Blind+Sight

Although considered somewhat controversial, it has been reported that although blind (due to destruction of the primary visual cortex), some individuals are able to indicate the presence or absence of a moving stimulus within the "blind" portion of their visual field and even differentiate between various objects, although not knowing what they are. Using forced choice procedures, although "blind" some patients can localize visual targets and perceive moving objects. In one case it has been reported that a patient who denied visual perception and with a diagnosis of cortical blindness, was able to correctly name objects, colors, famous faces, facial emotions, as well as read various single words with greater than 50% accuracy.

Visual preservation following area 17 lesions has been referred to as "blind sight". Although blind, these patients may avoid obstacles, and correctly retrieve desired objects, and thus appear to have some residual visual functions even though they verbally claim no conscious awareness of the visual stimulus and thus have no verbal awareness that they can see. Although the verbal aspects of consciousness have been disconnected from the visual cortex and claims it cannot see, the patient continues to behave as if visual input is still being received.

It has also been argued that "blind sight" is "useful visual function is preserved only when a critical amount of area 17 is spared. This view is in error, however as "blind sight" has also been demonstrated with evidence of no functional activity in area 17.

Possibly, although cortically blind, patients are able to recognize moving objects, or in some cases, correctly move about in space, due to the preservation of intact subcortical nuclei involved in visual orientation, i.e. via the so called "second (retino-collicular- pulvinar-extrastriate) visual system" which projects to temporal occipital junction, i.e. area V5.

In cases of disconnection and blind sight due to occipital lesions, complex visual input may still reach the temporal lobe, and may therefore be directed to the auditory areas via a secondary route so that associatd "feelings" of seeing something can be communicated, and in some cases, so that objects can be named, although the patient continues to deny visual perception.

Presumably, those who are "cortically" blind but demonstrate "blind sight" coupled, in some cases, with the ability to turn the head and orient, can accomplish these acts apparently because undamaged midbrain, thalamic, and neocortical tissues (e.g. temporal/parietal lobe) involved in visual functioning continue to function normally. Although "cortically blind" the eyes are functional, and visual impressions are transmitted via the optic nerve directly to the midbrain visual colliculus, and via the optic tracts and optic radiations to the lateral geniculate nucleus of the thalamus and other forebrain structures including the visual areas in the temporal and parietal lobe which in turn may transmit visual input to those islands of striate cortex which remain intact, but which are unable to communicate with the language areas of the brain, which, in failing to receive visual signals, claims to have no knowledge of the visual world. Nevertheless, visual processing continues outside of linguistic conscious-awareness, and patients may report that their behavior is guided by "non-visual feelings".

The fact that these and other patients with blind sight may have a feeling of seeing movement, are/or experience a desire to turn the head, and/or correctly reach out and grasp or move around objects, directly implicates the midbrain visual colliculus as this structure is able to detect movement and different gradations of light and shadow, and (via the lower brainstem) can direct head turning, groping and grasping, and even walking.

Humans, however, have evolved neocortex, and the primary visual receiving areas hierarchically analyze these subcortical visual signals which are then transferred to the adjoining visual association areas thereby forming complex visual associations. These complex associations are then transmitted to the language axis which then names and verbally describes what the individual sees. These visual impressions come to be associated with language and thus with linguistic consciousness. However, with massive injuries to the primary visual cortex, the language axis no longer receives visual input, and the patient reports that he or she is blind and cannot see, though, in some cases, they avoid obstacles and can reach for desired objects.

Thus patients with "blind sight" demonstrate at least two disconnected streams of mental activity, one of which utilizes language to deny visual experience other than through the experience of "feelings", and a second non-verbal form of subcortical or isolated neocortical mental activity that is capable of seeing and/or controlling movements of the body in response to certain visual stimuli. Moreover, these multiple modes of conscious-awareness, including those supporting blindsight may wax and wane.