To ascertain the primary improvement mechanism in speech therapy, we applied factor analysis to the improvement scores for 26 subtest items on the Standard Language Test of Aphasia (SLTA) of 267 aphasic patients whose mother tongues are Japanese. The improvement score was defined as the difference between the subtext score at the beginning of speech therapy and at the second evaluation. Our factor analysis derived six improvement factors, none of which was considered as corresponding to so-called general factor. Factor I was determined as representing "non-converting" language production and complex language information processing. Each one of Factors 2-6 was identified primarily by a particular modality, leading us to presume that Factors 2-6 represent instrumental faculties of language information processing. As Factor 1 appears to embody the entirety of the human language function and to manage symbols creatively and independently, we categorize this Factor as "the core factor of improvement in a recovery from aphasia" and stress its importance in speech rehabilitation.

Key words: speech therapy, Standard Language Test of Aphasia (SLTA), factor analysis, core factor of improvement in aphasia rehabilitation