An argument for discrete temporal frames in conscious perception
Once presented with a stimulus in our environment, how do we proceed to perceive it? Seemingly, we do not gather every insignificant detail about the world around us. Hence, is our perception of the world continuous, and our brains ignore unimportant stimuli? Or is it discrete, and we simply do not perceive certain stimuli? Intuition leads us to believe that perception is merely a continuous translation of the outside world into the mind. However, advancements in cognitive neuroscience are challenging prominent theories of continuous conscious perception in philosophy of mind. Investigations into oscillatory brain networks during perception increasingly support the hypothesis that perceptual consciousness is discrete.
In order to understand perceptual consciousness, we must understand how the brain perceives time and separate events in time. We instinctively agree with physics that time proceeds continuously. When we watch a car go down the road, it does not teleport from one place to the next — the car rolls forward continuously with no interruption. If perceptual consciousness really were continuous, then we should be able to distinctly identify truly discrete events as separate. Two discrete events presented to us must be consciously perceived as two discrete events.
Unique perceptual events have unique signatures in our brains. Doesburg et al. (2009) used EEGs to record these signatures and showed that large-scale phase-amplitude patterns in the gamma frequency band correlate with discrete moments of conscious perception. This finding is mirrored in both alpha and beta frequency bands (Baumgarten, 2015; Milton, 2016). Notably, Milton (2016) found that two discrete events can be perceived as either simultaneous or sequential depending on when they occur in relation to the phase of ongoing neuronal oscillations. Evidently, perceptual consciousness is influenced by rhythmic oscillations that may cause two discrete events to be perceived as one single event. While we receive a continuous stream of sensory data, what we consciously perceive depends on how our brain divides this stream into a series of discrete frames.
White (2016) argues against the hypothesis of conscious perception being a series of discrete temporal frames with two main objections:
“Evidence does not consistently support any proposed duration or range of durations for frames” (White, 2016).
A number of durations for discrete temporal frames have been proposed, ranging from 4.5 ms (Geissler, 2001) to 200 ms (Kozma, 2017). As White (2016) argues, we must be skeptical of the discrete frame hypothesis due to the variation in durations. He attributes the variation to result from researchers relying solely on evidence that agrees with their proposed frame duration. The focus on supporting evidence and disregard for inconsistent evidence causes the proposed durations to be uncertain. No single proposal is better than the others, so no definite conclusion can be made on the duration of perceptual frames. If this is the case, we must consider the fact that the discrete explanation of perceptual consciousness is inadequate, and perceptual consciousness may be continuous.
While many explanations for the discrepancy in frame durations exist, two of the most relevant ideas will be discussed. Firstly, brain function involves neural oscillations at various frequencies. For example, theta oscillations occur at 4–7 Hz (Nakayama, 2018), while beta oscillations occur at 13–30 Hz (Baumgarten, 2015). The cycle length of these neural oscillations defines discrete temporal frames. If these oscillations occur at various frequencies, then the resulting frame durations will also vary. Clearly, measurements taken across various frequency bands would not result in the same proposal of frame duration. Another explanation for the varying durations can be derived from research done by Herzog (2016). Their proposed conceptual framework stimuli posit that stimuli are first analyzed quasi-continuous and unconsciously. Once this unconscious processing is “completed”, the stimuli are rendered conscious at discrete moments in time. The study implicates that the duration of a frame and the temporal resolution of conscious perception are independent issues. A group of subjects may consciously perceive a stimulus at the same time and have different durations of frames, or conversely, consciously perceive a stimulus at different times and have the same duration of frames. Therefore, the varying durations of frames cannot falsify the idea of discrete frames in conscious perception.
Though the first response seems to be adequate, it needs to be investigated whether neural oscillations measured in the same frequency band really do result in the same proposed frame durations. As of now, there are not enough studies to indicate a statistically significant result. However, the implications made by Herzog (2016) seems to negate White’s objection entirely. If the duration of a frame and conscious perception are truly separate issues, then the discrepancies in durations do not matter. More work needs to be done to further confirm whether the duration of frames and conscious perception are truly independent. The two responses presented provide promising beginnings for the argument against White’s criticism.
“EEG waveforms provide evidence of periodicity in brain activity, but not necessarily in conscious perception” (White, 2016).
Periodic brain oscillations do not imply the existence of discrete frames in our perceptual consciousness. The current field of cognitive neuroscience largely agrees that perception wanes and waxes periodically in a given interval (Baumgarten, 2015; Doesburg, 2009; VanRullen, 2015). The oscillations form a continuous motion, so how can we derive discrete moments from a continuous wave? When exactly, on these oscillations, do we perceive and not perceive? Evidently, perception does not switch on and off discretely. Hence, it is more compelling that perception is just stronger at some times than others, rather than actually being discrete. It is incorrect to assume that brain oscillations directly determine discrete frames in conscious perception. White (2016) concludes with the argument that periodic oscillations in brain activity are not enough evidence to support the discrete frame hypothesis.
Contradicting evidence from recent studies lie in the wake of White’s objection. One study used imperceptible electrotactile stimuli in a temporal discrimination task to show how stimuli can reset neural oscillations and impact perception (Baumgarten, 2017). While previous studies may have shown that oscillations somehow influence perception, this study shows that unperceived stimuli can reset these oscillations. These stimuli systematically modulated the ongoing oscillations, which critically influenced the discrimination task. The participants’ responses showed conscious discrete perception. This causal evidence for discrete perception due to imperceptible stimuli implicates a discrete model of perceptual consciousness. In another study, subjects were shown a visual illusion in which a smoothly moving pattern is perceived as jumping periodically (Nakayama, 2018). The illusion induced rhythms in the theta-band frequency range, suggesting that slow theta rhythms are involved in converting continuous, dynamic, visual events to discretized perceptual experiences.
Although the study by Baumgarten (2017) does not provide clear causal evidence of discretized perception due to periodic oscillations, it successfully shows the two to be indirectly, but undoubtedly, linked — unconsciously perceived stimuli can bring about rhythmic perception by modulating neural oscillations. Additionally, Nakayama (2018) provides further evidence to support the discrete frame hypothesis with theta-band rhythms — truly continuous stimuli can seem perceptually discrete. More evidence and peer-review are needed to support the claims made by both authors, and more studies must be performed to strengthen the argument for discrete perceptual consciousness. However, both studies disprove White (2016) by successfully showing that periodic neural oscillations not only exist but that they play a critical role in discrete conscious perception.
While the evidence for discrete perceptual consciousness will undoubtedly grow, we cannot be quick to assume that the debate is over. Whether any of the aforementioned studies directly show that perception is organized in discrete temporal perceptual frames is highly contentious. As objections to the hypothesis and its responses are evaluated and reassessed, skeptics must be tolerant of new ideas and proponents must be open to critique. Nevertheless, the truth about perceptual consciousness will become more clear as research progresses.
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