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Abstract:
Background
: Neurological double dissociations suggest that the mental
dictionary of words is part of a temporal lobe declarative memory
system for facts, whereas grammatical rules are processed by a
frontal/basal ganglia procedural memory system for motor skills:
Patients with temporal lobe damage (posterior aphasia, Alzheimer's
disease) are worse producing past tenses of irregular (
dig-dug
) than regular (
look-looked
) verbs; those with frontal/basal-ganglia lesions (anterior
aphasia, Parkinson's disease) show the opposite pattern (Ullman et
al., 1997). This dichotomy may extend to the domain of reading:
Anterior aphasics often have difficulty following spelling-sound
correspondence rules (
bool
) , while being relatively spared at reading "irregular" words (
wool
) (phonological dyslexia) (Goodglass, 1993). Patients with
Alzheimer's diseases can show the opposite pattern (surface
dyslexia) (see Balota & Ferraro, 1993).
Motivation
: Recent evidence suggests the cerebellum may subserve cognition,
including the search or retrieval of lexical and semantic knowledge
(Schmahmann, 1997). We used the regular/irregular paradigm to probe
the role of the cerebellum in language.
Method
: A 38 year old monolingual man (W.W.), whose brain MR showed
diffuse cerebellar atrophy but was otherwise unremarkable, was
given 3 tasks: Past tense production ("Every day I
dig
a hole. Yesterday I _____ a hole"), plural production
(ÒHere is one
mouse
. Here are three ____Ó), and the reading out loud of words
with regular (
pool
) or irregular (
wool
) pronunciations.
Results
: W.W. was worse at irregulars than regulars: at producing past
tenses (5% vs. 22%, p<.05) and plurals (33% vs. 89%, p=.05), and
pronouncing words (67% vs. 92%, p<.05).
Conclusions
: Cerebellar atrophy can lead to a deficit of lexical memory,
while leaving rule processing relatively spared, in the domains of
both morphology and reading out loud. These dissociations
underscore a role for the cerebellum in the search or retrieval of
lexical knowledge, and suggest that it may be less important for
rule processing. The results strengthen the link between words and
facts, and further dissociate these from rule processing.
Grant support: Army DAMD17-93-V-3018
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