Rate effects are defined as a reduction in amplitude of an evoked brain response with increasing stimulus frequency. In auditory paired-stimulus paradigms, a smaller amplitude evoked response to the second stimulus at a latency of 50 ms has been proposed to index a preattentive sensory gating mechanism. The present study investigated the possibility that expectancy and/or attentional biases could alter evoked potentials associated with rate effects. EEG data were recorded from 30 channels while subjects received 240 trials of 1, 2 or 3 click stimuli (with successive stimuli being separated by 500-ms intervals). Half of the subjects knew (blocked condition) and half of the subjects did not know (mixed condition) how many stimuli they would receive on a given trial. Subjects in the blocked condition had a significantly larger rate effect than subjects in the mixed condition. This effect was present only for low-frequency components of the event-related brain potential (ERP; below 10 Hz) and occurred from 30 to 60, 90 to 160 and 190 to 260 ms after stimulus presentation (P1-N1-P2 complex). Mixed condition subjects also had larger contributions to their ERPs from temporal channels. These results suggest that the rate effect can be significantly altered by expectancy, and they are inconsistent with the thesis that ERPs near 50 ms in a paired-stimulus paradigm solely index a preattentive sensory gating mechanism.
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