Perceptual asymmetry on the dichotic fused words test and cerebral speech lateralization determined by the carotid sodium amytal test

Abstract

The rhymed fused dichotic words test was administered to 61 epileptic patients whose lesions were atrophic and predominantly unilateral. Subjects were categorized according to the side of speech representation, as determined by intracarotid sodium Amytal injection (left-hemisphere representation, 35 subjects; right-hemisphere representation, 4 subjects; bilateral representation, 22 subjects). Results indicated that 33 of the patients with left-hemisphere speech representation obtained right-ear advantages, and all four of the patients with right-hemisphere speech representation obtained left-ear advantages. The distribution of scores for patients with bilateral speech straddled the zero ear-advantage point, but overlapped both of the other two groups to some extent. Neither handedness, familial handedness, sex, nor side of epileptogenic focus had any significant effect on the observed asymmetries. It was also found that stimulus dominance effects have an important influence on the results, and must be taken into account in the interpretation of dichotic listening asymmetries. It is concluded that this test yields valid estimates of speech lateralization; it is also apparent that the magnitude of ear advantage may be an important variable insofar as the more extreme asymmetries appear to be exclusively associated with speech dominance of the contralateral hemisphere, whereas small-magnitude asymmetries are often associated with bilateral speech representation.