Observation of quantization effects in human auditory cortex.

Auditory evoked magnetic fields were recorded from five human subjects using the BTi whole head system (148 channels). The stimuli were simple tone bursts with different inter-stimulus intervals (ISI). The magnetic field was also recorded from the same subjects in place without stimulation to provide baseline 'noise' measurements. Magnetic field tomography (MFT) was applied to the average data and activation curves were computed from well circumscribed regions of interest (ROI). Objective statistical measures (Kolmogorov-Smirnov test) were then used to identify latency segments with distribution significantly different from the baseline distribution. For latency segments with significant activations the dependence of the activation strength on ISI was modeled by an exponentially saturating function. In earlier studies, the characteristic time (tau) in this function was interpreted as a measure of a memory trace for auditory tones. Our analysis identified distinct levels for tau: all subjects had the same lower two levels (0.50 s and 0.69 s). Attention modulation introduced considerably higher tau values in most subjects but it did not change the two 'ground state' tau levels. To our knowledge, this is the first demonstration of quantization effects in the macro-properties of brain activity, which were universal amongst the subjects we studied.