Abstract
Apparent motion is an illusion in which two sequentially presented and spatially separated stimuli give rise to the experience of one moving stimulus. This phenomenon has been deployed in various philosophical arguments for and against various theories of consciousness, time consciousness and the ontology of time. Nevertheless, philosophers have continued working within a framework that does not reflect the current understanding of apparent motion. The main objectives of this paper are to expose the shortcomings of the explanations provided for apparent motion and to offer an alternative explanation for the phenomenon.
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Notes
For English translation and commentary, see (Wertheimer 2012).
In partial cases of apparent motion, the experienced motion covers only part of the spatial gap between the locations of the stimuli. Apparent motion needs to be distinguished from (what is called) pure motion because they differ in two respects. Firstly, unlike apparent motion stimuli, the stimuli used for inducing the experience of pure motion do not have a small temporal gap between the offset of the first stimulus and the onset of the second stimulus. Secondly, while subjects report experiencing that something moves in the cases of apparent motion, in the pure motion experiments the movement is experienced (as experiential quality) but the subjects do not report seeing any object that moves (Steinman et al. 2000).
According to cinematic models of temporal experiences (Dainton 2010), also known as the zoëtrope conception of experience (Phillips 2011a), experiences are confined to momentary episodes and lack dynamic content (e.g. change, motion, and succession). A momentary episode of experiencing is not synonymous with the lack of dynamic content, however, because momentary experiences can have dynamic contents, too. For example, if one can have an experience of motion without seeing that any object moves (i.e. pure motion, discussed in the footnote 1 and in Hoerl 2012), such a phenomenal quality could be instantiated in a momentary experience and even on an instantaneous content (Arstila, The time of experience and the experience of time. forthcoming; Le Poidevin 2007). Accordingly, even though TOR view may appear to be most compatible with the idea that both the episodes of experience and their content are momentary, the view remains neutral concerning the cinematic models.
For this reason, the inheritance principle would be trivially true if the cinematic model is correct. Then again, the TOR view could still be incorrect and indeed it is if Dennett and Kinsbourne’s view is correct (Arstila, The time of experience and the experience of time. forthcoming).
The Paillard-Fraisse hypothesis assumed that the differences due to cortical processing of sensory signals are marginal. This assumption is contested in the described alternative explanation for apparent motion since it allows that the differences in the cortical processing lead to differences in experienced timing of events (e.g. by delaying the experience of the second stimulus).
Given that most philosophers (including those discussed in this paper) hold that our experiences as of motion are real, this idea will not be discussed here. For the reasons why Reid’s position is not usually held, see Dainton (2008a).
It should be noted that Hoerl (2012, 22–23) argues that his theory is incompatible with Muckli et al.’s work, but only if the “activity (of the form found by Muckli et al.) in a particular region of V1 implies conscious visual experience as of a stimulus in the location represented by that region.” As discussed above, this is how the results of studies concerning recurrent processing have been interpreted.
The weaker version of the subjective time view, the content-sensitive settling view (see Dennett and Kinsbourne 1992), maintains lesser separation between neural processing and experiences: although our experiences can be delayed, the experienced order of things is mainly the same as the order in which things are neurally processed. Since the content-sensitive mechanism does not generate any new experiential contents that are required for the filling-in to occur, it does not explain the experiences of apparent motion and it will not be elaborated on.
The immediate future does not refer to clairvoyance but one’s anticipation on what is about to happen next (as in listening a familiar melody).
Alternatively, one could base the theory on the temporal modes of presentation, in which case the felt “presentness” of the experienced things will decrease as the time passes. As Dainton has argued, however, attempts to explicate what this means in detail have led to various problems.
Calling this Orwellian revision is misleading, however, because originally the term (as used in Dennett and Kinsbourne 1992) referred to the revision of our memories of events, not rewriting the experiences on which the memories are based. Thus in the original meaning motion would not really be experienced. Then again, there are good reasons to think that motion is experienced in apparent motion experiments and if the earlier experiences can be rewritten after they have been experienced, perhaps Orwellian revision is close enough in meaning to warrant its use in these cases too.
Other problems with Grush’s theory, and any theory that deploys time markers, stem from the fact that the notion of the time marker remains underdeveloped. It is worth noting that this applies also if Grush’s theory is constructed to incorporate relative time markers (“before”, “later”) rather than absolute time makers (t0, t1, etc.) (Arstila, The time of experience and the experience of time. forthcoming).
This delay can be compensated for by predictive mechanisms, as in Grush’s view, but such compensation does not work for a surprising stimulus that is used in many apparent motion experiments.
This estimation is based on the peak latencies related to stationary stimuli presented to humans (Yoshor et al. 2007). The faster latencies that are sometimes reported are based on (i) the studies on macaque monkeys, which have faster latencies as a result of smaller brains and thus shorter neural pathways than humans, (ii) the pathway that processes motion, not features of stationary stimuli (see next section).
It takes 10–50 milliseconds for the information from V5 to reach V1 and the effect is strongest at 25 milliseconds (Pascual-Leone and Walsh 2001) or maybe only after 40 milliseconds (Silvanto et al. 2005). Assuming that it takes as long for information from V1 to reach V5 as it takes for the information from V5 to reach V1, the revision of the empty screen to motion takes at least 50 milliseconds.
This estimation is conservative for the following reasons. First, it assumes that the registration of the mere presence of the second stimulus in V1 is enough to trigger the revision of pre-experiential states. In other words, only simple features—e.g. contrast, orientation, chromatic information (not colors yet)—need to be processed in order for the revision to occur. If the revision requires that, say, the color, shape, and perceived location of the second stimulus are determined, then the latency would be longer. Second, this estimation assumes that the processing of the offset of the first stimulus, the empty screen, and the onset of the second stimulus take an equal amount of time. (E.g. the second stimulus is not processed slower than the offset of the first stimulus due to the fact that attention is drawn to the first stimulus.) Third, this estimation assumes that the pre-experiential state related to the empty screen is already completed at this point and hence the postulated extra delay (the waiting time) is as small as it needs to be. In practice this means that the extra delay is less than the interstimuli interval, which follows from the idea that the processing of the second stimulus does not need to reach the pre-experiential state in order for the revision to occur. Fourth, the interstimuli interval is 200 milliseconds. However, longer interstimuli intervals can produce at least partial experiences of apparent motion too and Phillips (2011b) argues that events taking place 600 milliseconds after a stimulus can mask the experience of a stimulus. In both cases, the delay would need to be much longer. (It should be noted that Phillips questions the need for a single universal delay. Then again, it is not obvious how our perceptual processes could know beforehand the required length of the delay.)
Dennett and Kinsbourne (1992) also argued that the required delay is empirically too long for the brain to hide or hold back the processed information, but they did not provide any details on the matter.
Both measurements vary depending on the stimulus complexity and the nature of the task. When a stimulus is simple and clearly visible (as in apparent motion experiments), VAN can occur already 100 milliseconds after the stimulus. Likewise, when we expect certain things to happen (for example, the second stimulus to be presented in apparent motion experiments), late positivity occurs 200 milliseconds after the onset of the stimulus and VAN before late positivity.
One could argue that the experience emerges much later than VAN or late positivity. Two things speak against this possibility. First, the experience cannot be inhibited with transcranial magnetic stimulation (TMS, which can be used to disrupt neural processing) that is given after the time specified by VAN or late positivity. Second, reaction time studies show that when subjects report seeing a stimulus by pressing a button, this reporting action begins almost immediately after VAN. That is, subjects can reliably report whether they perceived a stimulus or not a mere 200–300 milliseconds after the stimulus was presented.
Once processing in those areas which specialize in processing particular information has finished, they in turn activate V1 by means of feedback connections. As discussed above, such recurrent processing is thought to be crucial for any visual experience to occur. Adding to the idea of the independence of motion processing from the other features, the stimulation of only V5, and later V1 (when it matches the time course of recurrent processing), can give rise to the experiences of motion (Silvanto et al. 2005).
The feed-forward sweep reaching V4 continues on to later cortical areas, which are also central to the perception of fine details, whereas this does not hold for V5 and visual motion perception. Thus, these temporal differences only increase in later processing stages.
It is important to notice that this scheme, too, simplifies the complexity of visual processing. For example, in truth, the experience of motion could still continue at the time B is experienced. We might not notice this, however, because of the masking effects of the stronger neural activation related to B or because of the temporal resolution of the judgments we make about our experiences. (Also a new object representation draws attention away from the older object representations, in this case from the moving stimulus.)
It is worth noting that Moutoussis and Zeki’s studies also suggest that visual motion is processed slower than color and orientation contrary to what has been claimed here. This discrepancy is only apparent, however, because such effects only occur when the stimuli have been presented for some time. In the speed of visual processing studies, on the other hand, the stimuli are shown only briefly—as in the apparent motion studies too.
This explanation suggests that, with suitable exposure times and inter-stimulus intervals, apparent motion is experienced before the first stimulus. For example, if the first stimulus is shown for one millisecond and the inter-stimulus interval is 59 milliseconds, then the experience of motion might precede the experience of the first stimulus by 30–80 milliseconds (and the second stimulus by 90 milliseconds). Although this is not impossible—(to my knowledge) no one has ever tested this hypothesis—it is more probable that this prediction ignores the complexity of visual processing. For instance, in the provided example, information related to A arrives at V1 shortly before motion processing by V5 influences V1. Thus, the former is likely to mask the initiation of the motion. If one makes the inter-stimulus interval shorter, then the stimuli are likely to produce an experience of simultaneity and not apparent motion (when the onset of visual stimuli is less than 30–70 milliseconds, they are experienced to occur simultaneously). If the inter-stimulus interval is lengthened a little, then masking will no longer take place, but motion is not experienced to occur before the first stimulus either. Rather, A and the experienced initiation of the motion are likely to occur roughly at the same time.
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Arstila, V. Theories of apparent motion. Phenom Cogn Sci 15, 337–358 (2016). https://doi.org/10.1007/s11097-015-9418-y
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DOI: https://doi.org/10.1007/s11097-015-9418-y