[Journal für Philosophie & Psychiatrie, Juli 2011, Supplement]
4. Proposal of a general model of perception of date and duration on the basis of the theory of the time of presence
4.1. The issue of the reconstruction of date and duration in the time of presence
Date, like duration, can be reasonably hypothesised as being experienced on the basis of the feeling of the more or less great temporal remoteness/proximity of conscious phenomena. The fact of comparing episodes experienced as more or less remote (or proximate) from each other in the past or in the future may be a means of determining when and for how long they occurred or will occur. In other words, perceiving date and duration would consist in apprehending differences of intensity between the temporal factors, i.e., comparing the relative degree of "pastness" and "futureness" of the mental contents projected into the time of presence. According to this view, perception of date and duration is nothing but a modality of our ability to experience the past and the future: this is the capacity of apprehending the relationships between the mental contents we experience as belonging to the past or the future. In this respect, one could wonder if perception of the past and of the future per se is not in fact a psychological abstraction, i.e., if episodic memories are not always systematically dated or experienced as having some duration in the past and in the future, rather than being simply experienced as being "past" or "future".
Another possibility would be to assume that date and duration are mediated by another category of temporal factors, namely, qualities of "proximity"/"remoteness". In that case, representation of date and duration would not result directly from the apprehension of a differential feeling of pastness or futureness, but from the feeling that past or future events are close to or far from actual experience. These temporal factors would confer a special coloration to memory images allowing us to determine whether conscious phenomena we remember are old or recent episodes of conscious life, and whether those we envision are supposed to occur in a near or in a remote future. According to this view, our representation of the past and of the future is twofold: there is a domain of proximate conscious phenomena and a domain of remote conscious phenomena. By attributing a conscious phenomenon to one domain or to the other, we determine approximately the moment of its appearance in consciousness. This does not mean of course that temporal relationships between past and future conscious phenomena, and notably date, cannot be perceived more accurately. As in the previous model, one can hypothesize here as well that accurate estimation of date and perception of duration result from the comparison of the intensity differences between temporal factors.
Both theories are not exclusive from each others, and one could indeed imagine that the two presumptive categories of temporal factors are indeed involved in the perception of date and duration. Whatever that may be, the hypothesis of the proximity/remoteness qualities fits well with the distinction that psychologists and neuroscientists have established between short-term and long-term memory. It has been proposed that short-term memory and long-term memory are two functionally distinct forms of memory (Baddeley, 1986; Speer and al., 2003; Ryan and Cohen, 2004). Short-term (or working) memory is an essential cognitive function that allows us to re-experience very recent episodes of conscious life and to confront them with present events of conscious life. It ensures the short-term continuity of conscious experience and the maintenance "online" of information necessary to the performance of cognitive tasks. Besides short-term memory of the past that allows us to remember mental contents we have just experienced, there is also a short-term memory of the future that allows us to anticipate episodes of conscious life that are supposed to occur imminently (Addis and Schacter, 2008). In the first instance, contents retrieved would be associated with a judgment of "recency" (Eyler-Zorilla et al., 1996; Konishi et al., 2002; Hintzman, 2005), in the second instance with a judgment of "expectancy" (Haggard, 2005; Sarrazin et al., 2008). By contrast, one calls "long-term memory" the general ability to retrieve more or less remote events of one's own conscious life and to experience them as being "old" conscious phenomena. The notion of long-term memory is actually often confused with the more general concept of episodic memory, the type of memory involved in the recollection of episodes, i.e., spatially-defined mental contents that correspond to definite moments of the stream of consciousness (the issue of episodic memory is discussed in detail below). Episodic memory is now clearly demonstrated to be an autonomous form of memory that engages specific neural substrates, and it was shown to be as well prospective as retrospective: there is an episodic memory of the future as there is an episodic memory of the past. On the basis of the existing neuroscientific literature, one can evidence that remembering the past and envisioning the future are respectively mediated by a quality of pastness and a quality of futureness (see below). Episodic or long-term memory, or, more exactly, long-term episodic memory, ensures the long-term continuity of conscious experience, insofar as it allows us to apprehend the various autobiographical aspects of our past and future conscious life, chronologically and in relation to present facts we are currently experiencing (Markowitsch, 1995; Cabeza et al., 2002).
However, a growing number of studies tends to demonstrate that there is in fact no substantial difference between short-term and long-term memory and that brain areas recruited in both cases are significantly the same, i.e., those that have been classically recognized as mediating episodic memory in general (Cabeza et al., 2002; Ranganath and Blumenfeld, 2005, Jonides et al., 2008). Thus, there would be only a difference of degree, and not of nature, between remembering remote and proximate memories. Remoteness and proximity, in other words, would constitute the two poles of one same experiential continuum. Interestingly, neuroimaging studies contrasting remote and recent autobiographical memory suggest that our ability to determine the age of our memories consist in appreciating their differential recency rather than their differential remoteness. These studies have shown indeed that the activity level of some brain areas decreases as the subjective oldness of memories increases (Haist et al., 2001; Maguire et. al., 2001; Niki and Luo, 2002; Maguire and Frith, 2003; Piefke et al., 2003; Gilboa et al., 2004). One of the regions whose activity level appears to be negatively correlated to the age of memories is the hippocampus, a structure known to play a fundamental role in episodic memory processing. This result is consistent with traditional neuropsychological observations that hippocampal damages may result in a temporally graded retrograde amnesia, i.e., impair recent memories while relatively sparing remote ones (Squire, 1992; Bayley et al., 2003). Nevertheless, it has been demonstrated that the decrease of hippocampal activity according to the age of memories does not directly correlate with the degree of remoteness/recency, but with the degree of vividness of the memory images retrieved, the richness of details and emotionality being significantly higher for recent than for old memories (Graham et al., 2003; Gilboa et al., 2004; Moscovitch et al., 2005, 2006). Other interpretations have been proposed, as the fact that old memories are more widely distributed in the hippocampus than recent ones (Gilboa et al., 2004), or that episodic memory traces become progressively independent from the hippocampus as they are being transferred to neocortical structures (Squire, 1992). Ventrolateral prefrontal cortex and retrosplenial cortex are two other regions that have been shown to be all the less activated as memories are more remotely experienced (Maguire et al., 2001; Piefke et al. 2003; Gilboa et al., 2004). Ventrolateral prefrontal cortex is supposed to contribute to the contextual integration of memory traces and to the specification of retrieval cues (Maguire et al., 2001). The activity of retrosplenial cortex seems to vary independently from the vividness of memory images and is thought to mediate the appearance of generic visual representations by integrating memory images, emotions, and personally familiar information (Conway and Pleydell-Pearce, 2000; Pfieke et al., 2003; Moscovitch et al., 2005). Similar parametric modulation analyses have been carried out in the case of memory of future. Neuroimaging studies are here far less numerous and far less conclusive than those contrasting remote and proximate past autobiographical memory. A fMRI study by d'Argembeau et al. evidenced that the anterior ventromedial prefrontal cortex was comparatively more responsive to remote than to proximate emotionally-laden future memories, while proximate future memories elicited more activation in the caudate nucleus than remote ones (d'Argembeau et al., 2004). The anterior ventromedial prefrontal cortex was hypothesized to assign emotional value to representational contents associated to long-term goals while it was assumed that the caudate nucleus was involved in the planning of actions whose goal is supposed to be achieved in the near future. Moreover, Addis and Schacter showed in a fMRI study that the increasing remoteness of future memories (irrespective of the amount of details) correlated with a decreasing activity in parahippocampal gyrus and an increasing activity in the hippocampus (Addis and Schacter, 2008). They suggested that the hippocampal response reflects the increasing disparateness of the details that constitute remote future memories and the increasing efforts required to integrate these features in a consistent representational content. Albeit none of the authors concerned has explicitly considered this hypothesis, it cannot be excluded that there are neural regions involved in the appearance the feeling of recency per se, and, more generally speaking, that serve as a substrate for a hypothetical "quality of proximity".
Although not being inconceivable from the empirical point of view, the assumption that experience of temporal remoteness and proximity is mediated by a specific kind of cognitive factors remains epistemologically problematic. In comparison with the hypothesis of the pastness/futureness qualities, the hypothesis of the remoteness/proximity qualities has the inconvenient of multiplying the entities presumably involved in the perception of date and duration, as well as the conjectures regarding their exact number and nature. Therefore, it is questionable whether one should postulate the existence of a quality of proximity and of a quality of remoteness, or the existence of only one kind of quality, to explain the difference between short-term and long-term memory (as seen above, it seems more plausible to postulate solely the existence of a quality of proximity). Moreover, one may wonder whether there is a category of proximity/remoteness factors specific to retrospective memory and another one specific to prospective memory, or, conversely, just one single category of factors for both types of memory.
Whatever the temporal factors involved in perception of date and duration may be, it is possible to specify the general characteristics of the cognitive mechanism responsible for each of the two kinds of temporal experience. It was said above that date and duration are temporal relationships that can be perceived only insofar as they are reconstituted in actual experience: the corresponding mental contents are supposed to be "projected" into the time of presence and then differentiated by means of appropriate "symbols". The process of the symbolic reconstitution of date and duration implies that psychical phenomena encompassed in the time of presence are structured in a particular way. The contents of the episodes to be compared should be retrieved in a definite temporal order and accompanied with temporal factors whose intensity correlates with the relative moment of their appearance in past or future conscious life. Generally speaking, perception of date and duration should be regarded as the ability to apprehend in the same time a definite series of memory images and a given intensity pattern of temporal factors. Both kinds of temporal experience result from the manifestation of a unique fundamental cognitive mechanism; nevertheless, date perception and duration perception correspond each to the apprehension of a particular kind of temporal relationships and then should originate each in a special configuration of both memory images and temporal factor intensities in the time of presence. Perception of the relationships between autobiographical events, sometimes called "memory for time" or "temporal-order memory", has been investigated during the last two decades on the basis of behavioral, electrophysiological, and neuroimaging approaches (McAndrews and Milner, 1991; Friedman, 1993, 2004; Cabeza et al., 1997; Burt et al., 2000; Curran and Friedman, 2003; Skowronski et al., 2007; St.-Jacques et al., 2008). Our memory for time is nowadays commonly regarded as consisting in two main separate cognitive processes, a location-based process and a distance-based process. Locating autobiographical events in time would depend on the reconstruction of their temporal order, whereas estimating the distance that separate a definite past autobiographical event from current experience would be mediated by the relative feeling of recency/oldness inferred from the differential vividness of memory images ("memorial strength"). The two models of date perception and duration perception I propose here are an attempt to address systematically the issue of memory for the time and may serve as a theoretical and methodological guideline for further psychological and neuroscientific investigations.
4.2. Cognitive model of date perception
Generally speaking, dating a conscious phenomenon consists in evaluating the moment of appearance of an episode (or a sequence of episodes) in consciousness, that is to say, in locating an event of past or future conscious life relatively to other events of past, future or present conscious life. On the basis of what has been said previously, we can legitimately consider that date is given to us by the level of intensity of temporal factors: the more intensely the quality of pastness or futureness manifests itself in consciousness, the more remote are perceived the corresponding past or future memory images. Nevertheless, the degree of pastness/futureness of a memory image (its "temporal intensity") has only a differential value: it remains meaningless as long as it is not compared with the degree of pastness/futureness of other mental contents encompassed in the time of presence. A conscious phenomenon is never dated per se, but always in relation to other conscious phenomena. The temporal intensity of a memory image can be compared with the temporal intensity of other memory images (fig. 9 and 10).
- Figure 9: model of relative dating of past episodes
- Figure 10: model of relative dating of future episodes
In that case, the issue is to define the position of a past (or future) conscious phenomenon relatively to other past (or future) conscious phenomena, i.e., to know whether an episode occurred (or will occur) before or after other episodes of past (or future) conscious life. The comparison of the temporal intensity of various memory images projected successively into the time of presence allows us to determine the relative date of past and future conscious phenomena. But a memory image of a given intensity can also be put in relation with the events we are currently experiencing in the time of presence (fig. 11 and 12).
- Figure 11: model of absolute dating of past episodes
- Figure 12: model of absolute dating of future episodes
In that case, the point of comparison is not a mental content characterized by a higher or a lower degree or pastness/futureness, but a conscious phenomenon that is perceived as "present" in consciousness (i.e., with a "0 degree" of pastness/futureness). Here, the process of comparison in the time of presence allows us to evaluate the distance that separates any past or future episode from actual experience, and then to identify this episode as a particular moment of conscious life, that is, as a conscious phenomenon to which we can ascribe a definite position in the stream of consciousness. In other words, we succeed in estimating the absolute date of past and future conscious phenomena. The notion of the date is in reality an abstraction from the cognitive point of view. There is no genuine experience of date, since dating a single conscious event always depends on dating other single conscious events. Date can be apprehended only as an element of a more or less complex system of temporal relationships.
4.3. Cognitive model of duration perception
In contrast with date, duration consists in the apprehension of a whole system of temporal relationships: it can be experienced per se, as the content of a specific act of consciousness. Generally speaking, duration perception is nothing but the ability to experience the distance that separates two or more episodes (or sequences of episodes) of past and future conscious life. More precisely, we should be able to estimate the difference of pastness or futureness between consecutive conscious events on the basis of the successive retrieval of their memory images in the time of presence. Unlike date perception, duration perception results from the comparison of mental contents and temporal factors that are put in order in the time of presence, i.e., from the apprehension of definite temporal series of memory images. As seen above, for Stern, perceiving duration consists in apprehending segments of past or future conscious life that have been projected in a "compacted" form into the time of presence (fig. 4). The assumption that duration perception results from the integral reconstitution of more or less long sequences of episodes in actual experience seems little realistic from the cognitive point of view. One hardly sees, indeed, how contents of consciousness that extended over hours, months, or years could be encompassed while remaining unaltered in an interval of time as short as the time of presence. It is actually simpler and much more consistent to suppose that estimating the length of a segment of past or future conscious life results from the partial reconstitution of its content in actual experience, i.e., from the projection into the time of presence of a number of episodes characteristic of the considered sequence (fig. 13 and 14). The corresponding mental contents must also be supposed to be retrieved in the same or in the reverse order in which they appeared or are expected to appear in consciousness. Stern's second mistake regarding duration perception consists in believing that the latter is nothing but the fact of experiencing as a part of the past or the future a series of episodes of conscious life that has been projected into the time of presence. In reality, a series of episodes that is reconstituted and differentiated as a memory image is not apprehended as a succession of episodes of past or future conscious life, but only as an immediate succession of past or future conscious phenomena, that is, in fact, as one single episode of past or future conscious life. To be apprehended as successive experiential moments of past or future conscious life, the episodes that have been projected into the time of presence must differ from each other by their relative degree of pastness/futureness. The content of each episode must be accompanied by temporal factors whose intensity is correlative of the moment of its appearance in the past or in the future, i.e., of its position in the sequence of conscious life that have been reconstituted in the time of presence (fig. 13 and 14). When all is said and done, one can hypothesize that duration perception consists in the apprehension of a chronological or an anti-chronological series of episodic contents, and of an intensity gradient of pastness/futureness qualities.
- Figure 13: model of perception of duration of past episode sequences
- Figure 14: model of perception of duration of future episode sequences
4.4. The interdependence of date perception and of duration perception
Perception of date and perception of duration are in reality only the particular manifestations of our general capacity of experiencing temporal relationships in the past and the future. Both cognitive functions are closely related to each other and can hardly be disentangled from each other in conscious experience. Duration perception depends directly on our ability to date conscious phenomena. As shown above, it consists indeed in the comparison of the relative time of appearance of successive events in the stream of consciousness. If our ability to estimate the duration of a definite segment of conscious life is determined by our ability to date conscious phenomena relatively to each others, our ability to date them absolutely results itself from our ability to estimate duration. Perceiving the absolute date of a conscious phenomenon was said above to consist in allocating to the latter a definite position in the stream of consciousness. To do so, we are supposed to evaluate the distance that separates any event of past or future conscious life from the events that are currently occurring in actual experience, that is, in other words, to calculate the time that has flown between the appearance of a particular past or future episode and the present. Moreover, estimating the duration of a segment of conscious life does not mean that the latter is apprehended abstractly, independently from the global temporal organisation of conscious life. In the contrary, we should be able to date the sequences of episodes of which we estimate duration, i.e., to replace them respectively to the other events that constitute the stream of consciousness. As single episodes themselves, they are likely to be compared with other past or future conscious phenomena, and with conscious phenomena that are currently occurring in actual experience (fig. 15). When a sequence of episodes is projected into the time of presence, we apprehend not only the temporal intensity difference between the corresponding memory images, but also the temporal intensity difference between the latter and the other mental entities that may be encompassed in the time of presence. If the same duration is always inferred from the same intensity gradient, the same intensity gradient may result from the comparison of temporal factors with very different intensity levels. The intensity level of a definite gradient is correlative of the more or less great remoteness/proximity of the corresponding sequence of episodes in the past or the future. Sequences of episodes we perceive as having the same duration can nonetheless be experienced as being very different moments of past or future conscious life. Taken together, perception of date and perception of duration ensure the consistency of episodic memory and contribute to the emergence of a structured representation of our own conscious life. They play a critical role in the construction of self-identity and in the maintenance of the unity of consciousness beyond the perpetual renewal of actual experience.
Date perception and duration perception are two issues that were unsatisfactorily treated by Stern in his 1897 paper but that can be nonetheless explained in a perfectly consistent way on the basis of the theory of the time of presence. The explanatory power of Stern's model of time perception and the structure of consciousness appears actually much greater than Stern himself was willing to admit.
- Figure 15: model of absolute dating of a chronological sequence of past episodes
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