exercise (will be useful later): point at the dot

exercise (will be useful later): point at the dot

Blindsighted people have a region of their visual field in which they have no conscious visual experience: to them it is as if they are blind. When objects are presented to the affected area of their visual fields, they say that they cannot see them. However, blindsighted people are able to act on the basis of visual cues presented in this part of their visual field. For example, here is a diagram of the complete visual field of a blindsighted person.

The grey area represents the region affected by blindsight. This person can only consciously see the word ‘bank’. However, if we ask her to define ‘bank’, she is very likely to think of bank as a financial institution rather than as the kind of thing you find by a river:

this tendency is reversed if we put the word ‘river’ in her blind field. Although she cannot consciously see the word ‘money’, she does somehow discern the difference between this word and the word ‘river’.

Blindsighted people can also act on the basis of their blindsight, even though they do not consciously see anything. For example, if you flash a point of light somewhere in the affected part of their visual field, they see nothing. But if you ask them to point at the light by guessing where it is, they can point with surprising accuracy. With practice, blindsighted people can also reach for and pick up objects of different shapes and sizes. In fact, someone with blindsight can perform many non-reflexive actions guided by various kinds of visual stimuli in her region of apparent blindness. Blindsighted people can discern pattern, orientation, light wavelength, and the direction of moving objects. This is surprising because the blindsighted person does not consciously see the things she can point to or reach for. To her, the pointing and reaching seems just like guessing.

So blindsight enables people to discriminate visual stimuli. But consciousness is missing. To enjoy blindsight of objects is now a way of being conscious of those objects.

How is blindsight possible? From a purely neurological point of view, there is no mystery. Blindsight is caused by damage to the striate cortex (also known as V1), an area at the back of the brain. The striate cortex contains a kind of map of the visual field, and damage to part of the striate cortex causes blindsight in the corresponding part of the visual field. Although the striate cortex is the largest and most important region of the brain for visual processing, it isn’t the only target for optic nerves. Other parts of the visual system remain intact, so it is to be expected that damage to the striate cortex should leave some visual functioning intact. The brain damage that causes blindsight is completely different from damage to the retina or to the optic nerves.

For this reason, the original puzzle about blindsight wasn’t that people with damage to the striate cortex could act on the basis of visual information; it was that damage to their striate cortices should have such a dramatic effect on their vision. Research into the striate cortex was first done on monkeys and dogs. Early experimenters looked at what happens when you remove this part of the brain. At first they thought the striate cortex was not involved in vision at all, because when you remove this part of the brain animals recover the ability to make fine visual distinctions. But around 1884, Luigi Luciani noticed that animals without a striate cortex suffer from what he called “psychic blindness”. He observed that: although “visual sensation becomes perfect again [and] they are able to see minute objects, what they want is the discernment of things and a right judgement concerning their properties and their nature; they are deficient, in a word, of visual perception. For example, if small pieces of fig, mixed with pieces of sugar, are offered to them, they are incapable of choosing by sight alone but require to take the sugar in their hand and put it in their mouth in order to reassure themselves.” (Luciani 1884: 18) More recent experiments with monkeys have found the same thing: “After six years [one monkey] still does not know a carrot when she sees one, nor apparently can she recognise my face, despite an excellent ability to locate visual events in her environment and to avoid obstacles by vision alone” (Humphrey 1972: 684).

It was therefore puzzling that humans with damage to the striate cortex appeared to be completely blind in the affected regions of their visual fields (Weiskrantz 1986: §1). Psychologists searched for residual visual capacities in people with damaged striate cortex, and this led to the discovery of the phenomenon called blindsight. They expected to find some visual abilities, but they were surprised by what they found.

So there’s no mystery about blindsight from a purely neurological point of view. But blindsight is mysterious from the point of view of the person affected. As far as she is concerned, she doesn’t see anything. It comes as quite a surprise to her that she can point at and reach for things she can’t see: it must be a bit like discovering you’ve always had eyes in the back of your head.

Same idea but with more pathways and visual areas This is with V1 intact (V1 is top right)

Ok, let’s look at some data

Ok, let’s look at some data

Important thing here isn’t that we know the patient (G.Y.) is unaware; in fact G.Y. reports awareness in many cases. The key is that whether G.Y. reports awareness appears to be statistically unrelated to G.Y.’s discrimination performance.

Two measures: 2AFC for orientation (vertical or horizontal). And yes/no for awareness

G.Y. ‘was insistently instructed, and frequently reminded, that he was to signal unaware only when he had absolutely no sensation or feeling or experience of the visual event, and he repeatedly confirmed his conformance With this instruction’ \citep[p.~6122]{weiskrantz:1995_parameters}.

caption: ‘FIG. 2. Discrimination of horizontal vs. vertical movement (FR paradigm), as a function of stimulus contrast. The subject had to indicate (by guessing if necessary) whether the presented stimulus was moving horizontally or vertically by pressing the appropriate response key. He also had two commentary keys to use on every trial. Awareness refers to the percentage of trials on which the subject pressed the aware key. Correct when unaware refers to performance during those trials when the subject pressed the unaware key. The luminance of the test stimulus was held constant at 131 cd/m2, and background luminance in the blind hemifield was altered systematically thus changing the contrast of the stimulus. Speed was 150/s, and displacement was 200. Note the relative stability of the high level of performance indepen- dent of contrast but with a steep decline in percentage of aware responses at high background luminance level when the contrast decreased’ \citep[p.~6123]{weiskrantz:1995_parameters}

recall earlier when I asked you to point at the dot ...

In conclusion, ...