Scalp VEPs and intra-cortical responses to chromatic and achromatic stimuli in primates

We propose asimple method of monitoring separate visual pathways inlightly sedated monkeys using chromatic and achromatic gratings of low contrast, which are known to activate predominantly either parvo- or magno-systems. The scalp Visual Evoked Potentials (VEPs) are compared with simultaneously recorded intra-cortical VEPs which in turn are compared with multi-unit and single-unit responses. At isoluminance, the onset of low contrast, coarse chromatic square wave spatial profile gratings generates negative scalp VEPs which exhibit properties consistent with the activation of sustained, parvocellular-chromatic mechanisms (e.g. low-pass spatio-temporal characteristics). In monkeys, most components of chromatic onset VEPs have latencies comparable to neuronal activity within the supragranular layers of V1. Corresponding coarse achromatic gratings elicit positive VEPs which exhibit properties consistent with the activation of transient-type magnocellular mechanisms (e.g. temporal tuning to higher temporal frequencies) and which have a more complex morphology. Achromatic onset VEPs may contain early components of similar timing to activity recorded in monkey V1, but later components cannot be related to V1 generators; other sources are not known. Achromatic reversal VEPs are similar to achromatic onset, chromatic reversal and both chromatic and achromatic offset VEPs and all differ from chromatic onset VEPs. It is observed that early components of scalp-recorded chromatic-onset VEPs are related in time to some intra-cortical potentials. These VEPs are generated by low spatial frequencies and have low pass temporal characteristics. Other scalp potentials, apparently unrelated to V1 field potential activity must be generated by other sources such as extra-striate areas.