By Amélie Ravier and Lena Sannier
What is the blindsight?
Blindsight is an unconscious intuition of the characteristics of objects and spatial configurations. For example, it can be being able to point in the direction of an object we cannot see (Perenin & Jeannerod, 1975), being able to catch or dodge an object that is thrown at us, or being able to guess the shape and colour of an object without seeing or touching it (Cowey & Sterig, 2001).
Weiskrantz (1996), after studying the case of D.B., defines blindsight as the ability of a person who, after losing vision following a lesion, is able to locate and discriminate visual stimuli, without being perceptually aware of them. Weiskrantz (1998) made a distinction between type I blindsight when the person has unconscious residual abilities in the blind visual field, and type II when they are conscious: the patient has a vague impression that something is happening.
This ability, following blindness, can be found after a stroke, or in children after cerebral hypoxia at birth. It may affect one or both eyes. Indeed, sometimes the patient has lesions all over the visual cortex, resulting in what we call “total cortical blindness”. In this case, the blindsight is total. This essentially means that the person is blind, but their eyeballs are fully functional.
In the 1970’s, some studies have shown animals with both cortical blindness and blindsight, that have residual vision. Humphrey (1971) filmed a blind monkey, Helen, who was able to move freely between objects, on her own. However, she would bump into obstacles made of transparent Plexiglas, proving that her ability to navigate was based on visual cues, and not just on a physical feeling, fox example.
More recently, similar cases have been found in humans, such as the TN patient (De Gelder et al., 2008), who not only was able to avoid obstacles, but also to detect emotional faces outside of visual awareness.
Hypothesis
There are three hypotheses concerning blindsight.
It was initially thought that there could be “functional islands” in the visual area, and therefore that the entire visual cortex was not damaged: this was the cortical sparing hypothesis. However, total resection experiments of the visual cortex in both hemispheres in animals have shown that some have residual capacities. Even though these are animal studies, there are great similarities between the visual abilities of humans and monkeys, so we assume that this hypothesis is refuted.
According to the diffusion hypothesis, the light emitted by visual stimuli could diffuse from the blind field (i.e. what the patient does not see) into the visible field, which would explain the residual visual capacities. However, this hypothesis has been invalidated from an anatomical point of view.
Only the most recent hypothesis has been scientifically validated: the two visual systems hypothesis. In this one, ‘normal’ visual perception typically involves solely the retino-geniculo-striate pathway, but it is thought that in patients with blindsight (after damage to the primary visual cortex), vision may involve two secondary pathways that can be used when the normal one is damaged.
Furthermore, recent studies have shown vestigial structures that re-connect and start functioning again after several decades of living with a blindsight (see Bridge, 2008).
Evaluation and treatment
There is no specific evaluation of the blindsight, people generally realise that they have these residual capacities by accident. Nevertheless, perimetry makes it possible to determine the extent of the visual field amputation, which thus makes it possible to verify that the person is indeed totally “blind”. A neuro-visual assessment is also proposed to check whether there are differences according to the brightness of the room, the movements of a person, etc.
In terms of treatment, very little is offered. Patients are trained to direct their attention, in the form of saccades, towards a target located in their blind field so that they learn to detect and locate it.
However, it should be remembered that those patients with residual vision in cortical blindness are quite rare, as there are about 20% of blindsight positive patients (with these residual abilities), while blindsight negative patients (without residual abilities) represent 80% of people with cortical blindness (Ajina et al., 2015). This explains why very few treatments are offered.
To go further
D.B. (De Gelder et al., 2008)
Researchers found that D.B.’s blind vision outperformed his normal, unimpaired vision in the Gabor patch experiment. In this experiment, a stimulus is presented intermittently, and the patient has to say when it is displayed. This is a difficult test because there is little contrast, and even for a person with normal vision, it is a difficult test. But patient D.B. had a correct response rate of 87% in his blind visual field, whereas it was only 50% in his normal visual field (equivalent of responding randomly). The researchers do not really give an explanation for this improvement, since the patient did not participate in enough experiments to observe a training effect.
The case of Milena Canning
In 1999, Milena Canning becomes completely blind following a stroke that leaves her brain damaged. Six months later, she starts to perceive reflections, movements of flowing water, etc. Her ophthalmologist encourages her to develop movement-based strategies (using a rocking chair or making voluntary head movements) to improve her performances. In this short video, we can see her catch a ball that is thrown at her, and even ride a horse between poles.
Bibliography
Ajina, S., Pestilli, F., Rokem, A., Kennard, C., & Bridge, H. (2015). Human blindsight is mediated by an intact geniculo-extrastriate pathway. eLife, 4. https://doi.org/10.7554/eLife.08935
Bridge, H., Thomas, O., Jbabdi, S., & Cowey, A. (2008). Changes in connectivity after visual cortical brain damage underlie altered visual function. Brain : a journal of neurology, 131(Pt 6), 1433–1444.
Humphrey, N. (1974). Vision in a monkey without striate cortex: A case study. Perception 3 (3), 241-55.
Perenin, M. T., & Jeannerod, M. (1975). Residual vision in cortically blind hemiphields. Neuropsychologia, 13(1), 1–7.
Weiskrantz, L. (1996). Blindsight revisited. Current Opinion in Neurobiology, 6, (2), 215-220.
Weiskrantz, L. (1998). Consciousness and commentaries. In: Hameroff, S., Kaszniak, A., Scott, A. (Eds.), Towards a Science of Consciousness II—The Second Tucson Discussion and Debates. MIT Press, Cambridge, 371–377.
Keywords
- Blindsight: vision aveugle
- To dodge: éviter
- Visual cues: indices visuels
- Pathway: voie
- Brightness: luminosité