Blind vision

How do you locate an object even with eyes closed?

Published: Monday 15 December 1997

-- nerve cells help detect an object even if it has been seen just once. They are stimulated by vision, but continue to respond in the dark as if the stimulus is present and visible. M S A Graziano and his colleagues of the Department of Psychology, Princeton University, usa , have found the neurones in the region of a monkey's brain known as ventral premotor cortex ( pm v). It is an area of the cerebral cortex that guides the movement of a body ( Science , Vol 227, No 5323).

Nearly half of the neurones in the pm v respond to both touch and sight. In other words, the neurones have receptive fields ( rf s) for touch and vision. The receptive field for touch is an area of a body that excites a given neurone after being touched. The rf for vision is an area from which a visual stimulus excites the same neurone.

The visual rf is anchored to a body part and not to the eyes. It means that the visual rf does not move when eyes are rotated. If a visual rf is to remain active even after a stimulus is withdrawn, it can do so with the help of tactile rf .

The researchers conducted electrical recordings of the pm v of macaque monkeys. The monkeys were seated in chairs with their heads fixed and electrodes were positioned in their pm v. Then tactile rf s were plotted by stroking the skin and noting the region that caused maximal electrical response corresponding to one position of the electrodes. Visual rf s were similarly determined by presenting objects on a wand. Out of 153 single neurones in the pm v that were located, about 60 per cent responded to touch or sight. Of these, six responded only to visual stimuli, 34 only to touch, 55 to both and 11 to sound (in addition to vision and touch).

Graziano and his team outlined the visual rf of a neurone. When an object was moved into the visual rf but in dark the entire procedure being invisible (and inaudible), the neurone did not respond. When lights were switched on with a small time-lag, the neurone started showing signs of activity. And when the lights were switched off once again, the activity decreased but remained significantly higher than it was in the beginning.

This decreased level of activity persisted even when the object was removed. The monkeys could not have any idea that the object had disappeared. On the other hand, when the disappearance was confirmed by switching the lights on, all activity ceased abruptly.

It shows there is a significant time period when nothing is visible. But a neurone responds as if something is there. In contrast, to this behaviour, neurones in the primary visual cortex -- the chief cerebral centres for visual inputs -- respond only when an object is within their rf.

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