This neurological disorder causes some people to see despite believing they are totally blind.
His eyes work fine, they are 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 blind vision, a neurological disorder caused by brain damage that affects the ability to consciously represent the visual stimuli of the environment.
In this article we explain what blind vision is, how this concept arose, what its causes are, and how to differentiate it from other similar disorders.
Blind vision: definition and background
Blind vision ( blindsight ) is a term coined by the English psychologist, Lawrence Weiskrantz, which refers to the ability of some subjects to detect, locate and discriminate visual stimuli unconsciously. People with this disorder “see, without knowing that they see” ; that is, they do not consciously recognize the objects in front of them, even though they act as if, de facto, they were there.
The first investigations on the phenomenon of blind vision were carried out in animals, mainly monkeys, with the surgical removal of the brain regions responsible for vision (area V1). When deprived of these structures, the animals appeared to retain some visual abilities, such as the ability to detect contrast or to differentiate one object from another based on its shape.
Few neuroscientists believed that human beings could achieve normal vision with these damaged brain areas. Patients whose visual cortex had been destroyed showed total blindness, or so it appeared. In 1973, the team of the German psychologist Ernst Pöppel found that, although some of them lacked visual cortex and declared that they were unable to see objects, the eye movements of their eyes were directed towards them : it was evidence that their visual system was informing, in some way, of their existence.
But it was the work of Larry Weiskrantz and his colleagues in the early 1970s that finally convinced the scientific community that the phenomenon of blind vision deserved their full attention. In the experiments, the technique of forced choice was used (which forces patients to choose between defined options, instead of just asking what they see): patients had to choose between two possible colors or locations, while at the same time they asked them to guess which one was applicable to a visual object they said they couldn’t see.
The responses of some of the patients were found to be correct in a significant proportion; that is, more frequently than one might expect by chance. It was from then on that these people began to be labeled as blind vision patients.
Nowadays, it has been shown that people with blind vision can not only “intuit” the color or the location of objects, but also the orientation of lines or lattices, the moment of appearance or the expressions of faces. However, they cannot do it with other aspects such as detection of subtle nuances or complex movements.
Causes and brain structures involved
Blind vision occurs in a portion of our perceptual 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 processing 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 areas, are responsible for integrating the information from each sensory modality (in this case, sight).
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”, as 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.
Patients with blind vision have, therefore, damaged the final module of a complex visual processing circuit, which is insufficient by itself and without the rest of the sensory and subcortical structures but necessary, at the same time, for there to be conscious recognition of what we perceive.
The sensorimotor model of vision
The conventional model of structural failure in visual processing (which involves injury to various areas of the brain) implicitly assumes that vision consists of creating an internal representation of external reality, the activation of which would generate the conscious visual experience. However, it is not the only one that has been postulated to try to explain why a phenomenon such as blind vision occurs.
The ecological approach to visual perception proposed by psychologist James J. Gibson, considers that vision must be understood as a necessary tool for survival. According to Gibson, the real value of visual processing is in being able to identify and see with our eyes what is and where, so that we can avoid obstacles, identify foods or potential 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 to discriminate the relevant information, from among so many signals, to be able to manage a particular behavior.
At present, Gibson’s approach has been reformulated as the sensorimotor model of vision, in which concepts are borrowed 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 as a function of the motor (eg eye muscles or pupillary contraction) and sensory changes that accompany this visual experience, as well as the visual attributes of the objects that we perceive themselves.
The basic difference between the sensorimotor model and the conventional model is that the latter assumes that if a certain region of the brain (the primary visual cortex) fails or is missing, the internal representation disappears from conscious perception, with what this 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 reality would function as an external memory that is tested in the relationships between sensory stimuli and motor responses.
At the time of diagnosis, blind vision must be differentiated from other similar disorders such as double hemianopia, Munk psychic blindness, hysterical blindness and simulated blindness.
The patient has preserved macular and central vision, although he has vision in the form of a “rifle barrel”. This disorder can precede or follow blind vision.
Munk’s psychic blindness
The person has difficulty recognizing objects (visual agnosia), although he does retain the sense of visual awareness.
The patient is indifferent, but without anosognosia. The exams confirm that the vision is normal, even though the person reports partial or total vision problems.
The person invents his own ailment, in this case blindness, to assume the role of sick person (Münchhausen syndrome)
- Aldrich MS, Alessi AG, Beck RW, Gilman S. Cortical blindness: etiology, diagnosis and prognosis. Ann Neurol 1987; 21: 149-158.
- Brogaard, 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.