Optic aphasia is a selective disturbance in naming visually
presented objects despite intact visual processes and intact
ability to name objects presented in other modalities.
(Typically these patients have the left occipital cortical and
sub cortical lesions extending to the posterior part of the
corpus callosum, disconnecting the left hemisphere from visual
input). Modality-specific naming deficits, such as optic
aphasia, have been taken as evidence that semantics is organized
into distinct modality-specific subsystems. On this view, the
visual properties of an object are represented in a visual
semantic subsystem, its auditory properties (e.g., its sounds)
in an auditory semantic subsystem, and so forth (Allport,
1985). Consequently, the loci, spared structure will carry on
to operate normally, and the patient’s impairment will be
expressed in a relatively direct and straightforward manner.
This postulation follows from a notion of the cognitive
architecture as being modular in the sense of
being "informationally encapsulated" (Fodor, 1983) that is,
their inputs and outputs are highly constrained. Components
interact only when one has completed its processing, at which
point it makes the end product available to a relatively small
number of other components. If this were true, then the effects
of damaging one component should be relatively local.
McCarthy & Warrington, (1988) amongst others (e.g., Beauvois,
1982; Shallice, 1987) suggested that this modality-specific
DISSOCIATION indicates a disconnection between intact visual and
verbal semantic subsystems. The verbal semantics subsystem is
assumed to be intact because optic aphasics produce normal
language, and produce normal ability to comprehend oral words,
and respond suitably in reply to verbal definitions. The visual
semantic sub system was assumed to be intact since the patients
could mine the use of objects they could not name. McCarthy?? et
al (1988) argued that these contrasting patterns of deficits
provide evidence for a modality specific organisation of the
semantic system.
Thus, optic aphasia provided successful support for McCarthy and
Warrington’s (1988) modality-specific semantics.
It has been proposed however that these modality-specific
anomias does not point towards distinct semantic sub systems.
The neuroscienctific literature has put forward that these
conditions are rather a reflection of a partial disjointing
between modality specific structural descriptions of items and a
unitary modality neutral semantic system (Caramazza, 1990;
Riddoch et al., 1988). What I am dissatisfied with is that I
do not seem to get information about the quesiton I set out in
the outset. Do you have any ideas where I might lose myself?
ALl information about his topic is of interst. Especially with
regard to aphasia etc. Thanks., M.
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