| |
Abstract:
Abstract: It has been posited that the learning and use of
memorized words is subserved by temporal-lobe circuits previously
implicated in the learning and use of conceptual knowledge, whereas
the learning and processing of grammatical rules depends upon
frontal/basal-ganglia circuits implicated in the learning and
expression of skills and habits (Ullman et al., 1997). Evidence
suggests that the temporal-lobe memory system underlies irregular
past-tenses (dug), whereas the frontal/basal-ganglia system
normally underlies the compositional -ed-affixation of regulars
(walked). It was predicted that damage to frontal/basal-ganglia
circuits would not only impair -ed-affixation (Ullman et al.,
1997), but would also lead to a reliance on the temporal-lobe
system to memorize regular forms. To test this hypothesis, 9
agrammatic non-fluent aphasics with frontal damage and 5 fluent
aphasics with temporal damage were asked to read aloud regular and
irregular past-tense forms. The fluent aphasics, like healthy
subjects, showed past-tense frequency effects (positive
correlations between successful past-tense computation and
frequency) for irregulars, but not for regulars. In contrast, the
agrammatics showed frequency effects for regulars as well as
irregulars, suggesting both were memorized. Moreover, a positive
correlation between years post-lesion and frequency correlation
r-values was obtained only for the agrammatics and only with
regulars, suggesting that the agrammatics memorized regulars
following lesion-onset. The data underscore the relative plasticity
of the temporal-lobe system and its role in the recovery of
grammatical function.
|
