrease in response latency for repeated items, and was found for both visually and aurally cued word-generation tasks. Brain imaging using whole-brain functional MRI identified neural correlates of these effects. The principal effect of priming was to reduce neural activity within regions that were already being used to perform the word-generation tasks. Repeated word generation in response to visual cues was correlated with anatomically selective reductions in activity within the left frontal cortex along the inferior frontal gyrus and inferior temporal regions and, to a lesser degree, in specific earlier visual regions. These reductions were reversed when new items were presented, indicating that they were item-specific. Repeated word generation in response to aural cues also showed anatomically selective activity reductions within the left frontal and inferior temporal regions, indicating that these activity reductions were not dependent on the perceptual modality of the cue. The auditory cortex showed minimal repetition-related reductions. The presence of activity within left frontal regions that decreases as a function of item repetition for both visual and auditory cues suggests that these reductions may underlie an amodal repetition-priming effect existing at processing stages involving lexical/semantic search and access. The surprising finding that activity reductions in the inferior temporal cortex can be linked to repetition of either visual or auditory cues further suggests that these regions may be modulated in a top-down fashion during repetition priming, independent of (or in parallel with) stimulus-driven perceptual processes. Taken collectively, the data converge on a neural correlate of lexical/semantic priming. Amodal lexical/semantic processes, which may be triggered initially by modality-specific cues, proceed via an interaction between frontal and posterior brain regions. These interdependent regions show activity reductions that correlate with facilitated task performance when items are repeated.
priming; memory; learning; neuroimaging; frontal cortex; inferior temporal cortex; prefrontal cortex; implicit memory
ANOVA = analysis of variance; BA = Brodmann area; fMRI = functional MRI; SPGR = spoiled gradient echo
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Introduction |
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TopAbstractIntroductionMethodsIndividual experiments and...General discussionReferences |
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Brain imaging methods based on PET and functional MRI (fMRI) have been used recently to characterize the regions involved in word-retrieval tasks and to determine how the level of activity within these regions may change when items are repeated. Both PET and fMRI studies have shown that word-retrieval tasks that depend on meaning-based (semantic) decisions or phonological decisions are most often accompanied by increased activation in several spatially and functionally diverse regions (for reviews, see Wise et al., 1991; Petersen and Fiez, 1993; Warburton et al., 1996; Binder et al., 1997; Cabeza and Nyberg, 1997). In addition to increased activation in modality-specific brain regions involved in the initial perception of the stimuli used to cue task performance (e.g. activation in the visual cortex in the case of visually presented words or other items), word-retrieval tasks often activate regions of the left inferior frontal gyrus at or near Brodmann areas (BA) 44, 45 or 47, sometimes extending in an anterior and dorsal direction along the border of the prefrontal and motor cortex (BA 44 or 6). Activations in the anterior cingulate, right lateral cerebellum and left temporal cortex are also commonly observed.
These regions play some role in operating on and/or maintaining higher-level verbal representations. However, the specific role is unclear at present and may relate to aspects of phonology, lexical representations and/or their integration with semantics; subregions within the larger networks probably provide separate processing contributions. What is clear is that, across a wide range of tasks that demand elaboration upon verbal and semantic representations, this network of higher-level regions that includes the frontal cortex is consistently activated and is largely independent of the specific modality of the cue (e.g. Chee et al., 1999) or even whether the cue is a picture or visual word (e.g. Vandenberghe et al., 1996). Moreover, these regions can be modified by experience. When subjects retrieve the same words repeatedly or make the same semantic meaning-based decision on multiple occasions, activity within these regions is attenuated (Raichle et al., 1994
; Demb et al., 1995).
The observation that these regions can be modified by experience is particularly critical for the understanding of human memory. Behavioural indices, such as response latency and accuracy, have shown that when items are repeated during semantic or lexical retrieval (or decision tasks), performance on the repeated items is faster, more accurate, and/or biased—a phenomenon referred to as repetition priming. Expanding beyond the early studies of Raichle and colleagues and Demb and colleagues, mentioned above, numerous studies have shown that brain regions that are active during the initial performance of a semantic or lexical retrieval task are less active after repeated exposures to the items (Squire et al., 1992; Raichle et al., 1994; Buckner et al., 1995a; Demb et al., 1995; Blaxton et al., 1996; Gabrieli et al., 1996; Schacter et al., 1996). These reductions may reflect a neural correlate of repetition priming (Ungerleider, 1995; Schacter and Buckner, 1998; Wiggs and Martin, 1998). Repetition-related reductions have been observed in the frontal cortex and in the other regions mentioned above, as well as in earlier visual regions associated with perceptual processes.
Recent investigations have focused on understanding which specific brain regions show priming-related activity reductions and what kinds of task processes are correlated with these reductions. These explorations are, in part, motivated by the behavioural observation that repetition-priming effects can be reduced, but are typically not eliminated, when the perceptual characteristics of a repeated item are changed between study and test. For example, priming effects are often reduced when an item is presented first in the visual and then in the auditory modality, or when characteristics of an item within a given modality are altered (Roediger and McDermott, 1993; Schacter et al., 1993). Behavioural observations indicating that priming may often, to a greater or lesser degree, survive changes in the specific perceptual format of the stimulus suggest that priming involves both form-specific and non-specific or amodal mechanisms (Kirsner et al., 1989; Rajaram and Roediger, 1993), possibly involving processes occurring at multiple stages in a processing hierarchy (Roediger et al., 1999) and/or multiple brain regions within processing subsystems (Schacter and Tulving, 1994). Perceptual processing regions that are sensitive to the perceptual format of a stimulus and higher-level, conceptual processing regions that are relatively less influenced by the precise perceptual instantiation of a word or concept may both benefit from item repetition. That is, repetition priming may modify activity both within relatively early brain regions involved in perceptual processes and in regions occurring later in the hierarchy of processing, at more conceptual stages that promote lexical and semantic access. Anterior brain regions, including the left frontal cortex discussed above, provide a prime candidate for regions likely to be involved in conceptual aspects of repetition priming; more posterior modality-specific regions in the visual or auditory cortices may be involved in perceptual aspects of priming.
In the present study, two related questions were explored using functional neuroimaging techniques: (i) to what extent are brain regions that are believed to be involved in amodal (conceptual) processing modulated by repetition priming? (ii) to what extent are brain regions that are believed to subserve modality-specific processes modulated by repetition? These questions were addressed in a series of fMRI studies using both visually and aurally cued tasks. All of these studies involved word-generation tasks that have been used previously with PET. Because these word-generation tasks can cause motion artefacts when overt speech is employed, we adopted a covert procedure in which subjects generate words silently. Accordingly, a series of behavioural studies outside the MRI environment was also conducted to characterize the behavioural correlates of repetition priming in these covert tasks, and to build confidence that the paradigms imaged using fMRI yielded robust repetition-priming effects under covert word-generation procedures.
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Methods |
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TopAbstractIntroductionMethodsIndividual experiments and...General discussionReferences |
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Overview
Subjects performed word-generation tasks within the MRI environment or outside the MRI environment, in a behavioural setting typical of cognitive psychological studies. Cues to guide word generation (most often the beginnings of words or word stems) were presented in blocks. A given block consisted either entirely of new items (cues that were never previously presented in the experiment) or entirely of repeated items (cues that were presented previously, in earlier blocks of the experiment). Within the MRI environment, brain activity was indirectly measured using an fMRI sequence sensitive to blood oxygenation level-dependent contrast (Kwong et al., 1992
; Ogawa et al., 1992); all word generation was performed covertly. In the behavioural setting, voice onset latencies of word generation were measured to provide a behavioural index of repetition priming. Critically, in the behavioural setting both covert and overt word-generation procedures were used to determine their comparability.
Methods that are general to all of the studies are described first, followed by the methods and results pertaining specifically to each of the five behavioural and three fMRI studies. Table 1 provides an overview of the studies, including the type of study (behavioural or fMRI), the nature of the stimuli, the presentation modality, the word-generation task performed and the response mode (overt, covert or both). As indicated in Table 1, the studies were organized into three sets of experiments and subexperiments: experiments 1a–c focused on visual word-stem completion, experiments 2a–b compared visual word-stem completion with visual verb generation, and experiments 3a–c examined auditory word-stem completion.
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Subjects
All subjects were native speakers of English between the ages of 18 and 37 years. The number of participants included in the three fMRI experiments were as follows: eight in fMRI experiment 1 (six males); eight in fMRI experiment 2 (four males); and 12 in fMRI experiment 3 (six males). All fMRI subjects were right-handed, as assessed using the Edinburgh Handedness Inventory (Lezak, 1995
). For the behavioural studies, 12 naive subjects between the ages of 18 and 35 years served in each study. Informed consent was obtained prior to scanning or testing in a manner approved by the Human Studies Committee of the Massachusetts General Hospital.
Behavioural methods
Visual stimuli were presented using a PowerMacintosh (Apple Computer) connected to a Sharp 2000 colour LCD projector. For fMRI, stimuli were projected onto a screen attached to the head coil through a collimating lens. For the auditory study, subjects wore a custom-modified headphone connected to an amplifier outside the MRI suite. Sounds were presented by the PowerMacintosh computer and fed directly into the amplifier. In the behavioural setting, stimuli were displayed on a 17-inch AppleVision monitor. Voice onset latencies were recorded through a CMU button box (Carnegie Mellon University, Pittsburgh, Pa., USA). Sounds were presented using AppleDesign Powered Speakers (Apple Computer).
MR imaging methods
Scans were acquired on a 1.5 T General Electric scanner fitted with an echo-planar imaging upgrade (Advanced NMR systems, Wilmington, Mass., USA). Whole-brain imaging was performed using the standard General Electric quadrature head coil.
Subjects lay in the scanner with the head snugly surrounded by a pillow and cushions within the head coil to reduce movement. The scanning procedure involved first collecting anatomical images and adjusting the echo-planar shim (Reese et al., 1995). This non-functional portion of the session (~30 min) was directly followed by eight to 10 functional runs of 3.5 min each (<1 h 30 min).
Two relevant anatomical images were collected, including: (i) a sagittal localizer image [conventional T1-weighted spoiled gradient echo (SPGR) sequence, 60 contiguous 2.8 mm slices) and (ii) an echo-planar T1-weighted inversion recovery image aligned to the functional runs [TI (inversion time) = 1200 ms, 1.563x1.563 mm in-plane resolution].
Functional runs involved scanning over 16 or 17 transverse slices (7 mm, skip 1 mm between slices, 3.125x3.125 mm in-plane resolution) aligned to the plane intersecting the anterior and posterior commissures. The resulting 12.8 or 13.6 cm range of imaging was sufficient to image the entire brain, including the cerebellum, in almost every subject. Between 105 and 128 images per slice were acquired using a T2*-weighted asymmetrical spin echo sequence designed to reduce contributions from large vessels [TE (echo time) = 70 ms, 25 ms offset] (Baker et al., 1993). A set of four or more sequential images were generated and discarded before the critical image acquisition was begun in order to allow saturation of T1.
Generation of individual subject activation maps
All fMRI runs for an individual subject were averaged vertically, such that each time-point was the average of that same time-point across runs. For example, if six runs were collected from a subject, the images acquired at the 8 s time-point in each of the six individual runs were averaged to yield one set of images at 8 s, each image representing the mean of the six contributing images.
Runs averaged within a subject were used to construct a statistical activation map based on the non-parametric Kolmogorov–Smirnov statistic (Press et al., 1992). Either all task blocks were compared with fixation blocks, or the task blocks of novel items were directly compared with blocks of repeated items (similar to Buckner et al., 1998a, b; Wagner et al., 1998). Statistical maps were displayed using a pseudocolour scale superimposed on the high-resolution EPI (echo-planar imaging) T1-weighted inversion recovery image, which was acquired in-plane and aligned to the functional activation runs.
Generation of group activation maps in Talairach space
Each individual subject's data were transformed into the stereotaxic space of the Talairach and Tournoux atlas (Talairach and Tournoux, 1988) using procedures described previously (Buckner et al., 1998a, b). Once in atlas space, data were averaged across subjects. First, the interpolated SPGR images were averaged to yield a mean anatomy image. Secondly, the functional runs were averaged and used to construct activation maps in a manner identical to that used for individual subjects. In this instance, however, the interpolated and averaged SPGR images were used as an anatomical underlay.
Identification of peaks in standardized atlas coordinates and time-course generation
Coordinates of activation peaks were determined using an automated peak detection algorithm. Peaks were defined as the location of voxels (i) showing significant activation (P < 0.0001), (ii) showing more significant activation than neighbouring voxels, and (iii) occurring within a cluster of at least five significant voxels. These procedures have been set so as to identify few false positives in control data sets using the logic of Zarahn and colleagues (Zarahn et al., 1997; see also Buckner et al., 1998b). The location of the most significant voxel was selected in instances in which peaks were identified within 8 mm of each other. However, because of uncertainties in any statistical-threshold procedure owing to coloured-noise properties such as motion, the time-course of the signal change was characterized for all peaks of theoretical interest to determine whether the signal change followed the task paradigm (Binder and Rao, 1994).
Regions were defined around these peaks using a second automated procedure to track the signal change within that region over time. For this procedure, all contiguous significant voxels within 12 mm of the peak location were included in a 3D volume. Mean signal intensity within this volume was calculated for each image over the averaged functional runs, yielding a time-course for the signal change. The time-course was corrected for linear drift and temporally smoothed with a 6 s Hanning filter. The time-course was then scaled to percentage signal change.
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Individual experiments and results |
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TopAbstractIntroductionMethodsIndividual experiments and...General discussionReferences |
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Experiment 1a: visually cued overt word-stem completion
Repetition-priming effects were examined in an overt visually cued variant of the word-stem completion task. Subjects were presented visually with three-letter word stems, one at a time (e.g. `gre_' or `pur_'), and were instructed to generate aloud the first English language word completion that came to mind (e.g. `green' or `purple'). Word stems were selected from a pool of 288 unique three-letter word stems that had at least five English word completions. Word stems were presented centrally in white-on-black letters, in 36-point Geneva font. The stimulus duration was 2.0s, with 2.5s between the onsets of stimuli. Behavioural testing was conducted in experimental runs in which blocks of multiple stimuli were presented sequentially and repeatedly, across a given run (12 stimuli in each 30 s block). Each block contained stimuli that were presented in a different random order each time. The stem-completion blocks were separated by a 30 s period of visual fixation. In this manner, a series of word-stem completion blocks was performed with items within the blocks having been exposed for differing numbers of repetitions across blocks. In the first block the word stems were new (NEW), in the second they were repeated once (REP1), in the third they were repeated twice (REP2), etc. for a total of five blocks. Six runs were conducted for each subject. This paradigm is identical to fMRI experiment 1c except that, in the behavioural study, subjects responded overtly to each word-stem cue.
Experiment 1a: results
Clear repetition-priming effects were observed (Fig. 1). The mean reaction time for NEW word-stem completions (the first block) was 902 ms. Responding became progressively faster across repetitions, decreasing to 726 ms by the final block. A repeated measures ANOVA (analysis of variance) performed on the mean voice onset latencies for the five blocks showed a highly significant effect of repetition [F(4,44) = 16.57, P < 0.0001)].
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Examination of participants' response stereotypy revealed that, on average, subjects provided the same completions to the stems on 87% of the trials (SD = 0.09, range = 0.72–0.98; calculated as the sum of the most frequently repeated responses for each item). Furthermore, in this experiment and all subsequent behavioural experiments, significantly greater decreases in response times were found for those items where subjects provided the same response to the repeated cues than for those occasional items where they generated a novel response to the repeated cues (and which showed response times similar to those of never-presented NEW items). This suggests that priming depended on repetition of both the cue and the lexical/conceptual processes involved in generating the response.
Experiment 1b: visually cued overt and covert word-stem completion
In order to determine whether covert word-stem completion would yield repetition-priming effects similar to those of overt word-stem completion, a behavioural study was conducted using a within-subjects design in which, across runs, subjects generated words either overtly or covertly (three runs of each kind). For the covert runs, subjects did not speak aloud during the initial repetition blocks but silently generated a single-word completion for each cue as it was presented. The response was then switched to overt production after four item repetitions so that the repetition-priming effect could be assessed. As in the subsequent fMRI studies, which also involved covert rather overt word generation, the instructions to participants were specific, requiring subjects to covertly generate a response for each item that was presented.
In addition, to allow the estimation of voice-onset latencies for the new items in the covert runs, prior to the first block an additional set of new items was presented. For this prerepetition block, subjects responded overtly for both overt and covert word-production runs. This allowed estimation of the beginning of the voice-onset latency for subjects during the covert word-production runs but without participants ever overtly producing items in the critical covert blocks. Similarly, the final block (REP4) was performed overtly for both the overt and covert word-production runs, allowing the effect of item repetition to be assessed in both instances. For all runs (both overt and covert), participants were instructed, via a visual cue presented immediately before each block, whether the block was to be `spoken' or `silent'. The basic question was: would runs involving covert word production show the same item-repetition effects as those involving overt word production? Finally, a final overt block of repeated items (REP5) was added as a further control so that subsequent learning could be assessed in the covert blocks following the adoption of an overt response.
Experiment 1b: results
Again, clear repetition-priming effects were observed for overt word production (Fig. 2). Importantly, item repetition effects were also clearly observed for covert production; there was little difference between the covert and overt runs. The mean reaction time for completions to the NEW word-stems was 858 ms for the overt runs and was estimated to be 842 ms for the covert runs (based on the PRE repetition block). In the overt condition, responding became progressively faster across repetitions, reducing to 677 ms by the fourth repetition block. As shown in Fig. 2, covert repetition yielded a similar repetition-priming effect, decreasing to 649 ms by the fourth repetition. The final, overt fifth repetition block yielded only slightly faster latencies for both the overt and covert runs, indicating that the majority of the repetition-priming effect had been attained for both procedures.
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A repeated measures ANOVA performed on the mean voice-onset latencies for the overt blocks, and including only the items from the repetition phase (i.e. items that were the same across the blocks, or the conditions NEW, REP1, REP2, REP3, REP4 and REP5 from Fig. 2
) showed a highly significant overt repetition effect [F(5,55) = 19.91, P < 0.0001]. Importantly, a similar analysis for the covert blocks also showed an effect of repetition [considering only the PRE-NEW and REP4 blocks, F(1,11) = 124.96, P < 0.0001; considering PRE-NEW, REP4 and REP5, F(2,22) = 84.47, P < 0.0001]. Thus, by all measures used, the covert procedure yielded a repetition-priming effect that was robust and comparable with that of the overt production procedure. These findings suggest that (i) participants are generating words during the covertly cued blocks, and (ii) the processes underlying repetition priming occur under covert conditions.
Experiment 1c: fMRI of visually cued word-stem completion
The basic task paradigm was a covert variant of the word-stem completion task as used in behavioural experiments of the repetition-priming effect, and also as previously used with PET (Squire et al., 1992; Buckner et al., 1995a; Schacter et al., 1996) and fMRI (Buckner et al., 1996a; Ojemann et al., 1998) studies. Two types of fMRI run were studied. In the first six runs for each participant, 30 s blocks of word-stems were presented with all new items in each block. The goal of these runs was to determine the functional–anatomical brain regions recruited during the word-stem completion task, independently of item-specific repetition-priming effects. The TR (repetition time) was set to 2.0 s. The second type of run was identical to that of experiment 1a, and comprised the final four runs for each participant. For these runs, the same set of word-stem cues was presented repeatedly across a series of five blocks. The TR was set to 3.0 s. The overall presentation parameters were identical to those of experiment 1a, except that the participants performed covert word-stem completion as described in experiment 1b.
Word-stem completion was contrasted with a low-level reference task in which subjects simply visually fixated on a centrally presented cross-hair. Fixation occurred for 30s before and after each block of word-stem completion.
Experiment 1c: results
Main effect of task
A network of brain regions was active during word-stem completion when compared with fixation, confirming results from prior PET and fMRI studies (Buckner and Tulving, 1995; Ojemann et al., 1998) (Fig. 3, Table 2). This network included statistically significant activations within the visual cortex extending from medial regions that are likely to lie within the striate cortex to extrastriate regions to regions within the inferotemporal cortex; these extrastriate activations extended further in the anterior direction in the left than in the right hemisphere. Multiple frontal regions were active, including a dorsal frontal region within the inferior frontal gyrus near BA 6 and/or 44 and a ventral frontal (prefrontal) region within and lateral to the opercular cortex (at or near BA 44 and/or 45). Along the medial wall of the frontal cortex, activations were noted in supplementary motor area, extending into the anterior cingulate. The right lateral cerebellar cortex also showed prominent activation. A complete list of all active regions, their coordinates in Talairach and Tournoux atlas space (Talairach and Tournoux, 1988) and their significance can be obtained from the first author.
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Repetition effects
To determine the correlates of repetition priming, the mean fMRI signal intensity was tracked across the new and repeated item blocks for a subset of regions that were activated by the task during initial runs (in which no items were repeated). The findings are shown in Fig. 4
. Multiple regions, including those within the visual extrastriate cortex and the two regions within the left inferior frontal gyrus, decreased significantly for the repeated compared with the new items. Thus, the first three studies converge to demonstrate related correlates of repetition priming. Experiments 1a and 1b clearly revealed decreases in response latency as a function of repeated performance of the word-stem completion task, and the present experiment showed that these decreases were accompanied by reductions in the level of neural activation observed in specific regions of the pathway that is activated during word generation.
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However, the present experiment contained two confounding factors. First, it is possible that the gradual reduction in fMRI signal that we observed for the left frontal regions partly or entirely derives from non-specific factors, such as fatigue, that are unrelated to item repetitions. Secondly, examination of the time-course of the fMRI signal revealed a signal undershoot that caused shifts in the baseline that could account for part or all of the signal reductions (e.g. the component labelled b in Fig. 4
). Confidence in the item-specific nature of the effects that we observed would be increased if it could be shown that (i) the reductions were specific to the particular sets of items that were repeated and did not generalize to new (not previously presented) items, and (ii) similar magnitudes and patterns of reduction occur under conditions in which multiple new blocks of items are included at the beginning of the runs, such that the fMRI signal undershoot would be stable before presentation of the first repetition block. The first of these aims was addressed in experiment 2a, in which participants received both a modified version of the visual stem-completion paradigm and performed an additional verb-generation task (described below) in a behavioural setting; the second aim was addressed in experiment 2b, in which both the modified visual stem-completion and verb-generation paradigms were presented during functional imaging. Moreover, because new and repeated item blocks were examined within the same fMRI runs in experiment 2b, the separate item types could be compared directly in order to reveal the complete anatomy showing repetition-related modulation. It was not possible to perform this analysis in the present experiment.
Experiment 2a: overt visually cued word-stem completion and verb generation
Word-stem completion
In the modified visual word-stem completion paradigm, voice-onset latencies were examined when words were generated in response to visually presented word stems in a total of four runs. Each run comprised blocks of items that were repeated from previous blocks (similar to experiments 1a–c) but, in addition, they also included blocks of items at the beginning and the end of the run that were entirely new (never previously presented). Specifically, each run began with two blocks of new items, then items from the second block were repeated four times, after which there were two further blocks of new items (i.e. runs followed the following structure: NEW, NEW, REP1, REP2, REP3, REP4, NEW, NEW). The block length was decreased to 16 s and included seven items per block (stimulus duration = 1.5 s with 2.3 s between stimulus onsets). The key question was: will response latencies for the two blocks of new items at the conclusion of each run be similar to the response latencies for new items at the beginning of the run? If, as expected, response latencies abruptly increased upon changing from the repetition phase of the run to the presentation of new items, this finding would provide evidence for the item-specific nature of the behavioural facilitation effects we observed.
Verb generation
The observation in experiment 1c of repetition-related decreases in brain activation in amodal processing regions, particularly the left prefrontal cortex along the inferior frontal gyrus, suggests that the word-generation task reveals neural correlates of facilitated task processing in regions that are involved in higher-level aspects of lexical or semantic response generation and selection. However, in visually cued word-stem completion, retrieval occurs under conditions in which the stimulus cue (the three-letter word stems) provides comparatively direct perceptual support for word retrieval (but note that, because the word stems all allowed multiple completions, some selection as well as generation is clearly necessary). Examination of a task in which the stimulus cue does not provide any direct perceptual support for the response to be retrieved, and in which the relationship between the cue and the word to be generated is strictly semantic or conceptual in nature, would allow generalization of the findings from the stem-completion task. Investigating such a task would also establish more firmly that the repetition-related reductions that we observed are associated with repeated semantic, conceptual or lexical processing. A suitable task for these purposes is verb generation, which has been used previously in PET studies (e.g. Petersen et al., 1988; Wise et al., 1991; Raichle et al., 1994; Warburton et al., 1996). In the verb-generation task, participants are shown nouns (e.g. `dog') and are asked to generate a verb that might be associated with the noun (e.g. `walk' or `bark').
As for visually presented word stems, the effects of repeated generation of verbs (action words) associated with nouns were assessed in an identical behavioural study that paralleled the word-stem study in design. All subjects performed both visual word-stem completion and verb generation, counterbalanced for order across subjects.
Experiment 2a: results
Word-stem completion
From Fig. 5 it is clear that the effects of repetition in the visual word-stem completion task were largely specific to the sets of repeated items: the mean voice-onset latencies for the first new blocks were very similar to one another (1002 and 1008 ms, respectively). These latencies were also similar to the concluding blocks of new items within each run (1029 and 1016 ms, respectively). Only the blocks in which the items were repeated showing systematically decreasing response times, decreasing to 870 ms by the fourth repetition. A repeated measures ANOVA, performed on the mean voice onset latencies in which only the blocks from the phase where there was repetition of the same items were considered, showed a significant effect of repetition [F(4,44) = 8.63, P < 0.0001]. In contrast, an ANOVA comparing the mean voice-onset latency for the first two NEW blocks versus that for the last two NEW blocks, provided no evidence of a global or `item-independent' within-run practice or fatigue effect [F(3,33) < 1]; a similar conclusion could be drawn from a focused comparison of the average of the first two new blocks against the average for the last two new blocks [F(1,11) < 1]. These comparisons indicate that the effects of repetition in the visual stem-completion task are item-specific, i.e. observed when the particular stems are repeated, rather than as function of prolonged or additional performance of the task.
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As in the previous experiments, participants' responses showed a high level of stereotypy (mean = 86%, SD = 0.08, range = 0.73–0.97) .
Verb generation
Repetition effects in the verb-generation task were clearly present, and paralleled the pattern observed for the word-stem completion task (Fig. 6). Response times were longer for new items for which participants were required to generate an appropriate verb for the first time (mean voice-onset latencies of 1267 and 1253 ms for the PRE NEW and NEW items, respectively), but decreased with repeated performance of the task, decreasing to 998 ms by the fourth repetition. A repeated measures ANOVA performed on the mean voice-onset latencies for the blocks from the repetition phase, in which the same items were presented repeatedly (i.e. conditions NEW, REP1, REP2, REP3 and REP4), revealed a significant effect of repetition [F(4,44) = 38.83, P < 0.0001]. Plotting the number of repetitions against the mean response time revealed that, as for the word-stem completion task, performance facilitation was well described by a power function (Fig. 6).
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As found for the visual word-stem completion task, Fig. 6
shows that these decreases in response times were specific to the particular items that were repeated. The two measurements where new nouns were presented following the repetition blocks (POST) were accompanied by a clear increase in response latency, with response times (1226 and 1246 ms) that were very similar to the initial response times (1267 and 1253 ms). There was no difference in the average latency of responding to NEW items at the beginning of the runs versus NEW items at the conclusion of the runs [comparing all four blocks of NEW items, F(3,33) < 1; comparing the average of the first two NEW blocks against the average of the last two NEW blocks, F(1,11) < 1.8]. Consideration of participants' responses showed that the level of response stereotypy during the repetition phase was again considerable (mean = 84%, SD = 0.08, range = 0.67–0.96)
Experiment 2b: fMRI of covert visually cued word-stem completion and verb generation
Functional imaging was performed during covert tasks that were similar in format to those described in the behavioural experiment 2a. Both word-stem completion and verb generation were examined in separate runs. Prior to each run, specific instructions were given to perform all task blocks covertly. Ten functional runs (five for each task) were acquired per subject, each run consisting of 16 interleaved 16 s blocks (TR = 2 s). The ordering of which sets of runs came first (word-stem completion or verb generation) was counterbalanced across subjects. This design allowed several analyses beyond those possible in experiment 1c. First, each type of block (NEW or REP) could be independently compared with the same fixation blocks. Secondly, the NEW and REP blocks could be directly compared with each other to identify directly those regions that are modulated by item repetition. Thirdly, the repetition effects could be examined after a baseline state had been established, following the signal undershoot as observed in experiment 1c and many prior fMRI studies. The signal rebound could be examined for the NEW item blocks following REP blocks to determine whether the repetition-related effects were indeed item-specific.
Experiment 2b: results
Main effect of task for word-stem completion
Consistent with experiment 1c, a network of brain regions was again activated by word-stem completion (all NEW words), as contrasted with the low level-fixation control (Fig. 7, Table 2). These activations were located within multiple left frontal and prefrontal regions (along the inferior frontal gyrus), the visual striate and extrastriate cortex extending into the inferior temporal cortex, and the right lateral cerebellum, among others. For reasons that are not clear, these data contained more noise of high spatial frequency (note the speckled appearance of the data in Fig. 7).
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Main effect of task for verb generation
The findings from verb generation were extremely similar to those from word-stem completion. All of the activations generalized, including left prefrontal activation extending from both the inferior (near BA 44 and/or 45) and dorsal portions of the inferior frontal gyrus (BA 44 and/or 6) into more anterior prefrontal regions, including BA 47. Surprisingly, there were few differences between the word-stem completion and verb-generation tasks (Fig. 7A and B
). On the one hand, this suggests that the present findings are reliable across independent data sets from different tasks. On the other hand, word-stem completion and verb generation are different in terms of task demands (for discussion, see e.g. Buckner et al., 1995b). It is thus noteworthy that the two tasks activate similar anatomical regions, as revealed by bold-contrast fMRI. In the present study, however, a powerful means of comparing the word-stem completion and verb generation tasks was not available to address directly whether there were relatively subtle differences in the anatomical regions recruited by these two tasks. This is because the between-task comparison would be across runs. Results of this comparison were explored but yielded no notable differences. For purposes of further comparison of the repetition-priming effect, given that they were so similar, the two data sets were pooled in order to generate the best estimate of the repetition-priming effects. All effects noted were present and were very similar in both of the two data sets when they were considered separately.
Repetition effects
To address the question of repetition priming, regions in the left frontal cortex (centred on x = –37, y = 9, z = 37), the inferior temporal cortex (centred on x = –50, y = –58, z = –12) and the lingual gyrus at or near the primary visual cortex (centred on x = –12, y = –99, z = –12) were examined for regional signal intensity change over time (Fig. 8). All regions showed an effect of item repetition, with a qualitative decrease in magnitude as the repetitions increased. The signal intensities increased abruptly upon presentation of NEW items in the last two blocks, establishing that the signal reductions were related to item-specific factors, consistent with the behavioural repetition-priming effects observed in experiment 2a.
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Statistical activation maps from the direct comparison between the NEW item blocks and REP item blocks revealed the network of brain regions showing priming-related reductions (Fig. 7C
). Two notable findings emerged. First, confirming the observations from the regional analyses above, the left frontal and visual extrastriate regions extending into the inferior temporal cortex showed significant signal modulation between NEW and REP items, exhibiting greater activity during the NEW item blocks. Secondly, the network of regions showing significant reductions comprised only a subset of those regions activated by the task when compared with fixation. That is, a subset of regions activated by the word-stem and word-generation tasks showed significantly reduced activation in association with item repetition although, qualitatively, most regions showed some degree of modulation. The regions showing robust signal reductions included the left frontal and inferior temporal regions but, consistent with prior PET and fMRI studies, did not include regions at or near the primary motor cortex or many of the visual regions activated during the task. However, unlike our previous study using randomly intermixed event-related trials (Buckner et al., 1998a), the present study did reveal significant left-lateralized signal modulation at or near the primary visual cortex, perhaps reflecting a form of perceptual priming or decreased attention to items in the repeat blocks.
In general, the blocked-task paradigm used in the present study showed a subtle degree of repetition-related modulation in almost all of the time-courses that were examined. Moreover, blocked-trial modulation was also observed in the last new trial block in the POST repetition period within many regions, the second NEW trial block showing a signal reduction relative to the first (Fig. 8). This fMRI signal reduction for new item blocks paralleled the numerical trends in the behavioural findings in all but one of the POST blocks tested (Figs 5, 6, 9 and 10). The tentative conclusion, which will need to be explored further, is that two separate effects may exist: a non-specific blocked factor that perhaps relates to the subjects' anticipation of trials and general arousal level (possibly related to the participants' adoption of a general task `set,' in which the `set' subtly changes for blocks comprising repeated items relative to blocks comprising entirely new items), and an item-specific factor.
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An open question concerns the degree to which activity reductions reflect facilitation of (i) amodal, non-specific processes relating to lexical/semantic search and access or (ii) perceptual processes dependent on the modality and form of the cue. For regions such as those in the frontal cortex that have been associated with higher-level lexical and semantic processes, the strong hypothesis would be that reductions occur in these regions independent of cue modality. However, for other regions, such as the region in the inferior temporal cortex, the predictions are less clear. On the one hand, the inferior temporal cortex is contiguous with visual regions late in the ventral visual processing pathway, and activity reductions may reflect perceptual aspects of the task. On the other hand, auditory word processing tasks have activated these regions, suggesting that their role may be related to lexical/semantic aspects of the task and, hence, reflect amodal processes (Price et al., 1996
; Binder et al., 1997; Buchel et al., 1998). To explore which of these possibilities best characterizes activity reductions related to frontal and inferior temporal priming, a series of further studies was conducted using auditory cue presentation. The hypothesis was that auditory cues would show repetition-priming effects and that brain regions that showed overlapping repetition-priming effects between visually and auditorily cued word-stem completion would reflect amodal processes relating more to conceptual processes than to perceptual processes.
Experiment 3a: auditorily cued overt word-stem completion
Experiments 3a–c examined auditorily cued word-stem completion (e.g. Bassili et al., 1989; Schacter and Church, 1992) in a paradigm structured in the same manner as experiment 1c (16 s blocks). In this instance, however, all word stems (single syllables) were presented aurally (e.g. the sound `pur' or `tray') and participants generated completions based on the sounds (e.g. `perfect', `trait'). The goal of these studies was to generalize findings from the word-generation tasks outside the domain of purely visual stimulus presentation and to determine which brain regions involved in word generation were activated independently of cue modality and which were selective for cue modality. Experiments 3a and 3b were behavioural studies involving overt word generation and both overt and covert word generation respectively; experiment 3c involved fMRI imaging during covert performance of the auditorily cued word-generation task.
Possible auditory word-completion stems were selected using the Oxford Psycholinguistic Database. An initial pool of items was compiled such that all stems were unique and had at least five English language phonetic completions. A word corresponding to each stem was recorded in a male voice using MacRecorder and then edited (using SoundEdit Pro) to include only the first syllable. A normative study with 40 participants, aged 16–26 years, showed that the average correct completion rate for the 147 stems eventually selected, under untimed presentation conditions, was 88%.
The word stems were presented in five runs, preceded by a brief practice block. Each run consisted of eight blocks, each block comprising seven trials. The nature of the task blocks followed a pattern directly parallel to that of the visual word-stem completion and verb-generation tasks of experiment 2a. The actual presentation duration of the auditory word stems varied somewhat, depending on the item (i.e. the stems were self-terminating), with a stimulus presentation rate of one word stem every 2.29 s. Subjects visually fixated on a central fixation point during the entire run to reduce eye movements.
Experiment 3a: results
The average voice onset latencies for overtly generated items on the auditory cued task are shown in Fig. 9. The pattern is very similar to that previously observed for both visually cued word-stem completion and visually cued verb generation: responses to the NEW items at the outset of the runs (1403 and 1433 ms) were slower than those to the REP items, response latencies decreasing to 1298 ms by the fourth repetition but returning to the initial levels (1432 and 1390 ms) when entirely NEW items were once again presented. A repeated measures ANOVA, considering only items from the repetition phase that were the same across the blocks (i.e. NEW, REP1, REP2, REP3, REP4), revealed a highly significant effect of repetition [F(4,44) = 11.22, P < 0.0001]. Examination of the response times for the four blocks of NEW items also showed a trend towards an overall effect of block [F(3,33) = 2.88, P = 0.5]; however, from Fig. 9 it can be seen that this outcome primarily reflected relatively longer response latencies for the second and third NEW blocks relative to that for the first and last NEW blocks. Consistent with this, a focused comparison contrasting the mean for the first two blocks of NEW items in each run against the mean for the last two blocks of NEW items revealed no effect [F(1,11) < 1].
Consideration of participants' responses showed a considerable level of response stereotypy, with an average of 75% of the items repeated (SD = 0.13, range = 0.54–0.91).
Experiment 3b: auditorily cued overt and covert word-stem completion
The purpose of this experiment was to ensure that covert auditory stem completion also resulted in repetition priming. The experimental procedure was identical to that of experiment 2a except that, for some of the runs, participants responded covertly rather than overtly. The covert runs followed the same general pattern as the overt runs in the visually cued task: an initial `estimation' of the novel completion rate provided by a preobservation, then silent (covert) production until the fourth repetition, which was spoken, and ending with two blocks of new items, both requiring spoken responses. Each subject had two runs in which responding was entirely overt and three runs involving a covert response. As in the visually cued word-stem completion paradigm, participants were instructed, immediately before each block (via the presentation of a visual cue), whether the block was to be `spoken' or `silent.'
Experiment 3b: results
Clear priming effects of repetition on auditory word-stem completion were again observed (Fig. 10). For the overt runs, considering only the items from the repetition phase that were the same across the blocks (i.e. NEW, REP1, REP2, REP3, REP4), response latencies decreased from 1393 ms on the first presentation to 1224 ms on the fourth repetition [F(4,44) = 3.88, P = 0.009]. Most importantly, a similar decrement was observed for runs in which participants responded covertly, the estimated response latency for the PRE baseline items of 1449 ms decreasing to 1168 ms by the fourth repetition [F(1,11) = 27.96, P = 0.0003]. Response latencies to NEW items during the POST phase were similar to those for NEW items in the PRE phase, with similar increases in response times for the new items observed in runs that were preceded by overt or by covert response generation, thereby showing, in both instances, that the reduction in voice onset latencies was specific to the particular items that were presented.
The average level of stereotypy for the overt runs was very similar to that observed for the previous auditory stem-completion experiment, in which participants responded overtly for all runs (mean response stereotypy of 77%, SD = 0.14, range = 0.51–0.96).
Experiment 3c: fMRI of covert auditorily cued word-stem completion
Functional imaging was performed during covert word generation, using procedures similar in format to behavioural experiment 3a and similar in terms of fMRI format to experiment 2b. Again, prior to each run specific instructions were given to perform all task blocks covertly, in this instance blocks of auditorily cued word-stem completion. Five functional runs were acquired per subject, each run consisting of 16 interleaved 16 s blocks (TR = 2 s). Within all runs, word-stem completion blocks followed the same order as in experiment 2a (NEW, NEW, REP1, REP2, REP3, REP4, NEW, NEW), and regularly alternated with blocks of visual fixation (odd numbered blocks). Stimulus duration varied during task blocks, with 2.3 s between onsets (seven items per block).
Experiment 3c: results
Main effect of task
Multiple brain regions forming a network were active during auditory word-stem completion when compared with visual fixation (Fig. 11A, Table 2). Many of these regions had also been found to be activated during the visual word-stem completion and verb-generation tasks, including the left inferior frontal gyrus near the border of BA 6 and/or 44 extending ventrally into the inferior prefrontal cortex at or near BA 44, 45 and/or 47 near the frontal operculum. Activations were again noted in the supplementary motor area and the right lateral cerebellum.
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Quite distinct from the visual tasks, however, the auditory variant showed extremely robust activation in the bilateral auditory cortex. This activation was detected in spite of the loud sound of the scanner, which may have interacted with the word stems, consistent with earlier observations (Binder and Rao, 1994
).
Regions in the visual cortex extending into the inferior temporal cortex and parahippocampal gyrus showed two opposing effects that existed in spatially distinct regions. By and large, the visual cortex was either silent or had reduced activation compared with fixation (Fig. 12). Such reductions are distinct from those observed during item repetition. In this instance, the reductions are in relation to fixation and not relative to an active task baseline with an elevated activity level due to task demands. The reductions here probably (although not definitively) reflected a cross-modal suppression effect: strong activation in the auditory cortex may have been suppressing activity in certain visual regions (Haxby et al., 1994; Kawashima et al., 1995; Buckner et al., 1996b; for review, see Shulman et al., 1997). The visual regions showing activity reductions were extremely similar in spatial location to those previously observed with PET during auditory word presentation (Buckner et al., 1996b).
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The second effect observed in the putative visual cortex was surprising: a region in the inferior temporal cortex near the fusiform gyrus (x = –43, y = –52, z = –12) was increased in activation and spatially distinct from nearby regions of signal decrease that extended into the parahippocampal cortex and posteriorly into the visual extrastriate regions. The time-course of activation of this inferior temporal region, which is quite similar in location to the region showing activity reductions in the visually cued tasks, is shown in Fig. 13
. Thus, auditory word-stem completion increases activity within certain regions of the inferior temporal cortex in addition to a more widespread and general suppression effect across many regions of the ventral visual processing pathway. The clear opposing effects of the auditory word-stem completion task on the separate occipital-temporal regions shows that they are functionally dissociable regions (Fig. 12). The inferior temporal region behaved as an amodal processing region.
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Repetition effects
In a parallel analysis of the visual word-stem completion studies, regions in the left frontal cortex (centred on x = –40, y = 6, z = 34), the inferior temporal cortex site of activation increase (centred on x = –43, y = –52, z = –12) and the superior temporal gyrus at or near the primary auditory cortex (centred on x = –62, y = –18, z = 12) were examined for regional signal intensity change over time (Fig. 13
). Both the frontal and inferior temporal regions showed strong effects of item repetition similar in profile and magnitude to those observed in the earlier studies using visual cues. For both regions, the reductions in signal intensity were reversed upon presentation of NEW items, again indicating that the signal reductions related to item-specific factors. By contrast, the region near the primary auditory cortex showed no change that was associated with item-specific repetition effects. Thus, despite the finding that regions at or near the primary auditory cortex were the most robustly activated (in terms of both statistical significance and percentage signal change), these regions showed little or no modulation in relation to repetition priming. Direct comparison of the NEW with the REP item blocks confirmed all of these findings (Fig. 11B). |
General discussion |
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TopAbstractIntroductionMethodsIndividual experiments and...General discussionReferences |
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The present series of studies explored the effects of item repetition during a variety of lexical/semantic word-generation tasks. Tasks included visually cued word-stem completion, visually cued verb generation and aurally cued word-stem completion. In all instances, behavioural measurements of voice onset latency demonstrated that repetition of items decreased response latency (repetition priming). Reductions in response latency followed a stereotypic pattern that was predicted well by a power function. Blood oxygenation level-dependent-contrast fMRI measurements revealed that repetition of items was associated with decreased activity levels within specific brain regions that were activated by the tasks in their unprimed states, including the left frontal cortex near the inferior frontal gyrus and also the inferior temporal cortex. All correlates of item repetition were reversible upon presentation of novel items, indicating that the effects were item-specific. Taken collectively, the present findings support a neural correlate of repetition priming: repeated exposure to an it