La perception explicite du temps écoulé (la durée, l'ordre temporel¿) et les jugements implicites des dynamiques de notre environnement (percevoir le mouvement, la parole) nécessitent l'extraction des relations temporelles entre événements sensoriels. Alors que le temps physique est communément évalué en rapport à un référentiel externe (celui de l'horloge), le cerveau lui n'a pas accès à ce référentiel. Dans cette thèse, nous émettons l'hypothèse que le cerveau génère son propre référentiel temporel à partir des dynamiques neurales. Combinant la magnétoencéphalographie (MEG) aux données psychophysiques, les présents travaux suggèrent que les oscillations corticales sont impliquées dans l'encodage du temps perçu. Une première étude montre que la phase des oscillations corticales basse-fréquences peut encoder l'ordre temporel perçu entre événements sensoriels s'il y a entrainement neural, i.e. si l'activité cérébrale suit les régularités temporelles de la stimulation. L'implication des oscillations cérébrales en l'absence d'entrainement est testée dans une seconde expérience. Les résultats d'une troisième expérience suggèrent que l'entrainement neural n'a d'influence sur le traitement temporel des informations multisensorielles qu'à basse fréquence (1-2 Hz). Un dernier chapitre aborde le rôle de l'entrainement neural dans l'encodage des dynamiques du signal acoustique pour la perception de la parole. En conclusion, cette thèse suggère que le cerveau est capable de suivre la structure temporelle du monde extérieur, et que cet ajustement permet la construction d'un référentiel temporel interne pour la perception explicite et implicite du temps. / The timing of sensory events is a crucial perceptual feature, which affects both explicit judgments of time (e.g. duration, temporal order) and implicit temporal perception (e.g. movement, speech). Yet, while the relative external timing between events is commonly evaluated with a clock in physics, the brain does not have access to this external reference. In this dissertation, we tested the hypothesis that the brain should recover the temporal information of the environment from its own dynamics. Using magnetoencephalography (MEG) combined with psychophysics, the experimental work suggests the involvement of cortical oscillations in the encoding of timing for perception. In the first part of this dissertation, we established that the phase of low-frequency cortical oscillations could encode the explicit timing of events in the context of entrainment, i.e. if neural activity follows the temporal regularities of the stimulation. The implications of brain oscillations for the encoding of timing in the absence of external temporal regularities were investigated in a second experiment. Results from a third experiment suggest that entrainment does only influence audiovisual temporal processing when bound to low-frequency dynamics in the delta range (1-2 Hz). In the last part of the dissertation, we tested whether oscillations in sensory cortex could also ‘tag’ the timing of acoustical features for speech perception. Overall, this thesis provides evidence that the brain is able to tune its timing to match the temporal structure of the environment, and that such tuning may be crucial to build up internal temporal reference frames for explicit and implicit timing perception.

Le temps peut être à la fois une dimension structurant l’organisation de la perception et un contenu de la conscience, ce qui en fait un objet d’étude crucial pour comprendre la cognition humaine. Le but de cette thèse est de déterminer dans quelle mesure l’ordonnancement temporel de l’information est lié à l’ordre perçu (explicite ou implicite) des évènements. Le premier axe d’étude s’intéressant au temps comme contenu conscient montre que l’ordre temporel est un biais psychologique indépendant de l’attention. Une étude complémentaire en magnétoencéphalographie a révélé que les participants ayant les plus larges biais (c.à.d. que leur perception de la simultanéité correspond à une large désynchronisation physique) montrent aussi les plus larges fluctuations de la puissance des oscillations alpha spontanées lorsque la perception de l’ordre est contrastée pour un même délai physique. Ces oscillations sont donc proposées comme moyen pour compenser un biais individuel. Le deuxième axe de recherche explore comment le temps façonne implicitement la perception d’une séquence visuelle, par l’étude de la postdiction, c.à.d. quand des informations plus tardives influencent la représentation d’informations antérieures. Dans l’illusion du Lapin, un flash intermédiaire est mal localisé à cause de la régularité temporelle de la séquence. Nous avons montré que percevoir l’illusion active des régions parieto-centrales après la fin de la séquence. Ces résultats suggèrent que des régions de haut niveau pourraient contribuer à la reconstruction postdictive d’une séquence visuelle, en accord avec l’hypothèse que l’illusion dépend d’une connaissance préalable sur la vitesse d’un stimulus. / Time in the brain can be considered both as a dimension structuring the organization of perception and as a conscious content: this confers to time a particular flavor that makes it a paramount object of study to understand the basis of human cognition. The present thesis aims at empirically determining to which extent the temporal ordering of information relates to the perceived order of events, both implicitly and explicitly. First, we investigated how time may become an explicit conscious content by showing that temporal order is a psychological bias, independent of attention. A follow-up magnetoencephalography study revealed that participants having a large order bias (i.e. their perceived synchrony corresponds to a large physical asynchrony) also showed the largest fluctuations in ongoing alpha power when perceived orders were contrasted for a given physical asynchrony. Alpha oscillations are herein argued to be a means to compensate for an individual internal bias. Second, we investigated how time implicitly shaped the perception of visual sequences by studying postdiction, i.e. when late inputs strikingly influence the representation of earlier information. In the Rabbit illusion, the intermediate flash of a visual sequence is spatially mislocalized due to the temporal regularity of the sequence. We showed that parieto-frontal regions were more activated following the presentation of the full sequence when the illusion was perceived. These results suggest that high-order regions may contribute to the postdictive reconstruction of a visual sequence, consistently with the hypothesis that the illusion is shaped by prior knowledge on a stimulus speed.

Prior entry refers to the notion that attended stimuli are perceived sooner than unattended stimuli due to a speed up in sensory processing. The century long debate regarding the prior entry phenomenon’s existence has always been grounded in the degree to which the methods applied to the problem allow for cognitive response bias. This thesis continues that trend by applying the synchrony comparison judgment method to the problem of audiovisual prior entry. Experiment 1 put this method into context with two other common psychophysical methods – the temporal order judgment and the synchrony judgment – that have been applied to the prior entry problem. The results of this experiment indicated that the temporal order judgment method was out of step with the other two methods in terms of the parameter estimates typically used to evaluate prior entry. Experiment 2 evaluated and confirmed that a specific response bias helps explain the difference in parameter estimates between the temporal order judgment method and the other two. Experiment 3 evaluated the precision of the synchrony comparison judgment method. The results indicated that the method was precise enough to detect potentially small prior entry effect sizes, and that it afforded the ability to detect those participants with points of subjective synchrony that deviate substantially from zero. Finally, Experiment 4 applied the synchrony comparison judgment method to a prior entry scenario. A prior entry effect was not realized. Overall, this thesis highlights the drawbacks of all previous methods used to evaluate audiovisual perception, including prior entry, and validates the use of the synchrony comparison judgment. Further, due to the resistance of this method to response bias, this result now stands as the most convincing evidence yet against the prior entry phenomenon.

Encoding time is one of the most important features of the mammalian brain. The visual system, comprising almost half of the brain is of no exception. Time processing enables us to make goal-directed behavior in the optimum “time window” and launch a ballistic eye movement, reach/grasp an object or direct our processing resources (attention) from one point of interest to another. In addition, encoding time is critical for higher cognitive functions, enabling us to make causal inferences. The limitations of temporal individuation in the visual stream seem to vary across the visual field: the resolution gradually drops as objects become farther away from the center of gaze, where little differences were found in terms of resolution for objects in the upper versus lower visual field. This resolution of temporal individuation is vastly different from the resolution ascribed to spatial individuation. If individuation is mediated through attention, as some researchers have proposed, the general term ”attention” seems to possess different properties, at least regarding temporal and spatial processing. Next we looked at another aspect of encoding time: Temporal Order Judgments (TOJ), where animals had to judge the relative timing onset of two visual events. After training two monkeys on the task, we recorded from neurons in the lateral intraparietal area (LIP), while the animals reported the perceived order of two visual stimuli. We found that LIP neurons show differential activity based on the animal’s perceptual choice: when the animal reports the stimulus inside the receptive field of the neuron as first, the cells show an increased level of activity compared to when the animal reports he same stimulus as second. This differential activity was most reliable in the tonic period of the response \((\sim100 ms\) after stimulus onset). However, no difference in visual response latencies was observed between the different perceptual choices. The parietal cortex has previously been implicated in temporal processing based on patient studies as well as neuroimaging investigations. Physiological studies have also suggested the involvement of parietal area in encoding elapsed time. However, our study is the first to demonstrate parietal neurons encoding relative timing. / Psychology

The timing of sensory events is a crucial perceptual feature, which affects both explicit judgments of time (e.g. duration, temporal order) and implicit temporal perception (e.g. movement, speech). Yet, while the relative external timing between events is commonly evaluated with a clock in physics, the brain does not have access to this external reference. In this dissertation, we tested the hypothesis that the brain should recover the temporal information of the environment from its own dynamics. Using magnetoencephalography (MEG) combined with psychophysics, the experimental work suggests the involvement of cortical oscillations in the encoding of timing for perception. In the first part of this dissertation, we established that the phase of low-frequency cortical oscillations could encode the explicit timing of events in the context of entrainment, i.e. if neural activity follows the temporal regularities of the stimulation. The implications of brain oscillations for the encoding of timing in the absence of external temporal regularities were investigated in a second experiment. Results from a third experiment suggest that entrainment does only influence audiovisual temporal processing when bound to low-frequency dynamics in the delta range (1-2 Hz). In the last part of the dissertation, we tested whether oscillations in sensory cortex could also 'tag' the timing of acoustical features for speech perception. Overall, this thesis provides evidence that the brain is able to tune its timing to match the temporal structure of the environment, and that such tuning may be crucial to build up internal temporal reference frames for explicit and implicit timing perception.

Evidence suggests that human time perception is likely to reflect an ensemble of recent temporal experience. For example, prolonged exposure to consistent temporal patterns can adaptively realign the perception of event order, both within and between sensory modalities (e.g. Fujisaki et al., 2004 Nat. Neurosci., 7, 773-778). In addition, the observation that 'a watched pot never boils' serves to illustrate the fact that dynamic shifts in our attentional state can also produce marked distortions in our temporal estimates. In the current study we provide evidence for a hitherto unknown link between adaptation, temporal perception and our attentional state. We show that our ability to use recent sensory history as a perceptual baseline for ongoing temporal judgments is subject to striking top-down modulation via shifts in the observer's selective attention. Specifically, attending to the temporal structure of asynchronous auditory and visual adapting stimuli generates a substantial increase in the temporal recalibration induced by these stimuli. We propose a conceptual framework accounting for our findings whereby attention modulates the perceived salience of temporal patterns. This heightened salience allows the formation of audiovisual perceptual 'objects', defined solely by their temporal structure. Repeated exposure to these objects induces high-level pattern adaptation effects, akin to those found in visual and auditory domains (e.g. Leopold & Bondar (2005) Fitting the Mind to the World: Adaptation and Aftereffects in High-Level Vision. Oxford University Press, Oxford, 189-211; Schweinberger et al. (2008) Curr. Biol., 18, 684-688). / Wellcome Trust, College of Optometrists

Prior entry refers to the notion that attended stimuli are perceived sooner than unattended stimuli due to a speed up in sensory processing. The century long debate regarding the prior entry phenomenon’s existence has always been grounded in the degree to which the methods applied to the problem allow for cognitive response bias. This thesis continues that trend by applying the synchrony comparison judgment method to the problem of audiovisual prior entry. Experiment 1 put this method into context with two other common psychophysical methods – the temporal order judgment and the synchrony judgment – that have been applied to the prior entry problem. The results of this experiment indicated that the temporal order judgment method was out of step with the other two methods in terms of the parameter estimates typically used to evaluate prior entry. Experiment 2 evaluated and confirmed that a specific response bias helps explain the difference in parameter estimates between the temporal order judgment method and the other two. Experiment 3 evaluated the precision of the synchrony comparison judgment method. The results indicated that the method was precise enough to detect potentially small prior entry effect sizes, and that it afforded the ability to detect those participants with points of subjective synchrony that deviate substantially from zero. Finally, Experiment 4 applied the synchrony comparison judgment method to a prior entry scenario. A prior entry effect was not realized. Overall, this thesis highlights the drawbacks of all previous methods used to evaluate audiovisual perception, including prior entry, and validates the use of the synchrony comparison judgment. Further, due to the resistance of this method to response bias, this result now stands as the most convincing evidence yet against the prior entry phenomenon.

Previous research shows that integration of the senses is interchangeably dependent by  temporal neural mechanisms. One unsolved problem is how sensory timing differences in the brain is processed. In this systematic review (K = 18), audio-visual behavioral task paradigms are investigated with a focus on temporal binding window estimates. The results showed among other things that temporal integration is an adaptive neural process and that temporal acuity increases with age. Measurements between studies were sometimes incompatible which limited conclusions. Future studies should focus on standardizing operational parameters and compare within and between group designs.

After exposure to a short, constant delay between voluntary movement and sensory stimuli, temporal recalibration (TR) arises to realign asynchronous stimuli. The objective of this study was to determine if awareness of the temporal lag between a motor response (i.e., a keypress) and a sensory event (i.e., a visual flash) is necessary for TR to occur. We further investigated whether manipulating the motor and judgment tasks required modifies the influence of awareness on TR due to the cognitive processes engaged. Participants (n = 22) were randomly divided between two groups (Group 1: Aware and Group 2: Unaware). The Aware group was told of the temporal lag between their keypress and visual flash at the beginning of the experiment, whereas the Unaware group was not. All participants completed 8 blocks of trials, in which the motor tasks (e.g., a single or repetitive tap), judgment tasks (e.g., judging the order of the keypress in relation to the visual flash or judging whether the two stimuli were simultaneous or not), and temporal lag between keypress and visual flash (e.g., a 0 ms or 100 ms lag) varied. TR was determined by comparing judgments between corresponding blocks of trials in which the temporal lag was 0 ms to 100 ms. Results revealed that both the Aware and Unaware groups of participants demonstrated TR across both motor and judgment tasks, and that the magnitude of TR did not vary across Aware and Unaware participants or tasks. Thus, results of the present study revealed that awareness of a temporal lag does not influence the magnitude of motor-sensory TR achieved.

<p>Audio-visual temporal processing, and integration of auditory and visual information, is necessary for perceiving the world around us. Although previous research has indicated a slowing of temporal processing in older adults for unisensory stimuli, little work has examined the role of aging on multisensory temporal processing. The goal of this thesis is to use temporal-order judgment (TOJ) tasks to investigate age-related changes in audio-visual temporal processing. Overall, our results indicate that older adults do not demonstrate impairments on simple audio-visual TOJs, but they do exhibit deficits on more complex TOJ tasks. We also found no influence of spatial cues on TOJs for younger or older adults. Finally, we found age differences in complex TOJ tasks could not be explained by changes in the ability of older adults to detect a gap between sequential visual stimuli. The work in the thesis suggests that although there may be slowing in audio-visual temporal processing in complex situations, there are circumstances where audio-visual temporal processing is spared. By categorizing multisensory processing deficits in the elderly, we can aim to improve quality of life by preventing fails and perpetuating social interactions.</p> / Master of Science (MSc)