I've been struggling with this question for a while. I haven't looked at it deeply. It's more like a nagging ache that I've been meaning to examine closely at some point. Here's the basic observation/idea in the auditory domain:
-Observation #1: Neural oscillations tend to entrain in the phase of their response to periodic stimuli.
-Observation #2: Many natural sounds, such as speech, are quasi periodic.
-The Claim: Oscillation entrainment facilitates perception by synching periods of maximal neural sensitivity to temporal windows in the stimulus stream that contain the most useful information.
I like this idea. It is a reasonable idea. I don't know if it is correct. The alternative possibility, given the two observations is that neurons and neural networks oscillate just because--i.e., build a network with excitatory-inhibitory interactions, balance them so you don't get runaway excitation or constant suppression, and it tends to oscillate and will tend to reset it's phase with inputs (this is a fact, btw). On this alternative possibility, phase-locked oscillations are more of a by product of network design, not something that was selected for to enhance perception. In fact, it's conceivable that phase locking to a stimulus could actually interfere with neural signal processing, although not sufficiently to preclude the systems useful as a signal processor.
So what we need to show to decide whether oscillation entrainment is a design feature to enhance perception or whether it is a spandrel-like emergent property is examples of perceptual enhancement resulting from neural oscillation entrainment.
A recent paper by Henry & Obleser (Frequency modulation entrains slow neural oscillations and optimizes human listening behavior, PNAS) claims to do just this. They used an FM modulating stimulus and inserted gaps at various phases of the modulation. The listener's task was to detect the gaps. They found that listener performance was affected by the position of the gap relative to phase angle. Cool! But it wasn't consistent across subjects. Bummer! Different listeners had different phases that led to best performance. BUT, when they looked at the EEG signal oscillation phase, then a consistent relation was observed. Cool! Behavioral was tied to neural, not signal rhythmicity.
This sounds like a very nice and important result, which is why it's in PNAS. But again I have a nagging worry. Maybe it's nothing to get twisted up about but here it is: If the point of neural phase locking is to time neural sensitivity to stimulus rhythmicity, then shouldn't we worry if there is no consistent relation between stimulus phase and perceptibility? You only see the relation when you look at the neural oscillations themselves. Maybe different listeners just have different neural delays. But by half a stimulus cycle? Makes me worry.
So despite how appealing the theory, and how suggestive the growing collection of results is, I still worry that oscillations are doing all that much--at least on purpose.