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= Blindsight_ =
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Introduction
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Blindsight is the ability of people who are cortically blind to
respond to visual stimuli that they do not consciously see due to
lesions in the primary visual cortex, also known as the striate cortex
or Brodmann Area 17. The term was coined by Lawrence Weiskrantz and
his colleagues in a paper published in a 1974 issue of 'Brain'. A
previous paper studying the discriminatory capacity of a cortically
blind patient was published in 'Nature' in 1973.
The assumed existence of blindsight is controversial, with some
arguing that it is merely degraded conscious vision.
Type classification
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The majority of studies on blindsight are conducted on patients who
are hemianopic, i.e. blind in one-half of their visual field.
Following the destruction of the left or right striate cortex,
patients are asked to detect, localize, and discriminate amongst
visual stimuli that are presented to their blind side, often in a
forced-response or guessing situation, even though they may not
consciously recognize the visual stimulus. Research shows that such
blind patients may achieve a higher accuracy than would be expected
from chance alone.
'Type 1 blindsight' is the term given to this ability to guess--at
levels significantly above chance--aspects of a visual stimulus (such
as location or type of movement) without any conscious awareness of
any stimuli. 'Type 2 blindsight' occurs when patients claim to have a
feeling that there has been a change within their blind area--e.g.
movement--but that it was not a visual percept. The re-classification
of blindsight into Type 1 and Type 2 was made after it was shown that
the most celebrated blindsight patient, "GY", was usually conscious of
stimuli presented to his blind field if the stimuli had certain
specific characteristics, namely being of high contrast and moving
fast (at speeds in excess of 20 degrees field of view per second).
In the aftermath of the First World War, a neurologist, George
Riddoch, had described patients who had been blinded by gunshot wounds
to V1, who could not see stationary objects but who were, as he
reported, "conscious" of seeing moving objects in their blind field.
It is for this reason that the phenomenon has more recently also been
called the 'Riddoch syndrome'.
Since then it has become apparent that such subjects can also become
aware of visual stimuli belonging to other visual domains, such as
color and luminance, when presented to their blind fields. The ability
of such hemianopic subjects to become consciously aware of stimuli
presented to their blind field is also commonly referred to as
"residual" or "degraded" vision.
As originally defined, 'blindsight' challenged the common belief that
perceptions must enter consciousness to affect our behavior, by
showing that our behavior can be guided by sensory information of
which we have no conscious awareness. Since the demonstration that
blind patients can experience some visual stimuli consciously, and the
consequent redefinition of blindsight into Type 1 and Type 2, a more
nuanced view of the phenomenon has developed. Blindsight may be
thought of as a converse of the form of anosognosia known as Anton
syndrome, in which there is full cortical blindness along with the
confabulation of visual experience.
History
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Much of our current understanding of blindsight can be attributed to
early experiments on monkeys. One monkey, named Helen, could be
considered the "star monkey in visual research" because she was the
original blindsight subject. Helen was a macaque monkey that had been
decorticated; specifically, her primary visual cortex (V1) was
completely removed, blinding her. Nevertheless, under certain specific
situations, Helen exhibited sighted behavior. Her pupils would dilate
and she would blink at stimuli that threatened her eyes. Furthermore,
under certain experimental conditions, she could detect a variety of
visual stimuli, such as the presence and location of objects, as well
as shape, pattern, orientation, motion, and color. In many cases, she
was able to navigate her environment and interact with objects as if
she were sighted.
A similar phenomenon was also discovered in humans. Subjects who had
suffered damage to their visual cortices due to accidents or strokes
reported partial or total blindness. Despite this, when prompted they
could "guess" the presence and details of objects with above-average
accuracy and, much like animal subjects, could catch objects tossed at
them. The subjects never developed any kind of confidence in their
abilities. Even when told of their successes, they would not begin to
spontaneously make "guesses" about objects, but instead still required
prompting. Furthermore, blindsight subjects rarely express the
amazement about their abilities that sighted people would expect them
to express.
Describing blindsight
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Patients with blindsight have damage to the system that produces
visual perception (the visual cortex of the brain and some of the
nerve fibers that bring information to it from the eyes) rather than
to the underlying brain system controlling eye movements. The
phenomenon was originally thought to show how, after the more complex
perception system is damaged, people can use the underlying control
system to guide hand movements towards an object even though they
cannot see what they are reaching for. Hence, visual information can
control behavior without producing a conscious sensation. This ability
of those with blindsight to act as if able to see objects that they
are unconscious of suggested that consciousness is not a general
property of all parts of the brain, but is produced by specialized
parts of it.
Blindsight patients show awareness of single visual features, such as
edges and motion, but cannot gain a holistic visual percept. This
suggests that perceptual awareness is modular and that--in sighted
individuals--there is a "binding process that unifies all information
into a whole percept", which is interrupted in patients with such
conditions as blindsight and visual agnosia. Therefore, object
identification and object recognition are thought to be separate
processes and occur in different areas of the brain, working
independently from one another. The modular theory of object
perception and integration would account for the "hidden perception"
experienced in blindsight patients. Research has shown that visual
stimuli with the single visual features of sharp borders, sharp
onset/offset times, motion and low spatial frequency contribute to,
but are not strictly necessary for, an object's salience in
blindsight.
Cause
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There are multiple theories about what causes blindsight. The first
states that after damage to area V1, other branches of the optic nerve
deliver visual information to the superior colliculus, pulvinar and
several other areas, including parts of the cerebral cortex. In turn,
these areas might then control the blindsight responses.
Another explanation for the phenomenon of blindsight is that even
though the majority of a person's visual cortex may be damaged, tiny
islands of functioning tissue remain. These islands are not large
enough to provide conscious perception, but nevertheless enough for
some unconscious visual perception.
A third theory is that the information required to determine the
distance to and velocity of an object in object space is determined by
the lateral geniculate nucleus (LGN) before the information is
projected to the visual cortex. In a normal subject, these signals are
used to merge the information from the eyes into a three-dimensional
representation (which includes the position and velocity of individual
objects relative to the organism), extract a vergence signal to
benefit the precision (previously auxiliary) optical system, and
extract a focus control signal for the lenses of the eyes. The
stereoscopic information is attached to the object information passed
to the visual cortex.
More recently, with the demonstration of a direct input from the LGN
to area V5 (MT), which delivers signals from fast moving stimuli at
latencies of about 30 ms, another explanation has emerged. This one
proposes that the delivery of these signals is sufficient to arouse a
conscious experience of fast visual motion, without implying that it
is V5 alone that is responsible, since once signals reach V5, they may
be propagated to other areas of the brain. The latter account would
seem to exclude the possibility that signals are "pre-processed" by V1
or "post-processed" by it (through return connections from V5 back to
V1), as has been suggested. The pulvinar nucleus of the thalamus also
sends direct, V1 by-passing, signals to V5 but their precise role in
generating a conscious visual experience of motion has not yet been
determined.
Evidence of blindsight can be indirectly observed in children as young
as two months, although there is difficulty in determining the type in
a patient who is not old enough to answer questions.
Research
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Lawrence Weiskrantz and colleagues showed in the early 1970s that if
forced to guess about whether a stimulus is present in their blind
field, some observers do better than chance. This ability to detect
stimuli that the observer is not conscious of can extend to
discrimination of the type of stimulus (for example, whether an 'X' or
'O' has been presented in the blind field).
Electrophysiological evidence from the late 1970s has shown that there
is no direct retinal input from S-cones to the superior colliculus,
implying that the perception of color information should be impaired.
However, more recent evidence point to a pathway from S-cones to the
superior colliculus, opposing previous research and supporting the
idea that some chromatic processing mechanisms are intact in
blindsight.
Patients shown images on their blind side of people expressing
emotions correctly guessed the emotion most of the time. The movement
of facial muscles used in smiling and frowning were measured and
reacted in ways that matched the kind of emotion in the unseen image.
Therefore, the emotions were recognized without involving conscious
sight.
A study reported in 2008 asked patient GY to 'mis'state where in his
visual field a distinctive stimulus was presented. If the stimulus was
in the upper part of his visual field, he was to say it was in the
lower part, and 'vice versa'. He was able to misstate, as requested,
in his left visual field (with normal conscious vision); but he tended
to fail in the task--to state the location correctly--when the
stimulus was in his blindsight (right) visual field. This failure rate
worsened when the stimulus was clearer, indicating that failure was
not simply due to unreliability of blindsight.
Evidence in animals
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In a 1995 experiment, researchers attempted to show that monkeys with
lesions in or even wholly removed striate cortexes also experienced
blindsight. To study this, they had the monkeys complete tasks similar
to those commonly used for human subjects. The monkeys were placed in
front of a monitor and taught to indicate whether a stationary object
or nothing was present in their visual field when a tone was played.
Then the monkeys performed the same task except the stationary objects
were presented outside of their visual field. The monkeys performed
very similar to human participants and were unable to perceive the
presence of stationary objects outside of their visual field.
Another 1995 study by the same group sought to prove that monkeys
could also be conscious of movement in their deficit visual field
despite not being consciously aware of the presence of an object
there. To do this, researchers used another standard test for humans
which was similar to the previous study except moving objects were
presented in the deficit visual field. Starting from the center of the
deficit visual field, the object would either move up, down, or to the
right. The monkeys performed identically to humans on the test,
getting them right almost every time. This showed that the monkey's
ability to detect movement is separate from their ability to
consciously detect an object in their deficit visual field, and gave
further evidence for the claim that damage to the striate cortex plays
a large role in causing the disorder.
Several years later, another study compared and contrasted the data
collected from monkeys and that of a specific human patient with
blindsight, GY. His striate cortical region was damaged through trauma
at the age of eight, though for the most part he retained full
functionality, GY was not consciously aware of anything in his right
visual field. In the monkeys, the striate cortex of the left
hemisphere was surgically removed. By comparing the test results of
both GY and the monkeys, the researchers concluded that similar
patterns of responses to stimuli in the "blind" visual field can be
found in both species.
"DB"
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Researchers applied the same type of tests that were used to study
blindsight in animals to a patient referred to as "DB". The normal
techniques used to assess visual acuity in humans involved asking them
to verbally describe some visually recognizable aspect of an object or
objects. DB was given forced-choice tasks to complete instead. The
results of DB's guesses showed that DB was able to determine shape and
detect movement at some unconscious level, despite not being visually
aware of this. DB himself chalked up the accuracy of his guesses to be
merely coincidental.
The discovery of the condition known as blindsight raised questions
about how different types of visual information, even unconscious
information, may be affected and sometimes even unaffected by damage
to different areas of the visual cortex. Previous studies had already
demonstrated that even without conscious awareness of visual stimuli,
humans could still determine certain visual features such as presence
in the visual field, shape, orientation and movement. But, in a newer
study evidence showed that if damage to the visual cortex occurs in
areas above the primary visual cortex, the conscious awareness of
visual stimuli itself is not damaged. Blindsight shows that even when
the primary visual cortex is damaged or removed a person can still
perform actions guided by unconscious visual information. Despite
damage occurring in the area necessary for conscious awareness of
visual information, other functions of the processing of these visual
percepts are still available to the individual. The same also goes for
damage to other areas of the visual cortex. If an area of the cortex
that is responsible for a certain function is damaged, it will only
result in the loss of that particular function or aspect, functions
that other parts of the visual cortex are responsible for remain
intact.
Alexander and Cowey
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Alexander and Cowey investigated how contrasting stimuli brightness
affects blindsight patients' ability to discern movement. Prior
studies have already shown that blindsight patients are able to detect
motion even though they claim they do not see any visual percepts in
their blind fields. The study subjects were two patients who suffered
from hemianopsia--blindness in more than half of their visual field.
Both subjects had displayed the ability to accurately determine the
presence of visual stimuli in their blind hemifields without
acknowledging an actual visual percept previously.
To test the effect of brightness on the subject's ability to determine
motion they used a white background with a series of colored dots. The
contrast of the brightness of the dots compared to the white
background was altered in each trial to determine if the participants
performed better or worse when there was a larger discrepancy in
brightness or not. The subjects focused on the display for two equal
length time intervals and were asked whether they thought the dots
were moving during the first or the second time interval.
When the contrast in brightness between the background and the dots
was higher, both of the subjects could discern motion more accurately
than they would have statistically through guesswork. However, one
subject was not able to accurately determine whether or not blue dots
were moving regardless of the brightness contrast, but he/she was able
to do so with every other color dot. When the contrast was highest,
subjects were able to tell whether or not the dots were moving with
very high rates of accuracy. Even when the dots were white, but still
of a different brightness from the background, subjects could still
determine whether they were moving. But, regardless of the dots'
color, subjects could not tell when they were in motion when the white
background and the dots were of similar brightness.
Kentridge, Heywood, and Weiskrantz
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Kentridge, Heywood, and Weiskrantz used the phenomenon of blindsight
to investigate the connection between visual attention and visual
awareness. They wanted to see if their subject--who exhibited
blindsight in other studies--could react more quickly when their
attention was cued without the ability to be visually aware of it. The
researchers aimed to show that being conscious of a stimulus and
paying attention to it was not the same thing.
To test the relationship between attention and awareness, they had the
participant try to determine where a target was and whether it was
oriented horizontally or vertically on a computer screen. The target
line would appear at one of two different locations and would be
oriented in one of two directions. Before the target would appear an
arrow would become visible on the screen, sometimes pointing to the
correct position of the target line and less frequently not. This
arrow was the cue for the subject. The participant would press a key
to indicate whether the line was horizontal or vertical, and could
then also indicate to an observer whether or not he/she actually had a
feeling that any object was there or not--even if they couldn't see
anything. The participant was able to accurately determine the
orientation of the line when the target was cued by an arrow before
the appearance of the target, even though these visual stimuli did not
equal awareness in the subject who had no vision in that area of
his/her visual field. The study showed that even without the ability
to be visually aware of a stimulus the participant could still focus
his/her attention on this object.
"CB" and "SJ"
===============
Two separate studies involving the blindsighted patients "CB" and "SJ"
both showed that visually guided action can occur in the absence of
conscious perception. CB, a 75-year-old man blind on his left side,
was asked to reach with his hand towards a target while avoiding
various obstacles. These were placed on both his blind side and his
sighted side. Despite reporting no awareness of the placement (or even
presence) of the obstacles on the blind side, he was able complete
each trial without bumping into a single object. Similarly, SJ, a
37-year-old woman blind on her right side, was presented with objects
on both sides of her vision and asked to grab them. Even when she
couldn't see the object, she proved able to scale her grasp
accurately. Crucially, in both studies the task was then altered
slightly, introducing a two second delay between when the objects were
shown and when the participants completed the task. Their prior
ability to react to unseen objects completely disappeared, and they
ceased to exhibit any signs of blindsight, though their performance
remained mainly unaffected when it came to objects on their sighted
side.
Some scientists have argued that the dorsal pathway dedicated to
"vision-for-action" does not store information; rather, actions are
coordinated in real-time based on the visual information being
received at that very moment. This would explain why CB and SJ were
able to react to visual information presented on their blind side in
real-time but not after a delay. Indeed, a conclusion of the SJ study
was that action based on memory seems to rely on the ventral pathway
involved in perception, in a way that action based on objects in our
direct line of sight does not. This was backed up by an fMRI study in
which ventral stream activation usually linked to visual perception
was activated by actions carried out after an 18-s delay, despite the
participants being in complete darkness.
"TN"
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A potential weak point of case studies like the ones above is that the
participants were not completely blind, and therefore it is not out of
the question that their existing vision could have assisted them in
some way. So a particularly noteworthy patient is a man known as "TN",
who suffered two strokes at age 52 that resulted in the destruction of
the primary visual cortex (V1) in both hemispheres of the brain, and
hence complete loss of (conscious) sight. In one famous case,
researchers persuaded TN to walk down an obstacle-filled hallway,
without his cane or any prior knowledge of the layout. He was able to
navigate the full length of the hallway without hitting a single
object, at one point even hugging the wall to get past a trashcan.
TN has taken part in a number of other experiments looking at
blindsight. In one early study, he was shown images of expressive
faces. While he could not guess the gender or shape accurately, he
correctly guessed what emotion was shown at an above chance level.
Brain imaging showed significant activity in the right amygdala,
particularly in response to fearful faces, suggesting that the brain
can unconsciously process expressions of emotion.
Similarly, another experiment investigated the brain's sensitivity to
looming stimuli, which often indicate an incoming collision. This
sensitivity can involve heightened attention capture, but is also
thought to operate on a subconscious level, and has been observed in
monkeys and infants. In the experiment, both TN and a group of control
participants were shown a series of moving red dots, some of which
made looming motions. An fMRI scan of the control group showed that
the movement of the dots produced normal activation in the middle
temporal visual area (V5), known for its role in motion processing. In
TN, activation was found in response to both general motion and to
looming in particular; notably, however, this occurred primarily in
areas of the brain not associated with motion processing in healthy
individuals. Given that the specific V5 areas activated in the control
participants were mostly lesioned in TN, this unusual activation seems
to be a result of cortical plasticity.
Other cases
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Other cases refer to SL, GY and GR.
Brain regions involved
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Visual processing in the brain goes through a series of stages.
Destruction of the primary visual cortex leads to blindness in the
part of the visual field that corresponds to the damaged cortical
representation. The area of blindness - known as a scotoma - is in the
visual field opposite the damaged hemisphere and can vary from a small
area up to the entire hemifield. Visual processing occurs in the brain
in a hierarchical series of stages (with much crosstalk and feedback
between areas). The route from the retina through V1 is not the only
visual pathway into the cortex, though it is by far the largest; it is
commonly thought that the residual performance of people exhibiting
blindsight is due to preserved pathways into the extrastriate cortex
that bypass V1. However both physiological evidence in monkeys and
behavioral and imaging evidence in humans shows that activity in these
extrastriate areas, and especially in V5, is apparently sufficient to
support visual awareness in the absence of V1.
To put it in a more complex way, recent physiological findings suggest
that visual processing takes place along several independent, parallel
pathways. One system processes information about shape, one about
color, and one about movement, location and spatial organization. This
information moves through an area of the brain called the lateral
geniculate nucleus, located in the thalamus, and on to be processed in
the primary visual cortex, area V1 (also known as the striate cortex
because of its striped appearance). People with damage to V1 report no
conscious vision, no visual imagery, and no visual images in their
dreams. However, some of these people still experience the blindsight
phenomenon, though this too is controversial, with some studies
showing a limited amount of consciousness without V1 or projections
relating to it.
The superior colliculus and prefrontal cortex also have a major role
in awareness of a visual stimulus.
Lateral geniculate nucleus
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Mosby's Dictionary of Medicine, Nursing & Health Professions
defines the LGN as "one of two elevations of the lateral posterior
thalamus receiving visual impulses from the retina via the optic
nerves and tracts and relaying the impulses to the calcarine (visual)
cortex".
What is seen in the left and right visual field is taken in by each
eye and brought back to the optic disc via the nerve fibres of the
retina. From the optic disc, visual information travels through the
optic nerve and into the optic chiasm. Visual information then enters
the optic tract and travels to four different areas of the brain
including the superior colliculus, pretectum of the mid brain, the
suprachiasmatic nucleus of the hypothalamus, and the lateral
geniculate nucleus (LGN). Most axons from the LGN will then travel to
the primary visual cortex.
Injury to the primary visual cortex, including lesions and other
trauma, leads to the loss of visual experience. However, the residual
vision that is left cannot be attributed to V1. According to Schmid et
al., "thalamic lateral geniculate nucleus has a causal role in
V1-independent processing of visual information". This information was
found through experiments using fMRI during activation and
inactivation of the LGN and the contribution the LGN has on visual
experience in monkeys with a V1 lesion. These researchers concluded
that the magnocellular system of the LGN is less affected by the
removal of V1, which suggests that it is because of this system in the
LGN that blindsight occurs. Furthermore, once the LGN was inactivated,
virtually all of the extrastriate areas of the brain no longer showed
a response on the fMRI. The information leads to a qualitative
assessment that included "scotoma stimulation, with the LGN intact had
fMRI activation of ~20% of that under normal conditions". This finding
agrees with the information obtained from, and fMRI images of,
patients with blindsight. The same study also supported the conclusion
that the LGN plays a substantial role in blindsight. Specifically,
while injury to V1 does create a loss of vision, the LGN is less
affected and may result in the residual vision that remains, causing
the "sight" in blindsight.
Functional magnetic resonance imaging has launched has also been
employed to conduct brain scans in normal, healthy human volunteers to
attempt to demonstrate that visual motion can bypass V1, through a
connection from the LGN to the human middle temporal complex. Their
findings concluded that there was an indeed a connection of visual
motion information that went directly from the LGN to the V5/hMT+
bypassing V1 completely. Evidence also suggests that, following a
traumatic injury to V1, there is still a direct pathway from the
retina through the LGN to the extrastriate visual areas. The
extrastriate visual areas include parts of the occipital lobe that
surround V1. In non-human primates, these often include V2, V3, and
V4.
In a study conducted in primates, after partial ablation of area V1,
areas V2 and V3 were still excited by visual stimulus. Other evidence
suggests that "the LGN projections that survive V1 removal are
relatively sparse in density, but are nevertheless widespread and
probably encompass all extrastriate visual areas," including V2, V4,
V5 and the inferotemporal cortex region.
Controversy
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The results of some experiments suggest that blindsighted people may
be preserving some kind of conscious experience and thus they are not
fully blind. The criteria for blindsight has repeatedly changed based
on findings that challenge the original definition, which has led some
scientists to cast doubt on the existence of blindsight.
See also
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* Koniocellular cell
* Riddoch syndrome
* Two-streams hypothesis
* Visual agnosia
External links
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* [
https://news.bbc.co.uk/2/hi/health/7794783.stm Blind man navigates
maze]
*
[
https://www.psychologytoday.com/intl/blog/brain-sense/200909/seeing-without-sight
Blind man avoids obstacles when reaching]
License
=========
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License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Blindsight_
