A Scientific Approach to Conscious Experience, Introspection, and Unconscious Processing: Vision and Blindsight

Abstract

Although subjective conscious experience and introspection have long been considered unscientific and banned from psychology, they are indispensable in scientific practice. These terms are used in scientific contexts today; however, their meaning remains vague, and earlier objections to the distinction between conscious experience and unconscious processing, remain valid. This also applies to the distinction between conscious visual perception and unconscious visual processing. Damage to the geniculo-striate pathway or the visual cortex results in a perimetrically blind visual hemifield contralateral to the damaged hemisphere. In some cases, cerebral blindness is not absolute. Patients may still be able to guess the presence, location, shape or direction of movement of a stimulus even though they report no conscious visual experience. This "unconscious" ability was termed "blindsight". The present paper demonstrates how the term conscious visual experience can be introduced in a logically precise and methodologically correct way and becomes amenable to scientific examination. The distinction between conscious experience and unconscious processing is demonstrated in the cases of conscious vision and blindsight. The literature on "blindsight" and its neurobiological basis is reviewed. It is shown that blindsight can be caused by residual functions of neural networks of the visual cortex that have survived cerebral damage, and may also be due to an extrastriate pathway via the midbrain to cortical areas such as areas V4 and MT/V5.

Keywords: behaviorism; blindsight; conscious experience; pulvinar; superior colliculi; vision; visual system.

Figure 2

Location of area hMT+ (encircled) on the left hemisphere of the human brain. FL: frontal lobe, TL: temporal lobe, CB: cerebellum [148].

Figure 1

Schematic drawing of layers and cells in primate area V1. Red triangles: pyramidal cells with apical dendrites (dendritic spines are omitted). Red stars: stellate cells (dendritic spines are omitted). Black: basked cell contacting the perisomatic area of a pyramidal cell. Green: double bouquet cell contacting the apical dendrite of a pyramidal cell. Brown: chandelier cell contacting the dendrites of a pyramidal cell. Blue: konio cell (K) ending in a layer III blob. P: input from an LGN parvo cell contacting the dendrites of a pyramidal cell. M: input from an LGN magno-cell contacting the dendrites of a pyramidal cell. V2: axon of a layer IV pyramidal cell sending information to extrastriate visual area V2. MT: axon of a layer IV stellate cell sending information to area MT/V5.

Figure 3

Right superior colliculus (rSC) and right inferior colliculus (rIC) of the human brain. OL: occipital lobe, CB: cerebellum.