Visual evoked potentials and magnocellular and parvocellular segregation.

We have measured visual evoked potentials (VEPs) to luminance-modulated, square-wave alternating, 3-deg homogeneous disks for stimulus frequencies ranging from 1 Hz to 16.7 Hz. The aim of the study was to determine the range of frequencies at which we could reproduce the two-branched contrast-response (C-R) curves we had seen at 1 Hz (Valberg & Rudvin, 1997) and which we interpreted as magnocellular (MC) and parvocellular (PC) segregation. Low-contrast stimuli elicited relatively simple responses to luminance increments resulting in waveforms that may be the signatures of inputs from magnocellular channels to the visual cortex. At all frequencies, the C-R curves of the main waveforms were characterized by a steep slope at low contrasts and a leveling off at 10%-20% Michelson contrast. This was typically followed by an abrupt increase in slope at higher contrasts, giving a distinctive two-branched C-R curve. On the assumption that the low-contrast, high-gain branch reflects the responsivity of magnocellular-pathway inputs to the cortex, the high-contrast branch may be attributed to additional parvocellular activation. While a two-branched curve was maintained for frequencies up to 8 Hz, the high-contrast response was significantly compromised at 16.7 Hz, revealing a differential low-pass filtering. A model decomposing the measured VEP response into two separate C-R curves yielded a difference in sensitivity of the putative MC- and PC-mediated response that, when plotted as a function of frequency, followed a trend similar to that found for single cells. Due to temporal overlap of responses, the MC and PC contributions to the waveforms were hard to distinguish in the transient VEP. However, curves of time-to-peak (delay) as a function of contrast often went through a minimum before the high-contrast gain increase of the corresponding C-R curve, supporting the notion of a recruitment of new cell ensembles in the transition from low to high contrasts.

Possible contributions of magnocellular- and parvocellular-pathway cells to transient VEPs.

We have measured transient visual evoked potentials (VEPs) to low-contrast luminance stimuli favoring responses of magnocellular pathway cells and to low-contrast red-green stimuli favoring parvocellular cells. Stimuli were square-wave alternating, 3-deg homogeneous disks. Low-contrast stimuli modulated in luminance elicited relatively simple responses. For some observers, a negativity was present that saturated at low contrast. This may be the signature of inputs from magnocellular channels to the visual cortex. The slope of the contrast-response curve for low-contrast stimuli was about the same for all subjects. For medium contrasts, these contrast-response curves displayed an abrupt increase of slope. The shallower slope may reflect the responsivity of magnocellular-pathway inputs to the cortex, whereas the steeper slope may be caused by additional parvocellular activation. Contrast-response curves for the most sensitive waveforms of the isoluminant green-red modulation also showed two branches, although not as clearly as for luminance. This may indicate parvocellular-mediated activity for small chromatic differences, and a combination of parvocellular and magnocellular inputs for larger contrasts. Curves of time-to-peak response as a function of contrast often changed their monotonous behavior near the kink of the corresponding contrast-response curve, thus supporting the notion of a contribution from several mechanisms to the main waveforms.