Perceptual Oscillation:
A few years ago it would not have been considered appropriate by the leaders in the field to use the word “oscillation” to describe the irregular, variable and often-voluntary alternations of perceptual rivalry. The timing of rivalry was usually ignored, or even disguised by the “phase normalisation” process where individual variations in rivalry rate were factored out.
 
That has changed.

There is now recognition that there may be an underlying oscillatory process that forms a basis for the variety of perceptual rivalries.

 There is no agreement on the location of the oscillator, with Hugh Wilson’s “g” factor of cortical networks at one extreme and my own suggestion of a sub-cortical, interhemispheric oscillator at the other. Here are some of the new facts that have made it acceptable to talk about rivalry as a perceptual oscillation (numbers in brackets refer to my publication list):-

1. The pattern of oscillations can be predictably changed by caloric vestibular stimulation (157) and TMS (157,177). Since there are 2 perceptual alternatives and 2 hemispheres, there are 4 possible effects of the manipulations, but only the predicted one is seen. It is easier to manipulate the Left hemisphere than the Right, but in later experiments we succeeded in producing clear effects from Right hemisphere TMS also (177).

2. The frequency of rivalry oscillations is coupled in the same individual across different forms of rivalry. For example, a “fast switcher” on binocular rivalry will also be a “fast switcher” on Plaid Motion Rivalry (190). This is true despite the wide inter-individual variation in rivalry rate.

3. Motion-induced Blindness is a form of rivalry, even though it does not show all of the accepted outward attributes, such as the symmetrical pair of alternative percepts that allows one to guess which percepts will alternate. The identity of MIB with rivalry was shown using neuropharmacology (LSD produces identical changes in the timing of both binocular rivalry and MIB), a demonstration that Levelt’s second proposition also applies to to MIB and the frequency coupling phenomenon (175).

4. Rivalry timing has a neural mechanism that uses 5HT1a receptors, not the 5HT2a receptors that are prominent in visual cortex. This finding shifts the focus on the rivalry timer to the brainstem where 5HT1a receptors predominate (183).

5. Although rivalry rate is very stable in one individual, rare individuals with extensive training in meditation, can change their rivalry rate (189).

6. Rivalry oscillations cease (or become too fast to be noticeable) during laughter (186)