Hemispheric asymmetries in auditory event-related potentials (AERPs) to linguistic and nonlinguistic stimuli

Abstract

Differences in N1P2 amplitude of auditory event-related potentials (AERPs) recorded simultaneously from homologous sites overlying temporo-parietal regions in the left and right hemispheres in response to verbal and nonverbal stimuli were examined in normal human adults. Although previous studies have investigated interhemispheric differences in AERPs none has systematically examined the influence of handedness, gender and priming on hemispheric response. Various technical deficiencies and methodological shortcomings (e.g. inadequate assessment of subjects' handedness, and neurological and auditory status) are also apparent in these studies. In Experiment 1, two random sequences of blocked verbal and nonverbal stimuli were presented to 8 male and 8 female left- and right-handers. The expected pattern of laterality, namely, left hemisphere response greater than the right for verbal stimuli, and right hemisphere response greater than the left for nonverbal stimuli, occurred for right-handers. However, left-handers showed no consistent pattern, a finding in accord with the view that left-handers are more heterogenous than right-handers in terms of hemispheric lateralization, especially for language. There was no evidence that the laterality effect was more pronounced in men than in women. Unexpectedly, the laterality effect for right-handers was significant only for one order, namely, for verbal followed by nonverbal presentation. Experiment 2 compared blocked versus interspersed presentation of verbal and nonverbal stimuli. Subjects were 18 right-handed men. As predicted, there was a significant difference in the laterality effect between these conditions, the effect reaching significance only for the blocked, but not the interspersed, condition. This result was interpreted as evidence for the influence of priming on hemispheric asymmetries in the AERP. That is, blocked (but not interspersed) presentation would prime the hemisphere specialized for a particular type of stimulus (i.e. verbal versus nonverbal). Within the blocked condition, the laterality effect was significant only for nonverbal followed by verbal presentation. Experiment 3 again compared blocked and interspersed presentation of verbal and nonverbal stimuli. However, subjects (12 right-handed men) were instructed to count the occurrences of particular stimuli. It was expected that counting would optimize the priming effect, especially in the blocked condition. Although the laterality effect was again demonstrated there was no significant difference between the blocked and interspersed conditions. Counting may have overridden any dampening of the laterality effect associated with interspersed presentation. There was no significant difference in the laterality effect between the two blocked conditions. Since a possible source of the difference between the two orders within the blocked condition in Experiments 1 and 2 lay in a slight difference in electrode placement, AERPs in Experiment 3 were recorded simultaneously from both montages employed earlier. However, electrode placement did not modify the laterality effect. These experiments have established the auditory event-related potential as a robust electrophysiological indicator of hemispheric specialization for verbal and nonverbal processing in the intact brain. The results are in accord with the current model of the AERP which holds that the late components (e.g. N1P2) are modulated by the psychological characteristics of the subject (e.g. hemispheric organization), the psychological characteristics of the stimulus (e.g. verbal or nonverbal) and manipulation of the experimental task (e.g. priming).