Attention and auditory evoked responses to low-detectability signals

Comparisons were made between cortical evoked responses obtained under two conditions: (1) while Ss were reading, and (2) while they were attempting to count auditory signals. The amplitudes of evoked responses to low-detectability auditory stimuli were found to be approximately doubled when the Ss were required to count the number of stimuli, as compared to amplitudes recorded when they were reading. The duration of the response was also markedly increased. These increases in response amplitude and duration are considerably greater than those observed in earlierexperiments, where high-levelsignalswere used. Inter-S variability of the waveform of the averageevoked response was observed to be much less when the Ss counted the stimuli. In another experiment the level of the auditory signalwas varied over a range of approximately +4 to -4 decibels relative to the listeners’ behavioral thresholds. The per cent of signalswhich they counted varied from near-zero to 100, over this range, and the evoked response concurrently showed a variation from “unmeasureable” to approximately 8 microvolts.