Color imaging of the human middle latency response

Color imaging brain mapping techniques were applied to the middle latency response in 40 normal-hearing, neurologically intact adults. Stimuli included 9/sec and 41/sec clicks and 500 and 2000 Hz tone bursts. Wave Na had no specific focus of activity while wave Pa was largest in amplitude over the vertex and frontal lobes. Variables such as the baseline from which amplitude measures are made (prestimulus versus preceding wave) and reference electrode (ipsilateral mastoid versus C7) were assessed. The preceding trough (Na) yielded a more precise measure of Pa amplitude than the prestimulus baseline. The reference electrode location did not influence the topography of wave Pa, although response amplitudes were significantly larger with the noncephalic reference. Two averaging strategies were assessed: (1) averaging the entire pre- and poststimulus epoch point for point across individuals and (2) averaging the voltage of Pa at the latency of Pa for each individual. The z statistic was evaluated as a means for identifying clinically encountered abnormalities. The use of this measure was found to be problematic because such an analysis assumes normally distributed data while Pa amplitude approximates a gamma distribution. The z statistic may be more appropriate for the analysis of interhemispheric differences, which have a normal distribution with a mean close to zero. Responses obtained to 500 Hz stimuli tended to be larger than 2000 Hz-elicited responses. Of interest clinically, is that Pa amplitude was largest at electrode location Fz. The scalp topography and subsequent conclusions regarding 9 versus 41/sec responses differed depending on the data analysis strategy used, emphasizing the importance of these strategies. Future directions for this technology were discussed.