1/8. visual perception of motion, luminance and colour in a human hemianope.Human patients rendered cortically blind by lesions to V1 can nevertheless discriminate between visual stimuli presented to their blind fields. Experimental evidence suggests that two response modes are involved. patients are either unaware or aware of the visual stimuli, which they are able to discriminate. However, under both conditions patients insist that they do not see. We investigate the fundamental difference between percepts derived for the normal and affected hemifield in a human hemianope with visual stimuli of which he was aware. The psychophysical experiments we employed required the patient, GY, to make comparisons between stimuli presented in his affected and normal hemifields. The subject discriminated between, and was allowed to match, the stimuli. Our study reveals that the stimulus parameters of colour and motion can be discriminated and matched between the normal and blind hemifields, whereas brightness cannot. We provide evidence for associations between the percepts of colour and motion, but a dissociation between the percepts of brightness, derived from the normal and hemianopic fields. Our results are consistent with the proposal that the perception of different stimulus attributes is expressed in activity of functionally segregated visual areas of the brain. We also believe our results explain the patient's insistence that he does not see stimuli, but can discriminate between them with awareness.- - - - - - - - - - ranking = 1keywords = motion (Clic here for more details about this article) |
2/8. Non-conscious recognition of affect in the absence of striate cortex.functional neuroimaging experiments have shown that recognition of emotional expressions does not depend on awareness of visual stimuli and that unseen fear stimuli can activate the amygdala via a colliculopulvinar pathway. Perception of emotional expressions in the absence of awareness in normal subjects has some similarities with the unconscious recognition of visual stimuli which is well documented in patients with striate cortex lesions (blindsight). Presumably in these patients residual vision engages alternative extra-striate routes such as the superior colliculus and pulvinar. Against this background, we conjectured that a blindsight subject (GY) might recognize facial expressions presented in his blind field. The present study now provides direct evidence for this claim.- - - - - - - - - - ranking = 0.33333333333333keywords = motion (Clic here for more details about this article) |
3/8. Visual extinction for motion.PURPOSE: To alert clinicians to the heretofore undescribed visual behavioral phenomenon of visual extinction limited to motion. methods: Neuro-ophthalmological, neuropsychological and neuroimaging assessment of a 57-year-old man with vague visual complaints. RESULTS: Extinction limited to visual motion perception in the left hemifield was demonstrated. The visual defect was attributed to a lesion involving the right occipito-temporo-parietal region in the presence of a left posterior infarction. CONCLUSION: The importance of clinical examination for detection of this specific higher-order visual defect is stressed. The present observation may be helpful in understanding the effects of attention on visual perception and may have important implications for rehabilitation of patients with visual neglect.- - - - - - - - - - ranking = 1keywords = motion (Clic here for more details about this article) |
4/8. Training-induced cortical representation of a hemianopic hemifield.BACKGROUND: patients with homonymous hemianopia often have some residual sensitivity for visual stimuli in their blind hemifield. Previous imaging studies suggest an important role for extrastriate cortical areas in such residual vision, but results of training to improve vision in patients with hemianopia are conflicting. OBJECTIVE: To show that intensive training with flicker stimulation in the chronic stage of stroke can reorganise visual cortices of an adult patient. methods: A 61-year-old patient with homonymous hemianopia was trained with flicker stimulation, starting 22 months after stroke. Changes in functioning during training were documented with magnetoencephalography, and the cortical organisation after training was examined with functional magnetic resonance imaging (fMRI). RESULTS: Both imaging methods showed that, after training, visual information from both hemifields was processed mainly in the intact hemisphere. The fMRI mapping results showed the representations of both the blind and the normal hemifield in the same set of cortical areas in the intact hemisphere, more specifically in the visual motion-sensitive area V5, in a region around the superior temporal sulcus and in retinotopic visual areas V1 (primary visual cortex), V2, V3 and V3a. CONCLUSIONS: Intensive training of a blind hemifield can induce cortical reorganisation in an adult patient, and this case shows an ipsilateral representation of the trained visual hemifield in several cortical areas, including the primary visual cortex.- - - - - - - - - - ranking = 0.16666666666667keywords = motion (Clic here for more details about this article) |
5/8. Visuospatial and other "right-hemispheric" functions after long recovery periods in left-hemispherectomized subjects.Two subjects who had infantile brain damage and left hemispherectomies at the ages of 17 and 18 yr are assessed after 28 and 16 yr recovery periods. Both have intact language and verbal memory abilities. However, on functions normally mediated predominantly by the right hemisphere, including nonverbal memory, higher cognitive visuospatial skills, and complex extrapersonal orientation ability, they are severely impaired. Simple visuospatial perceptual and orientation abilities, emotional expression, and face recognition, also "right-hemispheric" functions, are normal. It is postulated that the right hemisphere, if isolated in infancy, has the potential to take over most cognitive functions. The pattern of intact and impaired abilities shown by these subjects allows a hierarchy of functionally important abilities to be constructed. The functions mediated by the solitary right hemisphere are those most essential for independent survival, and functions that are not represented are those least required.- - - - - - - - - - ranking = 0.16666666666667keywords = motion (Clic here for more details about this article) |
6/8. The Riddoch syndrome: insights into the neurobiology of conscious vision.We have studied a patient, G.Y., who was rendered hemianopic following a lesion affecting the primary visual cortex (area VI), sustained 31 years ago, with the hope of characterizing his ability to discriminate visual stimuli presented in his blind field, both psychophysically and in terms of the brain activity revealed by imaging methods. Our results show that (i) there is a correlation between G.Y.'s capacity to discriminate stimuli presented in his blind field and his conscious awareness of the same stimuli and (ii) that G.Y.'s performance on some tasks is characterized by a marked variability, both in terms of his awareness for a given level of discrimination and in his discrimination for a given level of awareness. The observations on G.Y., and a comparison of his capacities with those of normal subjects, leads us to propose a simple model of the relationship between visual discrimination and awareness. This supposes that the two independent capacities are very tightly coupled in normal subjects (gnosopsia) and that the effect of a VI lesion is to uncouple them, but only slightly. This uncoupling leads to two symmetrical departures, on the one hand to gnosanopsia (awareness without discrimination) and on the other to agnosopsia (discrimination without awareness). Our functional MRI studies show that V5 is always active when moving stimuli, whether slow or fast, are presented to his blind field and that the activity in V5 co-varies with less intense activity in other cortical areas. The difference in cerebral activity between gnosopsia and agnosopsia is that, in the latter, the activity in V5 is less intense and lower statistical thresholds are required to demonstrate it. Direct comparison of the brain activity during individual 'aware' and 'unaware' trials, corrected for the confounding effects of motion, has also allowed us, for the first time, to titrate conscious awareness against brain activity and show that there is a straightforward relationship between awareness and activity, both in individual cortical areas, in this case area V5, and in the reticular activating system. The imaging evidence, together with the variability in his levels of awareness and discrimination, manifested in his capacity to discriminate consciously on some occasions and unconsciously on others, leads us to conclude that agnosopsia, gnosopsia and gnosanopsia are all manifestations of a single condition which we call the Riddoch syndrome, in deference to the British neurologist who, in 1917, first characterized the major aspect of this disability. We discuss the significance of these results in relation to historical views about the organization of the visual brain.- - - - - - - - - - ranking = 0.16666666666667keywords = motion (Clic here for more details about this article) |
7/8. Blindsight and visual awareness.Some patients with damaged striate cortex have blindsight-the ability to discriminate unseen stimuli in their clinically blind visual field defects when forced-choice procedures are used. Blindsight implies a sharp dissociation between visual performance and visual awareness, but signal detection theory indicates that it might be indistinguishable from the behavior of normal subjects near the lower limit of conscious vision, where the dissociations could arise trivially from using different response criteria during clinical and forced-choice tests. We tested the latter possibility with a hemianopic subject during yes-no and forced-choice detection of static and moving targets. His response criterion differed significantly between yes-no and forced-choice responding, and the difference was sufficient to produce a blindsight-like dissociation with bias-sensitive measures of performance. When measured independently of bias, his sensitivity to static targets was greater in the forced-choice than in the yes-no task (unlike normal control subjects), but his sensitivity to moving targets did not differ. Differences in response criterion could therefore account for dissociations between yes-no and forced-choice detection of motion, but not of static pattern. The results explain why patients with blindsight are apparently more often "aware" of moving stimuli than of static stimuli. However, they also imply that blindsight is unlike normal vision near threshold, and that pattern- and motion-detection in blindsight may depend on different sets of neural mechanisms during yes-no and forced-choice tests.- - - - - - - - - - ranking = 0.33333333333333keywords = motion (Clic here for more details about this article) |
8/8. 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.- - - - - - - - - - ranking = 0.16666666666667keywords = motion (Clic here for more details about this article) |