SUSTAINED AND TRANSIENT MECHANISMS IN THE STEADY-STATE VISUAL-EVOKED POTENTIAL - ONSET PRESENTATION COMPARED TO PATTERN-REVERSAL

1. There are many reports of a region of reduced amplitude in the spat ial frequency characteristics of the occipital evoked potential genera ted by contrast reversing sinusoidal gratings. This effect is especial ly common at medium spatial frequencies and therefore shows as a notch in the mean amplitude vs spatial frequency function. 2. In this repor t we test the idea that the region of reduced amplitude responses may be due to cancellation of signals dominated by transient and sustained mechanisms. 3. Visual evoked potentials (VEPs) elicited by 8 Hz contr ast reversal are compared with those obtained using on-off stimulation at 8 and 16 Hz. Psychophysical contrast thresholds are obtained for t hese stimuli and the data indicate that 8 Hz contrast reversal is an e ffective stimulus for both transient and sustained mechanisms, whereas 16 Hz on-off is a poor stimulus for transient mechanisms. 4. The VEP data for these two conditions support this view; a principal component analysis of the VEP amplitude data shows that two statistically indep endent sources contribute to the 8 Hz contrast-reversal response and t hat the 16 Hz on-off response is derived from a single source. 5. The obtained factors exhibit spatial tuning as described for sustained and transient mechanisms. As reported previously, it is shown that for ma ny, but not all subjects, the 8 Hz contrast-reversing stimulus leads t o a pronounced notch in the mean amplitude vs spatial frequency respon se function indicative of the suspected interaction of sustained and t ransient channels. 6. In general, the 16 Hz on-off response does not s how the notch, reflecting the activity of a single channel. It is of p ractical interest that the 16 Hz on-off VEP data exhibit high interind ividual reliability and that their spatial tuning characteristics matc h those of the contrast sensitivity function. These properties suggest that for objective measurement of psychophysical thresholds on-off st imulation at 16 Hz is likely to yield more reliable data than contrast reversal.