Visual Agnosia: The Inability To Understand Visual Stimuli
I had stopped at a flower shop on the way to his apartment and had bought a slightly extravagant red rose for the buttonhole of his lapel. I took it off and gave it to him. He took it like a botanist or a morphologist who is given a specimen, not like a person who is given a flower.
– “About fifteen centimeters in length. “A rolled red shape with a green linear addition.”
-“Yeah. And what do you think you are?”
–“It’s not easy to say. It lacks the simple symmetry of geometric shapes, although it may have a superior symmetry of its own…it could be an inflorescence or a flower.”
P. acted exactly like a machine acts. It was not just that it showed the same indifference that a computer does to the visual world, but that it constructed the world as a computer constructs it, through distinctive features and schematic relationships.
(…)
I begin today’s entry with this fragment of a book by Oliver Sacks (“The Man Who Mistook His Wife for a Hat”) in which a case of visual agnosia which leads the protagonist of the story to a disintegrated vision of the world and different situations that, although comical, result in a serious problem of visual recognition.
Sight being our main sense, we are always shocked and shocked to read alterations in something as basic as perception. The brain, through its main window to the world – the eyes – shows us a simple and orderly image of the world around us.
This creation that our nervous system makes is shared, to a greater or lesser extent, by almost all of us. The bases of everything we call reality are in the light that hits our retinas and travels through the optic nerve in the form of a nervous impulse, to make synapses in the geniculate nucleus of the thalamus – a structure that we could consider a kind of cerebral toll in the which a large number of synapses are made – until we reach our primary visual cortex in the occipital lobe. But it would be a mistake to believe that this circuit, these three synapses, are what give meaning to the world in which we live. What prevents us from living in a chaotic or fragmented world, as in the case of P., is the function of gnosis.
Gnosis, from the Latin knowledge, refers to the ability to recognize objects, people, faces, spaces, etc. Furthermore, it is also the faculty that offers us a global and united perception of reality and not a schematic or “partial” one. Therefore, visual agnosia is the loss of this ability To better understand this process we will talk about the two main brain pathways that participate in this function. We will also talk about the types of agnosia most frequently described in the literature.
Visual perception: the way of what and where
As we said, information from the retina reaches our primary visual cortex after having synapsed in the thalamus. But the primary visual cortex is not informative in itself when it comes to recognition. It only processes the physical characteristics of what the retina perceives. That is: light, contrast, visual field, visual acuity, etc.
Thus, the primary visual cortex, Brodman area 17, only has raw information. It doesn’t tell us that we see a beautiful sunset or a dry leaf. So, What will it mean to recognize an object?
First of all, we must be able to see the object in question, making those three synapses in order to capture the physical information of the light that hits first the object and then our retina. In second place, we must integrate all this information to perceive it as a whole Finally, we will have to rescue from our memory the memory of that object already present in our memories and its name.
As we can see, this implies more than one source of information. In the brain, the cortex that is in charge of relating different types of information is called the associative cortex. To carry out the steps that we have described, we will need an associative cortex. Thus, the brain will need more synapses, and this is when the what and where pathways come into play.
1. Identification
The what pathway, or ventral pathway, goes to the temporal lobe and is responsible for the recognition and identification of objects It is the way that, if for example we see a big, green thing with thorns in the middle of the desert, it helps us identify it as a cactus and not as the Hulk.
It is not surprising that this pathway is located in the temporal lobe if we think that this is the main body responsible for memory functions. Therefore the path of that They are nervous projections that link the information in our retina with that in our memory. It is the synthesis of optical and limbic information.
2. Location
The way of whereor dorsal pathway, projects to the parietal lobe. It is the way responsible for locating objects in space ; perceive its movement and trajectory, and relate its location to each other. Therefore, it is the way that allows us to direct our movements effectively in a given space.
They are the neurons that allow us to follow with our eyes the direction taken by a tennis ball that is hit from one field to another. It is also the way that allows us to put a letter in a mailbox without making mistakes.
Different neurological disorders – heart attacks, head injuries, infections, tumors, etc. – may affect these pathways with the expected deficits depending on the affected region. As usual, these brain regions will not only be affected if their cortex is damaged, but also if the fibers that connect these areas with the primary visual cortex are affected.
Apperceptive visual agnosia
In this type of agnosia components of perception fail, and consequently, there is no recognition Perception is the faculty that integrates the physical characteristics of an object so that we can capture them as a three-dimensional whole.
In apperceptive visual agnosia this integration is severely affected and the patient shows deficits even in the recognition of the simplest shapes. These patients, when faced with the drawing of a hammer, will not know how to recognize it as a hammer. Nor will they know how to copy it or match it with another drawing of the same hammer. Despite everything, visual acuity is normal, as is the perception of light, darkness, etc. In fact, patients can even avoid obstacles when walking. However, the consequences for the patient are so dire that functionally they tend to appear almost blind with serious problems in their level of independence.
Some authors, in a very opportune manner, have paraphrased Saramago “there are blind people who do not see, and blind people who, seeing, do not see.” The case of a patient with apperceptive agnosia would be the second. These patients can recognize the object through another sensory modality such as touch – sometimes touching different parts of the object in question – or with contextual clues or descriptions from the examiner. Furthermore, these types of actions on the part of the examiner help to make a differential diagnosis and rule out that the anomia – inability to say the name of what is seen – is not due to a language deficit, for example.
It is a rare type of agnosia and has been described most frequently after bilateral infarctions of the posterior arterial regions, carbon monoxide poisoning, and in the posterior variant of Alzheimer’s disease. So that, It is caused by pathologies that affect occipitotemporal regions
Associative visual agnosia
In this type of agnosia, in addition to visual acuity, perception of color, light, contrast… perception is also preserved However, despite normal perception, recognition is affected. As in the previous case, when faced with a drawing of a hammer the subject will not know that it is a hammer, but in this case they will be able to match it with another drawing of a hammer. He may even copy the drawing or describe the object.
They may identify the drawing because of one of the details of the object represented. As a general rule, objects are more difficult to identify than real ones, possibly due to a contextual factor. Again, the rest of the sensory modalities can help with its recognition.
Associative agnosia seems to be due to the disconnection between the visual and limbic systems The substrate may be the bilateral lesion of the white matter (inferior longitudinal fasciculus) from the occipital associative cortex to the medial temporal lobe, which involves a disconnection of the visual and memory systems. That is why this agnosia is also called amnesic agnosia. The causes are similar to the case of apperceptive agnosia.
Other types of agnosia
There are many more types of agnosia and perception disorders Below I will quote some of them. I am simply going to make a small definition to identify the disorder,
1. Achromatopsia
It is the inability to distinguish colors. Patients who suffer from it see the world in shades of gray. A bilateral lesion of the occipitotemporal region appears secondarily. There are very few registered cases. If the injury is unilateral it will not cause symptoms. I highly recommend reading “Anthropologist on Mars” which tells the story of a case of achromatopsia. Plus, reading Oliver Sacks is always a pleasure. I show you a fragment of said case which will be much more explanatory of the disorder than my definition:
“Mr. people’s flesh, his wife’s flesh, his own flesh, an abominable gray; “flesh color” seemed to him “rat color” (…) he found meals unpleasant due to his dull, grayish appearance, and he had to close his eyes to eat “
2. Prosopagnosia
It is the inability to recognize faces of family members, previously known famous people, or even one’s own face in the mirror
Prosopagnosia is a specific deficit in face recognition and, therefore, we must rule out other types of agnosia for its diagnosis. In general, other functions such as reading are not affected.
They can also estimate whether they are human or primate faces and even recognize the emotional expression of the face in question. It should be noted that the deficits are more evident when recognizing photographs than when seeing the person in question, since there will be other contextual clues such as their movement. Also very interesting is the proposal by Damasio et al (1990) who would consider that prosopagnosia would not be so much a failure in face recognition, but rather the inability to identify individuality within a set of similar ones.
It is the inability to perceive moving objects It is frequently due to posterior occipitoparietal lesions. The first case of akinetopsia was described in 1983 in a 43-year-old woman who had suffered several bilateral cerebrovascular infarctions. The deficits seriously affected her level of independence. For example, she needed to touch the rim of the cup to know when she should pour coffee.
Some conclusions
I think it is not necessary to justify how basic the function of gnosis is for our life. In a way, our consciousness depends on what we see and the reality that makes up our brain This “reality”, manufactured by our circuits, is possibly very far from what reality as such is. Let’s think for a moment: when we see how someone speaks, generally what we see and what we hear have a synchronicity. That is to say, if a friend speaks to us, we should not see that he first moves his mouth and then we hear the sound, as if it were a poorly dubbed movie. But, on the other hand, the speed of light and the speed of sound are very different.
The brain, in some way, integrates reality so that we understand it in an orderly and logical way When this evil Cartesian genius fails, the world can take on a chaotic and aberrant tone. Like the fragmented world of P. or the colorless world of I. But is his world more unreal than ours? I think not, we all somehow live deceived by our brain. As if we were in the Matrix. A Matrix created by ourselves.
Patients like P. or I. have contracted pathologies that have made them distance themselves from the “reality” that the rest of human beings are used to sharing. Although these specific cases had happy endings characterized by personal improvement, in the usual vein of Oliver Sacks, it should be noted that not all cases are equally beautiful. Neurologists and neuropsychologists only see clinical manifestations of these pathologies and, unfortunately, on many occasions in these cases we are forced to adopt a “voyeur” attitude. That is to say, Many times we cannot do much more than follow the case and see how it evolves
Currently, pharmacological therapies for neurodegenerative disorders are of very limited usefulness. Science must develop new drugs. But neuropsychologists must develop new non-pharmacological therapies beyond classic cognitive stimulation. Centers such as the Guttmann Institute, specialists in neurorehabilitation, are putting great effort and dedication into this. My subjective opinion is that perhaps the new virtual reality therapies will mark the 21st century of neuropsychology. In any case, we must work on this or other options and not settle for just the diagnosis.
Books that narrate cases of Agnosia and that I highly recommend reading:
Textbooks:
Articles:
I highly recommend this article above. It is very well explained and very clear and concise.
