William Bechtel and Jennifer Mundale make the following comment regarding lesion studies and multiple realization:
Nevertheless, it is important to note
that in interpreting these deficits, researchers implicitly reject
multiple realization among human brains and assume that damage to a
brain area in anyone will result in a deficit to a particular
cognitive function that is performed by that area in undamaged
brains. (Bechtel & Mundale, 1999, p. 184).
This seems to be an overinterpretation of the implications of
localization research. For simplicity of exposition, let us consider
only one area, say, V1 and suppose, again only for the sake of
simplicity, that it has only one cognitive function. Finally,
suppose that V1 displays some variation in size and position from
individual to individual, but is constant enough to enable
investigators to assume that damage to V1 in one individual will lead
to comparable damage in another individual.
Let us concede what Bechtel and Mundale are pointing out and what
seems to be entirely correct. Let us say that V1 is univocally
localized. The point to observe now is that this univocal
localization is not the same thing as univocal realization.
Moreover, by itself, buying into univocal localization does not
amount to rejecting multiple realization. For it may still be that
there are any number of ways in which cells of V1 might be organized
to carry out the functions of V1. There might be lots and lots of
different ways of putting together nerve cells to form a region of
brain tissue that performs the function(s) of V1 whatever those
happen to be. Were this to happen, were there to be different ways
of assembling neurons so as to realize the function(s) of V1, this
would look to be a case of multiple realization. So, it appears that one can have
a psychological function be multiply realized, even if it is realized
in only one region of the brain. So, Bechtel and Mundale do not here have a compelling argument against the multiple realization of psychological functions.