Differential sensitivity of early and late components of the auditory cortical middle latency electrically evoked response to change of cortical stimulation site

Middle latency responses of the electrocorticogram to single pulse electrical stimulation at two sites (caudal, low frequency representation; rostral, high frequency respresentation) in the gerbil auditory cortex (field AI) were recorded from eight microwires implanted in the depth of cortical tissue at positions along the tonotopic axis between the stimulation electrodes. Sensitivity of the P1, N1, P2 and N2 peaks to changing of the site of stimulation was investigated. It was found that the early middle latency response components (P1, N1) showed a distinctly higher sensitivity to changes of the stimulation site compared to late components (P2, N2). It is argued that this finding lends support more to holographic coding schemes than topographic coding schemes both of which are candidates to explain previously reported successful perceptual discrimination in test animals using a prototype neuroprosthesis for the auditory cortex.