Breakdowns: Amnesia, Agnosia, Aphasia

Summary Prepared by Mark Domino, Zubin Tiku, and Litza Stark

The study of breakdowns is essential to cognitive science because if the mind/brain is domain-specific (as cognitive scientists assume), then breakdowns should reveal the domains and the interfaces between them. Though there are system-wide breakdowns (such as Alzheimer's disease, though even this has domain-specific effects), the examples given in class involve localized trauma. Though these "experiments of nature" are not surgically precise, the ones presented-- amnesia, agnosia, and aphasia-- are fairly specific to the domains they have damaged.

Amnesia is a general term for the breakdown of memory. Retrograde amnesia affects memory that existed prior to trauma, while anterograde amnesia affects the memory of the present and future. H.M., our example, suffered from both of these amnesias following surgery to reduce seizures. This surgery created a lesion in his hippocampus (supposed to be a gating mechanism between short-term and long-term memory) and amygdala (supposed to be resoponsible for emotional response).

H.M. lost memories of the two years preceeding his surgery. Though he maintained "short term" memory, this was quickly and permanently lost when he was distracted; hence, he could not learn new things and remember them over any substantial period of time. H.M. did show evidence that priming effects were present, and seemed to be able remember new information when it was repeatedly reinforced. H.M. seemed to be able to remember procedural information, but declared no knowledge of the procedure as an event he had experienced. The most striking example of this is his performance on the Tower of Hanoi puzzle; over repeated exposure, his skill with the puzzle increased, though he had no memory of ever doing it, let alone getting better at it. That H.M. can retain procedural memory ("how-to" knowledge) suggests that his disorder is linked to retreival, and not storage.

One possible explanation for his disorder is that he has retained procedural memory (how memory), but not declarative memory (what memory). Procedural memory should be impaired by his lack of declarative memory, however. Thus, he would have lost his ability to recall declarative information, but can still access related procedural memories. Another proposed explanation of H.M.'s disorder is that he has a "disorder of consciousness"; he has no conscious awareness of what he knows or experienced, even though he can sometimes use the information.

[Or HM has a disruption of explicit vs. implicit knowledge, or controlled vs. automatic. Note how all these explanaitons make reference to the mind-brain as a general computing device.]

A second example of specific modular breakdown is agnosia, which is a disorder of visual perception and recognition. Agnosia affects low-level and high-level processing, but not sensory organs. Apperceptive agnosia is the most severe form; those with this disorder can see only the most fundamental aspects of the seen world, such as color and texture. It is impossible for them to perceive objects. Simultanagnosia is another form of agnosia, and it comes in two forms: dorsal and ventral (depending on where the brain damage is). Dorsal agnosics have difficulty perceiving more than one object at once, since they cannot compute peripheral information, and this has far-reaching effects on their perceptive abilities. They are unable to perceive location, match, or count objects. Ventral agnosics are able to perceive more than one object, but they have difficulty extracting detail from objects, such that all vision is equivalent to peripheral vision. Thus, they are able to perceive location and to count objects. Another type of agnosia, distinct from those varieties described above, are associative agnosias, which are disorders affecting the larger knowledge and representational system. Associative agnosics have difficulty with naming specific types of objects.

[Note the ocnnection of "what" and "where" to these two streams of processing: "where" requires the larger field to judge objects' relative positions.]

The examples in the film [Stranger in the Mirror] we watched in class reveal the difficulty in diagnosing and categorizing agnosia. One man in the film could identify individual objects and pieces of objects, but had extreme difficulty informing cohesive wholes out of these pieces. His disorder could thus be categorized as a form of simultanagnosia, with certain characteristics of both dorsal and ventral varieties (he had trouble perceiving multiple objects, but could still perceive location, for example). The second man in the film was unable to recognize individual faces, nor did he notice or identify flowers by sight. He apparently had suffered a breakdown in his face recognition system, along with other specific visual knowledge.

Breakdowns in language abilities are aphasias, and there are many types, but two receive significant attention: Broca's aphasia and Wernicke's aphasia. Broca's aphasia affects morpho-phonological production; those with this disorder can understand the language and know what they want to say, but can't produce the words. They tend not to use verbs, but only nouns and noun phrases. [That is, it looks like a lexical/functional category split.] Subjects with Wernicke's aphasia have control of the normal structure of language, but speak in a meaningless jumble of pronouns and connecting words. [They also fail to understand and generally have disruption of lexical-semantic abilities, including naming and specific semantic features.]