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Abstract:
Accounts of neurological disorders often posit damage to a
specific functional pathway of the brain. Farah (1990) has proposed
an alternative class of explanations involving partial damage to
multiple pathways. We explore this explanation for optic aphasia, a
disorder in which severe performance deficits are observed when
patients are asked to name visually presented objects, but
surprisingly, performance is relatively normal on naming objects
from auditory cues and on gesturing the appropriate use of visually
presented objects. We model this highly specific deficit through
partial damage to two pathways--one that maps visual input to
semantics, and the other that maps semantics to naming responses.
The effect of this damage is superadditive, meaning that tasks
which require one pathway or the other show little or no
performance deficit, but the damage is manifested when a task
requires both pathways (i.e., naming visually presented objects).
Our model explains other phenomena associated with optic aphasia,
and makes testable experimental predictions.
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