Is blindsight an effect of scattered light, spared cortex, and near-threshold vision?

Abstract

Blindsight is the term commonly used to describe visually guided behaviour elicited by a stimulus falling within the scotoma (blind area) caused by a lesion of the striate cortex. Such “vision” is normally held to be unconscious and to be mediated by subcortical pathways involving the superior colliculus. Blindsight is of considerable theoretical importance since it suggests that destriate man is more like destriate monkey than had been previously believed and also because it supports the classical notion of two visual systems. It is also of potential clinical importance, since it has been claimed recently that systematic practice in blindsight can lead to the recovery of normal visual function in patients with cortical lesions. From a review of the literature it is concluded that all of the phenomena of blindsight can be attributed either to light scatter into unimpaired parts of the visual field or to residual vision resulting from spared striate cortex. The possible contribution f other factors is also considered. It is concluded that blindsight studies have generally failed to control for such nonblindsight interpretations partly because of poor methodology and partly because of difficulties in defining the term “blindsight.”Experiments were carried out to investigate the extent to which subjects can exhibit performance similar to blindsight when they are using scattered light as a cue. This was done both with hemianopic subjects (by manipulating the amount of scattered and direct light coming from a stimulus) and with normal subjects (by presenting targets within their blind spots). Good blindsight performance was observed when only scattered light was available as a cue to the subjects. It is therefore concluded that an adequate case for blindsight has not been made. It is probably impossible to demonstrate the existence of blindsight on purely behavioural grounds. What is required is the establishment of relationships between visual function and independent anatomical evidence.

Failure to handle more than one internal representation in visual detection tasks

“…They will then ignore the weaker internal responses generated in the affected areas. Criterion shifts under these neurological conditions have been invoked (12,13) and actually observed (14).…”

Introspective physicalism as an approach to the science of consciousness

“…He shows that belief in the hypothesis of perception without awareness was widespread prior to Ericksen's (1960) influential critique, after which confidence in the hypothesis was slow to recover. He did not foresee that, shortly after the publication of his second book on subliminal perception (Dixon, 1981), similar concerns would again throw the field into controversy (Campion, Latto, & Smith, 1983;Holender, 1986;Merikle, 1984;Schacter, 1989;Shanks & St. John, 1994). A third wave of confidence has arrived.…”

Neural Substrate of Spatial Memory in the Superior Colliculus after Damage to the Primary Visual Cortex

“…Thus, we confirmed that the visual field corresponding to the lesion anatomically estimated by the postoperative MRI was functionally affected. In our previous study, we also examined the possibility that the monkeys used light scattering of the visual stimuli presented in the affected visual field (Campion et al, 1983;Gross et al, 2004). We examined whether they could respond to a 1.52°v…”