ScienceMade Research-Style Blog
By Emily Sun
Seeing, perceiving, and interpreting. Many of us are able to perform these tasks so seamlessly on a day-to-day basis without even realizing. How can you tell your mother is not your next door neighbor Fred? Or that your bedroom is not your living room? These questions may seem absurd for the majority of us-- comical, even. We just seem to know. But how?
Close your eyes. If you can imagine the layout of your house, or the stuffed animals sitting on your bed, then your brain has successfully formed a cognitive map of this location. This is because your recognition of objects and locations rely heavily on stored sensory information in certain areas of the brain. When trauma or exposure to toxins causes damage to these regions, this may render your memories of these locations and objects useless, resulting in a rare but serious disorder called visual agnosia.
Acquired Topographical Disorientation (TD) is a specific type of visual agnosia where one cannot orient oneself due to severe problems recognizing previously known visual input. There are many possible symptoms of this condition, but the most common variations are (1) the inability or heightened difficulty in perceiving familiar locations, commonly accompanied by further difficulty in facial recognition, called prosopagnosia (2) the ability to orient oneself in familiar pathways but difficulty in learning new pathways from one location to another (3) successful recognition of an object but inability to glean a sense of direction from these visual cues [2].
A patient with TD may have perfect mental recall of a particular location, yet still lack the ability to successfully navigate towards or within that location due to impaired object-association skills, or the ability to make connections between a physical object and its significance. The reasoning behind this strange phenomenon may be traced back to the complex organization of spatial memory, memory crucial for orientation and navigation. This would explain why in TD, even though a patient’s ability to see is perfectly normal, there is usually damage found in regions of the brain responsible for processing certain visual cues.
Take the fusiform gyrus, for example. It is a structure located between the temporal and occipital lobes of the brain (see diagram on right) thought to play a particularly large role in categorization and recognition of people and objects, particularly in a section called the fusiform face area (FFA) [6] . Although the name suggests that the FFA is specifically dedicated to facial recognition, several studies suggest otherwise [5]. Scientist Isabel Gauthier and her team tested eleven car experts and eight bird experts on the recognition of various cars and birds, respectively. Interestingly, the team found that there was activation of the FFA in the participants [1]. This seemingly intertwined relationship between object and facial recognition may explain why sufferers of TD experience some difficulty recognizing faces and landmark disorientation.
Another region found to play a pivotal role in TD is the lingual gyrus, which is primarily in charge of visual processing and memory-encoding. The lingual gyrus plays an essential role in a task-based pairing of words to a given image, which is largely based on memory retrieval rather than logical problem-solving [3]. That is, a patient with TD may have no problem knowing what a given object is from past experiences, but is unable to retrieve the required memory that matches the object input, demonstrated clearly in Figure 1. As shown by the figure, a patient with demonstrated associative visual agnosia was told to copy a series of four objects and animals and identify them to the best of their ability. The patient copied each image to near perfection, indicating that this was not a problem with their eyes nor was this a problem with memorizing and understanding the given images. If asked, the patient was knowledgeable as to what a key, pig, bird, and train was because they had previously encountered that stimuli. Rather, this inability to interpret the given objects suggests that the patient has problems with object-association.
Interestingly, the patient drawing the train remarks that “the larger vehicle is being pulled by the smaller one” [4]. This specific observation sheds light on the patient's difficulty connecting the details of the image to form one cohesive and meaningful image. Thus, a probable explanation is that the patient suffered damage on both sides of the brain, inhibiting his ability to perceive objects with situational relevance.
Today, the adult treatment of TD remains limited and focuses largely on coping mechanisms. Unfortunately there are virtually no treatment options for children who are born with or acquire this condition at a young age because they have not adequately developed spatial learning and memory. We often take for granted everyday object-association and assessment because our brains do the majority of the work for us. Hopefully understanding these complex processes behind ~seemingly~ simple actions can help us feel grateful for everything we are capable of.
References
[1] Gauthier I, Skudlarski P, Gore JC, Anderson AW (Feb 2000). "Expertise for cars and birds recruits brain areas involved in face recognition". Nat. Neurosci. 3 (2): 191–7.
[2] Kim JS. 25 – Posterior Cerebral Artery Disease. In: Stroke: Pathophysiology, Diagnosis, and Management. Sixth Edition; 393-412
[3] Leshikar, E. D., Duarte, A., & Hertzog, C. (2012). Task-Selective Memory Effects for Successfully Implemented Encoding Strategies.
[4] 18:The Organization of Cognition. The Organization of Cognition | Principles of Neural Science, Fifth Editon | AccessNeurology | McGraw-Hill Medical.
[5] Peelen MV, Downing PE. Selectivity for the human body in the fusiform gyrus. J. Neurophysiol. 2004;93 (1): 603-8.
[6] Tyler LK, Chiu S, Zhuang J. Objects and categories: feature statistics and object processing in the ventral stream. J Cogn Neurosci. 2013;25 (10): 1723-1735.