The visual image in mind and brain.
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Animals↗
Biomedical subjects
S Zeki
Publications and source records attributed to S Zeki.
A thought experiment with positron emission tomography.
This paper describes a thought experiment. The experiment supposes that the technique of positron emission tomography (PET), as we known it today, was available in 1920 and had been applied then to a study of the visual cortex of man. The 'results' of the experiment show that such an approach would have generated new concepts about the functioning of the visual cortex, and that PET can therefore be considered to be an hypothesis-generating technique.
History, 20th Century↗
Cerebral akinetopsia (visual motion blindness). A review.
Cerebral akinetopsia is a syndrome in which a patient loses specifically the ability to perceive visual motion following cortical lesions outside the striate cortex. There has been only one good case of akinetopsia in the published literature. Yet that case was immediately accepted by the neurological world. In this, cerebral akinetopsia differs markedly from cerebral achromatopsia, the evidence for which was strongly contested for the better part of a century (Zeki, 1990). This article complements the one on cerebral achromatopsia, traces the history of akinetopsia and enquires into why it was so much more readily acceptable than achromatopsia.
Animals↗
A direct demonstration of functional specialization in human visual cortex.
We have used positron emission tomography (PET), which measures regional cerebral blood flow (rCBF), to demonstrate directly the specialization of function in the normal human visual cortex. A novel technique, statistical parametric mapping, was used to detect foci of significant change in cerebral blood flow within the prestriate cortex, in order to localize those parts involved in the perception of color and visual motion. For color, we stimulated the subjects with a multicolored abstract display containing no recognizable objects (Land color Mondrian) and contrasted the resulting blood flow maps with those obtained when subjects viewed an identical display consisting of equiluminous shades of gray. The comparison identified a unique area (area V4) located in the lingual and fusiform gyri of the prestriate cortex. For motion, blood flow maps when subjects viewed moving or stationary black and white random-square patterns were contrasted. The comparison identified a unique area located in the region of the temporo-parieto-occipital junction (area V5). We thus provide direct evidence to show that, just as in the macaque monkey, different areas of the human prestriate visual cortex are specialized for different attributes of vision. The striate cortex (V1) and the contiguous visual area (V2), which in the monkey brain feed both the homologous areas, were active in all 4 conditions. This pattern of activity allowed us to use an extension of the approach to assess the functional relationship between the 3 areas during color and motion stimulation. This is based on an hypothesis-led analysis of the covariance structure of the blood flow maps and promises to be a powerful tool for inferring anatomical pathways in the normal human brain.(ABSTRACT TRUNCATED AT 250 WORDS)
Adult↗
A century of cerebral achromatopsia.
This review is an enquiry into why the early clinical evidence for a colour centre in the cerebral cortex of man was so successfully dismissed for the best part of a century. The imperfection of this evidence cannot be the reason, for the same evidence that was rejected earlier is accepted today. Instead, it was because the prevalent concepts of vision as a function, and of the role of the cerebral cortex in it, dominated facts and prevented acceptance of evidence showing a specialization for colour in the visual cortex. It was only after those concepts were overthrown by the demonstration of functional specialization in the visual cortex of the primate that the evidence for a colour centre in the human brain became acceptable. Today, our new knowledge of the colour areas and pathways in the primate brain allows us to give a more complete account of the pathophysiology of cerebral achromatopsia in man.
Brain↗
Parallelism and functional specialization in human visual cortex.
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Color Perception↗
The colour centre in the cerebral cortex of man.
Anatomical and physiological studies have shown that there is an area specialized for the processing of colour (area V4) in the prestriate cortex of macaque monkey brain. Earlier this century, suggestive clinical evidence for a colour centre in the brain of man was dismissed because of the association of other visual defects with the defects in colour vision. However, since the demonstration of functional specialization in the macaque cortex, the question of a colour centre in man has been reinvestigated, based on patients with similar lesions in the visual cortex. In order to study the colour centre in normal human subjects, we used the technique of positron emission tomography (PET), which measures increases in blood flow resulting from increased activity in the cerebral cortex. A comparison of the results of PET scans of subjects viewing multi-coloured and black-and-white displays has identified a region of normal human cerebral cortex specialized for colour vision.
Cerebral Cortex↗
The functional logic of cortical connections.
Patterns of anatomical connections in the visual cortex form the structural basis for segregating features of the visual image into separate cortical areas and for communication between these areas at all levels to produce a coherent percept. Such multi-stage integration may be a common strategy throughout the cortex for producing complex behaviour.
Animals↗
[Brodmann's area 18 and area 19].
This article, written on the 60th anniversary of Korbinian Brodmann's death, discusses his schematic cytoarchitectonic chart of the cerebral cortex in relation to the brain sections from which it was derived and draws attention to the discrepancies between the two.
Animals↗
Segregation of pathways leading from area V2 to areas V4 and V5 of macaque monkey visual cortex.
V5 and V4 are areas of macaque monkey prestriate visual cortex that are specialized for involvement in different aspects of visual perception, namely motion for V5 (refs 1-4) and colour vision, with other possible functions, for V4 (refs 2, 5-9). Thus, it is unlikely that they should be fed the same information for further processing, yet both receive a strong input from patches of the upper layers of V2 (refs 10, 11), the area immediately adjoining the primary visual cortex, V1. V2, however, seems to comprise functionally distinct subregions, which can be revealed by staining the tissue for the mitochondrial enzyme cytochrome oxidase. Here we report that V4 and V5 are connected with separate cytochrome oxidase-defined subregions of V2, suggesting that cortical pathways dealing with motion and colour perception are segregated in their passage through V2, and reinforcing evidence for functional specialization in the visual cortex.
Animals↗