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Biomedical subjects

A Cowey

Publications and source records attributed to A Cowey.

At least 91 records · Page 5Linked to original sources
PubMed1995

There's more to colour than meets the eye.

Patients with cerebral achromatopsia, a perceptual disorder caused by ventromedial occipital brain damage, can be completely unable to arrange colours in chromatic sequence and fail most conventional tests of colour blindness. A possible explanation for cerebral achromatopsia is that the colour-opponent parvocellular (P) channel has been selectively and totally destroyed at the level of visual cortex, leaving vision to be mediated by the broad-band magnocellular (M) channel. The persistence of normal occipital visually evoked potentials, and preserved sensitivity to isoluminant chromatic gratings indicates that if this hypothesis is correct the destruction must occur beyond the striate cortex. We have shown that an achromatopsic subject can detect chromatic borders and construct shape from colour, and that he can even perceive the apparent direction of motion of a phase shifted isoluminant chromatic grating where perceived direction depends on knowing the sign of the colour diffence, i.e., which colour is which in the stripes. This and other evidence suggests that perhaps only one part of the cortical P channel has been destroyed. Does the critical area involved in achromatopsia correspond to cortical area V4 of monkeys, often implicated in processing wavelength? When Visual Area 4 is totally ablated in monkeys they have only a mild colour discrimination impairment and easily solve the colour ordering and colour selection tasks that an achromatopsic patient finds impossible. However, monkeys with ventromedial damage rostral to Area V4 do perform like achromatopsic patients, suggesting that the role of V4 in the perception of colour is still unclear and that the colour area of the human brain does not correspond to area V4.

Animals↗
PubMed1995

Visual perception and phenomenal consciousness.

In the (re-)animated debate on consciousness we focus on three questions: Who has consciousness? What is its neuronal basis? What is its function? Regarding the first, we suggest that consciousness is exclusive to living organisms able to distinguish self from non-self. It may be restricted further to organisms who possess a repertoire of overt and covert behaviour which can be voluntarily modified and suppressed. This requires an intermediary neuronal net mediating between sensory input and behavioural output. What are the properties of this net which distinguish unalloyed information processing per se from conscious representation? To tackle this second question, we use the visual system and the functional losses that result from lesions at its different levels, and differentiate a reflexive, a phenomenal, and a consciously accessible stage of visual processing. We suggest that the latter two represent two distinct aspects of consciousness. Blindsight, a neurological example of visual processing in the absence of phenomenal vision, could help to elucidate the neuronal basis of phenomenality, and the special role of striate cortex. Like the patients, our monkeys with unilateral striate cortical removal show evidence not just of residual visual processing, but of the same absence of phenomenal vision, opening routes to further exploring the details of its neuronal implementation. The second aspect, conscious access to presently or previously processed information, is likely to require higher cortical structures, and may depend on the stage of phenomenal representations. In patients with blindsight, both aspects are lost, and it is conceivable that a loss of phenomenality generally causes a loss of conscious accessibility. One important function of phenomenal representations, our third question, would then be to allow conscious retrieval and manipulation of currently processed or formerly stored information, enabling organisms to consciously think and plan.

Animals↗
PubMed1995

Nasal and temporal retinal ganglion cells projecting to the midbrain: implications for "blindsight".

We placed pellets of horseradish peroxidase in the superior colliculus of four macaque monkeys and retrogradely labelled the retinal ganglion cells of both eyes. The ratio of labelled cells in the contralateral nasal retina and the ipsilateral temporal retina was no different from the ratio found after implants in the optic nerve, which label the entire afferent pathway. Our finding therefore invalidates the proposal that prominent differences in the properties of "blindsight" in monocular nasal and temporal visual fields arise from differences in the projection from the nasal and temporal retina to the midbrain. We also measured the size of the soma and dendritic field of the labelled ganglion cells (mostly gamma cells) and compared them with those of alpha and beta cells that project to the dorsal lateral geniculate nucleus. Soma size was very close to that of beta cells at all eccentricities but was much smaller than that of alpha cells. Dendritic field size was significantly larger than that of beta cells but was smaller than that of alpha cells. The number of primary dendrites was counted for cells labelled from the midbrain and in samples of alpha and beta cells labelled from the optic nerve. At eccentricities of 3-7 mm there was a consistent and prominent difference between beta and gamma cells. The results show that at intermediate eccentricities even ganglion cells whose distal dendrites are too poorly labelled to reveal their morphological class can never the less be categorized as alpha, beta or gamma by using a combination of soma size and number of primary dendrites. This is particularly useful when attempting to classify retinal ganglion cells following microinjections into selected target nuclei of optic axons.

Animals↗
PubMed1994

Visual form discrimination from luminance or disparity cues: functional anatomy by PET.

With the purpose of elucidating the functional fields involved in the discrimination of visual form based either on luminance or binocular disparity cues, we used PET to measure changes in regional cerebral blood flow (rCBF) in ten volunteers while they performed visual discrimination tasks. The averaged standardized subtraction images (delta rCBF) were analysed for statistically significant changes between the form tasks and their reference tasks. Twenty cortical fields in the visual association areas and the prefrontal cortex were engaged by the discrimination of visual form based upon disparity cues, whereas only four fields showed increased activity during the discrimination of visual form created by luminance cues. The only functional field activated in both conditions was in the left fusiform gyrus. The present findings extend our earlier observations, namely that disparate functional networks of activated fields in the human brain can perform the discrimination of visual form perceptually defined by different visual cues.

Adult↗
PubMed1994

Visual form discrimination from color or motion cues: functional anatomy by positron emission tomography.

To explore the extent to which various cortical functional pathways are involved in processing and analyzing different types of information that yield the same perceptual entity, we mapped anatomical structures in the human brain participating in the discrimination of visual forms mediated either by motion or color cues. Changes in regional cerebral blood flow were measured in 10 young male volunteers with positron emission tomography and with [15O]butanol. During the measurements, the subjects performed four visual discrimination tasks (form-from-motion, motion alone, form-from-color, and color alone discrimination). The individual regional cerebral blood flow images were standardized in shape and size with the help of a computerized brain atlas. Subtraction images were determined and averaged across data from all subjects. The resulting images were analyzed for statistically significant changes between specific and reference tasks. The discrimination of form by means of motion cues activated functional fields bilaterally in the inferior and lateral occipital gyri, in the lingual, anterior cingulate, middle frontal and orbitofrontal gyri, and in the left fusiform and right inferior temporal gyri. Form discrimination by color cues resulted in activation bilaterally in the inferior temporal, lateral occipital, and orbitofrontal gyri, the left precuneus and intraparietal sulcus, and the right precentral gyrus. The regions engaged in the two kinds of form discrimination did not overlap, demonstrating that differences in visual forms mediated by color or motion cues are processed and analyzed by disparate networks of functional fields in human cerebral cortex.

Adult↗
PubMed1994

How lateralised is visuospatial neglect?

Seventeen patients with left visuospatial neglect caused by cerebral infarction undertook the six subtests of the Behavioural Inattention Test (BIT) as soon as possible following their neurological event. The group mean results on line, letter and star cancellation tasks revealed a linear decrement in performance from right to left on the displays. However, subsequent analysis of patients' cancellations revealed that this finding was not representative of individual performance. Many patients in fact made both marked left- and right-sided omissions on the three cancellation tasks. This was not merely a transitory phenomenon because it remained present at weekly follow-up intervals. Patients' performance on a laterally extended version of the star cancellation task was also investigated. These findings strongly suggest that patients with visuospatial neglect can initially present with a diverse range of performance deficit. Many patients, in particular those with the lowest BIT scores, showed a consistent pattern in the results on letter and star cancellation, providing evidence that their inattention consistently included a considerable section of right-sided (including the extreme right of displays) as well as left-sided space.

Aged↗
PubMed1994

Left visuo-spatial neglect can be worse in far than in near space.

We tested five patients with marked left-sided visuo-spatial neglect and two control subjects on a test of line bisection. A series of horizontal lines was presented to each subject, who had to indicate the centre with a projection light-pen. All five patients misplaced the centre to the right, in accordance with their left-sided neglect. However, in all five the angular displacement was greater for lines well beyond reach, than for lines of identical angular size within reaching distance. This result, precisely because it is opposite to that of a previous report, supports the conclusion that there are separate dissociated neural systems concerned with the perception of, and response to, stimuli in near and far space.

Aged↗
PubMed1994

Retinal ganglion cells labelled from the pulvinar nucleus in macaque monkeys.

In order to study the distribution and morphological classes of retinal ganglion cells that can be retrogradely labelled from the pulvinar nucleus, we made two iontophoretic injections of horseradish peroxidase into the pulvinar in each hemisphere of five macaque monkeys. The retrogradely labelled ganglion cells projecting to or through the pulvinar nucleus were examined in retinal whole-mounts. They comprise all three major ganglion cell classes. Primate gamma cells formed the great majority of classifiable cells and, like the primate alpha cells that were found in much smaller numbers, they were already known to send axons to the superior colliculus and to the pretectal complex. In contrast, the primate beta cells were hitherto thought to project solely to the dorsal lateral geniculate nucleus. This primate beta cell projection to an extrageniculate target could account in part for the substantial number of primate beta cells that escape transneuronal retrograde retinal degeneration following striate cortical ablation, and might contribute to the residual visual sensitivity that survives destruction of striate cortex and the degeneration of the lateral geniculate nucleus.

Animals↗
PubMed1994

On the role of parvocellular (P) and magnocellular (M) pathways in cerebral achromatopsia.

We assessed the ability of an achromatopsic patient to detect and discriminate colour and form concealed in a static or dynamic checkerboard display where the luminance differences among adjacent squares were randomly assigned. There were no conditions under which he could discriminate two very different saturated colours from each other. Nevertheless, he could discriminate chromatic from luminance boundaries in static displays when the colour defining the boundary was saturated and the achromatic boundaries all had similar luminance contrast, i.e. varied over a narrow range. However, he could not readily detect chromatic boundaries from among many achromatic boundaries that differed widely in luminance contrast. In addition, he was able to detect chromatic boundaries even when they were concealed by dynamic random luminance masking. His ability to pick out chromatic borders was abolished when desaturated colours were used. However, he was singularly proficient at detecting coloured forms in static or dynamic displays even when the saturation of the colours of which the form was composed were such that they were rendered invisible when concealed as a single square in a checkerboard. This implies that signals about chroma are still available in extracting shape. The patient performed flawlessly when asked to indicate the direction of motion of a horizontal red/green isoluminant grating which was phase shifted by 90 degrees in either direction, demonstrating unequivocally that he has access to the sign of colours that he nevertheless does not perceive.

Brain Diseases↗
PubMed1993

Preferential representation of the fovea in the primary visual cortex.

The retinal fovea, which corresponds to the central degree or so of vision, is spatially over-represented in the visual cortex. It is about 0.01% of retina area, but at least 8% of the striate cortex. Does this simply reflect an equivalently uneven distribution of ganglion cells in the retina, or is the cortical representation of the fovea preferentially expanded? The answer hinges on the resolution of long-standing discrepancies between the retinal and cortical magnification factors. We approached the problem in a different way, using a retrograde transneuronal tracer from cortex to retina to relate directly the number of ganglion cells projecting to marked areas of striate cortex. We report here that ganglion cells near the fovea were allocated 3.3 to 5.9 times more cortical tissue than more peripheral ones, and conclude that the cortical representation of the most central retina is much greater than expected from the density of its ganglion cells.

Animals↗
PubMed1993

Visual discrimination impairments following lesions of the superior temporal sulcus are not specific for facial stimuli.

Six rhesus monkeys took part in an experiment on visual learning. In three of the monkeys the part of the superior temporal sulcus in which many of the cells respond selectively to some aspect of faces was removed, while the remaining three animals served as unoperated controls. In Experiment 1 they learned a series of two-choice visual discriminations between patterns. The animals with lesions of the superior temporal sulcus were markedly impaired. The discriminations were of two types: in the first, the discriminanda differed in shape (e.g. Y and 3), while in the second they differed only in their orientation (e.g. ). Unlike animals with lesions to the neighbouring inferior temporal cortex who are impaired on shape but not orientation discriminations, animals with lesions of the superior temporal sulcus were equally impaired on both types of discrimination. In Experiment 2 the same six animals learned a series of discriminations between discriminanda which consisted of photographs of pairs of eyes. Each discrimination was between a set of eyes which looked directly at the viewer and a set in which the gaze was laterally averted to varying degrees. Again, animals with lesions of the superior temporal sulcus showed a marked impairment. We conclude that this impairment may be a general impairment in two-choice visual discrimination learning, rather than a selective impairment in discrimination of eye gaze. This result warns against a simple interpretation of the function of this area as a "face area", concerned only, or chiefly, with the perception and significance of parts of the body, notably faces, and their movements.

Animals↗
PubMed1992

The role of the 'face-cell' area in the discrimination and recognition of faces by monkeys.

Cortical neurons that are selectively sensitive to faces, parts of faces and particular facial expressions are concentrated in the banks and floor of the superior temporal sulcus in macaque monkeys. Their existence has prompted suggestions that it is damage to such a region in the human brain that leads to prosopagnosia: the inability to recognize faces or to discriminate between faces. This was tested by removing the face-cell area in a group of monkeys. The animals learned to discriminate between pictures of faces or inanimate objects, to select the odd face from a group, to inspect a face then select the matching face from a pair of faces after a variable delay, to discriminate between novel and familiar faces, and to identify specific faces. Removing the face-cell area produced no or little impairment which in the latter case was not specific for faces. In contrast, several prosopagnosic patients were impaired at several of these tasks. The animals were less able than before to discern the angle of regard in pictures of faces, suggesting that this area of the brain may be concerned with the perception of facial expression and bearing, which are important social signals in primates.

Animals↗
PubMed1992

Defensive responses to looming visual stimuli in monkeys with unilateral striate cortex ablation.

A number of residual visual functions including detection, localization and discrimination of visual stimuli, have been demonstrated in the "blind" fields of monkeys and human patients following damage to striate cortex. We report here that avoidance movements of the head can also be elicited from monkeys with unilateral striate cortex ablations when a "looming" stimulus is presented within the hemianopic and presumably "blind" field. The possible role of the retinofugal projection to superior colliculus in the mediation of these defensive head movements is discussed.

Animals↗