MEG alpha activity decrease reflects destabilization of multistable percepts

Abstract

Multistable stimuli offer the possibility to investigate visual awareness, since they evoke spontaneous alternations between different perceptual interpretations of the same stimulus and, therefore, allow to dissociate perceptual from stimulus-driven mechanisms. In the present study, we used an ambiguous motion paradigm and compared endogenous reversals of perceived motion direction which occur spontaneously during constant ambiguous stimulation with exogenous reversals that were induced externally by changes of stimulation. Contrasting the two conditions allowed to investigate processes that trigger endogenous reversals, since the related activity should be absent in the exogenous reversal condition. We employed ambiguous dot patterns which can easily be transformed to present two stable motion directions in order to induce exogenous pattern reversals. Whole-head MEG was recorded from 10 subjects. We analyzed event-related fields (ERFs) and oscillatory activity in the alpha and gamma ranges. The results showed P300-like slow waves in response to both endogenous and exogenous reversals reflecting the conscious recognition of pattern reversals. Analyses in the gamma-band did not reveal any significant modulations. The alpha activity showed different time courses for endogenous and exogenous reversals. While the exogenous alpha activity decreased in temporal relation to the pattern reversal, the endogenous alpha activity displayed a continuous decrease starting in the time interval preceding the reversal. This time course of the endogenous alpha activity is consistent with a bottom-up approach to figure reversals, since it reflects a process of destabilization of the actual percept until a switch of visual awareness occurs. 

Electrophysiological correlates of flicker-induced color hallucinations

“…The finding that good perceptual performance is related to low alpha power has also been shown by Hanslmayr et al (2005). A variety of studies have shown that decreasing alpha power precedes the transitions between alternative perceptual representations of a multistable visual stimulus (Isoglu-Alkaç et al, 2000;Müller, Federspiel, Fallgatter, & Strik, 1999;Müller et al, 2005), findings confirmed in using magnetoencephalographic (MEG) measurements by Strüber and Herrmann (2002). Evidence for the potential of alpha activity for binding separate neural processes has been provided by von Sarnthein (2000, also von Stein, Chiang, &König, 2000) who have reported long-range fronto-parietal interactions in the theta and alpha frequency band during working memory retention and mental imagery.…”

“…A multidimensional scaling of co-occurrent hallucinatory experiences in Becker and Elliott (2006) ple; see Becker & Elliott for a full description of the pattern of inter-relationships between colors), implying the impossibility of these colors to be experienced at the same time. In this respect, the experience of hallucinatory color may be similar to the perception of multistable figures (Müller et al, 2005;Strüber & Herrmann, 2002), where one interpretation of the figure is deactivated and replaced by another interpretation. The decrease of alpha power preceding the report of hallucinatory color may reflect such a transition.…”

“…As detailed in the introduction, specific changes of alpha activity have been reported during the perception of multistable figures (see for example, Müller et al, 2005 andHerrmann, 2002). Strüber and Herrmann (2002) suggest that alpha activity during the perception of one interpretation of a multistable figure constantly decreases until it reaches a critical threshold following which a perceptual switch occurs and a second interpretation of the figure is perceived.…”

“…Strüber and Herrmann (2002) suggest that alpha activity during the perception of one interpretation of a multistable figure constantly decreases until it reaches a critical threshold following which a perceptual switch occurs and a second interpretation of the figure is perceived. In the brain, this might be achieved by means of a decrease in alpha band synchronization, which would allow neurons to synchronize at other frequencies (such as those in the gamma band), responsible for the coding and representation of a new percept or event.…”

9. Johann Wolfgang von Goethe (1749–1832)

“…Using magnetoencephalographic (MEG) measurements, a continuous decrease of endogenous alpha activity was shown to precede the reversal between two stimulus interpretations [156]. In addition to changes in alpha activity, a role for gamma band activation during the perception of multistable figures has been claimed [157].…”