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Performance of two hemispherectomized subjects on a dichotic binaural frequency fusion test Feick, Elizabeth Anne

Abstract

This study investigates the performance of two hemispherec-tomized subjects and ten normal subjects on a dichotic binaural frequency fusion (DBFF) test and on a competing dichotic message test. The DBFF test was designed to examine whether binaural integration of two complementary frequency segments of the same word, dichotically presented, necessitates the presence of two intact hemispheres. The competing dichotic message test was presented to provide a measure of the extent of strengthening of ipsilateral pathways in the hemispherectomized subjects. The DBFF test consisted of three fifty-word CNC lists which were processed through two band-pass filters and recorded on a two--channel magnetic tape. The test consisted of two binaural conditions. In the Dichotic A condition, the high band was delivered to the left ear and the low band to the right. The Dichotic B condition was the reverse of the first. For each condition 50 phonetically balanced (PB) words were presented and the subject was required to repeat the word in a 4 second interval between words. The competing dichotic message test consisted of 15 sets of three pairs of words, one of each pair being presented simultaneously to either ear, using stereophonic head-phones. The subject was required to repeat as many words from each set as possible. The Z scores, measuring the deviation in standard deviation units of the raw scores of the operated subjects from the mean scores of the normals indicated that the removal of a hemisphere did not significantly decrease the scores of two hemispherectomized subjects on a DBFF test. Removal of a hemisphere, however, decreased the scores of the hemispherectomees on the competing dichotic message test in one of the ears -- specifically the ear contralateral to the removed hemisphere. A comparative analysis of how the central auditory nervous system (CANS) of a hemispherectomized subject might process a complementary dichotic message (exemplified by the DBFF test), as opposed to a competing dichotic message, provides an interesting basis for a discussion on the nature of the "biological detector" of speech elements in the CANS.

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