Lateralization-specific adaptation in auditory cortical evoked potentials: Comparison with frequency-specificity.

'Opponent channels model' (OCM) is the widely accepted model for cortical representation of sound lateralization. Stimulus-specific 'release from adaptation' (RFA) in cortical responses has been used in previous studies to test the predictions of this model. However, these attempts were shown to be prone to confounds of spurious responses such as those to auditory motion and sound onset. The present study aims to determine whether a multiple-adaptor RFA algorithm could be employed for relatively confound-free quantification of the population response of lateralization-specific auditory cortical neurons, and provide useful data for estimation of the OCM hemifield tuning curves. Two experiments were conducted on 12 volunteers with normal hearing. In Exp.1, quadruple tone pips of either low or high frequency were presented as adaptor, followed by a single tone pip of either frequency as probe. In Exp.2, tone pips were replaced with dichotic click train pips with left-leading and right-leading interaural time difference (ITD). Frequency- and ITD-specific RFA in cortical responses N1 and P2 was quantified using global field magnitude difference between ERPs to mismatched and matched adaptor-probe pairs. RFA level measured was lower for ITD mismatch than frequency mismatch. Nonetheless, it allowed measurement of ITD-specific cortical neurons' population response, without any spurious response confound. We proposed a method for extraction of ITD-specific response magnitude from the N1 response to a lateralized sound. Using it, one can reliably measure the activity of lateralization-specific cortical neurons, i.e. elicited by moderate ITD changes. This allows estimation of hemifield tuning curves in OCM using ERP data.