Individual simple and complex cells are associated with 1
F and 2
F outputs, respectively. As previously discussed, EEG measures a population response, and, as simple cells responding to both polarities are present within the visual system, we see mostly responses at 2
F. But, 1
F signals driven by simple cells are still present within the visual system. IM occurs at combinations of the self-term responses in binocular conditions only.
Figure 5 shows boxplots of the average response at first-order IM terms (i.e., 1
F2–1
F1 = 2 Hz and 1
F2+1
F1 = 12 Hz) and second-order IM terms (i.e., 2
F2–2
F1 = 4 Hz and 2
F2+2
F1 = 24 Hz). Here, we found that simple cells must be involved in binocular combinations because we observed large and significant IM responses at combinations of 1
F compared with noise (2 Hz,
p ≤ 0.008; 12 Hz,
p ≤ 0.007), but not at combinations of 2
F. This finding differs noticeably from the self-term responses, which are present at the second harmonics. As such, the binocular combinations that we measure may be driven primarily by simple cells rather than complex cells.
In summary, we measured robust stimulus–driven responses in both eyes. These responses were primarily at the second harmonic of the input frequency (2F), as we would expect for contrast-reversing stimuli. However, we do see some evidence of response at the first harmonic (1F) which may be due to asymmetries in early ON/OFF pathways or stimulus edge effects. We also measured strong binocular interactions. When the inputs were binocular, the F1 and F2 responses were significantly diminished (consistent with a binocular suppression computation) and IM terms appeared to arise largely in the simple cells responding at 1F. Finally, the overall strength of suppression expressed relative to the monocular responses did not appear to depend on spatial frequency or chromaticity.