The analysis of stimulus probability inside and outside the focus of attention, as reflected by the auditory N₁ and P₃ components.

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Abstract
Subjects (12) performed a selective 2-channel auditory frequency discrimination while their evoked responses were averaged concurrently in both the attended and rejected channels. The probability of a target tone relative to a standard tone was set at either 1 in 4, 1 in 8, or 1 in 16. Target detection and guessing rates were modeled with the maximum likelihood estimation technique. The auditory N1 component was suppressed for all stimuli in the rejected channel at all levels of probability, the suppression being greatest when target stimuli were most probable. Despite this suppression, the rejected channel responded to reduced stimulus probability with N1 augmentation. P3 amplitude increased linearly as targets were made rarer, but only in the attended channel; the rejected channel did not respond to changes in the probability of rare tones. When targets were most frequent (1 in 4) P3 amplitude in the attended channel dropped to the same level as P3 in the rejected channel, despite maximal N1 selectivity and adequate performance of the task. Selective attention is a necessary, but not sufficient condition for a selective P3 response. Apparently, attended inputs are modified selectively on the basis of location and pitch prior to the generation of N1, and also some information about stimulus probability is processed both inside and outside the focus of attention at this level. The results with respect to P3 point to a different probability-sensitive mechanism whose operation is contingent upon focal attention.

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