Flash stimulation elicits a visual evoked potential (VEP) as part of the electroencephalogram (EEG). This VEP is known to contain strong oscillatory activity around 120 Hz, which ceases 100 ms after the flash. It was unclear so far whether this time limit represents an averaging artifact due to loss of intertrial phase coherence or indicates a veridical cessation. Here we present results obtained from single-trial analysis of the EEG. These show that the oscillations exhibit virtually perfect phase locking and do in fact cease around 100 ms after the stimulus. Thus, the cessation of oscillatory activity in the VEP is not due to increasing intertrial phase jitter. Comparison with simultaneous retinal recordings exclude the possibility of direct crosstalk from the retina, but suggest that the oscillations are propagated from the retina to the cortex with a time lag of 48 ms.
