Studies with auditory stimuli have established in humans that a mismatch potential (MMP) is elicited whenever a deviant stimulus is substituted for a standard stimulus in a train of monotonous standard stimuli presented at rates > 0.25 Hz. The MMP in humans is localized in the auditory cortex and is known as mismatch negativily, from its polarity in scalp recordings. It is hypothesized to reflect the operation of sensory memory and to be a necessary component of the auditory orienting response.

To examine the generality of MMPs we used a visual mismatch paradigm with pond turtles (Pseudemys scripta) while recording with electrode arrays (200 μm spacing) from near surface and deep visual projection areas within the forebrain and optic tectum. Standard stimuli were 10-sec trains of diffused strobe flashes presented at rates of 1-6 Hz against backgrounds of 2-11 lux. Deviant stimuli were brighter or dimmer flashes that followed the last standard flash. MMps were separated from visual evoked potentials by subtracting the response to the last standard flash of the train from the response to the same flash (bright or dim) when delivered as a deviant. Comparisons were also made with evoked potentials to isolated bright or dim flashes, that is, equal in frequency (1 per 12 sec) to the deviants but without intervening standard flashes.

At tectal loci bright and dim deviants elicited net positivities that reached statistical significance in the period between 141 ± 8 and 184 ± 12 msec after the deviant stimulus (mean ± SEM). Earlier components in the tectal responses correlated with the intensity of the stimulus rather than its deviance. In the case of the bright deviants the early waves (P50-P75) were larger in amplitude. Forebrain recordings showed a similar although broader period of net positivity, associated with deviance, between 129 ± 8 and 195 ± 12 msec. Deviants, delivered as isolated flash responses evoked larger early components (100-140 msec). In separate experiments with cortical epipial electrodes, a condition somewhat more comparable to scalp recording, MMPs were similar in latency but had a negative polarity.

Regression analyses revealed a relationship between the amplitude (base-to-peak) of the MMF' and the degree to which the standard response had declined with repeated stimulation. Rate decrement, as measured by the isolated (long ISI) flash response minus the last standard response, was a significant predictor of MMP amplitudes (r2 = 0.37, tectum; r2 = 0.31, forebrain), whereas standard response amplitudes alone were not (r2 = 0.09; r2 = 0.06).

MMPs are present in nonmammals plurally, that is, at different levels of the visual system, at least as early as the tectum. The existence of subcortical MMPs caution against assigning a primary or exclusive role to those recorded from the cortex.

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