Steady state visual evoked potentials in the alert primate.

Macaque monkeys were trained to fixate a small spot while we recorded epidural steady state visual evoked potentials (SSVEP) in response to counterphase modulated sinusoidal gratings. This led to the following results: (1) the SSVEP can show either broad or narrow spatial frequency tuning, depending on electrode location, temporal frequency, contrast and method of analysis. (2) The SSVEP can also show narrow temporal frequency tuning, as narrow as 0.5 octave at half height. (3) Contrast functions relating VEP amplitude to log contrast were highly nonlinear. We propose that they are the composite of at least two distinct linear functions, one with a shallow slope for low contrasts and one with a significantly steeper slope for the higher contrasts. (4) Extrapolation of the low contrast function to zero voltage can lead to an excellent match with psychophysical functions. A similar extrapolation of the high contrast function, however, leads to a contrast value much higher than the psychophysical threshold. These findings suggest that the SSVEP can reflect the activity of two distinct neural mechanisms responsive to pattern stimulation. The degree to which either mechanism is evident determines the spatial and temporal frequency tuning of the VEP.