Residual Visual Capacities Research Articles

Blindsight revisited

Some human patients with lesions to their primary visual (striate) cortex (V1) demonstrate residual visual capacity, but without acknowledged perceptual awareness. This phenomenon has been termed blindsight. Recent results from work on blindsight patients suggest that it is unlikely to be attributable to intact residual areas (tags) of V1. Previous research has reported that blindsight patients can retain the ability to detect monochromatic light and grating stimuli, and to discriminate orientation and direction of movement in their ‘blind’ fields. These findings have been joined by reports that these patients also are sensitive to, and are able to discriminate, wavelength in the absence of any experience of ‘colour’. This reveals that retinal pathways other than those to the striate cortex are crucially involved in vision. Conditions can be controlled for obtaining either acknowledged awareness or unawareness of discrimination of the direction of a small moving target in blindsight patients. This potentially offers the possibility to determine whether there are structures uniquely involved in visual awareness. Monkeys lacking V1 also clearly demonstrate residual visual capacity, and some evidence exists that they also experience ‘blindsight’.

Motion specific responses from a blind hemifield.

In a previous study we showed that fast moving stimuli activate V5, an area specialized for motion, at very short latencies through a pathway that reaches it without passing through V1. Using the same technique of visual evoked responses, we have tested our conclusions by studying patient GY, whose V1 is damaged but whose V5 is intact. In spite of the contralateral hemi-blindness due to his V1 lesion, GY has a residual visual capacity that allows him to perceive, consciously, fast but not slow moving stimuli presented in his affected hemifield. By stimulating GY's 'blind' hemifield and comparing the responses with those obtained from normal subjects, we were able to study the relative contribution of V1 and V5 to the visual evoked response to motion in normal subjects. We found that GY's early response to fast motion is preserved and correlates with activity elicited in control subjects over area V5, while slow motion, pattern offset, and pattern reversal stimuli failed to elicit responses in GY. The results confirm our previous conclusions: namely, that the early part of the motion evoked response is generated in area V5 and that signals reach this area through a dynamically parallel pathway that bypasses area V1. They go on to demonstrate that neurophysiological activity in the prestriate cortex correlates with the conscious visual perception of motion.

Response selectivity of neurons in area MT of the macaque monkey during reversible inactivation of area V1.

1. Behavioral results in the monkey and clinical studies in human show remarkable residual visual capacities after a lesion of area V1. Earlier work by Rodman et al. demonstrated that visual activity can be recorded in the middle temporal area (MT) of the macaque monkey several weeks after a complete lesion of V1. These authors also tested the effect of a reversible block of area V1 on the visual responses of a small number of neurons in area MT and showed that most of these cells remain visually responsive. From the results of that study, however, it is difficult to assess the contribution of area 17 to the receptive-field selectivity of area MT neurons. To address this question, we have quantitatively measured the effects of a reversible inactivation of area 17 on the direction selectivity of MT neurons. 2. A circular part of the opercular region of area V1 was reversibly inactivated by cooling with a Peltier device. A microelectrode was positioned in the lower layers of V1 to control the total inactivation of that area. Eighty percent of the sites recorded in the retinotopically corresponding region of MT during inactivation of V1 were found to be visually responsive. The importance of the effect was assessed by calculating the blocking index (0 for no effect, 1 for complete inactivation). Approximately one-half of the quantitatively studied neurons gave a blocking index below 0.6, illustrating the strong residual responses recorded in many neurons. 3. Receptive-field properties were examined with multihistograms. It was found that, during inactivation of V1, the preferred direction changed for most neurons but remained close to the preferred direction or to its opposite in the control situation. During inactivation of V1, the average tuning curve of neurons became broader mostly because of strong reductions in the response to directions close to the preferred and nonpreferred. Very little change was observed in the responses for directions at 90 degrees to the optimal. These results are consistent with a model in which direction selectivity is present without an input from V1 but is reinforced by the spatial organization of this excitatory input. 4. Residual responses were found to be highly dependent on the state of anesthesia because they were completely abolished by the addition of 0.4-0.5% halothane to the ventilation gases. Finally, visual responses were recorded in area MT several hours after an acute lesion of area 17.(ABSTRACT TRUNCATED AT 400 WORDS)

Cortical Visual Impairment in Young Children with Multiple Disabilities

Teachers of visually handicapped children are seeing an increased frequency in referrals of young, multiply handicapped children with cortical visual impairment. The use of the residual visual capacity in these children is related to their ability to neurologically process and understand environmental sensory information. The complex interaction of the visual process, central nervous system functioning, and environmental stimuli has major implications for effective educational planning.

The Ferrier Lecture, 1989 - Outlooks for blindsight: explicit methodologies for implicit processes

In primates the retina is connected with different targets in the brain via several parallel pathways, the largest of which is that going to the lateral geniculate nucleus of the thalamus and thence to the striate cortex, the geniculo-striate pathway. When this route is damaged in man, apparent blindness in a corresponding part of the visual field occurs, despite the integrity of the other parallel pathways. In animals, it has been demonstrated by conventional behavioural forced-choice techniques that extrastriate routes can sustain a variety of visual discriminations. Comparable discriminations are also possible in some human subjects with geniculo-striate damage when forced-choice 'guessing' techniques are used. 'Blind-sight' refers to those subjects who state that they are unaware of the visual stimuli, even when performing discriminations at high levels of proficiency. Extensions of this approach are reviewed, especially to spectral sensitivity and movement discrimination. But residual capacities can also be assessed without requiring guessing responses, thereby avoiding issues of differential response criteria and other practical difficulties. Effects of 'unseen' stimuli in the cortically blind field on the visible perception of concurrent stimuli in the intact field can be measured. Also, positive reactions of the autonomic nervous system, such as the galvanic skin response, can be recorded to visual stimuli presented in the blind field. Recent evidence demonstrates that the pupil in normal adult subjects is systematically sensitive to structural and chromatic features of visual stimuli. Pupillometry reveals specific changes and residual capacities in visual-field defects of adult patients with striate cortical damage. Thus non-verbal and sensitive methods are available that permit the comparative study of normal and residual visual capacity in human infants, adults and infra-human animals.

Visual discrimination in the absence of visual cortex.

Normal rats and rats with devascularization lesions ranging from subtotal removals of striate cortex (Area 17) to complete removal of neocortex were trained in a horizontal/vertical stripe discrimination for a liquid reinforcer. Subgroups of animals were identified on the basis of size and location of lesion (with particular reference to striate cortex) as Subtotal, Striate, Posterior and Decorticate. Some animals in all of the lesion groups were able to acquire the discrimination, but there was a direct relationship between lesion size and number of training trials. Those animals which reached criterion on the original discrimination were trained on a second horizontal/vertical discrimination under either transfer or reversal conditions using 'rotated obliques' stimuli. Performance on this second discrimination indicated that animals from all lesion groups had been using visual stimuli based on stripe orientation in the original problem. Members of all lesion groups solved the rotated obliques problem under the transfer condition, though the speed and completeness with which they did so was again inversely related to lesion size. These data show high levels of visual competence in the absence of visual cortex even when stimuli thought to detect form discrimination are used and thus reinforce the view that superior colliculus may be a more significant visual area for the rat than was previously assumed. They also support other observations that animals do not use residual visual capacities without extensive experience and appropriate motivation.

Visual discrimination in hemispherectomized patients

Four hemispherectomized patients and two control subjects (Ss), matched in age and IQ, participated in a pattern (2D) and an object (3D) discrimination task. Ss had to indicate whether pairs of stimuli, presented simultaneously in both hemifields, were the same or different. The control Ss performed well on all tasks. Although the experimental Ss remained at chance level in the 2D task, two of them showed significant above chance performances on both 3D conditions. It was concluded that interfield comparisons are possible and that alternate visual structures (e.g. the superior colliculi), in conjunction with the remaining cerebral hemisphere, are responsible for these residual visual capacities.

Residual Visual Capacities in a Case of Cortical Blindness

A case of cortical blindness resulting from a dense ischemic lesion of both calcarine cortices (as seen on CAT Scan) was studied up till the seventh month after the initial stroke. By using mainly forced-choice procedures, similar to those previously used for testing hemianopic subjects, we were able to demonstrate the reappearance of some visual capacities, even though the patient still behaved as if completely blind in everyday life and the lesion remained as it was first seen. First, an ability to detect moving stimuli reappeared then bright flashes could be detected. At last, the patient could localize flickering spots approximately, by pointing with his hand, despite the fact that he did not really see them. As was hypothezised in cases of unilateral occipital lesions, such residual vision would likely to be subserved by extrageniculostriate pathways.