Colorful wave
Thus, while a single spatiotemporal wave of activity offers an attractive possibility for coordinating cortical activity, the existence of stimulus-evoked traveling waves with sufficient spatial scale to span the cortical hierarchy has never been directly demonstrated. Other studies identified a reflective boundary between the primary and the secondary visual cortex 24. Some studies attempted to identify spatiotemporal waves that span multiple cortical sites and concluded that sensory stimuli trigger two independent cortical waves, which travel along the horizontal fiber network in each site 25, 27. Most studies in this line of work, however, focused on a single cortical area rather than inter-area communication. Both spontaneous and stimulus-evoked traveling wave-like phenomena have been identified using voltage sensitive dyes and neurophysiological recordings from brain parenchyma in the primary and higher-order visual areas 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34. Indeed, traveling waves recorded directly from the cortical surface have different speeds and propagation patterns compared to their EEG counterparts 20, 21, 22, 23. However, due to the low spatial resolution of the EEG, the interpretation of these findings is unclear. Early EEG work identified traveling waves in the feedforward and feedback directions 15, 16, 17, 18, 19. Do pairwise feedforward and feedback interactions give rise to a single coherent assembly that coordinates activity among the different cortical regions involved in processing sensory stimuli? How does the brain coordinate the feedforward and feedback processing given the significant differences in timescales? One possibility for a neurophysiological process that could coordinate activity amongst multiple regions in the processing hierarchy is a spatiotemporal traveling wave. Pairwise interactions between oscillations in different cortical sites during processing of sensory stimuli raise several fundamental questions. Thus, consistent with their presumed behavioral roles, feedback signaling utilizes slower temporal oscillations compared to feedforward channels. By analyzing individual pairwise interactions between neural oscillations present at different areas of the primate cortex, many prominent studies have shown that feedforward processing involves gamma oscillations, whereas feedback signaling uses alpha (8–12 Hz) oscillations 1, 2, 4, 13, 14. The role of neuronal oscillations in coordinating neuronal activity has been a subject of intense investigation, especially in primate vision. Thus, it is thought that feedback modulation evolves on a slower time scale relative to feedforward processing 1, 2, 12.įeedforward–feedback interactions between the different cortical regions involved in sensory processing must be coordinated to give rise to integrated percepts situated in the behavioral context. Formulating predictions about the next sensory stimulus or deciding which stimulus to pay attention to requires temporal integration 1, 9, 10, 11. Feedback processing, in contrast, involves top-down influences such as attention, prediction, and context 7, 8. Feedforward processing involves bottom-up assembly of abstract stimulus representations in higher-order areas from simple receptive fields in the primary cortex 7, 8. Feedforward and feedback signaling contribute to the hierarchical processing of sensory stimuli, creating predictions and attaching behavioral context to the sensory world 1, 2, 3, 4, 5, 6, 7.