Previous studies of human contrast adaptation employing visually
evoked potentials (VEP) have revealed contradictory results,
namely, either a reduction or an enhancement in VEP amplitude.
In a cross-adaptation experiment, we explored the possibility
that differences in the temporal frequency of adapting and test
patterns played a role. Phase-reversing checkerboard stimuli
[1-deg check size, temporal frequency 8.5 or 17 reversals
per second (rps)] served as adaptation and test pattern
with contrasts of 0 or 97%. In 13 subjects, we recorded both
retinal (PERG) and cortical (VEP) steady-state responses
simultaneously. In a balanced block design, all four combinations
of the temporal adaptation and test frequencies were employed.
Contrast adaptation reduced the PERG amplitude by about 20%
in every temporal condition (P < 0.001). The VEP
amplitude was strongly affected by adaptation, but the effect
differed in magnitude and sign depending on condition: With
identical adaptation and test frequency, amplitude was reduced
by 15% (P = 0.07) at 8.5 rps and by 38% at 17 rps (P
< 0.05). Adapting at 8.5 rps and testing at 17 rps had a tiny (14%)
insignificant effect, whereas adapting at 17 rps and testing at 8.5 rps
revealed an amplitude enhancement of 27% (P < 0.05). These
strong temporal cross-adaptation effects (in the VEP, but not in the PERG)
suggest that the adaptable cortical mechanisms (gain control) can be narrowly
tuned in their temporal properties. A sizable adaptation effect can even
change its sign when varying the temporal frequency by a factor of two.
This finding resolves contradictions between previous VEP adaptation studies
and reconciles them with psychophysical findings.