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)
Cowey, 2010 figure 2
Ok, let’s look at some data
Weiskrantz et al, figure 2
Ok, let’s look at some data
Weiskrantz et al, figure 2
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}
\citet{weiskrantz:1995_parameters} show that a blindsighted patient’s
visual discimination capacities are unaffected by whether the patient is
aware of stimuli.
recall earlier when I asked you to point at the dot ...