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.
Valberg
A
A
Institute of Physics, Section of Biophysics, Norwegian University for Science and Technology, Trondheim, Norway.
Rudvin
I
I
D003118:Color Perception; D015350:Contrast Sensitivity / Q000502:physiology; D004594:Electrophysiology / Q000379:methods; D005074:Evoked Potentials, Visual / Q000502:physiology*; D006801:Humans; D010286:Paraventricular Hypothalamic Nucleus / Q000502:physiology*; D010775:Photic Stimulation; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology; D013377:Substantia Innominata / Q000502:physiology*; D014793:Visual Cortex / Q000166:cytology / Q000502:physiology; D014795:Visual Pathways / Q000166:cytology / Q000502:physiology*