Blindsight


Blindsight, a term coined by Lawrence Weiskrantz in the early 1970s that refers to visual perception in the absence of subjective experience (Blackmore & Troscianko, 2018) is the phenomenon where patients perceive, non-consciously, visual stimuli without being aware of having done so (Revonsuo, 2010). Lesions to the primary visual cortex, V1, as well as complete or partial cortical blindness can lead to blindsight. The visual area in which the patient is blind corresponds to that area in the primary visual cortex, the brain region that distributes the visual information coming from the eye, and the lateral geniculate nucleus of the thalamus (Schmid et al., 2010). And the larger the lesion in the primary visual cortex, on which the visual field is organized as a map, the larger the missing field in the visual perception.

If there is a V1 lesion in the left hemisphere, the entire right perceptual field is gone; bilateral lesions in the primary visual cortex lead to complete cortical blindness (Rahm, 2018). From these findings, the word “blind” in blindsight comes. The word “sight” comes from a paradigm called forced-choice task where patients with V1 lesions were asked to report whether or not they had seen a light flashed in the visual field in which they are cognitively blind; they choose between “light” or “no light” where there is no alternative for uncertainty. In some cases, the patients must even point with their finger to an area where they assume the light was shown, which remarkably enough was surprisingly accurate.

The limits to this paradigm, however, seem to be that the patients only code for fundamental aspects of the stimuli such as spatial location, if the stimulus is present or not, and simple movements.

The explanation seems to be that this more simple information to some extent passes through the lateral geniculate nucleus of the thalamus, which then directly sends ipsilateral projections to the lateral parietal cortex without passing through the primary visual cortex; stimuli of the emotional variety, such as threat and danger, passes through the pulvinar of the thalamus onto the amygdala without passing the primary visual cortex (LeDoux & Pine, 2016). When patients with V1 legions react to this kind of stimuli, the term is affective blindsight (Panksepp, 2017). 

However, in both cases, the patients failed to ascribe to the stimuli various forms of meaning and or identity. An insight that has led Thomas Polger, professor of philosophy at the University of Cincinnati, to conclude that consciousness plays a significant role in the accuracy of human perception, writing that this “gives us no reason to doubt that consciousness is implicated in (rather than merely correlated with) our visual processing” (Polger, 2017, p, 88).

References:

Blackmore, S., & Troscianko, E. (2018). Consciousness: an introduction. 3rd edition. Routledge: Oxon.

LeDoux, J. & Pine, D. (2016). Using Neuroscience to Help Understand Fear and Anxiety: A Two-System Framework. American Journal of Psychiatry, 173(11), pp.1083–1093.

Panksepp, J. (2017). ‘Affective Consciousness’ in Schneider, S., & Velmans, M. (ed.) The Blackwell Companion to Consciousness. 2nd Edition. John Wiley & Sons Ltd: West Sussex, pp. 141-156.

Polger, T. (2017). ‘Rethinking the Evolution of Consciousness’ in Schneider, S., & Velmans, M. (ed.) The Blackwell Companion to Consciousness. 2nd Edition. John Wiley & Sons Ltd: West Sussex, pp. 77-92.

Rahm, C. (2018). Neurovetenskaplig psykiatri. Stockholm, Sverige: Natur & Kultur.

Revonsuo, A. (2010). Consciousness — The Science of Subjectivity. Psychology Press. Abingdon, England: Routledge.

‌Schmid, M.C. et al. (2010). Blindsight depends on the lateral geniculate nucleus. Nature (London), 466(7304), pp.373–377.