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Is Consciousness a Stream? An Update

A new experiment shows perception is discrete, not continuous

About a month ago, at the Brains Blog, I published a post on the question of whether consciousness, specifically our perceptual experience, flows continuously or is discrete. It’s an ancient question, one that has been debated in both Western and Indian philosophy. A fascinating new experimental study, just published in the Proceedings of the National Academy of Sciences by German neuroscientists Thomas J. Baumgarten, Alfons Schnitzler, and Joachim Lange, provides novel evidence that the stream of perceptual consciousness is discrete.

To appreciate the study, it helps to have some background.

The great psychologist and philosopher William James introduced the metaphor of the “stream of consciousness” into modern Western thought. In 1890 he wrote: “Consciousness… is nothing jointed; it flows. A ‘river’ or a ‘stream’ are the metaphors by which it is most naturally described. In talking of it hereafter, let us call it the stream of thought, of consciousness, or of subjective life.”

Centuries before in a completely different culture, the Indian Buddhist philosophical school known as Abhidharma argued that the mental stream appears to be continuous only to the untrained observer. A deeper psychological analysis reveals it to be made up of discontinuous moments of awareness.

As I discussed at the Brains Blog and in Chapter 2 of my book, Waking, Dreaming, Being: Self and Consciousness in Neuroscience, Meditation, and Philosophy, recent experiments and older pioneering ones suggest that the Abhidharma philosophers were right. These experiments indicate that what we experience as the continuous flow of perception may be composed of discrete, rhythmic cycles.

One reason this idea is interesting is that it implies that temporal perception—how you perceive events as ordered in time—depends not just on time as measured by a clock but also on how your brain orders events according to its own internal rhythms. For example, two flashes of light with the same time interval between them can be perceived as happening either at the same time or as one after the other, depending on when they occur in relation to ongoing brain rhythms known as neuronal oscillations. In other words, whether you perceive the flashes as simultaneous or sequential depends not just on their clock time but also on how your brain makes sense of them according to its own internal rhythms.

Empirical evidence for this idea, however, has been hard to come by. An early pioneering experiment by Francisco Varela and his colleagues showed that whether two lights are perceived as simultaneous or sequential depends not just on the time interval between them but also on when they are presented in relation to the phase of the brain’s ongoing alpha rhythm. Presenting two flashes of light always with the same time interval between them, but at different phases of the alpha rhythm, resulted in noticeably different temporal perceptions. This finding, however, has proved difficult to replicate, both by Varela in a later experiment and by other scientists. In addition, all the experiments testing these ideas have come from the study of vision and not from other perceptual modalities.

Until, that is, the new study by Baumgarten and colleagues. These scientists used somatosensory perception, specifically the tactile perception of two electrical pulses delivered to the index finger and separated in time by approximately 25 milliseconds. Would the internal neuronal oscillations influence whether the experimental participants perceived the stimuli as one pulse or as two pulses? The answer was yes. Specifically, the scientists found that perceiving the stimuli as one pulse or as two pulses depended on when the pulses arrived in relation to the ongoing neuronal oscillations in the alpha and lower-beta frequency band (8-20 Hz) in an area of the brain called the primary somatosensory cortex. The basic idea is that if two sequential stimuli fall within one oscillation cycle, they are perceived as one, but if they fall within separate oscillation cycles, they are perceived as two. In other words, anything that falls within one cycle is perceived as happening "now;" anything that straddles more than one cycle is perceived as happening sequentially. In this way, neuronal oscillation cycles define discrete perceptual moments.

Baumgarten and colleagues note that these results support Varela’s basic finding and proposal from many years ago. As I describe at the Brains Blog and in Waking, Dreaming, Being, Varela was actually inspired by the Abhidharma idea of discrete “mind moments” when he devised his experiment. Besides showing that ancient ideas about the mind from another culture can be productive for cognitive science, the Abhidharma inspiration casts light on the relationship between these scientific findings and our subjective experience. In a section titled “Contradicting Subjective Experience,” Baumgartner and colleagues write: “personal experience does not intuitively match with a discrete sequencing approach, but, rather, resembles a seamlessly updated percept.” The Abhidharma philosophers would agree, but would go on to point out that the discrete sequencing of perception can be made experientially available to someone whose mind has been trained in meditation and who knows what to look for. They would argue that rather than taking these scientific findings as contradicting our subjective experience, we should take them as a reason to look deeper into our experience in a meditative way.