fMRI signatures of perceptual echoes in early visual cortex

Abstract

The visual Impulse Response Function (IRF) can be estimated by cross-correlating random luminance sequences with concurrently recorded EEG. It typically contains a strong 10Hz oscillatory component, suggesting that visual information reverberates in the human brain as a “perceptual echo” (VanRullen & Macdonald, Curr. Biol. 2012). The neural origin of these echoes remains unknown. To address this, we conducted an fMRI experiment. Instead of concurrently recording EEG and fMRI, which often leads to artifacts and spurious correlations, we recorded EEG and fMRI in two separate sessions. In both sessions, a disk whose luminance followed a random (white noise) luminance sequence was presented in the upper left quadrant. Participants (N=16) detected a rare near-threshold target embedded in the disk. Individual IRFs were derived from the EEG session. Then, these IRFs were used as “response templates” (Brüers & VanRullen, eNeuro, 2017) to reconstruct an estimate of the EEG during the fMRI session, by convolution with the corresponding random luminance sequences. The 7–14Hz (alpha) envelope of the reconstructed EEG was finally used as an fMRI regressor, to determine which brain voxels co-vary with the oscillations elicited by the luminance sequence, i.e. the “perceptual echoes”. The reconstructed envelope of EEG alpha (but not other frequencies like delta=2–4Hz, theta=4–8Hz or beta=14–28Hz) was significantly correlated with BOLD responses in V1 and V2. Surprisingly, this correlation was visible outside, but not within the directly (retinotopically) stimulated region. We tentatively interpret this lack of alpha modulation as a saturation effect, since the overall stimulus-induced BOLD response was inversely related, across voxels, to the signal variability over time. In conclusion, our results suggest that perceptual echoes originate in early visual cortex, driven by wide-spread activity in V1 and V2, not retinotopically restricted but possibly reflecting the propagation of a travelling alpha wave (Lozano-Soldevilla & VanRullen, bioRxiv 2018).