Neural Correlates of an Auditory Afterimage in Primary Auditory Cortex

The Zwicker tone (ZT) is defined as an auditory negative afterimage, perceived after the presentation of an appropriate inducer. Typically, a notched noise (NN) with a notch width of 1/2 octave induces a ZT with a pitch falling in the frequency range of the notch. The aim of the present study was to find potential neural correlates of the ZT in the primary auditory cortex of ketamine-anesthetized cats. Responses of multiunits were recorded simultaneously with two 8-electrode arrays during 1 s and over 2 s after the presentation of a white noise (WN) and three NNs differing by the width of the notch, namely, 1/3 octave (NN1), 1/2 octave (NN2), and 2/3 octave (NN3). Both firing rate (FR) and peak cross-correlation coefficient (ρ) were evaluated for time windows of 500 ms. The cortical units were grouped according to whether their characteristic frequency (CF) was inside (“In” neurons) or outside (“Out” neurons) a 1-octave-wide frequency band centered on the notch center frequency. The ratios between the FRs and the ρs for each NN and the WN condition and for each group of neurons were then statistically evaluated. The ratios of FRs were significantly increased during and after the presentation of the NN for the “In” neurons. In contrast, the changes for the “Out” neurons were small and most often insignificant. The ratios of the ρ values differed significantly from 1 in the “In–In” and “In–Out” groups during stimulation as well as after it. We also found that the ρs of “Out” neurons were dependent on the type of NN. Potentially, a combination of increased ρ and increased FR might be a neurophysiological correlate of the ZT.