AMPLITUDE AND PHASE VARIATIONS OF HARMONIC COMPONENTS IN HUMAN ACHROMATIC AND CHROMATIC VISUAL-EVOKED POTENTIALS

Occipital visual evoked potentials (VEPs) were recorded in response to low-contrast, low spatial-frequency chromatic, and achromatic grating s. Fast Fourier Transform (FFT) and time-domain analysis were used to reveal differences in harmonic content and amplitude of chromatic and achromatic response components over a wide range of temporal frequenci es. The chromatic ON/OFF VEP is dominated by the fundamental component indicating that onset and offset responses are different. This type o f response is typical of neurons with sustained type response characte ristics. Conversely, the achromatic onset VEP contains a predominant s econd harmonic component in addition to the fundamental. This similari ty between onset and offset responses suggests that transient mechanis ms are responsible for the generation of achromatic components. Freque ncy analysis of VEPs elicited by phase-reversing stimuli reveals that all of the response energy is concentrated at the second harmonic of t he stimulating frequency. The magnitude of the second harmonic compone nt is maximal for achromatic stimuli and undergoes a distinct minimum for isoluminant, chromatic stimuli. This behavior indicates that under the stimulus conditions used, magnocellular neurons with transient ch aracteristics dominate the reversal VEP.