Hemispheric lateralization and acoustic change complex in individuals with normal hearing

Abstract

The specific objectives formulated were to compare the latency and amplitude of acoustic change complex (ACC) for /sa/ and /si/ in terms of (1) ipsilateral and contralateral recording sites and (2) laterality in the right and left ears among male and female participants. Fifteen right-handed individuals (seven male, eight female) with normal hearing in the age range of 18–25 years participated in the study. The ACC were recorded by placing non-inverting electrodes on the cortex (C3 and C4) and a reference electrode on the ipsilateral mastoid of the test ear. To record the ACC, the stimulus (/sa/ or /si/) was presented separately to the right and left ears through an insert phone. The order of stimulus presentation and ear of stimulation were counter balanced across the participants. It was found that the mean latencies of ACC components were larger for /sa/ than for /si/, but significant difference noted in N1. Further, the mean latencies of N1 and P2 were shorter at the ipsilateral site when the right ear was stimulated and also at the contralateral site when the left ear was stimulated, suggesting rightward lateralization. The mean latencies of 2N1 and 2P2 were shorter at the ipsilateral site when the left ear was stimulated and at the contralateral site when the right ear was stimulated, indicative of leftward lateralization. However, the amplitude of the ACC components did not follow any trend among the stimuli at each recording site, presented at either ear. Further, female participants had a shorter latency and larger amplitude of ACC than the male participants, though not significant. The finding of the current study provides an insight into the utility of the ACC in detecting the subtle changes in ongoing stimulus. The rightward lateralization to the onset portion of stimulus and leftward lateralization to the transition portion of stimulus is due to the inherent rate mechanism at the cortical level.