Semir Zeki

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)

Functional organization of a visual area in the posterior bank of the superior temporal sulcus of the rhesus monkey.

1. Anatomical studies have shown the cortex of the posterior bank of the superior temporal sulcus to receive a projection from visual cortical areas, including areas 17, 18 and 19. In this paper the response of single neurones in this area to simple visual stimulation is reported. Ten monkeys were studied.

The neural basis of romantic love

The neural correlates of many emotional states have been studied, most recently through the technique of fMRI. However, nothing is known about the neural substrates involved in evoking one of the most overwhelming of all affective states, that of romantic love, about which we report here. The activity in the brains of 17 subjects who were deeply in love was scanned using fMRI, while they viewed pictures of their partners, and compared with the activity produced by viewing pictures of three friends of similar age, sex and duration of friendship as their partners. The activity was restricted to foci in the medial insula and the anterior cingulate cortex and, subcortically, in the caudate nucleus and the putamen, all bilaterally. Deactivations were observed in the posterior cingulate gyrus and in the amygdala and were right-lateralized in the prefrontal, parietal and middle temporal cortices. The combination of these sites differs from those in previous studies of emotion, suggesting that a unique network of areas is responsible for evoking this affective state. This leads us to postulate that the principle of functional specialization in the cortex applies to affective states as well.

Response properties and receptive fields of cells in an anatomically defined region of the superior temporal sulcus in the monkey

A clutch brake unit suitable for use in conjunction with a heavy duty clutch assembly for retarding the motion of the driven clutch shaft. The unit is actuated by the clutch release mechanism which causes the unit to be compressed between a movable member and a stationary member as the clutch is being released. A pair of discs, adapted to positively engage the shaft in sliding relationship therewith, are initially engaged by the members in friction driving contact and forced together against the biasing pressure of a wave washer mounted therebetween. The discs are independently mounted for compression within a relatively rigid housing. When the discs are compressed to a predetermined depth, the two members engage the housing thereby preventing further loading of the shaft.

Uniformity and diversity of structure and function in rhesus monkey prestriate visual cortex.

1. Recordings were made from single neurones, or small clusters of cells, in five prestriate visual areas of rhesus monkey cortex. The cells were studied for their binocularity, as well as for their orientational, motion and colour preferences. In all, 1500 cells were studied, 250 cells for each of the areas V2, V3, V3A and the motion area of the posterior bank of the superior temporal sulcus, and 500 cells for V4. All the cells referred to in this study can be placed in one prestriate area or another unambiguously. 2. The great majority of cells in all areas were binocularly driven, without monocular preferences. Within each area, there were cells that either preferred binocular stimulation markedly, or were responsive to binocular stimulation only. The ocular interaction histograms for all areas are remarkably similar when tested at a fixed disparity. 3. Over 70% of the cells in areas V2, V3 and V3A were selective for orientation. The receptive fields of cells were larger in V3 and V3A than in V2. By contrast, less than 50% of the cells in V4 and the motion area of the superior temporal sulcus were orientation selective. 4. Directionally selective cells were found in all areas. But they were present in small numbers (less than 15%) in areas V2, V3, V3A and V4. By contrast, 90% of the cells in the motion area of the superior temporal sulcus were directionally selective. 5. 8% of the cells in V2 had opponent colour properties. Cells with such properties were not found in V3, V3A or in the motion area of the superior temporal sulcus. By contrast, 54% of the cells in the V4 complex had opponent colour properties. 6. It is argued that despite its uniformity in cytoarchitectural appearance and in ocular interaction patterns, there is a functional division of labour within the prestriate cortex. Evidence for this is seen not only in the different concentrations of functional cell types in distinct areas of the prestriate cortex, but also in the differential anatomical and callosal connexions of each area.

The architecture of the colour centre in the human visual brain: new results and a review*

We have used the technique of functional magnetic resonance imaging (fMRI) and a variety of colour paradigms to activate the human brain regions selective for colour. We show here that the region defined previously [ Lueck et al. (1989) Nature, 340, 386–389; Zeki et al. (1991) J. Neurosci., 11, 641–649; McKeefry & Zeki (1997) Brain, 120, 2229–2242] as the human colour centre consists of two subdivisions, a posterior one, which we call V4 and an anterior one, which we refer to as V4α, the two together being part of the V4‐complex. The posterior area is retinotopically organized while the anterior is not. We discuss our new findings in the context of previous studies of the cortical colour processing system in humans and monkeys. Our new insight into the organization of the colour centre in the human brain may also account for the variability in both severity and degree of recovery from lesions producing cerebral colour blindness (achromatopsia).

Colour coding in rhesus monkey prestriate cortex

One of the striking features o f the anatomical organisation o f the prestriate cortex in the monkey is the mosaic o f sub-areas into which it may be divided on the basis of the afferent input, efferent output and inter-hemispheric connections of its individual partsl , 5-s. Such a mosaic organisation no doubt reflects a functional division of labour within the prestriate cortex for handling the various parameters o f vision, the emphasis on any particular function in any particular area being presumably dictated by the organisation of the afferent input to that area. The topographically organised pointto-point input f rom the lateral geniculate nucleus to area 17 is reflected functionally in the detailed region-by-region form analysis that this area performs 4. On the other hand, the overlapping, convergent input f rom area 17 to the cortex o f the posterior bank of the superior temporal sulcus is reflected functionally in the generalisation for receptive field position and the emergence o f movement as a critical stimulus parameter 2. In this paper, we report briefly the response properties o f units to simple visual stimulation in another prestriate area, the fourth visual area (V4) which we have already defined anatomically 7. This area lies in the anterior bank of the lunate sulcus dorsally and, because of the complicated gyral changes, emerges ventrally in the posterior bank of the inferior occipital sulcus 7. It receives an input f rom areas 18 and 19 but this projection does not appear to be very precisely defined topographically. We have recorded f rom 77 single units in this area, in 8 monkeys, using tungsten-inglass microelectrodes, and in every case the units have been cotour coded, responding vigorously to one wavelength and grudgingly, or not at all, to other wavelengths or to white light at different intensities. The animals were anaesthetised with sodium pentobarbital and paralysed with gallamine triethiodide (5 mg/kg/h). A hole was drilled in the skull over the appropriate

The Organization of Connections between Areas V5 and V1 in Macaque Monkey Visual Cortex

Area V5 or MT of primate extrastriate visual cortex is specialized for involvement in the analysis of motion and receives input from two layers, 4B and 6, of the striate cortex or V1. Injections of horseradish peroxidase ‐ wheatgerm agglutinin into V5 reveal a patchy distribution of labelled cells and axonal terminals in layer 4B, suggesting the presence of a segregated and functionally specialized subsystem within the layer. The patches are similar in size and frequency to the cytochrome oxidase blobs of layers 2 and 3, but bear little systematic relationship to them. V5‐efferent cells in layer 6, however, tend to avoid the cores of the blobs.

Cortical projections from two prestriate areas in the monkey

In the visual system from retina to cortex the diversity of cell types as judged by their electrophysiological response properties may be explained by the repeated convergence of one group of cells upon another, using both excitatory and inhibitory mechanismsL Such a convergence starts within the layers of the retina itself 3. It continues from the retinal ganglion cells to the neurones of the lateral geniculate nucleus 13 and thence to layer 4 of the striate cortex 5-7. In the striate cortex, interaction occurs within the columns between the cells of the different layers 6. Throughout this retinal-geniculate-striate cortical system and then on to the anatomically defined areas 18 and 191,16 and in spite of the repeated convergence that seemingly occurs, a gross topography is maintained anatomically. This anatomical topography correlates with the physiology in the sense that cells in any particular part of these structures will be excited only if the appropriate stimulus is presented in the appropriate, restricted part of the visual field2,6,12. This anatomical arrangement appears to dissolve at the posterior bank of the superior temporal sulcus which has been shown to receive afferents from large parts of the primary visual cortex is. Such a convergence at first sight appears to represent a step in obtaining cells with very much wider receptive fields 4, enabling them to respond to wider parts of the visual field--a logical anatomical substratum to the fact that the visual world is seen as one continuous whole and not as a mosaic of discrete parts. But although the striate cortex sends a convergent input to the cortex of the posterior bank of the superior temporal sulcus, it also sends a topographically organised input to areas 18 and 191,16. This raises the possibility that there might be two independent pathways between the striate cortex with its precise topographical organisation and the cortex of the posterior bank of the superior temporal sulcus with its apparently eroded topographical organisation. One pathway, already demonstrated1,S,16, is, is a direct one between striate cortex and the superior temporal sulcus. The other one might be an indirect pathway, through areas 18 and 19 and consisting possibly of more than one step at each of which the topographical organisation becomes less sharply defined. In pursuing this work on the

The relationship between cortical activation and perception investigated with invisible stimuli

The aim of this work was to study the relationship between cortical activity and visual perception. To do so, we developed a psychophysical technique that is able to dissociate the visual percept from the visual stimulus and thus distinguish brain activity reflecting the perceptual state from that reflecting other stages of stimulus processing. We used dichoptic color fusion to make identical monocular stimuli of opposite color contrast “disappear” at the binocular level and thus become “invisible” as far as conscious visual perception is concerned. By imaging brain activity in subjects during a discrimination task between face and house stimuli presented in this way, we found that house-specific and face-specific brain areas are always activated in a stimulus-specific way regardless of whether the stimuli are perceived. Absolute levels of cortical activation, however, were lower with invisible stimulation compared with visible stimulation. We conclude that there is no terminal “perceptual” area in the visual brain, but that the brain regions involved in processing a visual stimulus are also involved in its perception, the difference between the two being dictated by a higher level of activity in the specific brain region when the stimulus is perceived.