The mechanism of verbal short-term memory and its impairment : from an investigation of retention strategy

Abstract

     The present study reports a case of selective impairment of verbal short-term memory. The patient was a 59-year-old non-right-handed male native speaker of Japanese. He demonstrated no signs of aphasia even immediately after the onset of left-hemisphere infarction. Standardized examinations of intelligence, attention, and frontal lobe function revealed the patient's performances were within normal range and clinical observations indicated that his visual-spatial cognition and memories of daily activities were preserved. In contrast, his digit span scores were limited to three or four.
     In-depth examinations of his repetition revealed the following ; (1) repetition of meaningful sentences with phonological sequences identical to those used in the digit span test was better than that of the digit span test ; (2) when each set of digits in the digit span test was presented as a single number (i. e., “274 (two hundred seventy four)” for “2,7,4”) repetition performance was better compared to that of the digit span test ; (3) recall of high-frequency two-syllable word sequences was better than recall of two-syllable nonsense word sequences consisting of the same syllables.
     The findings suggest a possibility that the capacity of verbal short-term memory is determined by the number of chunks and processing efficiency of the chunks rather than the amount of segmental phonological information, which is consistent with previous reports. Further discussion on the effects of stimulus materials and presentation rate were provided and a clinico-anatomical examination suggested anatomical basis of verbal short-term memory was in the vicinity of the transverse temporal gyrus and operculum temporale of the dominant hemisphere.