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R W Kentridge

Publications and source records attributed to R W Kentridge.

17 recordsLinked to original sources
PubMed2004

Spatial attention speeds discrimination without awareness in blindsight.

An intimate relationship is often assumed between visual attention and visual awareness. Using a subject, patient GY, with the neurological condition of "blindsight" we show that although attention may be a necessary precursor to visual awareness it is not a sufficient one. Using a Posner endogenous spatial cueing paradigm we showed that the time our subject needed to discriminate the orientation of a stimulus was reduced if he was cued to the location of the stimulus. This reaction-time advantage was obtained without any decrease in discrimination accuracy and cannot therefore be attributed to speed-error trade-off or differences in bias between cued and uncued locations. As a result of his condition GY was not aware of the stimuli to which processing was attentionally facilitated. Attention cannot, therefore be a sufficient condition for awareness.

Adult↗
PubMed2004

Chromatic edges, surfaces and constancies in cerebral achromatopsia.

We tested achromatopsic observer, MS, on a number of tasks to establish the extent to which he can process chromatic contour. Stimuli, specified in terms of cone-contrast, were presented in a three-choice oddity paradigm. First we show that MS is able to discriminate the magnitude of chromatic and luminance contrast, but performance is inferior to that of normal observers. Moreover, MS can discriminate isoluminant borders of different chromatic composition. These abilities are not the result of unintended luminance differences and are abolished when chromatic borders are masked by sharp luminance change. In simple displays, local cone-contrast signals can make a significant contribution to surface colour appearance in normal observers. In more complex displays, the perception of a surface's colour becomes largely independent of the local contrast to its background, via processes presumed to be similar to the edge integration and anchoring stages of Land's Retinex algorithm. We show that in simple displays the percepts of both MS and normal observers are dominated by local chromatic-contrast. But, although the percepts of normal observers change in line with the predictions of retinex theory in more complex displays, those of MS do not, remaining dominated by local contrast signals. We conclude that MS has lost the ability to perform edge integration and that this loss is closely related to his absence of colour experience.

Adult↗
PubMed2004

Covert processing of visual form in the absence of area LO.

The patient D.F., who suffers from severe visual form agnosia, has been found to have a bilateral lesion of area LO, an area known to be intimately involved in the perception of object shape. Despite her perceptual impairment, however, D.F. retains residual form processing abilities that can provide distal visuomotor control, for example in the configuration of her grasp when reaching to pick up objects of different shapes and sizes. This dissociation has been interpreted as reflecting the sparing of a dedicated system for processing the physical properties of objects solely for purposes of guiding action. Here we test this hypothesis in two studies designed to examine whether or not spared shape processing capacities might be revealed under other kinds of indirect test conditions. First, we exploited the fact that a redundant shape cue will speed search for a coloured stimulus within an array, and vice versa. Unlike our control subjects, D.F. showed no facilitation effect of either kind. Second, we used two Stroop tasks in which single coloured uppercase letters were presented. Our intention was to determine (a) whether naming the colour would be influenced by whether the letter was the initial letter of the correct or incorrect colour name (e.g. 'R' or 'G'); and (b) whether the reverse might be true, that is that D.F.'s guesses at letter identity might be influenced by their colour. We found no evidence for a Stroop effect of the former (standard) kind in D.F., but we did find evidence for reverse-Stroop effects. This result may reflect a partial sparing of ventral stream areas specialised for letter-form processing.

Agnosia↗
PubMed2004

The primacy of chromatic edge processing in normal and cerebrally achromatopsic subjects.

The local chromatic contrast between surfaces in a visual scene plays an important role in theories of color perception. Our studies of cerebral achromatopsia suggest that this contrast signal is computed independently of the more complex processes such as edge integration and anchoring. We report a study in which we attempted to determine whether local-contrast signals also drove behavior in normal subjects. We sought to reduce the role of edge integration and anchoring by using stimuli whose background varied very gradually in color from top to bottom. The local chromatic contrast of patches relative to such backgrounds depends upon the position at which they are presented. It is therefore possible for patches with identical spectral composition to have opposite contrasts. We constructed stimuli in which two of three vertically arranged discs had the same contrast while the third had opposite contrast. The stimuli were also constructed so that the contrast-odd disc and one of the other two had identical spectral composition while the third disc had different composition. We used these stimuli in an attentional task where, after a brief delay, a letter discrimination target was presented in the location of one of the discs. Attention should automatically be attracted to the odd disc in such a display. Normal observers were faster at making the letter discrimination when the target appeared at the contrast-odd as opposed to spectrally odd location. We conclude that local chromatic contrast, but not raw spectral composition, is accessible to normal observers at an appropriate stage in visual processing to drive attention.

Adult↗
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↗
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

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

Fine grain of the neural representation of human spatial vision.

It is widely held that in human spatial vision the visual scene is initially processed through visual filters, each of which is responsive to narrow ranges of image spatial frequencies. The physiological basis of these filters are thought to be cortical neurons with receptive fields of different sizes. The grain of the neural representation of spatial vision is much finer than had been supposed. Using laser interferometry, which effectively bypasses the demodulation of the optics of the eye, we measured discrimination of, and adaptation to, high spatial frequency laser interference fringe patterns. Spatial frequency discrimination was good right up to the visual resolution limit (average Weber fractions of 0.13 at 50 c/deg). Both contrast and spatial frequency matches made after adapting to extremely fine interference fringes strongly suggested that there existed even finer, relatively unadapted, filters (mechanisms with small receptive fields). The smallest cortical receptive fields processing spatial information in human vision are so small that they can possess receptive field centers hardly wider than single cone photoreceptors.

Adaptation, Physiological↗
PubMed1995

Independent contributions of the orienting of attention, fixation offset and bilateral stimulation on human saccadic latencies.

In a series of experiments we examined the effects of the endogenous orienting of visual attention on human saccade latency. Three separate manipulations were performed: the orienting of visual attention, the prior offset of fixation (gap paradigm) and the bilateral presentation of saccade targets. Each of these manipulations was shown to make an independent contribution to saccade latency. In experiments 1 and 2 subjects were instructed to orient their attention covertly to a location by a verbal pre-cue; targets could appear in the attended hemifield (valid) or in the non-attended hemifield (invalid) together with a no-instruction (neutral) condition. Saccades were made under fixation gap and overlap conditions, to either single target or two bilaterally presented targets which appeared at equal and opposite eccentricities in both hemifields. The results showed a large increase (cost) of saccade latency to invalid targets and a small non-significant decrease (benefit) of saccade latency to valid targets. The cost associated with invalid targets replicates the "meridan crossing effect" shown in manual reaction time experiments and is consistent with the hemifield inhibition and premotor models of attentional orienting. The use of a "gap" procedure produced a generalised facilitation of saccade latency, which was not modified by the prior orienting of visual attention. The magnitude of the gap effect was similar for saccades made to attended and non-attended stimulis. This suggests that the gap effect may be due to ocular motor disengagement, or a warning signal effect, rather than to the prior disengagement of visual attention. When two targets were presented simultaneously, one in each hemifield, saccade latency was slowed compared with the single target condition. The magnitude of this slowing was unaffected by the prior orienting of visual attention or by the fixation condition. The slowing was examined in more detail in experiment 3, by presenting targets with brief offset delays. The latency increase was maximal if the two targets were presented simultaneously and decreased if the distractor appeared at short intervals (20-80 ms) before or after the saccade target onset. If the non-attended stimulus was presented at greater intervals (160, 240 ms) before the saccade target, then a facilitation effect was observed. This demonstrates that the onset of a distractor in the non-attended hemifield can have both an inhibitory and a facilitatory effect on a saccade production.

Attention↗
PubMed1993

Evidence for longevity differences between left handed and right handed men: an archival study of cricketers.

STUDY OBJECTIVE: The aim was to examine the relationship between handedness and longevity. DESIGN: This was an archival (retrospective) survey of a cohort of adult men who had played 'first-class cricket'. SETTING: The United Kingdom PARTICIPANTS: The subjects consisted of all of the deceased players included in an encyclopedia of 'first-class cricket' whose bowling hand had been recorded (n = 3165). The study also considered a further 2314 players, born before 1951 but still alive at the time the book was published (1984). MEASUREMENTS AND MAIN RESULTS: Using the bowling hand as an indicator of handedness it was possible to compare the lifespans of 2580 right handed men and 585 left handed men. The average life spans of the two groups differed by 25 months (right = 65.62, left = 63.52), a highly significant difference (p = 0.006). An examination of cause of death (where noted) strongly indicated that the left handed men were more likely to die prematurely in accidents or in warfare. As a consequence, when these unnatural deaths were removed from the sample the longevity difference between the right handers and left handers was considerably reduced. There was no evidence that these results related to any longitudinal change in the proportion of right handers to left handers across the time course of the sample. CONCLUSION: The study found clear evidence that left handedness was associated with a decrease in longevity among a cohort of adult, athletic men. A major factor responsible for this result seemed to be a differential likelihood of accidental death or death during warfare.

Aged↗
PubMed1992

Lesions of the fornix but not the amygdala impair the acquisition of concurrent discriminations by rats.

Rats with lesions in either the fornix, the amygdala, or both were compared with control animals on the acquisition of three different concurrent object discrimination tasks. In the first task the animals received one trial per day on each of six pairs of stimulus objects ('spaced' condition). In the second task the animals received four trials per day on each of six stimulus pairs ('standard' condition), and in the last task the animals received 36 trials on each of two stimulus pairs in just a single day ('massed' condition). Animals with fornical lesions were impaired on all three conditions. In contrast, the amygdala lesions only affected the 'massed' condition and then only when the animals had to select the 'non-preferred' stimulus. Although animals with combined amygdala and fornical lesions were impaired on all three conditions there was no evidence that their deficit was greater than that in the animals with lesions restricted to just the fornix. In view of the evidence that concurrent discrimination learning offers an appropriate test for anterograde amnesia these findings are seen as consistent with the notion that the hippocampus, but not the amygdala, is critically involved in the mnemonic processes disrupted by amnesia.

Amygdala↗
PubMed1991

Amygdaloid lesions and stimulus-reward associations in the rat.

Rats with amygdaloid lesions were trained on learning set tasks designed to tax stimulus-reward associations. Lesions centred in the medial and ventral half of the amygdala had no effect on the acquisition of two object discriminations but did impair successive reversals of the second discrimination. The same lesions had no effect, however, on the acquisition of a spatial win-stay lose-shift task which taxed one-trial place-reward associations. In a second experiment it was found that lesions in the central and basolateral regions of the amygdala disrupted performance of the same spatial win-stay lose-shift task although, as before, acquisition was unaffected. Taken together these findings support a role for the amygdala in stimulus-reward associations and indicate that it may be particularly important when differing values of reward must be distinguished.

Amygdala↗
PubMed1990

Cross-modal matching by amnesic subjects.

The present study examined the performance of two groups of amnesic subjects on a cross-modal identification task. It was found that subjects with Korsakoff's disease did not differ from alcoholic controls on their ability to match the tactile feel of an arc with the visual appearance of the full circle from which the arc was taken. The postencephalitic subjects were, however, impaired on this same task. All groups performed normally on two intramodal control tasks. The postencephalitic group, like the Korsakoff subjects, were also poor at identifying common objects from tactile cues. The results are consistent with the notion that limbic regions in the temporal lobe are important for cross-modal associations.

Adult↗
PubMedDate not supplied

Handedness and longevity: archival study of cricketers.

OBJECTIVE: To test whether handedness is associated with a change in longevity. DESIGN: Archival survey. SETTING: British Isles. SUBJECTS: All first class cricketers born before 1961 whose bowling hand was specified (right, n = 5041; left, n = 1132) in a comprehensive encyclopaedia. MAIN OUTCOME MEASURES: Bowling hand and life span. RESULTS: Regression analysis of the 5960 players born between 1840 and 1960 (3387 dead, 2573 alive) showed no significant relation between mortality and handedness (P = 0.3). Left handedness was, however, associated with an increased likelihood of death from unnatural causes (P = 0.03, log hazard 0.37, 95% confidence interval 0.04 to 0.70). This effect was especially related to deaths during warfare (P = 0.009, log hazard 0.53, 0.13 to 0.92). CONCLUSION: Left handedness is not, in general, associated with an increase in mortality.

Cause of Death↗