Faster than the brain's speed of light: Retinocortical interactions differ in high frequency activity when processing darks and lights

Visual processing of dark visual stimuli has been hypothesized to occur faster relative to bright stimuli. We investigated the timing, processing, and propagation of neural activity in response to darks and lights, operationalized as light offset and onset, in the human visual system by recording electroretinography (ERG) simultaneously with magnetoencephalography (MEG) in humans. We discovered that dark onset resulted in 75-95 Hz retinal activity that we call the dark retinal oscillatory potential, occurring with the same latency as the analogous but more broadband (55-195 Hz) oscillatory potential at light onset. Both retinal oscillations coupled with subsequent cortical activity of corresponding bandwidths, but cortical responses for darks indeed occurred earlier than for lights. Darks therefore propagate from retina to cortex more quickly than lights, potentially resulting from a thalamic advantage. Furthermore, we found that this propagation is effectuated by high frequency retinocortical coupling of narrow bandwidth for darks but wide bandwidth for lights.