Figure 1. Rhythmic Attentional Scanning (RAS).

Rhythmic Attentional Scanning (RAS) hypothesis: Two stimuli, A and B, activate two corresponding neuronal groups, A and B, in a lower visual area. Those groups have convergent projections onto neurons in a higher visual area. In the neuronal groups of the lower area, the two stimuli induce two gamma rhythms, illustrated as red and blue excitability time courses. A theta rhythm, illustrated on the bottom by a green line, rhythmically resets the phase of both gamma rhythms, indicated by vertical gray lines (displaced to the right between lower and higher area to reflect inter-areal delay). After each reset, one of the lower-area gamma rhythms wins the competition to entrain the gamma rhythm in the higher area, whereas the other lower-area gamma rhythm loses. The competition might be influenced by the frequencies of the two lower-area gamma rhythms: In each theta cycle, one of those rhythms is slightly faster; After each theta-rhythmic reset, the faster gamma provides input to the higher visual area that arrives slightly before input from the slower gamma; The earlier input can drive the higher area neurons and trigger inhibition, which shuts out the later input. Yet, this frequency-dependent competition is not essential for RAS; the core point is that in each theta cycle, one gamma rhythm, corresponding to one stimulus, entrains the higher area and thereby provides selective routing of that stimulus to the higher area. While the RAS hypothesis is illustrated for two stimuli, it can apply to more. Note that it does not need to apply to all stimuli that are contained in the entire visual field, but it merely needs to apply to the number of stimuli that actually compete with each other due to convergence onto a postsynaptic target neuron.