There is converging evidence that aging causes a progressive decline in the central processing of speech and that this decline is greater for left-ear than for right-ear input. In the present paper we investigated, by means of a dichotic sentence identification paradigm, some parameters of the “left ear effect.” We analyzed the clinical records of 366 individuals, 203 males and 163 females, to whom the Dichotic Sentence Identification (DSI) Test had been administered as part of routine audiometric assessment. Subjects ranged in age from B to 91 yr. The DSI test was always carried out in two modes: free report (FR) and directed report (DR). In the FR mode the subject reported what was heard in both ears. In the DR mode the subject reported only what was heard in one precued ear. In half of the trials the right ear was precued, in the other half the left ear was precued. Findings confirm a progressively larger right-ear advantage, or left-ear deficit, with increasing age. We document this effect in both the FR and DR modes, then demonstrate that the effects cannot be attributed to interaural asymmetries in threshold sensitivity. Comparison of male and female data suggest a gender difference in the effect of age on the left-ear deficit. Males show a larger effect then females in both modes of test administration. Finally, we propose a model of dichotic listening performance that attempts to explain ear asymmetry as the linear combination of an auditory/ structural component and a task-related/cognitive component. We then show how these hypothetical components change with age in the present sample.
Address for correspondence: James Jerger, 11922 Taylorcrest Rd., Houston, TX 77024-300.
Accepted September 30, 1993; Accepted January 28, 1994.
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