Biomedical subjects

C A Heywood

Publications and source records attributed to C A Heywood.

At least 19 recordsLinked to original sources

PubMed2001

The regional cortical basis of achromatopsia: a study on macaque monkeys and an achromatopsic patient.

Previous experiments have revealed total loss of colour vision following removal of all inferior temporal cortex, a condition akin to complete cerebral achromatopsia in humans. Whether less extensive ablation genuinely impairs colour perception without abolishing it or retards learning involving coloured stimuli is contested. We therefore tested macaque monkeys, with total removal of temporal areas TEO and TE but sparing rostral and perirhinal temporal cortex and the upper bank of the superior temporal sulcus. Compared with three monkeys with lateral parietal ablations, the monkeys with TEO/TE lesions were impaired at learning and retention of simultaneous two-choice colour discriminations and with a nine-choice oddity discrimination whether the coloured target was embedded among grey distracters of the same luminance or among isoluminant coloured distracters. However, their performance was superior to that of an achromatopsic human subject and to that previously measured in monkeys with much larger temporal lobe ablation. They were only mildly impaired at nine-choice oddity discrimination for grey stimuli where the grey target was brighter than the grey distracters. The impairment could be exacerbated or alleviated by altering the colour of the background of the displays and by static and dynamic luminance masking of the entire display in a manner that indicates that the colour deficit reflects a change in perception rather than a disorder of learning and memory. It resembles central dyschromatopsia in human subjects but falls short of achromatopsia.

Animals↗

PubMed2000

Metacognition and awareness.

It is tempting to assume that metacognitive processes necessarily evoke awareness. We review a number of experiments in which cognitive schema have been shown to develop without awareness. Implicit learning of a novel schema may not involve metacognitive regulation per se. Substitution of one automatic process by another as a result of the inadequacy of the former as circumstances change does, however, clearly involve metacognitive and executive processes of error correction and schema selection. We describe a recently published study in which we serendipitously discovered that a blindsight subject could change the schema with which he processed cue information in orienting spatial attention task without reporting any awareness of this change, or of the cues and targets which respectively directed and were the object his attention.

Attention↗

PubMed1999

Attention without awareness in blindsight.

The act of attending has frequently been equated with visual awareness. We examined this relationship in 'blindsight'--a condition in which the latter is absent or diminished as a result of damage to the primary visual cortex. Spatially selective visual attention is demonstrated when information that stimuli are likely to appear at a specific location enhances the speed or accuracy of detection of stimuli subsequently presented at that location. In a blindsight subject, we showed that attention can confer an advantage in processing stimuli presented at an attended location, without those stimuli entering consciousness. Attention could be directed both by symbolic cues in the subject's spared field of vision or cues presented in his blind field. Cues in his blind field were even effective in directing his attention to a second location remote from that at which the cue was presented. These indirect cues were effective whether or not they themselves elicited non-visual awareness. We concluded that the spatial selection of information by an attentional mechanism and its entry into conscious experience cannot be one and the same process.

Accidents, Traffic↗

PubMed1999

Effects of temporal cueing on residual visual discrimination in blindsight.

We tested the ability of a blindsight patient, GY, to identify in which of two locations a target was presented in a spatial two-alternative forced choice paradigm (spatial 2AFC). On each trial the subject was asked to make a second manual response indicating whether he had had any awareness of an event occurring during the trial. A cue, presented at the fixation location, could signal the 0.4 s period over which the target appeared within the 10 s duration of each trial. Targets of three contrasts, 93, 43 and 22% were used. We found that GY's ability to discriminate the location of targets in his blind field remained significantly above chance, with and without cueing, for each contrast. Cueing, did, however, significantly improve his performance for low contrast targets. When he performed a similar task with near threshold contrast targets in his spared visual field his discrimination was at chance unless the presentation of targets was cued, despite his reporting more awareness for these stimuli than he did for low-contrast stimuli in his blind field. These results are compared with those previously reported in monkeys who received lesions to their visual cortices as infants or adults. We conclude that (1) GY's blindsight is qualitatively different from near-threshold normal vision. (2) In common with infant-lesioned monkeys his blindsight remains even in the absence of temporal cues. (3) Residual vision is subject to modulation by attentional processes, or arousal, associated with temporal cueing.

Adult↗

PubMed1999

Saccade selection in visual search: evidence for spatial frequency specific between-item interactions.

We present two experiments in which subjects were required to make a saccade to a target amongst distractors. Targets were oriented Gabor patches. Analysis of errors, when subjects fail to make a saccade to the target, showed two interesting features. First, most error saccades were directed towards a distractor and not to the blank space between distractors. This suggests that although the location of the target may not be encoded correctly, the locations of the items in the display are encoded. Second, when the display items were all of the same spatial frequency, a long-range effect occurred whereby the likelihood of an error saccade in a specific direction decreased systematically as the distance from the target increases. This systematic influence of the target location extended over practically the whole display. The long-range effect appeared whenever all display items had the same spatial frequency and showed little dependence on the spatial frequency of the display items. However, when the items had different spatial frequencies the long-range effects were absent.

Humans↗

PubMed1998

Visual form discrimination from texture cues: a PET study.

With the purpose of localising the cerebral cortical areas participating in the discrimination of visual form generated exclusively by texture cues, we measured changes in regional cerebral blood flow (rCBF) with positron emissions tomography (PET) and 15O-butanol as the tracer. The subjects performed two odd-one-out discrimination tasks: a form-from-texture discrimination task (in which a visual form was defined by differences in texture) and its reference task, the discrimination of texture. During task performance, activated fields were present bilaterally in the primary visual cortex and its immediate extrastriate cortex, the right lateral occipital gyrus, bilaterally in the fusiform and superior temporal gyri and posterior parts of the superior parietal lobules, along the medial bank of the right intraparietal sulcus, and in the right supramarginal gyrus. Other fields were found in the cingulate and prefrontal cortex. The findings demonstrate that the discrimination of visual form as defined by texture engages cortical fields that are widely distributed ion the human brain. In the visual cortex, the activated fields are present in both the occipito-temporal and occipito-parietal visual areas. These results suggest that the perception and discrimination of forms in the visual system requires the joint-activation of neuronal populations in the visual cortex.

Adult↗

PubMed1998

Cortical color blindness is not "blindsight for color".

Cortical color blindness, or cerebral achromatopsia, has been likened by some authors to "blindsight" for color or an instance of "covert" processing of color. Recently, it has been shown that, although such patients are unable to identify or discriminate hue differences, they nevertheless show a striking ability to process wavelength differences, which can result in preserved sensitivity to chromatic contrast and motion in equiluminant displays. Moreover, visually evoked cortical potentials can still be elicited in response to chromatic stimuli. We suggest that these demonstrations reveal intact residual processes rather than the operation of covert processes, where proficient performance is accompanied by a denial of phenomenal awareness. We sought evidence for such covert processes by conducting appropriate tests on achromatopsic subject M.S. An "indirect" test entailing measurement of reaction times for letter identification failed to reveal covert color processes. In contrast, in a forced choice oddity task for color, M.S. was unable to verbally indicate the position of the different color, but was surprisingly adept at making an appropriate eye movement to its location. This "direct" test thus revealed the possible covert use of chromatic differences.

Blindness, Cortical↗

PubMed1998

The effect of lesions to cortical areas V4 or AIT on pupillary responses to chromatic and achromatic stimuli in monkeys.

We measured the pupillary response to achromatic and chromatic grating stimuli in left and right visual hemifields of two rhesus monkeys, who were trained to fixate the centre of a screen. After removing the rostral inferior temporal cortex of one hemisphere, the response to chromatically modulated gratings in the contralateral hemifield was abolished, whereas the response to the luminance modulated grating was unaffected. In one of the monkeys, in which area V4 of the other hemisphere was also removed, there was no effect on the pupillary response to either kind of grating presented in the hemifield contralateral to the V4 lesion. The results show that the cortical contribution to the response of the pupil to purely chromatic changes is mediated by rostral temporal cortex, not by area V4.

Animals↗

PubMed1998

Form and motion from colour in cerebral achromatopsia.

Patients with cerebral achromatopsia, resulting from damage to ventromedial occipital cortex, cannot chromatically order, or discriminate, hue. Nevertheless, their chromatic contrast sensitivity can be indistinguishable from that of normal observers. A possible contributor to the detectability of chromatic gratings is the subadditive nature of certain colour combination such that mixtures of, for example, red and green (yielding yellow) appear dimmer than expected from the simple addition of luminances. This subadditivity is believed to reflect colour-opponent interactions between the outputs of long- and medium-wavelength cones. We performed a first-order compensation for such subadditivity in chromatic gratings and demonstrated that their detection was still not abolished in an achromatopsic patient. In addition, we used a two-alternative forced-choice procedure with an achromatopsic patient, who was required to judge the apparent relative velocity of two drifting gratings with different degrees of compensation for subadditivity. It is well known that isoluminant gratings, constructed by adding a red and green sinusoidal grating of identical peak luminances in antiphase, appear to drift substantially slower than an achromatic grating with the same velocity. Adding 2f luminance compensation to an isoluminant grating of spatial frequency f, resulted in an identical minimum of perceived velocity at a compensation contrast of 5% in both achromatopsics and normal observers. Furthermore, while compensation for subadditivity did not substantially compromise grating detection at low contrasts, such correction severely affected motion detection. Saccadic eye movement accuracy and latency were also measured to uncompensated chromatic, compensated chromatic and achromatic targets. We conclude first that subadditivity, resulting from colour-opponent P-channel processes, influences motion judgements. The ability to extract motion from chromatic differences alone is little, if at all, different in achromatopsic and normal vision. Second, the paradoxical detection of sinusoidally modulated chromatic gratings in achromatopsic patients is not merely a result of subadditivity. Third, saccadic latency, but not accuracy, to chromatic targets is affected by luminance compensation. Finally, and more generally, wavelength processing continues to contribute to several aspects of visual processing even when colour is not perceived.

Adult↗

PubMed1996

Contrast sensitivity in one-eyed subjects.

The effects of early monocular form deprivation on the developing mammalian visual system, and the anatomical and physiological consequences of early monocular enucleation, suggest that the remaining eye of human subjects who had the other eye removed early during development might be capable of supernormal performance. To test this inference, the achromatic contrast sensitivity of the remaining eye of subjects who had the other eye removed at different ages after birth was compared with that of normal subjects tested under monocular and binocular conditions. The results show that all subjects who had an eye removed during early development had a higher contrast sensitivity than the better eye of control subjects. Furthermore, the earlier in development that the eye was removed, the lower the spatial frequency at which contrast sensitivity is enhanced compared with measurements made in the better eye of control subjects, and the larger the range of spatial frequencies over which contrast sensitivity is supernormal.

Adolescent↗

PubMed1996

Behavioural and electrophysiological chromatic and achromatic contrast sensitivity in an achromatopsic patient.

OBJECTIVES: In cases of incomplete achromatopsia it is unclear whether residual visual function is mediated by intact striate cortex or results from incomplete lesions to extrastriate cortical visual areas. A patient with complete cerebral achromatopsia was tested to establish the nature of his residual vision and to determine the integrity of striate cortex function. METHODS: Behavioural contrast sensitivity, using the method of adjustment, and averaged visually evoked cortical potentials were measured to sinusoidally modulated chromatic and achromatic gratings in an achromatopsic patient and a normal observer. Eye movements were measured in the patient using a Skalar infrared monitoring system. RESULTS: The patient's chromatic contrast sensitivity was normal, indicating that despite his dense colour blindness his occipital cortex still processed information about spatial variations in hue. His sensitivity to achromatic gratings was depressed particularly at high spatial frequencies, possibly because of his jerk nystagmus. These behavioural results were reinforced by the nature of visually evoked responses to chromatic and achromatic gratings, in which total colour blindness coexisted with an almost normal cortical potential to isoluminant chromatic gratings. CONCLUSIONS: The results show that information about chromatic contrast is present in some cortical areas, and coded in a colour-opponent fashion, in the absence of any perceptual experience of colour.

Adult↗

PubMed1995

Cerebral achromatopsia in monkeys.

In human cerebral achromatopsia, extrastriate cortical damage produces a severe or complete loss of colour vision, with relative sparing of non-chromatic vision. The critical lesion appears to be in a medial occipito-temporal area, occupying the lingual and caudal fusiform gyri; positron emission tomography has shown that this cortical region is one of several activated in normal human observers during colour vision tasks. Attempts to find an analogous 'colour centre' in the cortex of monkeys have not been successful. In particular, ablation of cortical area V4, sometimes thought on physiological grounds to be more involved in wavelength and colour coding than any other visual cortical area, produces only mild impairments in colour discrimination. In the present study we tested the colour vision of monkeys after cortical ablations that mainly or entirely spared area V4. One group of monkeys (group AT) received ablations in the temporal lobe anterior to area V4, and a second group (group MOT) received ablations in a medial occipito-temporal area roughly corresponding in cranial location to the lesion that produces human cerebral achromatopsia. The animals in group MOT showed no impairment of their colour vision. Group AT, in contrast, had a severe impairment in chromatic vision, with a relative sparing of non-chromatic vision. Their behaviour was indistinguishable from that of a human patient with total cerebral achromatopsia who had been tested on the same tasks. These results show that area V4 in macaque monkeys is not analogous, and probably not homologous, to the human colour centre. Instead, they suggest that the area of the monkey's brain corresponding to the colour area in the human brain is in the temporal cortex, anterior to area V4.

Animals↗

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

Ettlinger revisited: the relation between agnosia and sensory impairment.

The concept of agnosia as a higher order functional impairment, which can occur in the absence of low level visual perceptual deficits, continues to provoke debate. This controversy is complicated by the fact that, on close examination, agnosic patients do tend to have some perceptual difficulties. Thus the issue centres around the question as to whether these deficits play a causal part in the aetiology of agnosia or whether they are functionally independent, with both impairments resulting from the substantial cerebral lesions involved in agnosia. In 1956, Ettlinger published a study in which he compared the performance of patients with visual recognition deficits and patients with posterior brain lesions whose recognition abilities were intact. He argued that visual perceptual problems could not explain the recognition deficit in agnosia as he saw far worse perceptual impairments in patients who did not experience any problems in visual recognition. Although the logic of Ettlinger's argument is not disputed, some criticisms have arisen concerning the study, such as the fact that his experimental group did not include a truly object agnosic patient. In addition, Ettlinger's visual-sensory assessment can no longer be considered comprehensive in the light of present day knowledge of the cerebral visual apparatus. This study therefore investigated three (prosop)agnosic patients and five patients with unilateral brain lesions without recognition deficits on an extensive battery of visual sensory tests. The results support Ettlinger's original claim that (in some cases) agnosia cannot be explained as resulting from lower level visual impairments.

Adult↗

PubMed1995

Perception and memory: action and interaction.

Receptive field properties of neurons in, and the effects of cortical ablation of, inferior regions of the macaque temporal lobe reveal their role in the visual representation of objects. However, changes in receptive field properties, as a result of visual experience with specific objects or patterns, suggest that cells encode both sensory and mnemonic features of a visual stimulus. Thus, in addition to selectivity for the visual qualities of a stimulus, response properties of cells indicate their involvement with mechanisms of visual associative and visual recognition memory. Recently, ablation studies have extended the putative role of these neurons in memory. These results suggest that the anterior inferotemporal cortex not only plays a role in recognition memory by signaling novelty or familiarity and in coding for visual associative memory but also modifies responses of neurons to the stimuli themselves, playing a part in the visual learning that underlies sensory classification of complex visual discriminanda.

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

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↗