Blindsight: What Accounts for Blindsight:

Blindsight: What Accounts for Blindsight:

cortical preservation or subcortical routes?

Blindsight is defined as the residual visual ability of patients with damage to primary the primary visual cortex of the brain to react reliably to the emotional valence of stimuli presented to their blind visual fields and whose presence and properties they are unable to report or in short, it is the ability to process visual information without conscious awareness. Although, neuro-anatomical explanation of this phenomenon is still not fully understood.

It has been debated that blindsight can be explained by partial sparing of primary visual cortex, with sufficient preservation of cortical processing for stimuli vs. functional integrity of this subcortical visual pathway. Cortical preservation says that there are "islands" of spared cortex that accounts for the ability of the patient to still "see" despite being unconscious to the surrounding world. but, despite the explanations, cortical preservation theory has been ruled out based on behavioral and neuroimaging findings. This is because only little number of patients has sparing of the cortical tissue. But it was also found out that most of the patients with occipital cortex damage, and this includes those with preservation of cortex, have intact superior colliculi. On the other hand, visual information transmitted through the retinotectal pathway is projected to extra striate visual cortex. This is said to be sufficient enough to drive visually guided behavior without awareness.

These two theories have been debated upon as to how to explain the phenomenon that cortically blind patients can still do forced choice on visual discrimination tasks even though they are unaware of the visual stimulus.

There have been studies that show that the superior colliculus is involved in different forms of blindsight. Rafal et. al. (1990) studied three patients with dense homonymous hemianopia and examined whether extrageniculate vision can explain unconscious processing. The patients made saccadic eye movements or manual button-press responses under monocular conditions to see targets on the ipsilesional side of space. During the study, a distractor was presented in the contralesional, blind hemifield. Patients's saccadic latencies were significantly delayed in comparison to the no distractor trial. This effect was more evident for distractors in the temporal hemifield, which has more projections into the superior colliculus than the nasal hemifield. Thus it was concluded that the retinotectal pathway leading to the superior colliculus was involved with the unconscious processing of distractors.

But other studies failed to replicate the same effect for a bigger set of patients. An example of this is a study done by Walker et. al. (2000), were they suggest that the indirect measure of blindsight may not be as robust and detectable in all patients with damaged occipital cortex with preservation of the superior colliculus. Thus, other explanation to blindsight had been studied.

Involvement of the subcortical pathway have been studied by de Gelder et. al. (2005). Patient GY suffered an occipital lesion at age 7, and subcortical pathway has been mostly documented in this patient, thinking that post-lesion and experience-dependent plasticity has taken place. There have also been animal studies in rats which showed the role of the midbrain structures. In this case, even without the contribution of the primary sensory cortices, there were still analysis of the affective value of auditory and also, visual stimuli among the tested rats. Also, in the subcortical pathway, visual information are said to be transmitted through the retinotectal pathway and then projected to the extrastriate visual cortex. This is sufficient enough to drive visually guided behavior even if the patient is unaware of the visual stimuli. A function involving oculomotor processes of the superior colliculus has been suggested. This was based on the demonstration of the accurate localization with saccadic eye movements. Retinotectal or secondary visual pathway was suggested to mediate some remaining visual functions in patients with cortical blindness, this pathway project through the pulvinar and then into the dorsal stream of the extrastriate cortex. Also, this pathway has been recommended to be involved with the vision of the cortically blind patients. Not only does this pathway generate accurate visually guided saccades to the unseen stimuli, the patients also can accurately point towards the stimuli which were present within their scotoma.