Effects of stimulus orientation on spatial frequency function of the visual evoked potential

Visual performance is better in response to vertical and horizontal stimuli than oblique ones in many visual tasks; this is called the orientation eff ect, In order to elucidate the electrophysiological basis of this psychophy sical effect, we studied the effects of stimulus orientation on the amplitu des and latencies of visual evoked potentials (VEPs) over different spatial frequencies of the visual stimulation. VEPs to sinusoidal gratings at four orientations (certical, horizontal, and oblique at 45 degrees and 135 degr ees) with eight spatial frequencies (0.5-10.7 cycles/deg) at reversal rates of 1 Hz and 4 Hz were recorded in nine subjects. At 1-Hz stimulation, the amplitude and latency of P100 were measured, At 4-Hz stimulation, VEPs were Fourier-analyzed to obtain phase and amplitude of the second harmonic resp onse (2F). At 1-Hz stimulation, P100 latencies were decreased for oblique s timuli compared with those for horizontal and vertical stimuli at lower spa tial frequencies. Conversely, those for oblique stimuli were increased comp ared with those for horizontal and vertical stimuli at higher spatial frequ encies. At 4-Hz stimulation, spatial tuning observed in 2F amplitude of the oblique gratings shifted to lower spatial frequencies when compared with t hose of vertical stimulation. The alteration of the VEP spatial frequency f unction caused by the oblique stimuli was in good agreement with the orient ation effect observed in psychophysical studies. Our study may have a clini cal implication in that VEP testing with stimuli in more than one orientati on at slow and fast temporal modulations call be useful in evaluating neuro logical disease affecting the visual system.