What is blindsight, and how can people respond to what they can't see?

Blindsight is the capacity of some people with damage to the striate cortex, also called the primary visual cortex or area V1, to detect and even locate visual stimuli shown in the blind part of their visual field. Discrimination of movement, flicker, wavelength and orientation can also be present. These abilities are not accompanied by conscious awareness, so the person says they see nothing. [1]

The term was coined by Lawrence Weiskrantz and colleagues in 1974. The word blind reflects the patients' claim not to see the stimuli, while sight refers to their remaining ability to localize, detect and discriminate them. [2]

Direct methods ask patients to guess whether, where or which of a few stimuli appeared in the blind field. Patients may be prompted by a sound or a visible cue and then be pushed to reach, move their eyes or press different buttons. Indirect methods have patients respond to targets in the sighted field while an extra stimulus appears in the blind field, to see whether it changes their responses, often their reaction times. Skin conductance and pupil responses can also be affected by unseen stimuli. Both main approaches need many trials, and they usually, though not always, give similar results. [2]

Researchers distinguish two types. In type 1, the patient has no awareness of the stimulus yet predicts aspects of it at a better-than-chance rate. In type 2, the patient reports some awareness of a change, such as movement, but does not consciously perceive it as seeing. A more recently described and still debated category, blindsense, involves a non-visual feeling that a stimulus is present without accurate performance in forced-choice tasks. [3] [4]

Blindsight is not simply normal vision with the experience removed. The Current Biology review notes that patients may localize targets by hand very well yet fail completely to judge the direction of motion in random dot patterns, while detecting motion in single targets or coarse gratings almost perfectly. Unlike normal vision at threshold, making a stimulus stronger does not make it visible, unless it spills light into the sighted field. [2]

The pathway from the lateral geniculate nucleus to V1 does not work in people with blindsight. The APA dictionary says the cause is debated: one proposal is that the abilities rely on a route through the superior colliculus, and another is that they reflect residual vision from surviving striate cortex. [1]

A separate dispute concerns what blindsight shows about consciousness. One model holds that it is perception without consciousness. The other holds that it is degraded conscious vision. These mirror a broader question of whether unconscious cognition truly exists or is only a weak form of conscious processing. The 2022 review by Derrien and colleagues presents both models and argues that sensitive methods are needed to separate perception from awareness. The search results do not say which view most specialists favour. [4]