Hierarchy of cortical responses underlying binocular rivalryRESULTSDISCUSSIONDISCUSSIONMETHODSFigure 1 Rival stimuli, percepts and corresponding regions of primary visual cortex (V1).Figure 2 Temporal sequences of fMRI responses, averaged across trials for one observer in each experimental condition and each visual area.Figure 3 fMRI response latency differences averaged across observers.Figure 4 Traveling waves of cortical activity, averaged across four observers, in each visual area.Figure 5 Control experiments.Table 1 Effects of attention on fMRI response amplitudes and latencies ACKNOWLEDGMENTSCOMPETING INTERESTS STATEMENTReferencesHierarchy of cortical responses underlyingbinocular rivalrySang-Hun Lee1, Randolph Blake2& David J Heeger3During binocular rivalry, physical stimulation is dissociated from conscious visual awareness. Human brain imaging reveals a tightlinkage between the neural events in human primary visual cortex (V1) and the dynamics of perceptual waves during transitions indominance during binocular rivalry. Here, we report results from experiments in which observers’ attention was diverted from therival stimuli, implying that: competition between two rival stimuli involves neural circuits in V1, and attention is crucial for theconsequences of this neural competition to advance to higher visual areas and promote perceptual waves.When your eyes view dissimilar patterns, you experience a perceptualillusion called binocular rivalry. Rather than melding into a stable,single image, the two patterns compete for visibility, with one tem-porarily dominating perception for several seconds, only to be replacedin awareness by the other. What makes this phenomenon remarkable isthe dissociation between constant physical stimulation and fluctuatingperceptual experience. Because of this dissociation, binocular rivalryprovides a compelling means for studying visual awareness. Despite animpressive volume of work on rivalry, however, central questionsremain unanswered concerning the neural processing underlyingthis beguiling phenomenon. Particularly controversial are theroles of primary visual cortex (V1) in rivalry1–6and, consequently,in awareness7–9.To address this controversy, we have capitalized on a compellingaspect of rivalry: during transitions in perceptual state, one typicallysees a traveling wave in which the perceptual dominance of one patternemerges locally and expands progressively as it renders the otherpattern invisible. Previous psychophysical experiments provided indir-ect evidence for the involvement of V1 in the propagation of thesetraveling waves10, and functional magnetic resonance imaging (fMRI)experiments established a tight linkage between the dynamics ofperceptual waves during rivalry and the spatiotemporal dynamics ofV1 activity11. Specifically, traveling waves of cortical activity propagateover subregions of V1 that correspond topographically to perceptualwaves, and the spatiotemporal dynamics of cortical waves co-vary withthe propagation speed of those perceptual waves. But are these corticalwaves also seen in other, extrastriate visual areas, and if so, how doesthat activity relate to that measured in V1?Here, we carried out experiments that used attention as a tool todissociate the conscious perception of traveling waves during rivalryfrom the automatic neural processing underlying the initiation andpropagation of these waves. We measured activity in human visualcortex with fMRI while observers viewed rival stimuli. When attentionwas diverted from the spatial location of the perceptual waves, thespatiotemporal dynamics of neural activity differed across the hierarchyof visual cortical areas; waves of cortical activity were preserved inprimary visual cortex (V1), whereas they were eliminated in visual areaV2 and even reversed in V3.RESULTSHuman observers viewed a dichoptic display that was designed toinduce perceptual waves (Fig. 1). The rival images comprised alow-contrast carrier grating (viewed by one eye) and a high-contrastmask grating (viewed by the other eye). Each was restricted to anannular region of the visual field, centered on the point of fixation.Exploiting the susceptibility of binocular rivalry to transient stimula-tion12, we triggered a switch in perceptual dominance through a briefand abrupt increase in contrast in a small region (either at the top orthe bottom of the annulus) of the otherwise low-contrast carriergrating. When presented while the carrier grating was suppressedfrom vision, this contrast pulse typically ignited a perceptual travelingwave; observers perceived the carrier emerging from suppression,with its visibility first arising at the location of the triggeringpulse and progressively erasing the high-contrast mask grating fromvisual awareness10,11.During each fMRI scanning session, observers were instructedto maintain fixation at the center of the display while carrying outeither a perceptual latency task or a diverted attention task. Eachobserver performed a large number of repeated trials (384–576)for each task. In the perceptual latency task (Fig. 1a), observers directedattention (without moving their eyes) to the rival gratings in thenear-periphery of the visual field. When a perceptual wave reached atarget area (marked by nonius lines), observers pressed a key,thereby providing a measure of the arrival time of the perceptualwave. The two monocular rival gratings disappeared when the keywas pressed.Received 18 April; accepted 18 June; published online 15 July 2007; doi:10.1038/nn19391Department of Psychology, Seoul National University, Shillim, Gwanak, Seoul 151-746, South Korea.2Department of Psychology, Vanderbilt University, 512 Wilson Hall,Nashville, Tennessee 37203, USA.3Department of Psychology and Center for Neural Science, New York University, 6 Washington Place, New York, New York 10003, USA.Correspondence should be addressed to S.-H.L. ([email protected]).1048 VOLUME 10[NUMBER 8[AUGUST 2007 NATURE NEUROSCIENCEARTICLES© 2007 Nature Publishing Group http://www.nature.com/natureneuroscienceIn the diverted attention task (Fig. 1b), the dichoptic rival gratingsand the sequence of events were identical to those in the perceptuallatency task, except that a rapid series (one every 190 ms) of small,colored letters and numbers appeared at fixation. The onset of theletters was simultaneous with the onset of the carrier grating, and theoffset of the rival gratings and letters was determined by