In 1890 William James introduced the metaphor of the “stream of consciousness” into Western psychology: “Consciousness… is nothing jointed; it flows. A ‘river’ or ‘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.”
Over a thousand years before, the same image figured prominently in the Buddhist philosophical tradition known as the Abhidharma. There the Buddha is portrayed as saying: “The river never stops: there is no moment, no minute, no hour when the river stops: in the same way, the flux of thought” (quoted in Louis de la Vallée Poussin, “Notes sur le moment ou ksnana des bouddhistes”).
For both James and the Abhidharma, mental states don’t arise in isolation from each other; rather, each state arises in dependence on preceding states and gives rise to succeeding ones, thus forming a mental stream or continuum. However, James and the Abhidharma have different views about the nature of the mental stream.
According to James, although the mental stream is always changing, we experience these changes as smooth and continuous, even across gaps or breaks. The gaps and changes in quality that we do feel or notice—for example, when we wake up from a deep sleep—don’t undermine the feeling that our consciousness is continuous and whole. And the gaps and changes in quality that we don’t notice aren’t felt as interruptions because we’re not aware of them.
The Abhidharma philosophers agree that the mental stream is always changing, but they argue that it appears to flow continuously only to the untrained observer. A deeper examination indicates that the stream of consciousness is made up of discontinuous and discrete moments of awareness. Part of this deeper examination comes from philosophical analysis; part of it comes from meditation practice (the exact relationship between the two being a matter of debate among Buddhist scholars).
If the classical Abhidharma philosophers were alive today, they might wonder whether experimental psychology and neuroscience would have anything to say about these matters. Is there any scientific evidence for measurable discrete moments of experience in the “stream” of consciousness?
In 1979, when I was sixteen, I took part in an experiment that neuroscientist Francisco Varela devised to investigate whether perception is continuous or discrete. I’d never been to a neuroscience lab, and the prospect of seeing my own brain waves was enticing. Francisco and I set off from Sixth Avenue and 20th Street, where we lived at the Lindisfarne Association, to the New York University Brain Research Laboratories at 550 First Avenue. I sat in a dark room with electrodes fixed to my scalp and watched two small lights flash on and off. My task was to say whether the lights were simultaneous or sequential, or whether there was one light moving from left to right.
It’s well known in experimental psychology that there is a certain minimal window of time within which two successive events will be consistently perceived as happening at the same time. For example, if you’re shown two successive lights with less than about 100 milliseconds between them, you’ll see the lights as simultaneous. If the interval is slightly increased, you’ll see one light in rapid motion. If the interval is further increased, you’ll see the lights as sequential. These phenomena of “apparent simultaneity” and “apparent motion” have sometimes been interpreted as supporting the idea of a discrete “perceptual frame,” according to which stimuli are grouped together and experienced as one event when they fall within a period of approximately 100 milliseconds.
If perception is discrete—if it unfolds as a succession of perceptual frames with a gap between each frame and the next one—then we can make the following prediction: whether two distinct events will be judged as simultaneous or sequential depends not just on the time interval between them but also on the relation between the timing of each event and the way perception falls into discrete and successive frames, that is, the ongoing process of perceptual framing. In particular, two events with the same time interval between them can be perceived as simultaneous on one occasion and as sequential on another occasion, depending on their temporal relationship to perceptual framing: if they fall within the same perceptual frame, they’re experienced as simultaneous, but if they fall in different perceptual frames, they’re experienced as sequential. In short, what you perceive as one event happening “now” depends not just on the objective time of things but on how you perceptually frame them.
It was precisely this idea that Francisco wanted to test. Already in his early years as a young scientist, Francisco’s research was strongly motivated by a vision of the brain as a self-organizing system with its own complex internal rhythms. (Although popular today, this vision was a small minority view in the 1970s, when most scientists thought of the brain as a sequential-processing computer.) These rhythms, he believed, bring forth meaningful moments of perception in a fluctuating and periodic way. Francisco was also intrigued by the parallels between the Abhidharma notion of discrete “mind moments” and the neuroscience view of discrete perceptual frames created by the brain’s self-generated rhythms. A month or so before my visit to the NYU Brain Research Lab, Francisco and I had talked about the Buddhist idea of “mind moments” and the gaps between them as we walked to the old Paragon Book Gallery on East 38th Street, where he bought a hard-to-find copy of Louis de la Vallée Poussin’s classic French translation of Vasubandhu’s Treasury of Abhidharma. It was only after the experiment that Francisco told me what he really wanted to do was measure a “mind moment.”
In the experiment, Francisco recorded the brain’s ongoing EEG alpha rhythm and used it to trigger when the two lights flashed on and off. The hypothesis was that seeing the lights as either simultaneous or in apparent motion would depend on when they occurred in relation to the phase of the ongoing alpha rhythm. Like a surfer catching a wave, if the lights arrived at a certain point of the repeating alpha cycle, they would be seen as simultaneous, but if they missed the wave, they’d be seen as in apparent motion. In other words, presenting two flashes of light always with the same time interval between them, but at different phases of the alpha rhythm, would result in noticeably different perceptions.
The results supported the hypothesis: when the lights were presented at the positive peak of the alpha rhythm, they were almost always seen as in apparent motion, but when they were presented at the negative peak (the opposite phase), they were seen as simultaneous. In the published study, a figure showed my visual performance along with that of two other participants (see the bar labeled “ET” in the figure below from the original paper). At an interval of 47 milliseconds between flashes, my discrimination between simultaneity and apparent motion was at a chance level, but there was a change in the probability of my perceiving the lights as simultaneous when they were presented at either the positive or the negative peak of my ongoing alpha rhythm.
Unfortunately, these promising results have proved difficult to replicate, both by Francisco in a follow-up study and by other scientists today. Nevertheless, the experiment is widely cited as precisely the kind of experiment that would be needed to demonstrate definitively the discrete nature of perception; furthermore, new and more sophisticated studies are extending and deepening this line of research into the relationship between electrical brain rhythms and the pulsing character of perceptual awareness. For example, recent experiments show that whether a visual stimulus is consciously detected or not depends on when it arrives in relation to the phases of the brain’s ongoing alpha (8–12 Hz) and theta (5–7 Hz) rhythms (see also this study). You’re more likely to miss the stimulus when it occurs during the trough of an alpha wave; as the alpha wave crests, you’re more likely to detect it.
The moral of these new studies isn’t that perception is strictly discrete, but rather that it’s rhythmic; it happens through successive rhythmic pulses (an idea James also proposed), instead of as one continuous flow. Like a miniature version of the wake-sleep cycle, neural systems alternate from moment to moment between phases of optimal excitability, when they’re most “awake” and responsive to incoming stimuli, and phases of strong inhibition, when they’re “asleep” and least responsive. Moments of perception correspond to excitatory or “up” phases; moments of nonperception to inhibitory or “down” phases. A gap occurs between each “up” or “awake” moment of perception and the next one, so that what seems to be a continuous stream of consciousness may actually be composed of rhythmic pulses of awareness.
These ideas are needed to interpret the significance of some recent studies on the neural and behavioral effects of “mindfulness” meditation practice. In the well-studied cognitive psychology task known as the “attentional blink,” participants need to identify two visual targets (T1 and T2 in the figure below) presented within less than 500 milliseconds of each other in a rapid sequence of other visual stimuli.
Participants usually detect the first target but often miss the second one. It’s as if their attention blinks after they notice the first target, and the second one goes by in that instant. Neuroscientists Heleen Slagter, Antoine Lutz, and Richard Davidson investigated whether meditation practitioners would show improved performance on the “attentional blink” task after a three-month intensive retreat in Theravāda Buddhist Vipassanā or “insight” meditation. This type of mindfulness meditation cultivates both focused attention, whereby you learn to sustain your awareness on a given object, such as the sensations of the breath, and open awareness, whereby you learn to be open and attentive to whatever arises in experience from moment to moment. The scientists compared the performance of the practitioners on the attentional blink task before and after the retreat, and they also compared the performance of the practitioners with that of a control group of novices who were interested in meditation, took a one-hour Vipassanā meditation class, and were asked to practice for twenty minutes each day for a week before the experiment. After the three-month retreat, the attentional blink of the practitioners was significantly reduced, that is, the practitioners showed significantly improved detection of the second target (compared to the novice group, who also showed improvement). This improvement was also correlated in the practitioners (but not in the novices) with EEG measures showing more efficient brain responses to the first target. Furthermore, the individuals who showed the largest decrease over time in the neural activity they required to detect the first target also showed the greatest improvement in detecting the second target. Thus, a more efficient neural response to the first target seems to facilitate detecting the second one.
But there’s more. It’s well known that electrical brain rhythms in the theta frequency range (5–7 Hz) shape the rhythmic pulses of perception and attentional sampling. Slagter, Lutz, and Davidson found that intensive Vipassanā meditation practice affected these theta rhythms in ways that were linked to improved performance on the attentional blink task.
First, for both the meditation practitioners and the novices, the neural oscillations in the theta frequency range “phase-locked” to the targets when the targets were consciously perceived. If you think of the incoming stimuli and the ongoing brain activity as making up a partner dance, then the brain stays in step with its stimulus partners by matching its activity at a certain frequency and phase to their arrival. The scientists determined that whenever the targets were consciously seen, the brain had stayed in step with them by matching the phase of its theta oscillations to their occurrence.
Second, the scientists found that in the practitioners, but not in the novices, the theta phase-locking to the second target increased following intensive meditation. The brain got better at staying in step with the second target. More precisely, there was a reduction in the variability of theta phase-locking from trial to trial, which is to say that the brain’s matching of its theta waves to the second target became more precise and consistent. Furthermore, the individuals who showed the largest decrease in the neural processes required to detect the first target also showed the greatest increase in theta phase-locking to the second target. In this way, more efficient neural responses to the first target were linked to greater neural attunement to the second target. That is, individuals who required fewer resources to stay in step with the first target were also better at staying in step with the second one.
These studies using the attentional blink task indicate that intensive Theravāda Vipassanā meditation improves attention and affects brain processes related to attention. In recent years, other studies using other tasks and a wide range of meditation styles have shown that mindfulness meditation improves perceptual sensitivity and strengthens the abilities to sustain attention on a chosen object and to remain open to the entire field of awareness from moment to moment. One route by which these changes may happen is that Vipassanā meditation may fine-tune the theta oscillations that shape the stream of sensory events into rhythmic pulses of conscious perception.
So is perceptual consciousness a “stream”? Yes, in the sense that it seems to flow, but no, if “flow” means “uniformly and continuously.” Instead, the flow is rhythmic, with variable dynamic pulses.
You might object, however, that if Vipassanā meditation changes experience and how the brain operates, then we have no right to assume that ordinary, premeditative consciousness is not uniform. Maybe premeditative consciousness is uniform and Vipassanā meditation changes it. Given this possibility, it’s unwarranted to project onto premeditative experience how experience seems after meditative training.
This objection is important. As a general policy, we must avoid the fallacy of projecting qualities from later trained experience onto earlier untrained experience. In the present case, however, we have independent data from psychology and neuroscience that ordinary perception and attention exhibit rhythmic pulses, at least in certain respects or under certain conditions. We also have data about the electrical brain rhythms linked to these pulses. Given these findings, as well as the findings from the Vipassanā meditation studies on how meditation affects the same cognitive functions and electrical brain rhythms, it seems legitimate to conclude that mindfulness meditation can reveal and sensitive you to rhythmic and pulsing aspects of awareness that you may ordinarily overlook. So both James and the Abhidharma philosophers were right.