Selective attention and the auditory vertex potential. Effects of signal intensity and masking noise.

A randomized sequence of tone burst was delivered to subjects at short inter-stimulus intervals (mean ISI of 333 msec), with the tones originating from one to three spatially and frequency-specific channels. The subject's task was to count the tones in one of the three channels at a time, ignoring the other two, and press a button after each tenth tone. In different conditions, tones were given at high (60 dB SL) and low (20 dB SL) intensities and with or without a background white noise to mask the tones. The N1 component of the auditory vertex potential was found to be larger in response to attended-channel tones than in relation to unattended tones. This selective enhancement of N1 was minimal for loud tones presented without noise and increased markedly for the lower tone intensity and in noise-added conditions. The selectivity of attention was measured physiologically in this multichannel listening task was thus greater when tones were faint and/or different to detect.