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William Stern on the "psychical time of presence". Historical and theoretical study of a cognitive model of time perception and autonoetic consciousness

David Romand
[Journal für Philosophie & Psychiatrie, Juli 2011, Supplement]

3. Critical examination of the theory of the time of presence

3.1. Historical and conceptual significance of Stern's views for the theories of time perception and memory

The theory of the apprehension of temporal relationships advocated by Stern appears in many respects very close to that proposed by James few years earlier. It is worth mentioning that Stern just mentions James without bothering to discuss his argument, except about the specific (and relatively marginal) question of the duration of the specious present. But there is no doubt that Stern's psychical time of presence is much indebted to the ideas on time experience developed in The Principles of Psychology. Nevertheless, the notion of the psychical time of presence should not be regarded as a mere re-appropriation, but rather as a deepening and a systematization of the notion of the specious present. James' specious present is in reality a relatively vague notion that leaves unanswered a number of significant issues. It is also unclear whether James conceives the perception of time as the result of an independent act of thought, or simply as a moment of the stream of consciousness. Moreover, even if he expressly evokes the notion that complex time relationships are apprehended symbolically in the specious present, James does not really develop the question of the reconstruction of past and future temporal episodes in actual experience. Without trying to call into question the importance of James' pioneering investigations, one should recognize that the theory of the specious present does not have the coherence of the theory of the psychical time of presence. The theory of the time of presence appears indeed as a remarkably consistent conceptual scheme that answers both simply and accurately the difficult issue of the perception of time.

First, Stern gives a convincing solution to the problem of the experience of succession of conscious phenomena, a solution in any case indisputably more convincing than those proposed until then. He had the great merit of having shown that there is no other possible way of experiencing the succession of conscious phenomena than apprehending the actual flow of mental contents in consciousness. The succession relationships are given in the stream of consciousness: they do not depend on conscious phenomena themselves, but only on the order of the events of the external world and those of the nervous system that determine their appearance. Successiveness is a property of the psychical life that cannot be represented symbolically, i.e., reconstructed in consciousness under the form of a system of non-successive relationships. The only means of experiencing it is also to apprehend it directly through a temporally-extended act of thought. Neither Herbart's theory of the reproduction of the series, nor Lotze's theory of the temporal signs, nor any other theory falling within "the dogma of instantaneity", can explain our ability to be aware of the flow of mental contents. Rightly, Stern highlights the arbitrariness and the inconsistency of the theories based on the postulate that experience of successiveness originates from the reconstruction of temporal relationships through an instantaneous act of consciousness. By contrast, the theory of the psychical time of presence is based on the provocative assumption that temporal extensiveness cannot be perceived but intuitively.

Secondly, Stern succeeds in clearing up another fundamental aspect of perception of time: the apprehension of the past and of the future. In this respect, the theory of the psychical time of presence appears as an important contribution to the theory of episodic memory, considered both as the ability to re-experience mental phenomena that occurred before in consciousness (retrospective memory), and the ability to "pre-experience" those that are supposed to occur in a further stage (prospective memory). Stern's model of memory is based on a new conception of remembering and memory images. In accordance with some contemporary authors, Stern maintains that remembering cannot be simply regarded, as psychologists usually do, as a phenomenon of reproduction of mental phenomena. Memory images, as he argues, are not only representations that have reappeared in consciousness. The fact that a mental content is reproduced (retrieved) does not imply that this mental content can be recognized, i.e., identified as something that has been previously experienced. The issue of the recognition (Wiedererkennen) was promoted in the 1880s by the Danish psychologist Harald Høffding (1843-1931). According to Høffding, memory images distinguish from novel representations on the basis of a "quality of familiarity" (Bekanntheitsqualität): once reproduced, representations must be supposed to be endowed with a psychical characteristic indicating that they have been already encountered before (Høffding, 1889, 1892, 1893) [17]. Høffding remains in reality quite ambiguous insofar as he does not succeed in conceiving recognition as a notion clearly distinct from reproduction. The quality of familiarity is in his opinion a property that is inherent to memory images, resulting from the memorization process itself, and not a psychical phenomenon of its own. Lotze was the first to express overtly the notion that the ability to recognize representations has in itself nothing to do with their reproduction. As seen above, he ascribes to a category of psychical phenomena that he terms "temporal signs" the experiential coloration that differentiates reproduced representations from novel ones. The assumption that there are mental entities that confer a particular temporal meaning to memory images is taken up by Stern. But unlike Lotze's temporal signs, the entities postulated by Stern do not have the function of indicating the relative position of representations in time, i.e., their order of appearance in consciousness. They are symbols that allow us to interpret the sequences of representations contained in the time of presence as being past or future episodes of conscious life. The notion that memory images are differentiated experientially on the basis of a special "feeling" (Gefühl) or "sensation" (Empfindung) that is independent from the retrieval process is advocated at the period by other German psychologists (Külpe, 1893, 1897; Lipps, 1898, 1902; Wundt, 1911). Stern distinguishes in reality two types of temporal factors: those that mediate the experience of the past (he terms after Høffding "quality of familiarity"), and those that mediate the experience of the future. There are also two types of memory images, both being the expression of the same mechanism of retrieval (fig. 6 and 7). The concept of the "memory of the future" was hardly new in German psychology. As soon as 1849, Theodor Waitz (1821-1864) had postulated the existence of a "feeling of expectancy" (Gefühl der Erwartung) to explain our capacity of anticipating the appearance of representations in consciousness (Waitz, 1849). It was shown above that Ehrenfels regards expectancy images as being mediated by a specific structural quality that is different from the quality which is supposed to mediate recollections (Ehrenfels, 1890; see also Schumann, 1898). But Stern goes further all other German theorists in their explanation of past and future perception, insofar as he succeeds in systematising a number of conceptual innovations proposed during the 19th century, and in integrating them into a consistent model of time experience and consciousness. Whether experienced as past or future, memory images result from the projection of mental contents into the time of presence. Stern does not assert that mental contents are "reproduced" but "projected". Indeed, for him, retrieval does not just consist in the reappearance, but in the re-elaboration of what was experienced before. Past and future memory images are past episodes of conscious life which are reconstructed more or less faithfully in actual experience. By definition, psychical phenomena that are no longer or not yet in consciousness cannot be experienced but indirectly. Memory images are in fact perceived intuitively as any other content of actual experience. As such, mental contents that have been projected into the time of presence do no differ experientially from any other content of current psychical life. Past and future memory images would be perceived as present events of conscious life if they were not differentiated by the corresponding temporal factors, i.e., symbolically interpreted as being non-actual acts of consciousness. The model of memory advocated by Stern accounts not only for our ability to travel mentally in the past and in the future, but also for our capacity of comparing the episodes of conscious life we experienced previously and we expect to experience, with those we are currently experiencing as being present. From such a model, Stern succeeds in explaining how past, present, and future acts of consciousness can be confronted at any time to each others in actual experience in order to ensure the consistency to conscious experience beyond the succession of experiential moments.


Figure 6: general model of perception of the past after Stern

Figure 7: general model of perception of the future after Stern

3.2. Time of presence, stream of consciousness, continuity and unity of conscious experience

Generally speaking, the theory of the psychical time of presence appears as an attempt to resolve the question of the temporal continuity of conscious experience. The model proposed by Stern accounts for the two aspects of the problem: it explains both the immediate experiential continuity, i.e., our ability to perceive the succession of mental contents in consciousness, and the short-term and long-term experiential continuity, i.e., our ability to perceive the relationships between current experience and any other past or future episodes of conscious life. In other words, it explains how the unity and the consistency of the self can be maintained through the time despite the perpetual renewal of conscious phenomena. In this respect, the theory of the time of presence is clearly in line with the reflections of contemporary authors as Stumpf or Ehrenfels on the foundations of the unity of consciousness. The time of presence is conceived by Stern as an act of thought that ensures the apprehension of the relationships between mental contents. As seen above, he explicitly maintains in his 1898 monograph on "the apprehension of changes" that the time of presence is mediated by a particular "structural quality" (but curiously not in his 1897 paper dedicated to the issue of the time of presence). Some theorists of the stream of consciousness claimed that the continuity of conscious experience can be explained only if one admits that the successive occurrences of the time of presence overlap each other (Broad, 1938, 1959; Dainton, 2000, 2003). According to this model, each act of consciousness shares a segment with the act of consciousness that precedes it and the act of consciousness that follows it (fig. 8).


Fig. 8: Broad's first version of the «overlap model», after Dainton (Dainton, 2000, p. 138)

There is no leap between the experiential moments that compose the stream of consciousness: each one is still partially contained in the one that just preceded and already partially contains the one that shall immediately follow. Such a hypothesis encounters a number of difficulties, notably the fact that being conscious of a single content of consciousness at different times is thoroughly counterintuitive (for a detailed criticism of the overlap model, see Gallagher, 2002). Notwithstanding these theoretical difficulties, the main problem of the overlap hypothesis lies in the fact that it brings absolutely nothing new to the theory of the time of presence. The fact of postulating an overlap between the acts of consciousness permits to explain how the immediate past is retained and how the immediate future is anticipated, in other words how the immediate continuity of conscious experience is maintained. But this is a problem that the theory of the time of presence has already resolved in postulating the existence of temporally-extended acts of consciousness. Regarding the question of the short-term and long-term continuity, the overlap hypothesis finds no answer at all. Thus, the overlap hypothesis must be considered at best as a useless hypothesis. The overlap model is based on the implicit assumption that conscious experience is continuous only insofar as the stream of consciousness itself is continuous. The continuity of conscious experience and the continuity of the stream of consciousness are in reality two distinct issues. The maintenance of the unity and of the consistency of the self through the time does not imply that consecutive acts of consciousness are systematically connected with each other, i.e., that current experience always relates to what has been just experienced before and to what is just experienced after. There is no reason to think that the stream of consciousness is actually continuous. Unlike the continuity of conscious experience, the continuity of the stream of consciousness is an assumption that is neither necessary nor even probably acceptable from a theoretical point of view. In conclusion, Stern's discontinuous model of the stream of consciousness appears to be the one that resolves most parsimoniously the question of the continuity of conscious experience.

3.3. Problematic aspects and limits of Stern's model

Despite unquestionable scientific qualities, the theory of the psychical time of presence is far from being beyond any criticism from the epistemological point of view. The model of time experience and consciousness proposed by Stern is marred by a number of obscurities and difficulties that should be pointed out, and, as far as possible, cleared up and resolved. These problems may emerge in part from the fact that Stern expounded his views on the time of presence only in one single and rather short text. Even if he continued to refer to the issue of the time of presence in his later works, he did not try to refine or develop (neither theoretically nor experimentally) the general guidelines he highlighted in his 1897 paper. Thus, the thesis of the time of presence as expounded by Stern appears actually more as a theoretical draft than as a full-achieved theory, which justifies all the more so as the criticisms, the solutions, and the theoretical developments I suggest below. The problematic points I highlight relate in reality to very fundamental issues regarding cognition and theory of knowledge. Since most of these issues are currently not investigated experimentally, I have resigned myself to considering them speculatively or on the basis of merely theoretical arguments. Whatever are criticisms which may be addressed to the theory of the time of presence and even if all difficulties cannot be necessarily overcome, the global consistency of Stern's views remains unaltered. This imperfectness must be regarded as a proof of scientificity and the condition of further theoretical and experimental developments.

3.3.1. The issue of intuitiveness

The major criticism that can be formulated against the theory of the psychical time of presence is that it accounts too simply, if not too simplistically, for a number of fundamental and very complicated cognitive issues. Stern's model of cognition pretends to explain too many things on the basis of one single principle to be entirely satisfying from the epistemological point of view. Single-handedly, the elementary act of consciousness called "psychical time of presence" is presumed, not only to mediate experience of temporal extensiveness and the appearance of extensive mental contents in general, but also to ensure the unity of conscious experience and to determine the fundamental structure of the stream of consciousness. For Stern, all these manifold functional properties depend on our original capacity of intuiting the psychical phenomena and the relationships between psychical phenomena which are encompassed in the time of presence. By nature, the time of presence allows us to apprehend directly the totality of psychical activity which occurs during some interval of time. Our awareness of the content of the time of presence is not mediated by any other cognitive phenomenon, and then is not supposed to require any further psychological explanation. In this respect, the theory of the time of presence is reminiscent of a number of traditional psychological and metaphysical conceptions which consider extensiveness and unity of experience as a product of "the power of imagination" or of "the creative synthesis" (Wundt, 1910, 1911), or simply as a "form of intuition" (Kant, 1968/1965). Stern can be criticized here for having proposed a pseudo-explanatory theoretical scheme in which the time of presence is assimilated to a kind of an obscure or a "miraculous" psychical power.

However, this does not mean that the cognitive model he proposes must be rejected as being devoid of any scientific value. As shown above, the theory of the time of presence constitutes actually a particularly elegant answer to the question of time perception and continuity of consciousness. Stern's intuitionist model does not only appear to be the most consistent (and perhaps the only possible) solution to the difficult problem of the perception of successiveness and the immediate continuity of conscious experience; it also appears to be the starting-point of a very fruitful program of research regarding the other aspects of time perception and the long-term continuity of experience. The notion of the time of presence must be seen as a highly valuable working hypothesis, particularly powerful from the heuristic point of view. Despite the epistemological difficulties emphasized above, there is actually no reason to consider Stern's doctrine of time perception and consciousness as being fundamentally misleading. Whatever would be the case, it seems important not to interpret the theory of the time of presence too dogmatically. In particular, one should avoid the tendency that consists in reifying the concept of the time of presence and considering it as a substantial psychical entity. I should be kept in mind that, in his 1897 paper, Stern proposes only a theoretical draft, and that he remains vague and elusive on many critical issues regarding the nature and the properties of the cognitive phenomenon he hypothesises. Stern's notion of the time of presence should be accepted provisionally as an unavoidable working hypothesis, likely to be cleared up and revised through further theoretical and empirical investigations.

3.3.2. Ontological difficulties

The theory of the time of presence is also problematic from the ontological point of view. Indeed, Stern assumes implicitly the existence of two categories of mental entities: on the one hand, an extensive psychical phenomenon, i.e., the time of presence; on the other hand, intensive psychical phenomena, i.e., the variety of sensory and structural data which build the content of the time of presence. The time of presence is defined as a temporally-extended act of consciousness, whereas the elements which are encompassed in it are supposed to occur instantaneously. This dichotomy between extensive and intensive psychical phenomena may sound strange. Stern can be criticized here for having divided cognitive activity artificially into two kinds of components of a completely different nature. One does not see why the various elements constitutive of conscious life should not all share the same fundamental properties. Assuming the existence of both intensive and extensive psychical phenomena does only imply that one should distinguish two forms of cognitive activity, but also that cognitive activity is liable to exist in two completely different ways. An intensive psychical phenomenon can occur at any moment with a given magnitude. This is an infinitely short act of thought which does not exist but punctually in time. An intensive psychical phenomenon can be said to exist insofar as it is objectively present in consciousness. Conversely, an extensive psychical phenomenon (that is, the time of presence) does not exist but diachronically: its existence can be defined only in time. This is a unitary and homogeneous segment which cannot be decomposed into any temporal part. An extensive psychical phenomenon is never objectively present in consciousness, but has nonetheless some "presentness" as long as it exists. The existence of an extensive psychical phenomenon corresponds to the manifestation of a subjective present. Intensive and extensive psychical phenomena are characterized each by a special form of presentness, i.e., a particular way of existing actually. Both phenomena can be regarded as belonging each to a distinct ontological domain in which existence and temporality are not defined in the same way. This view is rather counterintuitive, but perhaps one should admit such counterintuitiveness if he/she wants to resolve the paradox of the psychology of time, i.e., to explain how representations which do not appear simultaneously in consciousness can nevertheless be experienced together within a single act of consciousness. It should be noted that at no time Stern overtly assumes that mental phenomena which are encompassed in the time of presence are entities of an intensive nature. Nevertheless, such a view appears to be the logical conclusion of his claim that the time of presence is a specific mental phenomenon which differs from the others for being temporally-extended. The notion that all kinds of psychical phenomena, except the time of presence, are unextended mental entities seems to contradict directly the principle advocated by Stern that our contents of consciousness have always some duration. In reality, Stern's criticism of "the dogma of instantaneity" does not concern psychical phenomena per se, but psychical phenomena as they are actually experienced in consciousness, i.e., as they are apprehended in the time of presence. The elements constitutive of our mental contents remain inseparable only insofar as they are structured together by one same temporally-extended act of consciousness. Abstracted from the time of presence, they appear as psychical entities devoid of any extensiveness and of any relation of continuity between each other. Stern's conception of the nature of mental phenomena remains in fact probably much closer to the mainstream conceptions of the time than he actually pretends. Like most 19th century German psychologists, Stern regards the contents of consciousness as the result of the combination of qualitatively-defined psychical elements which occur with some intensity.

3.3.3. The issue of the discretness of the time of presence

Another epistemological difficulty of Stern's cognitive model is the assumption that the stream of consciousness is a succession of discrete experiential moments. As discussed above, postulating a structural discontinuity of the stream of consciousness is not inconsistent with the principle of the continuity of conscious experience, and as I have previously demonstrated, such a conception is probably the most satisfying option from the theoretical point of view. In fact, if the model proposed by Stern is problematic, it is more from a philosophical than from a psychological point of view, insofar as it contradicts the principle of continuity as such. It appears indeed particularly counterintuitive to assume that the stream of consciousness consists in clear-cut segments and that conscious experience progresses by leaps from an experiential moment to another. We do not clearly see why the time of presence, as a natural phenomenon, should not fall under the principle of continuity. By way of a reply, one may argue that the time of presence is precisely the precondition of continuity experience, and that in this respect it is not supposed to be itself endowed with a property characteristic of the conscious phenomena. It should be noted that the question of the discreteness of the time of presence is not expressly discussed by Stern. It is only by inference that one can determine the conception of the stream of consciousness advocated by Stern.

3.3.4. Conditions and modalities of the actualization of the time of presence

Generally speaking, the theory of the time of presence raises a number of questions regarding the nature of the psychical mechanisms involved - questions that Stern leaves totally or partially unanswered. One of these questions concerns the conditions of the appearance and unfolding of the time of presence. Stern conceives the time of presence as a temporally-extended act of consciousness but completely disregards the question of how such an act is performed. One could indeed imagine that the time of presence is determined progressively as the mental contents it encompasses flow in consciousness; conversely, we could imagine that it remains totally undetermined as long as it has not attained its full extension. Such explanation is not actually convincing. Stern does not try to explain how the act of apprehension is performed, probably because this issue does not make sense from the ontological point of view. The time of presence and the intensive mental phenomena which compose its content were said above to have two completely different modes of existence, i.e., to be characterised each by a specific form of temporality. Duration is to the time presence what instantaneity is to intensive mental phenomena: the way of being actually present in consciousness. In contrast to the content apprehended, the act of apprehension is thus not likely to be determined through time. The time of presence does not "appear" nor "unfold", but simply "is" or "exists" as long as the elements constitutive of its content flow in consciousness. If one accepts Stern's notion that extensiveness is perceived intuitively, then the time of presence cannot be regarded as a psychical process nor the result of a psychical process, but as an elementary mental property which is spontaneously given as a whole over some interval of time.

Another important implication of his theory that he does not mention at all is the problem of the incessant rebuilding of the time of presence. As seen above, Stern conceives (implicitly) the stream of consciousness as a succession of discrete non-overlapping experiential moments which are supposed to follow up one another as long as the state of awakeness is maintained. Although Stern does not tackle the problem, it can reasonably be assumed that the time of presence appears anew immediately right after it has disappeared, i.e., that the experiential moments follow up each other contiguously. If not, the acts of consciousness should be regarded as being interrupted by "experiential gaps", which seems hardly sustainable from the epistemological point of view. If the stream of consciousness is actually composed of contiguous elements, then one should explain how "jointed" psychical entities can nevertheless constitute separated experiential moments. More generally speaking, this raises the question of the nature of the neural mechanisms responsible for the cyclicality of the time of presence (this point is developed below).

Finally, Stern does not totally clear up the question of whether we are aware in the same way of all mental phenomena we apprehend together in the time of presence. Stern's assumption that the time of presence results in the apprehension of a "homogeneous" content suggests that he does not accept James's notion that the specious present has "fringes", i.e., that the more marginally conscious phenomena are experienced in the specious present, the less clearly they are apprehended (see above). Though not explicitly discussed by Stern, this homogeneity assumption is in accordance with the notion that the time of presence is an elementary and indivisible act of consciousness. It is also consistent with the interpretation proposed below that duration variations of the time of presence are purely quantitative changes which leave it unaltered from the qualitative point of view.

3.3.5. The issue of the duration of the presence time

The question of the duration of the time of presence is another problematic aspect of Stern's theory. As seen before, the time of presence does not have for Stern any definite duration but may vary considerably according to the psychological conditions. Such a view may appear discrepant with the conception of the time of presence as an elementary and a qualitatively-defined psychical phenomenon. Indeed we can hardly argue that the properties of the time of presence are altered by psychical or physiological contingencies. A solution would be to consider duration variations of the time of presence as purely quantitative changes which would leave the latter unaltered from the qualitative point of view. As an extensive (temporally-extended) mental phenomenon, the time of presence would vary in "extensity", in contrast to intensive (instantaneous) mental phenomena which are supposed to vary in intensity. Thus, we could imagine that the duration of the time of presence is correlative of the level of activity of a specific neural substrate. Whatever that may be, Stern does not discuss the theoretical significance of the duration variations of the time of presence, nor the cognitive and neural mechanisms which could explain them. He only mentions various psychical factors likely to influence them (nature of the mental contents, direction of the apprehension, and strength of the psychical energy).

3.3.6. The neural correlates of the time of presence and the binding problem

Though not directly of the concern of Stern's strictly psychological approach, the question of the neural correlates is another important and problematic implication of the theory of the time of presence. As a precondition of conscious experience, the time of presence is supposed to be necessarily involved in the manifestation of every act of consciousness. Thus, the corresponding neural substrate should be active during the vigil state in general, i.e., contribute to the basal brain activity. Moreover, since the time of presence encompasses any other mental phenomenon in order to ensure the fundamental unity of consciousness, the correlative brain activity should itself encompass at any time a variety of functional territories which may be at very different locations in the brain. The question of the neural correlates of the time of presence raises a number of critical theoretical problems. The time of presence should be mediated by a very special substrate, i.e., a ubiquitous substrate which does not correspond to any cerebral region in particular and whose pattern of activation must be incessantly modified in order to fit with the activity of the other functional territories. It is hard to conceive how a neural substrate which overlaps virtually all other functional territories of the brain can itself be endowed with a functional specificity of its own. Generally speaking, we face in this regard the question of the nature of the cerebral mechanism capable of ensuring the binding of the elements that compose our experiential content, and the perceptual unity of actual experience.

Another difficulty inherent to the question of the neural correlates of the time of presence regards the very nature of the mental phenomenon to be correlated. Unlike the other mental phenomena, the time of presence is a temporally-extended psychical entity which has moreover the property of being immediately and systematically rebuilt right after it has elapsed. As suggested in the previous paragraph, the duration of the time of presence may depend on the quantitative variations of the activity of the corresponding neural substrate. More precisely, the extensity of the time of presence may be assumed to vary according to the intensity of its correlative brain process, as the intensity of some modular cognitive functions was proved to correlate directly with the intensity of activity of their substrate (O'Craven and Kanwisher, 2000; Moutoussis and Zeki, 2002). Nevertheless, it seems completely counterintuitive to ascribe the appearance of an extensive mental phenomenon to the manifestation of a physical phenomenon of an intensive nature. Indeed, we can hardly see how something which has no duration could correlates with something which exists only in time. Therefore, it seems more reasonable to hypothesise that the manifestation of the time of presence correlates with the appearance of a neural phenomenon which is itself temporally-extended, that is, of an extensive nature. This hypothesis has moreover the merit of explaining easily the origin of the duration variations of the time of presence. Indeed, we could imagine that the latter depends on the more or less great capacity of neural activity to propagate over or through the brain surface. According to this view, the more this activity is expanding, the more the time of presence is extending and then lasts. Assimilating the neural origin of time of presence to a spatial-temporal evolution of brain activity is in fact problematic. Indeed, the fact that mental contents flow in consciousness does not necessarily imply a change of the nature of representations, and then of the neural territories activated. As a consequence, neural activity mediating the appearance of the time of presence should occur without necessarily changing location.

Another puzzling issue regarding the neural correlates of the time of presence is the question of the origin of its presumptive cyclicality. It was seen above that the time of presence is supposed to be recreated as soon as it disappears, and that this cyclical process should continue as long as the state of awakeness is maintained. What is the physiological process capable of mediating a cognitive activity that recurs with a so remarkable exactness? Here we naturally think about neurophysiological research and theoretical reflections regarding the role of cerebral rhythms in the spatiotemporal coordination of neuron population firing and the appearance of a unified conscious experience (Pöppel, 1988; Pöppel and Schwender, 1993; Schillen and König, 1994; Knoblauch and Palm, 2001; Doesburg and al., 2005; Elliott and al., 2006).

More generally speaking, the solution to the different questions regarding the anatomo-physiological basis of the time of presence is probably to be found in the neurocognitive studies on the binding problem. In the last two decades, many neuroscientists have tried to resolve the binding problem, i.e., the question how perceptual features which are processed at different places and times in the brain can be "bound" together in order to build up one unitary representation, and to determine the nature of structures and mechanisms that contribute to the emergence of a "global cognitive workspace" (Treisman, 1996, 1999; Baars, 1997; Revonsuo, 1999; Revonsuo and Newman 1999; Riesenhuber and Poggio, 1999; Roskies, 1999; Singer, 2001; Whitney, 2009). The various programs of research on the binding problem should allow us to demonstrate the existence of the time of presence phenomenon and to specify its functional characteristics. We can hope to prove the validity of Stern's assumptions on the basis of the existing literature and to test them directly by means of appropriated experimental protocols. The theory of the time of presence is likely to be supported by neuroscientific studies on the binding problem, but also to serve them as a general interpretative framework. Stern's great merit is to have understood that the binding problem is twofold and that the question of the unity of actual experience consists in reality in two separate neurocognitive issues: 1/ the question of how we succeed in structuring the content of the episodes of our conscious life, and 2/ the question of how we succeed in apprehending this content as a whole. The first aspect of the binding problem relates to the question of the association of the elements that compose the objects, the scenes, and the sequences of events we are aware of. According to the model proposed by Stern, the consistency of the content of actual experience results from our capacity of combining in a definite order mental phenomena that follow up each other in the time of presence. The neuroscientific foundations of such an associational dynamics are to be found in the so-called monitoring processes, i.e., the variety of brain mechanisms whose function is to select at any time the mental phenomena that should appear in consciousness by controlling the activity level of the corresponding neural substrates (Fuster, 1997; Fletcher et al., 1998; Ranganath et al., 2004; Petrides, 2005; Simons et al., 2005). Besides this "structural binding" that ensures the harmonious combination of experiential features, we should assume the existence of an "experiential binding" that allow us to perceive the relationships between the representations that have been temporally put in order, and then to apprehend the resulting representational series, that is, the mental content structured, as a unique conscious representation. The psychical time of presence is the cognitive phenomenon that is supposed to mediate the experiential unity of every manifestation of conscious life. Structural binding (associational processes) and experiential binding (time of presence) are two complementary cognitive mechanisms that are in charge of the construction of the functional unit of consciousness, namely, of the episode.

3.3.7. The issue of perception of date and duration: date and duration are not experienced intuitively

Finally, I would like to criticize Stern's assumption that every kind of temporal relationship, except past and future, is simply intuited in the time of presence. For Stern, perceiving the various properties of the "sense of time" depends on the way representations flow through consciousness and does not require any special psychological explanation. Duration, rhythm, velocity, acceleration, etc., are supposed, like succession, to result from the direct comparison of mental phenomena which form the content of the time of presence (see above). Such an assumption is in my opinion too simple to be plausible. My aim here is not to rehearse the general criticisms I raised above against Stern's intuitionist conception of time perception. Even if we admit the notion that temporal extensiveness, i.e., basically, the succession of mental contents, is experienced intuitively, there is no reason to think that it should be so for more complex temporal properties. Indeed it is hard to conceive how conscious phenomena as specific as perception of duration, rhythm, velocity, or acceleration, may simply emerge from the apprehension of a segment of the stream of consciousness without being mediated by a specific cognitive process.

The intuitionist assumption appears particularly irrelevant to account for our ability to perceive the date and the duration of conscious phenomena. Stern explicitly considers duration as being intuited in the time of presence, whereas he does not say a word about date perception. As mentioned above, James claims on the contrary that both date and duration are symbolically apprehended from representations which are reproduced into the specious present - yet without giving any detail about the nature of the cognitive process involved (see above). Clearly, such a view fits well the theory of the projection and it is actually surprising that Stern does not try to extend to experience of date and duration his model of the perception of the past and the future. Estimating the position in time of the different moments of conscious life and the distance which separates them from each another is an important aspect of our ability to maintain the unity and the continuity of conscious experience. The scope and performances of self-consciousness would be indeed very restricted if the contents of past and future episodes were merely retrieved to be differentiated by temporal factors in the time of presence. In itself, retrieval and temporal differentiation simply allows us to become aware that the contents encompassed in the time of presence are events that occurred in the past or will occur in the future, but not to interpret memory images as being definite moments of the stream of consciousness. Over our general capacity of experiencing the past and envisioning the future, we should be able to attribute a particular temporal significance to past and future episodes that are reconstituted in the time of presence. More exactly, we should be able to apprehend the relationships that any past (future) event has with the other past (future) events of our conscious life, and with the events that are currently experienced in actual conscious life. The fact of determining the order of appearance and the succession of episodes in the stream of consciousness allows us to experience the temporal organization of our own past and future conscious life. Generally speaking, date perception and duration perception have to do with the possibility of evaluating the relative proximity/remoteness of memories we apprehend in the time of presence.

Stern's propensity to consider experience of duration as being simply intuited, and not perceived symbolically, is all the more surprising as the insists on the other hand on the fact that the projection phenomenon may be a means of reconstituting long experiential sequences in the time of presence. According to this hypothesis (see above), the latter can be projected as a whole into the time of presence in a shortened form. Stern explains here how one can re-experience and pre-experience during a very brief interval of time periods of past and future conscious life which last in reality much longer. But he leaves actually totally unanswered the question of how we succeed in determining the effective duration of the shortened experiential sequences which are projected into the time of presence. Once projected into the time of presence and differentiated by the corresponding temporal factors, a sequence of conscious episodes is simply experienced as a succession of past or future events, and not as some mentally-travelled distance. Wrongly, Stern seems to consider that the duration of conscious phenomena is directly given to us by the fact of reconstituting in the time of presence the various elements constitutive of an experiential sequence. He does not realize apparently that the reconstitution of successive episodes does not give us a representation of the duration of a sequence, but a representation of the sequence itself. Similarly, the order of successive contents experienced as being past or future does not indicate the date on which these contents occurred or will occur during conscious life, but only their moment of appearance in the time of presence.

Date and duration of conscious phenomena cannot be directly apprehended from, respectively, the position and the level of compaction of the episodes in the time of presence. The projection and the compaction of mental contents permit to reconstitute the relations of successiveness between various moments of the stream of consciousness, but they do not allow us to know when they occurred or will occur, nor how extended they were or will be. Like past and future, date and duration are two kinds of temporal relationships whose experience depends on the projection of past and future conscious phenomena into the time of presence, and which must be reconstituted symbolically in actual experience, i.e., reconstructed by means of temporal factors which allow us to interpret in a particular way the contents projected. The cognitive processes which mediate the perception of date and duration are obviously more complex than those mediating the perception of past and future, even if the nature of the factors involved is probably not fundamentally different in both instances.

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