Ij Murray

Electrophysiological Correlates of Chromatic-Opponent and Achromatic Stimulation in Man

Low spatial frequency gratings, modulated in luminance, elicit VEPs which are similar for stimulus onset, offset and reversal; they respond to a transient change in contrast, and are usually dominated by a positive wave. Conversely, VEPs generated by gratings modulated in hue along R-G or B-Y axes of the chromatic plane are qualitatively different from those derived from the luminance axis: grating onset elicits VEPs dominated by a negative wave, different from the reversal VEPs and consistent with a contribution of sustained (tonic) mechanisms.

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

We propose a simple method of monitoring separate visual pathways in lightly 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.

Photoreceptor topography and cone-specific electroretinograms

It is implicit in many cone-specific ERG studies that the amplitude is proportional to the numbers of cones stimulated. The objective of these experiments was to test this idea by comparing ERGs obtained from different areas of the retina with histological data on cone-density distributions. The histology (Curcio et al., 1990) shows that the cumulative number of cones in the human retina increases exponentially with stimulus diameter between 0- and 40-deg eccentricity. L-, M-, and (L+M) cone-driven 30-Hz ERGs were obtained from a series of stimuli with one of the following configurations: (1) Circular stimuli of different angular subtense up to 70-deg diameter. (2) Annuli with 70-deg outer diameter but variable inner diameter. (3) Annuli of constant area but increasing eccentricity. Cone contrasts were equalized for each stimulus condition. The modulated and nonmodulated regions of the screen had the same mean hue and luminance. The data suggest that the L+M cone ERG amplitude increases with stimulus diameter in direct proportion to the estimated number of cones stimulated. Furthermore, the total L+M responses appear to be predicted from individual L and M responses by simple linear summation for both the disc and annular stimuli.