Blind vision: causes and symptoms of 'seeing without knowing what to see'

His eyes work fine, they're intact. But they say they see nothing. And they really see, without knowing that they see. This curious phenomenon is what happens to people who suffer from blindsight, a neurological disorder Caused by brain damage that affects the ability to consciously represent visual stimuli from the environment.

In this article we explain what blindsight is, how this concept arose, what its causes are, and how to differentiate it from other similar disorders.

• Related article: "Cortical blindness: causes, symptoms and treatment"

Blindsight: Definition and Background

blindsight (blindsight) is a term coined by the English psychologist, Lawrence Weiskrantz, which refers to the ability that some subjects have to detect, locate and discriminate visual stimuli in a unconscious. People with this disorder "see without knowing what they see"; that is, they do not consciously recognize the objects in front of them, even though they act as if they were, in fact, there.

The first investigations into the phenomenon of blindsight were carried out in animals, mainly monkeys, with surgical removal of the brain regions responsible for vision (the area V1). When deprived of these structures, the animals seemed to retain some visual abilities, as the ability to detect contrast or to differentiate one object from another based on its shape.

Few neuroscientists believed that normal vision could be achieved in humans with these damaged brain areas. The patients whose visual cortex had been destroyed were totally blind, or so it seemed. In 1973, the team of the German psychologist Ernst Pöppel verified that, although some of them lacked a visual cortex and declared that they were incapable of seeing objects, the ocular movements of his eyes were directed towards them: It was the evidence that his visual system was informing, in some way, of their existence.

But what ended up convincing the scientific community that the blindsight phenomenon deserved his full attention were the works of Larry Weiskrantz and his colleagues in the early 1980s. seventy. The forced choice technique was used in the experiments. (which forces patients to choose between defined options, rather than just asking what they see): patients had to choose between two possible colors or locations, while asking them to guess which one applied to a visual object they claimed they could not see.

The answers of some of the patients turned out to be correct in a significant proportion; that is, with a greater frequency than would be expected by chance. It was from then on that these people began to be labeled as blindsighted patients.

Currently, it has been shown that blindsighted people They can not only "intuit" the color or location of objects, but also the orientation of lines or grids, the moment of appearance or facial expressions.. However, they can't do it with other things like detecting subtle nuances or complex motion.

Causes and brain structures involved

Blind vision occurs in a portion of our perceptive organs: the scotoma or blind spot. This phenomenon occurs when there is damage or injury to the occipital lobe, and more specifically to the primary visual cortex (V1)., which is responsible for the processing of visual stimuli.

When we receive information from an object through the retinas of our eyes, it travels from the ganglion cells of the optic nerve to various subcortical structures that, acting as relay zones, are responsible for integrating the information from each sensory modality (in this case, the view).

At the subcortical level, visual information passes through structures such as the medulla oblongata, the midbrain, and the lateral geniculate nucleus of the thalamus. At this level, we are not yet aware of what we have "seen", since the information has not yet reached the higher cortical levels. However, this can influence our behavior, as occurs in cases of blind vision, in which the person sees, without knowing what he sees.

Blindsighted patients therefore have damaged the final module of a complex visual processing circuit, which is insufficient by itself. and without the rest of sensory and subcortical structures but necessary, at the same time, for there to be a conscious recognition of what we perceive.

• You may be interested in: "Visual cortex of the brain: structure, parts and pathways"

The sensorimotor model of vision

The conventional model of structural failure in visual processing (involving damage to various areas of the brain) assumes, implicitly, that vision consists of creating an internal representation of external reality, the activation of which would generate the visual experience aware. However, it is not the only one that has been postulated to try to explain why a phenomenon such as blindsight occurs.

The ecological approach to visual perception proposed by psychologist James J. gibson, he believes that vision must be understood as a necessary tool for survival. According to Gibson, the real value of visual processing lies in being able to identify and see with your eyes what is there and where, so we can avoid obstacles, identify food or possible threats, achieve goals, etc.

All this work of "visual deduction" would be carried out by the retina in interaction with multiple environmental signals. And the key would be in discriminate the relevant information, among so many signals, to be able to manage a particular behavior.

Today, Gibson's approach has been reformulated as the sensorimotor model of vision, borrowing concepts from the ecological approach and it is postulated that vision is an activity to explore our environment based on sensorimotor contingencies, not a representation that we create internally.

What does this mean? That vision does not only imply the reception of information through our eyes; This information is shaped and transformed based on motor changes (p. eg eye muscles or pupillary contraction) and sensory that accompany said visual experience, as well as by the visual attributes of the objects that we perceive.

The basic difference between the sensorimotor model and the conventional model is that the latter assumes that if a In a certain region of the brain (the primary visual cortex), the internal representation disappears from conscious perception, thereby it implies; on the contrary, for the sensorimotor approach, the external world would not be remembered in the mind of the person who perceives it and the reality would function as an external memory that is tested in the relationships between sensory stimuli and responses motor.

differential diagnosis

When diagnosed, blindsight must be differentiated from a number of other disorders such as double hemianopsia, Munk's psychic blindness, hysterical blindness, and mental blindness. simulated.

double hemianopsia

The patient has preserved macular and central vision, although it has a vision in the form of a "gun barrel". This disorder may precede or follow blindsight.

Munk's psychic blindness

The person has difficulty recognizing objects (visual agnosia), although they do retains the sense of visual awareness.

hysterical blindness

The patient is indifferent, but without anosognosia. The tests confirm that the vision is normal, despite the fact that the person reports partial or total vision problems.

simulated blindness

The person invents his own ailment, in this case blindness, to assume the role of sick (Münchhausen syndrome)

Bibliographic references:

• Aldrich MS, Alessi AG, Beck RW, Gilman S. Cortical blindness: etiology, diagnosis and prognosis. Ann Neurol 1987; 21: 149 - 158.
• Brogard, B. (2011). Are there unconscious perceptual processes. Consciousness and Cognition, 20, 449-463.
• O'Reagan, J. & Noë, A. (2001). A sensorimotor account of vision and visual consciousness. Behavioral and Brian Sciences, 24, 939-973.