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We feel that we perceive our environment in real-time, despite the constraints imposed by neural transmission delays.
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Due to these constraints, the intuitive view of perception in real-time is impossible to implement.
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I propose a new way of thinking about real-time perception, in which perceptual mechanisms represent a timeline, rather than a single timepoint.
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In this proposal, predictive mechanisms predict ahead to compensate for neural delays, and work in tandem with postdictive mechanisms that revise the timeline as additional sensory information becomes available.
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Building on recent theoretical, computational, psychophysical, and functional neuroimaging evidence, this conceptualization of real-time perception for the first time provides an integrated explanation for how we can experience the present.
[cf. 2013] We feel that we perceive events in the environment as they unfold in real-time. However, this intuitive view of perception is impossible to implement in the nervous system due to biological constraints such as neural transmission delays. I propose a new way of thinking about real-time perception: at any given moment, instead of representing a single timepoint, perceptual mechanisms represent an entire timeline. On this timeline, predictive mechanisms predict ahead to compensate for delays in incoming sensory input, and reconstruction mechanisms retroactively revise perception when those predictions do not come true. This proposal integrates and extends previous work to address a crucial gap in our understanding of a fundamental aspect of our everyday life: the experience of perceiving the present.
[Keywords: perception, time, prediction, real-time, neural delays]
…Postdiction reconstructs the perceptual past: A key feature of this proposal is that the perceptual timeline can be updated, revised, reinterpreted, and overwritten as new information (sensory or otherwise) becomes available. This means that the subjective experience of past events can be affected by later events. Importantly, in this account, these postdictive mechanisms do not violate the law of causality because it is the represented past, not the physical past, that is revised.
Figure 1 illustrates how this allows the presentation of a second disc to affect the perception of events leading up that event in the Colour Phi effect. In this phenomenon, observers view 2 differently colored discs presented in different positions in quick succession (Figure 1A). This creates the percept of a single disc jumping from one position to the other, changing color midway. As in Box 1, rows in Figure 1B indicate the contents of the perceptual timeline at the 3 (physical) timepoints t0, t1, and t2. Broken squares indicate timepoints for which sensory input is not yet available, and asterisks mark the represented present.
At t0, the first available sensory evidence indicates that a disc has been detected. This is represented at the appropriate moment. Future representations may also be activated, depending on prior expectations of the disc’s duration. At t1, subsequent sensory evidence suggests the disc was an isolated flash. Any earlier prediction is discarded and empty space is represented for the moments following the flash. When the second disc is detected at t2, the timeline as a whole is postdictively reinterpreted as a moving disc. The timeline is revised, such that the disc is represented in intervening locations at intermediate moments.
…A final implication of the current proposal is that there is no hard natural boundary between perception and memory. Rather, there is a continuum between the 2: as perceptual representations become older, they become degraded, compressed, or summarised, gradually becoming experiences of a past event in a way that is typically called episodic memory. This continuum between perception and memory is consistent with previous discussions of consciousness more broadly26 and postdiction specifically [14], where retroactive revisions of past events are known to take place on timescales ranging from subsecond [14,27] to months or years [28].