Blind vision: causes and symptoms of ‘seeing without knowing you can see
His eyes work fine, they’re intact. But they say they don’t see anything. And they really see, not knowing what they see. This very 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 visual stimuli from the environment.
In this article we explain what blind vision is, how this concept arises, what its causes are, and how it differs 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 who suffer from this disorder “see, without knowing that they see” ; that is, they do not consciously recognize the objects before them, even though they act as if, in fact, they were there.
The first research into the phenomenon of blind vision was carried out on animals, mainly monkeys, with the surgical removal of the brain regions responsible for vision (the V1 area). When deprived of these structures, animals seemed 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 humans could ever have normal vision with these damaged brain areas. Patients whose visual cortex had been destroyed showed total blindness, or so it seemed. In 1973, the team of German psychologist Ernst Pöppel found that, although some of them lacked a visual cortex and claimed to be unable to see objects, the eye movements of their eyes were directed towards them : it was evidence that their visual system was somehow informing them of their existence.
But what finally convinced the scientific community that the phenomenon of blind vision deserved their full attention was the work of Larry Weiskrantz and his colleagues in the early 1970s. The experiments used the technique of forced choice (which forces patients to choose between definite options, instead of just asking what they see): patients had to choose between two possible colours or locations, while being asked to guess which one was applicable to a visual object they claimed not to be able to see.
The responses of some of the patients were found to be correct to a significant extent; that is, more often than would be expected by chance. It was only after that that these people began to be labelled as blind patients.
Today, it has been shown that people with blind vision can not only “intuit” the color or location of objects, but also the orientation of lines or grids, the moment of appearance or the expressions of faces . However, they cannot do this with other aspects such as the detection of subtle nuances or complex movements.
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 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 which, acting as relay zones, are responsible for integrating the information of each sensory mode (in this case, sight).
At the subcortical level, visual information passes through structures such as the spinal bulb, the midbrain and the lateral geniculated 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 upper cortical levels. However, it can influence our behaviour, as in the case of blind vision, where the person sees without knowing that he or she 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 a conscious recognition of what we perceive.
The sensorimotor model of vision
The conventional model of structural failure in visual processing (involving injury to various areas of the brain) implicitly assumes that vision is about 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 there and where, so that we can avoid obstacles, identify food or possible threats, achieve goals, etc.
All this “visual deduction” work would be done by the retina in interaction with multiple environmental signals. And the key would be to discriminate the relevant information, among so many signals, in order 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 postulating that vision is an activity to explore our environment based on sensorimotor contingencies, not a representation we create internally.
What does this mean? That vision does not only involve the reception of information through our eyes ; that information is shaped and transformed by the motor (e.g. eye muscles or pupil contraction) and sensory changes that accompany such a visual experience, as well as by the visual attributes of the objects we perceive themselves.
The basic difference between the sensory-motor 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 all that this implies; on the contrary, for the sensory-motor approach, the external world would not be remembered in the mind of the perceiving person and reality would function as an external memory that is tested on the relationships between sensory stimuli and motor responses.
When diagnosed, blind vision must be distinguished from a number of other similar disorders such as double hemianopsia, Munk’s psychic blindness, hysterical blindness and sham blindness.
The patient has preserved his macular and central vision , although he has a vision in the form of a “rifle barrel”. This disorder may precede or follow blind vision.
Munk’s psychic blindness
The person has difficulty recognizing objects (visual agnosia), although he or she does retain the sense of visual awareness .
The patient is indifferent, but without anosognosy . Tests confirm that vision is normal, even though the person reports partial or total vision problems.
The person invents his own illness , in this case blindness, to assume the role of the 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.