No abstract

Functional imaging studies identified a motion-sensitive area (V5/MT+) in the vicinity of the posterior branch of the inferior temporal sulcus that has no correlate in any classical cytoarchitectonic map. The aim of the present study was to identify a cytoarchitectonic correlate of this region in 10 human postmortem brains and to provide a probability map of this area. Observer-independent mapping revealed an area, hOc5 (h for human, Oc for occipital lobe), that has a broad layer III, a high cell density in layer II/III, and a low one in layer V. Most of area hOc5 is found in the depths of the anterior occipital sulcus and the anterior parts of either the inferior lateral occipital or the inferior occipital sulcus. After 3-dimensional reconstruction and registration to a standard reference space, a probability map of the area measured the individual variability of its size and location. The mean spatial locations of area hOc5 are -43, -73, 10 (left) and 49, -70, 11 (right). The locations and their relationships to sulci strongly suggest that hOc5 is the cytoarchitectonic correlate of human V5/MT+. This hypothesis was supported by comparing the cytoarchitectonic probabilistic map with results from a functional imaging study.

The structural correlates of gender differences in visuospatial processing are essentially unknown. Our quantitative analysis of the cytoarchitecture of the human primary visual cortex [V1/Brodmann area 17 (BA17)], neighboring area V2 (BA18), and the cytoarchitectonic correlate of the motion-sensitive complex (V5/MTϩ/hOc5) shows that the visual areas are sexually dimorphic and that the type of dimorphism differs among the areas. Gender differences exist in the interhemispheric asymmetry of hOc5 volumes and in the righthemispheric volumetric ratio of hOc5 to BA17, an area that projects to V5/MTϩ/hOc5. Asymmetry was also observed in the surface area of hOc5 but not in its cortical thickness. The differences give males potentially more space in which to process additional information, a finding consistent with superior male processing in particular visuospatial tasks, such as mental rotation. Gender differences in hOc5 exist with similar volume fractions of cell bodies, implying that, overall, the visual neural circuitry is similar in males and females.

The dorsal visual stream consists of several functionally specialized areas, but most of their cytoarchitectonic correlates have not yet been identified in the human brain. The cortex adjacent to Brodmann area 18/V2 was therefore analyzed in serial sections of ten human post-mortem brains using morphometrical and multivariate statistical analyses for the definition of areal borders. Two previously unknown cytoarchitectonic areas (hOc3d, hOc4d) were detected. They occupy the medial and, to a smaller extent, lateral surface of the occipital lobe. The larger area, hOc3d, is located dorso-lateral to area V2 in the region of superior and transverse occipital, as well as parieto-occipital sulci. Area hOc4d was identified rostral to hOc3d; it differed from the latter by larger pyramidal cells in lower layer III, thinner layers V and VI, and a sharp cortex-white-matter borderline. The delineated areas were superimposed in the anatomical MNI space, and probabilistic maps were calculated. They show a relatively high intersubject variability in volume and position. Based on their location and neighborhood relationship, areas hOc3d and hOc4d are putative anatomical substrates of functionally defined areas V3d and V3a, a hypothesis that can now be tested by comparing probabilistic cytoarchitectonic maps and activation studies of the living human brain.

Illusory contours provide a striking example of the visual system's ability to extract a meaningful representation of the surroundings from fragmented visual stimuli. Psychophysical and neurophysiological data suggest that illusory contours are processed in early visual cortical areas, and neuroimaging studies in humans have shown that Kanizsa-type illusory contours activate early retinotopic visual areas that are also activated by real contours. It is not known whether other types of illusory contours are processed by the same mechanisms, nor is it clear to what extent attentional effects may have influenced these results, as no attempt was made to match the salience of real and illusory stimuli in previous imaging studies. It therefore remains an open question whether there are any brain regions specifically involved in the perception of illusory contours. To address these questions, we have used 15O-butanol positron emission tomography (PET) and a novel kind of illusory contour stimulus that is induced only by aligned line ends. By employing a form discrimination task that was matched for attention and stimulus salience across conditions we were able to directly contrast perception of real and illusory contours. We found that the regions activated by illusory contour perception were the same as those activated by real contours. Only one region, located in the right fusiform gyrus, was significantly more strongly activated by perception of illusory contours than by real contours. In addition, a principal component analysis suggested that illusory contour perception is associated with a change in the correlation between V1 and V2. We conclude that different kinds of illusory contours are processed by the same cortical regions and that these regions overlap extensively with those involved in processing of real contours. At the regional level, perception of illusory contours thus appears to differ from perception of real contours by the degree of involvement of higher visual areas as well as by the nature of interaction between early visual areas.

To date, the delineation of the human visual "motion area" still relies on functional paradigms originally devised to identify monkey area MT. Using fMRI, we have identified putative human area V5/MT+ in normals by modelling the BOLD responses to alternating radially moving and stationary dot patterns. Functional activations were compared with cytoarchitectonic probability maps of its putative correlate area hOc5, which was calculated based upon data from histological sections of ten human post-mortem brains. Bilateral visual cortex activations were seen in the single subject dynamic versus stationary contrasts and in the group random-effects analysis. Comparison of group data with area hOc5 revealed that 19.0%/39.5% of the right/left functional activation was assigned to the right/left hOc5. Conversely, 83.2%/53.5% of the right/left hOc5 was functionally activated. Comparison of functional probability maps (fPM) with area hOc5 showed that 28.6%/18.1% of the fPM was assigned to hOc5. In turn, 84.9%/41.5% of the area hOc5 was covered by the respective fPM. Thus, random-effects data and fPMs yielded similar results. The present study shows for the first time the correspondence between the functionally defined human V5/MT+ and the post-mortem cytoarchitectonic area hOc5.

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The microstructural correlates of the functional segregation of the human lateral occipital cortex are largely unknown. Therefore, we analyzed the cytoarchitecture of this region in ten human post-mortem brains using an observer-independent and statistically testable parcellation method to define the position and extent of areas in the lateral occipital cortex. Two new cytoarchitectonic areas were found: an anterior area hOc4la and a posterior area hOc4lp. hOc4la was located behind the anterior occipital sulcus in rostral and ventral portions of this region where it occupies the anterior third of the middle and inferior lateral occipital gyri. hOc4lp was found in caudal and dorsal portions of this region where it extends along the superior and middle lateral occipital gyri. The cytoarchitectonic areas were registered to 3D reconstructions of the corresponding brains, which were subsequently spatially normalized to the Montreal Neurological Institute reference space. Continuous probabilistic maps of both areas based on the analysis of ten brains were generated to characterize their inter-subject variability in location and size. The maps of hOc4la and hOc4lp were then used as seeds for meta-analytic connectivity modeling and quantitative functional decoding to identify their co-activation patterns and assignment to functional domains. Convergent evidence from their location, topography, size, functional domains and connectivity indicates that hOc4la and hOc4lp are the potential anatomical correlates of the functionally defined lateral occipital areas LO-1 and LO-2.