by Maddie Critz '20
“Blindsight”. A word that may seem like only an oxymoron to you, but to a room full of neurologists, the word “blindsight” may incite groans of frustration or, perhaps, an argument.
Blindsight is a cognitive phenomenon that has stumped and divided neuroscience research for the past 40 years. The phenomenon occurs when a clinically blind person expresses evidence of cognitive processing of visual stimuli that they simply cannot see. A partially blind person may demonstrate blindsight when they are unaware of a stimulus shown in their blind field; however, they can guess above-chance about it.
One of the most famous examples of blindsight is that of Patient GY (name withheld for privacy). GY was entirely blind on one half of his visual field. The damage that caused this was located in the visual cortex, the main sight-processing center in the occipital lobes of his brain. The researchers GY was working with expected him to be entirely unaware and unable to detect stimuli in his blind field. However, they found that when they presented moving black dots and certain geometric shapes in GY’s field of blindness, he remained unaware of the stimulus (i.e. he could not tell that something was there at all), but when forced to guess about the movement of the dots, he was correct 75% of the time.
Confused? So were the researchers. But there were other cases of blindsight including blind patients with damage to their visual processing systems being able to move their eyes to a target in their blind fields without being aware of the target (study) A blind patient named DB could correctly guess types of animals and insects when photos were shown in his blind field (study). For 30 years, neuroscientists have been fascinated and divided by this seemingly impossible phenomenon.
Proponents of the existence of blindsight suggest that alternate routes of processing can bypass the visual cortex (the center for object recognition) and project directly to comprehensive areas, completely uprooting what neuroscientists have mapped for the visual system. This can explain the above-chance discrimination of stimuli without awareness; neurally, the stimulus bypasses the damaged region that would recognize it, and is immediately comprehended somewhere else instead. Using this system theory, researchers have suggested two routes for visual processing: a “high-road” that includes slow processing, recognition, and comprehension of a stimulus and a “low-road”, which bypasses centers of awareness and works much faster. You, a healthy subject, might use the “high-road” when shown a picture of a cat and asked to name the animal. Blind patients with damage to their visual processing “high-road” may rely on the “low-road” to do the processing observed in blindsight.
Researchers who do not believe in this two-route theory of blindsight have other explanations. Some propose that awareness is necessary for visual processing. They suggest the phenomenon is caused by light scatter in the retina that skewed the data, or perhaps intact areas of the visual processing center remaining and picking up the slack caused by the damage. Some claim that patients GY and DB exhibited response bias that made them “aware” of the stimulus, thwarting the experiments.
Though divisive, blindsight has changed the way that we study blindness and the visual system, proving that physiological testing and brain imaging aren’t enough to determine the full extent of any damage, and that behavioral studies such as these mentioned must be integrated in order to get a fuller picture of a patient's abilities and residual processing.
Recently, studies have been conducted using TMS (transcranial magnetic stimulation) to temporarily disrupt parts of the visual system in order to create the symptoms of blindsight and gain a greater understanding of the interconnectedness of awareness and discrimination. Blindsight cases may shed more light on neuroplasticity, the brain’s ability to employ undamaged neurons to do the work of damaged neurons. Blindsight could provide an example of the extreme effects of neuroplasticity, and could be used to make a blueprint for treatment and rehabilitation of blind patients.
Overview of Blindsight:
Ajina S, Bridge H. Blindsight and Unconscious Vision: What They Teach Us about the Human Visual System. Neuroscientist. 2017 Oct 1;23(5):529–41