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.