Trenton A. Jerde

Prioritized Maps of Space in Human Frontoparietal Cortex

Priority maps are theorized to be composed of large populations of neurons organized topographically into a map of gaze-centered space whose activity spatially tags salient and behaviorally relevant information. Here, we identified four priority map candidates along human posterior intraparietal sulcus (IPS0–IPS3) and two along the precentral sulcus (PCS) that contained reliable retinotopically organized maps of contralateral visual space. Persistent activity increased from posterior-to-anterior IPS areas and from inferior-to-superior PCS areas during the maintenance of a working memory representation, the maintenance of covert attention, and the maintenance of a saccade plan. Moreover, decoders trained to predict the locations on one task (e.g., working memory) cross-predicted the locations on other tasks (e.g., attention) in superior PCS and IPS2, suggesting that these patterns of maintenance activity may be interchangeable across the tasks. Such properties make these two areas in frontal and parietal cortex viable priority map candidates.

Dissociable systems of working memory for rhythm and melody.

Specialized neural systems are engaged by the rhythmic and melodic components of music. Here, we used PET to measure regional cerebral blood flow (rCBF) in a working memory task for sequences of rhythms and melodies, which were presented in separate blocks. Healthy subjects, without musical training, judged whether a target rhythm or melody was identical to a series of subsequently presented rhythms or melodies. When contrasted with passive listening to rhythms, working memory for rhythm activated the cerebellar hemispheres and vermis, right anterior insular cortex, and left anterior cingulate gyrus. These areas were not activated in a contrast between passive listening to rhythms and a non-auditory control, indicating their role in the temporal processing that was specific to working memory for rhythm. The contrast between working memory for melody and passive listening to melodies activated mainly a right-hemisphere network of frontal, parietal, and temporal cortices: areas involved in pitch processing and auditory working memory. Overall, these results demonstrate that rhythm and melody have unique neural signatures not only in the early stages of auditory processing, but also at the higher cognitive level of working memory.

Maps of space in human frontoparietal cortex

Prefrontal cortex (PFC) and posterior parietal cortex (PPC) are neural substrates for spatial cognition. We here review studies in which we tested the hypothesis that human frontoparietal cortex may function as a priority map. According to priority map theory, objects or locations in the visual world are represented by neural activity that is proportional to their attentional priority. Using functional magnetic resonance imaging (fMRI), we first identified topographic maps in PFC and PPC as candidate priority maps of space. We then measured fMRI activity in candidate priority maps during the delay periods of a covert attention task, a spatial working memory task, and a motor planning task to test whether the activity depended on the particular spatial cognition. Our hypothesis was that some, but not all, candidate priority maps in PFC and PPC would be agnostic with regard to what was being prioritized, in that their activity would reflect the location in space across tasks rather than a particular kind of spatial cognition (e.g., covert attention). To test whether patterns of delay period activity were interchangeable during the spatial cognitive tasks, we used multivariate classifiers. We found that decoders trained to predict the locations on one task (e.g., working memory) cross-predicted the locations on the other tasks (e.g., covert attention and motor planning) in superior precentral sulcus (sPCS) and in a region of intraparietal sulcus (IPS2), suggesting that these patterns of maintenance activity may be interchangeable across the tasks. Such properties make sPCS in frontal cortex and IPS2 in parietal cortex viable priority map candidates, and suggest that these areas may be the human homologs of the monkey frontal eye field (FEF) and lateral intraparietal area (LIP).

Cerebral cortical mechanisms of copying geometrical shapes: a multidimensional scaling analysis of fMRI patterns of activation

We used multidimensional scaling (MDS) to characterize the integrative neural mechanisms during viewing and subsequently copying nine geometrical shapes. Human subjects initially looked at a central fixation point (“rest” period), then looked at a geometrical shape (“visual” period) which they copied without visual feedback (“copying” period). BOLD signal was recorded from voxels in 28 cortical areas (14 from each hemisphere) using a 4 Tesla magnet. For each voxel, signal ratios of “Visual versus Rest” (VR), and “Copy versus Visual” (CV) were calculated and used to construct two sets of Euclidean distance dissimilarity matrices for the nine shapes, with separate matrices defined for each region of interest (ROI) across subjects. The relations of perceptual and motor aspects of the shapes to MDS dimensions and specific ROIs were assessed using stepwise multiple regressions. The optimal individually scaled (INDSCAL) solutions were 2-dimensional. For the VR condition, MDS dimensions were significantly associated with the presence of crossing in a shape (Dimension 1), and with perimeter, height, cycles, peak segment speed, and horizontal symmetry (Dimension 2). ROIs most prominently associated with these dimensions essentially comprised the medial frontal lobe bilaterally, the inferior frontal gyrus bilaterally, and the left intraparietal sulcus (Dimension 1), and visual areas, including the calcarine sulcus and cuneus bilaterally (Dimension 2). These results document the expected involvement of visual areas and support the hypothesis advanced on the basis of previous findings (Lewis et al. 2003a) that a motor rehearsal of the upcoming shape copying is occurring during this visual presentation period. For the CV condition, practically one motor feature (number of segments drawn) dominated both dimensions, with a secondary engagement of horizontal symmetry in Dimension 1. The right postcentral gyrus, right intraparietal sulcus, right superior parietal lobule and right inferior parietal lobule contributed mostly to Dimension 1; the superior frontal gyrus bilaterally, right middle frontal gyrus, left postcentral gyrus, left inferior parietal lobule contributed mostly to Dimension 2; and the left superior parietal lobule and left intraparietal sulcus contributed to both dimensions approximately equally. CV BOLD activation of ROIs contributing to Dimension 1 (or to both dimensions) was significantly associated with the number of shape segments drawn. Since the direction of movement differs in successively drawn shape segments, the number of segments (minus one) equals the number of changes in the direction of movement. We conclude that this fundamental spatial motor aspect of drawing geometrical shapes is the critical variable, independent of the particular shape drawn, that dominates cortical activation during copying.

True associations between resting fMRI time series based on innovations

We calculated voxel-by-voxel pairwise crosscorrelations between prewhitened resting-state BOLD fMRI time series recorded from 60 cortical areas (30 per hemisphere) in 18 human subjects (nine women and nine men). Altogether, more than a billion-and-a-quarter pairs of BOLD time series were analyzed. For each pair, a crosscorrelogram was computed by calculating 21 crosscorrelations, namely at zero lag ± 10 lags of 2 s duration each. For each crosscorrelogram, in turn, the crosscorrelation with the highest absolute value was found and its sign, value, and lag were retained for further analysis. In addition, the crosscorrelations at zero lag (irrespective of the location of the peak) were also analyzed as a special case. Based on known varying density of anatomical connectivity, we distinguished four general brain groups for which we derived summary statistics of crosscorrelations between voxels within an area (group I), between voxels of paired homotopic areas across the two hemispheres (group II), between voxels of an area and all other voxels in the same (ipsilateral) hemisphere (group III), and voxels of an area and all voxels in the opposite (contralateral) hemisphere (except those in the homotopic area) (group IV). We found the following. (a) Most of the crosscorrelogram peaks occurred at zero lag, followed by ± 1 lag; (b) over all groups, positive crosscorrelations were much more frequent than negative ones; (c) average crosscorrelation was highest for group I, and decreased progressively for groups II-IV; (d) the ratio of positive over negative crosscorrelations was highest for group I and progressively smaller for groups II-IV; (e) the highest proportion of positive crosscorrelations (with respect to all positive ones) was observed at zero lag; and (f) the highest proportion of negative crosscorrelations (with respect to all negative ones) was observed at lag = 2. These findings reveal a systematic pattern of crosscorrelations with respect to their sign, magnitude, lag and brain group, as defined above. Given that these groups were defined along a qualitative gradient of known overall anatomical connectivity, our results suggest that functional interactions between two voxels may simply reflect the density of such anatomical connectivity between the areas to which the voxels belong.

Perception and action selection dissociate human ventral and dorsal cortex

We test theories about the functional organization of the human cortex by correlating brain activity with demands on perception versus action selection. Subjects covertly searched for a target among an array of 4, 8, or 12 items (perceptual manipulation) and then, depending on the color of the array, made a saccade toward, away from, or at a right angle from the target (action manipulation). First, choice response times increased linearly as the demands increased for each factor, and brain activity in several cortical areas increased with increasing choice response times. Second, we found a double dissociation in posterior cortex: Activity in ventral regions (occipito-temporal cortex) increased linearly with perceptual, but not action, selection demands; conversely, activity in dorsal regions (parietal cortex) increased linearly with action, but not perceptual, selection demands. This result provides the clearest support of the theory that posterior cortex is segregated into two distinct streams of visual processing for perception and action. Third, despite segregated anatomical projections from posterior ventral and dorsal streams to lateral pFC, we did not find evidence for a functional dissociation between perception and action selection in pFC. Increasing action, but not perceptual, selection demands evoked increased activation along both the dorsal and the ventral lateral pFC. Although most previous studies have focused on perceptual variables (e.g., space vs. object), these data suggest that understanding the computations underlying action selection will be key to understanding the functional organization of pFC.

The search for the neural mechanisms of the set size effect

The set size effect in visual search refers to the linear increase in response time (RT) or decrease in accuracy as the number of distractors increases. Previous human and monkey studies have reported a correlation between set size and neural activity in the frontal eye field (FEF) and intraparietal sulcus (IPS). In a recent functional magnetic resonance imaging study, we did not observe a set size effect in the superior precentral sulcus (sPCS, thought to be the human homolog of the FEF) and IPS in an oculomotor visual search task (Ikkai et al., 2011). Our task used placeholders in the search array, along with the target and distractors, in order to equate the amount of retinal stimulation for each set size. We here attempted to reconcile these differences with the results from a follow‐up experiment in which the same oculomotor visual search task was used, but without placeholders. A strong behavioral set size effect was observed in both studies, with very similar saccadic RTs and slopes between RT and set size. However, a set size effect was now observed in the sPCS and IPS. We comment on this finding and discuss the role of these neural areas in visual search.

A compact and realistic cerebral cortical layout derived from prewhitened resting-state fMRI time series: Cherniak's adjacency rule, size law, and metamodule grouping upheld.

We used hierarchical tree clustering to derive a functional organizational chart of 52 human cortical areas (26 per hemisphere) from zero-lag correlations calculated between single-voxel, prewhitened, resting-state BOLD fMRI time series in 18 subjects. No special “resting-state networks” were identified. There were four major features in the resulting tree (dendrogram). First, there was a strong clustering of homotopic, left-right hemispheric areas. Second, cortical areas were concatenated in multiple, partially overlapping clusters. Third, the arrangement of the areas revealed a layout that closely resembled the actual layout of the cerebral cortex, namely an orderly progression from anterior to posterior. And fourth, the layout of the cortical areas in the tree conformed to principles of efficient, compact layout of components proposed by Cherniak. Since the tree was derived on the basis of the strength of neural correlations, these results document an orderly relation between functional interactions and layout, i.e., between structure and function.