3/22/2006

Synchrony and "Perception's Shadow"

Some of the earliest solid evidence for direct involvement between synchronous neural firing and cognitive tasks comes from a 1999 Nature article by Rodriguez, George, Lachaux, Martinerie, Renault & Verela. Here the authors argue that cycles of desynchronization and then re-synchronization are an organizing principle of neural computation - they even imply that the stages of this cycle reflect the essential "information processing components" of cognition.

The authors presented ten subjects with images of faces, either upside down or upright; the stimuli used made the patterns appear essentially meaningless unless they were upright (for example, the picture at the start of this article is actually an upside-down face!). The authors performed a time frequency analysis of EEG activity while subjects were viewing these stimuli and pressing one of two keys to indicate whether they had seen a face.

Two gamma-activity peaks were induced by the stimuli: one 36 Hz peak at 230 ms post stimulus onset, with a second 40 Hz peak at 800 ms post stimulus onset. The first was significantly stronger in trials where faces were detected, while the second was somewhat stronger in trials where no faces were seen. Even more surprising, they found a complex pattern of synchronization and desynchronization but only in trials where faces were seen:

1) A significant increase in synchrony occured at 200-260ms after stimulus onset centered on left parieto-occipital and frontotemporal areas [this is thought to reflect perceptual processing]
2) This was followed by a marked relapse into asynchrony at 500 ms, at bilateral parietal and occipitotemporal areas [this is thought to "set the stage" for motor processing]
3) In both trial types, synchrony reemerged at around 700 ms (on average, and centered on right temporal and central regions.) [this is thought to reflect motor processing]

One finding which is particularly surprising is that the highest levels of gamma activity co-occurred with desynchronization (not synchronization, as is often assumed to be the case).

One cannot explain these results in terms of volume conduction, or spurious synchronization, because only 7% of the synchronous oscillations occurred between neighboring electrodes, and because phase-locking was significantly more prevalent in trials where faces were observed. Yeung et al. have pointed to methodological problems with studies that assume that phasic peaks in time-averaged EEG signals are the result of phase-synchronization between previously uncorrelated signals (they can just as easily be interpreted as phasic bursts of activity). This criticism does not apply to this study, because they performed trial-by-trial comparisons of phase differences between electrodes after wavelet analysis.

The authors conclude that these results suggest "that a transition between two distinct cognitive acts (such as face perception and motor response) should be punctuated by a transient stage of undoing the preceding synchrony and allowing for the emergence of a new ensemble, through cellular mechanisms that remain to be established."

It should be noted that this evidence is merely correlational, not causal, which is a criticism that is frequently leveled against arguments for the importance of synchrony. The best causal evidence for the involvement of synchrony comes from Burle, B. and Bonnet, M. (2000) "High-speed memory scanning: a
behavioral argument for a serial oscillatory model
" which showed that one can decrease the efficacy of visual search processes by interfering with gamma-frequencies. Nonetheless, the study described above provides good evidence, in conjunction with research cited in previous posts (below) that "synchrony cycles" are a reliable part of successful cognitive processing.

Related Posts:
Entangled Oscillations
Anticipation and Synchronization
Gamma Synchrony

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