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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Schizophrenia patients have abnormalities of auditory information processing, theoretically associated with dysfunction of neuronal excitation. Auditory paired-stimuli (S1-S2) paradigms are used to evaluate the nature of these abnormalities. It is unknown whether patients' abnormalities during S1-S2 paradigms are attributable to specific hemispheric differences in cortical processing. The present studies used whole head magnetoencephalography and monaural or binaural versions of the paired-stimuli paradigm to evaluate auditory processing among 38 schizophrenia and 38 normal subjects. The strengths of auditory-evoked brain responses over time were quantified using distributed source reconstructions with L2 minimum norm constraint and realistic head models. For left ear stimuli, schizophrenia and normal groups did not differ on either left or right hemisphere activity over auditory cortex. For right ear and binaural stimuli, schizophrenia patients had less activity over left auditory cortex from 80 to 120 ms post-stimulus but did not differ from normal on activity over right auditory cortex. Additionally, in response to monaural stimulation, schizophrenia patients had significantly less activity than normal over right temporal parietal junction from 60 to 120 ms post-stimulus. These data are consistent with four propositions about schizophrenia: (i). right auditory cortex is functioning normally; (ii). processing of simple auditory stimuli is abnormal in left auditory cortex, probably specifically in supra-granular layers; (iii). auditory localization abilities are deficient; and (iv). auditory cortex abnormalities are not a function of deficient hemispheric communication because they are evident early in processing as long as stimuli are delivered directly to left hemisphere.