The polarity inversion of scalp potentials evoked by upper and lower half-field stimulus patterns: latency or surface distribution differences?

Evoked potentials to patterned stimulation of the upper and lower half of the visual field are generally inverted in polarity. Two conflicting proposals have been made to explain this effect, both based on surface distribution studies of pattern-reversal and/or pattern-onset VEPs. The first suggests that this polarity inversion is due to differences in surface distribution of corresponding components of constant latency; the second that it is due to differences in the latencies of peaks of similar surface distributions in the upper and lower half-field responses. Experimental evidence is here presented which supports the first explanation for the case of the pattern-onset VEPs. These results, which illustrate how different components in the same response can be identified from the selective adaptation effects of pre-exposure to outline patterns, show that there is no difference in latency of components of corresponding properties in the upper and lower half-field VEPs.