Event-related potentials in macaque monkey during passive and attentional processing of faces in a priming paradigm.

Epidural event-related potentials (ERPs) were recorded in monkeys using a priming (S1-S2) paradigm. One juvenile and 2 young adult macaque monkeys (Macaca fascicularis) viewed tachistoscopically presented pairs of faces in four possible combinations: human-human, monkey-monkey, monkey-human, human-monkey. Faces were presented upright to untrained subjects. Subsequently, one adult monkey was trained to associate the occurrence of a monkey-monkey pair of faces with juice reward. In a separate control experiment, the same stimuli were presented upside down. In both the upright and inverted conditions, adult monkey ERPs exhibited a P1-N1-P2 complex of peaks in the 400 ms following stimulus presentation. However, only upright faces elicited a prominent, widely distributed negativity (N4) in the 400-800 ms interval and a long-duration negativity (LDN) in the last 400 ms of the 1,400-ms epoch. N4 was earlier in latency, larger in amplitude, and larger over left hemisphere in response to monkey faces compared to human faces. It was also sensitive to the preceding or priming stimulus, exhibiting larger amplitudes when S1 matched S2 than when it did not. Trained monkey waveforms differed from untrained monkey responses in terms of the appearance of an N2-P3 complex. P3 was bilaterally symmetrical, larger in magnitude, and delayed in latency in response to monkey faces. The differences in components in the two conditions suggest that monkey ERPs can distinguish between passive and attentional processing of faces. N4 appears to reflect the recognition of conspecifics, is sensitive to priming, and occurs primarily over left hemisphere. Rewarded presentation of faces results in P3-like components that may reflect the novelty, meaningfulness, or active nature of the response. The differences in distribution between P3 and N4 suggest that they do not share the same neural sources.