Blind Vision: Causes And Symptoms Of ‘seeing Without Knowing What You See’
His eyes work well, they are intact. But they say they don’t see anything. And they really see, without knowing what 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 in the environment.
In this article we explain what blindsight is, how this concept arises, what its causes are, and how to differentiate it from other similar disorders.
Blindsight (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 what 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 research on the phenomenon of blindsight was 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 humans could achieve normal vision with these brain areas damaged. Patients whose visual cortex had been destroyed showed total blindness, or so it seemed. In 1973, German psychologist Ernst Pöppel’s team found that, although some of them lacked a visual cortex and declared that they were unable to see objects, the eye movements of his eyes were directed towards them : it was evidence that his visual system was informing, in some way, of their existence.
But what finally convinced the scientific community that the phenomenon of blindsight deserved its full attention was the work of Larry Weiskrantz and his colleagues in the early 1970s. 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 being asked to guess which one applied to a visual object that they said they couldn’t 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 blindsight 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 lattices, the moment of appearance or facial expressions However, they cannot do so with other aspects such as detecting subtle nuances or complex movements.
Causes and brain structures involved
Blindsight 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 about 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 object. 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.” , since the information has not yet reached the higher cortical levels. However, it can influence our behavior, as occurs in cases of blindsight, in which the person sees without knowing what he or she sees.
Patients with blind vision have damaged, therefore, 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 conscious recognition to exist. of what we perceive.
The sensorimotor model of vision
The conventional model of structural failure in visual processing (which involves a lesion in several areas of the brain) implicitly assumes that vision consists of creating an internal representation of external reality, the activation of which would generate conscious visual experience. 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 , considers that vision must be understood as a necessary tool for survival. According to Gibson, the real value of visual processing is being able to identify and see with our eyes what is and where, so that 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, 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 involve receiving information through our eyes ; This information is shaped and transformed based on the motor (e.g., eye muscles or pupillary contraction) and sensory changes that accompany said visual experience, as well as the visual attributes of the objects we perceive.
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.
Differential diagnosis
When diagnosed, blindsight must be differentiated from another series of similar disorders such as double hemianopsia, Munk’s psychic blindness, hysterical blindness and simulated blindness.
Double hemianopia
The patient has preserved macular and central vision , although it has a “rifle barrel” shaped vision. This disorder may precede or follow blindsight.
Munk’s psychic blindness
The person has difficulty recognizing objects (visual agnosia), although preserves the sensation of visual awareness
hysterical blindness
The patient appears indifferent, but without anosognosia Exams confirm that vision is normal, even though the person reports partial or complete vision problems.
Simulated blindness
The person invents his own illness in this case blindness, to assume the role of the sick (Münchhausen syndrome)
