The Hidden Spatial Dimension of Alpha: 10-Hz Perceptual Echoes Propagate as Periodic Traveling Waves in the Human Brain

Abstract

Summary: EEG reverse-correlation techniques have revealed that visual information processing entails a ∼10-Hz (alpha) occipital response that reverberates sensory inputs up to 1 s. However, the spatial distribution of these perceptual echoes remains unknown: are they synchronized across the brain, or do they propagate like a traveling wave? Here, in two experiments with varying stimulus locations, we demonstrate the systematic phase propagation of perceptual echoes. A single stimulation in the upper visual field produced an “echo traveling wave” propagating from posterior to frontal sensors. The simultaneous presentation of two independent stimuli in separate visual hemifields produced two superimposed traveling waves propagating in opposite directions. Strikingly, in each sensor, the phase of the two echoes differed, with a phase advance for the contralateral stimulus. Thus, alpha traveling waves sweep across the human brain, encoding stimulus position in the phase domain, in line with the 70-year-old “cortical scanning” hypothesis (Pitts and McCulloch, 1947). : Alpha oscillations are key players in visual processing. Lozano-Soldevilla and VanRullen demonstrate that the ∼10-Hz component of perceptual echoes behaves as a traveling wave, coding stimuli presented in different visual hemifields with consistent phase differences, like the beam of a radar scanning the field. Keywords: alpha rhythm, EEG, visual evoked potentials, perceptual cycles, traveling wave, impulse response function