Alpha-band echoes evoked by contrast and luminance changes emerge in and travel out from early visual cortex

How stimulus properties are processed in the human brain over time is critical to how we engage in dynamic everyday environments. To understand how changes in basic stimulus properties relate to changes in human electrical brain activity over time, previous work has estimated the brain's temporal response function (TRF) by cross-correlating random luminance sequences with electroencephalogram (EEG) signals at various lags to approximate the brain's response to temporal changes in luminance. Using this technique, it was found that luminance changes produce long-lasting "echoes" in the alpha frequency range. However, the neural origin of these echoes and the precise stimulus features that induce them have not been extensively studied. We measured TRFs in response to luminance and contrast changes. Additionally, the fact that EEG responses generated in the primary visual cortex (V1) have a unique pattern of polarity reversal depending on the visual field location (with upper stimuli projecting negatively and lower projecting positively) allowed us to test whether the TRFs generated from upper or lower visual field stimulation were counter-phased, as would be expected if the echoes were generated within V1. We found a luminance echo lasting ∼1 s in the alpha frequency and contrast echoes lasting only around 300 ms. For both stimuli, the TRF was initially counter-phased between upper and lower visual fields but quickly became in phase after ∼100 ms. Our findings demonstrate the existence of contrast (in addition to luminance) echoes in the alpha band, which appear to emerge from V1, perhaps as a traveling wave.