In recent years, the number of unilateral cochlear implant (CI) users with functional residual-hearing has increased and bimodal hearing has become more prevalent. According to the multi-source speech perception model, both bottom-up and top-down processes are important components of speech perception in bimodal hearing. Additionally, these two components are thought to interact with each other to different degrees depending on the nature of the speech materials and the quality of the bottom-up cues. Previous studies have documented the benefits of bimodal hearing as compared with a CI alone, but most of them have focused on the importance of bottom-up, low-frequency cues. Because only a few studies have investigated top-down processing in bimodal hearing, relatively little is known about the top-down mechanisms that contribute to bimodal benefit, or the interactions that may occur between bottom-up and top-down processes during bimodal speech perception. The research described in this dissertation investigated top-down processes of bimodal hearing, and potential interactions between top-down and bottom-up processes, in the perception of temporally interrupted speech. Temporally interrupted sentences were used to assess listeners' ability to fill in missing segments of speech by using top-down processing. Young normal hearing listeners were tested in simulated bimodal listening conditions in which noise band vocoded sentences were presented to one ear with or without low-pass (LP) filtered speech or LP harmonic complexes (LPHCs) presented to the contralateral ear. Sentences were square-wave gated at a rate of 5 Hz with a 50 percent duty cycle. Two factors that were expected to influence bimodal benefit were examined: the amount of linguistic context available in the speech stimuli, and the continuity of low-frequency cues. Experiment 1 evaluated the effect of sentence context on bimodal benefit for temporally interrupted sentences from the City University of New York (CUNY) and Institute of Electrical and Electronics and Engineers (IEEE) sentence corpuses. It was hypothesized that acoustic low-frequency information would facilitate linguistic top-down processing such that the higher context CUNY sentences would produce more bimodal benefit than the lower context IEEE sentences. Three vocoder channel conditions were tested for each type of sentence (8-, 12-, and 16-channels for CUNY; 12-, 16-, and 32-channels for IEEE), in combination with either LP speech or LPHCs. Bimodal benefit was compared for similar amounts of spectral degradation (matched-channels) and similar ranges of baseline performance. Two gain measures, percentage point gain and normalized gain, were examined. Experiment 1 revealed clear effects of context on bimodal benefit for temporally interrupted speech, when LP speech was presented to the residual-hearing ear, thereby providing additional support for the notion that low-frequency cues can enhance listeners' use of top-down processing. However, the bimodal benefits observed for temporally interrupted speech were considerably smaller than those observed in an earlier study that used continuous speech. In addition, unlike previous findings for continuous speech, no bimodal benefits were observed when LPHCs were presented to the LP ear. Experiments 2 and 3 further investigated the effects of low-frequency cues on bimodal benefit by systematically restoring continuity to temporally interrupted signals in the vocoder and/or LP ears. Stimuli were 12-channel CUNY sentences presented to the vocoder ear, and LPHCs presented to the LP ear. Signal continuity was restored to the vocoder ear by filling silent gaps in sentences with envelope-modulated, speech-shaped noise. Continuity was restored to signals in the LP ear by filling gaps with envelope-modulated LP noise or by using continuous LPHCs. It was hypothesized that the restoration of continuity in one or both ears would improve bimodal benefit relative to the condition in which both ears received temporally interrupted stimuli. The results from Experiments 2 and 3 showed that restoring continuity to the simulated residual-hearing or CI ear improved bimodal benefits, but that the greatest improvement was observed when continuity was restored to both ears. These findings support the conclusion that temporal interruption disrupts top-down enhancement effects in bimodal hearing. Lexical segmentation and perceptual continuity were identified as factors that could potentially explain the increased bimodal benefit for continuous, as compared to temporally interrupted, speech. Taken together, the findings from Experiments 1-3 provide additional evidence that low-frequency sensory information can provide bimodal benefit for speech that is spectrally and/or temporally degraded by improving listeners' ability to make use of top-down processing. Findings further suggest that temporal degradation reduces top-down enhancement effects in bimodal hearing, thereby reducing bimodal benefit for temporally interrupted speech as compared to continuous speech.

L'entrée visuelle rétinienne est discontinue. D'une part les saccades causent un énorme mouvement de l'image sur la rétine 3 à 4 fois par seconde, qui devrait résulter en un floutage des hautes fréquences spatiales et une forte impression de mouvement. D'autre part, les clignements des yeux induisent une diminution temporaire drastique de la luminance toutes les 3 à 4 secondes. Dans des conditions de vision écologiques, ces conséquences visuelles des saccades et des clignements des yeux ne sont pas consciemment perçues et le monde extérieur semble continu et net : deux phénomènes que l'on peut désigner sous le terme d'omission saccadique et d'omission des clignements des yeux. Dans cette thèse, nous avons voulu mieux comprendre comment le système visuel s'accommode de ces interruptions et quels sont les mécanismes qui contribuent à la continuité perceptive autour des saccades et des clignements des yeux. Deux principaux éléments pourraient contribuer à ces omissions : l'entrée visuelle elle-même et un mécanisme extra-rétinien qui informerait le cerveau de l'interruption à venir qui agirait en modifiant le traitement de l'information autour des saccades et des clignements des yeux. Dans une première série d'expériences, nous avons étudié les caractéristiques du masquage du smear saccadique, c'est à dire dans quelle mesure la présence d'images pré et post saccadiques nettes permet de rendre compte de l'omission du smear saccadique. Plus précisément, nous avons élaboré une méthode de mesure objective du masquage du smear et examiné son étendue spatiale et son origine périphérique ou centrale. A l'aide de cette nouvelle méthode, nous avons répliqué les résultats de masquage du smear et mis en évidence que ce masquage a lieu après le site d'interaction binoculaire et résiste à des séparations spatiales entre smear et masque jusqu'à 6 deg. Dans une deuxième étude nous avons comparé la sensibilité à des réseaux sinusoïdaux de basse fréquence spatiale autour des saccades et en fixation lorsque l'entrée visuelle simule les conséquences visuelles des saccades. De plus, nous avons cherché à établir si la plus importante diminution de la sensibilité observée pour de vraies saccades en comparaison des saccades simulées peut être expliquée par les propriétés cinématiques des mouvements oculaires. L'objectif de la troisième étude était de déterminer si le masquage est suffisant pour rendre compte de l'absence de percept de mouvement autour des saccades. Pour ce faire, nous avons présenté en fixation, un stimulus dont le contenu fréquentiel est similaire à celui des scènes naturelles. Ce stimulus était présenté en mouvement avec un profil similaire à celui d'une saccade. Il pouvait être précédé et suivi de l'image statique avant et après le mouvement. Les résultats indiquent que l'amplitude du mouvement perçu diminue considérablement en présence des masques, sans toutefois annuler totalement tout percept de mouvement pour des longues durées de masques. Dans une dernière série d'études nous nous sommes intéressés à la perception de la durée autour des clignements des yeux. Dans la première expérience nous avons quantifié la contribution de la durée d'un clignement des yeux à la durée d'une période d'obscurité plus longue et dans la seconde expérience, nous avons étudié la perception de la durée d'un objet interrompu ou non par un clignement des yeux. Les résultats de ces deux expériences suggèrent l'implication d'un mécanisme extra-rétinien qui supprime la durée perçue de la période d'obscurité causée par les clignements des yeux mais pas la durée des objets visuels chevauchés par le clignement. Pris dans leur ensemble ces résultats précisent notre compréhension des contributions relatives des mécanismes rétiniens et extra-rétiniens à l'omission saccadique et l'omission des clignements des yeux. / The retinal input is discontinuous. On the one hand saccades, that occur 3-4 times a minute, cause a huge motion of the image on the retina that should result in smearing of the high frequencies of the image and perceived motion. On the other hand eye blinks induce drastic transient decreases in luminance every 3-4 seconds. Under real-world conditions, those visual consequences of saccades and blinks are barely noticed and the world appears continuous and sharp: two phenomena that can be referred to as saccadic and blink omission. In this thesis we were interested in understanding how the visual system deals with these interruptions and which mechanisms contribute to perceived continuity around saccades and blinks. Two main elements could contribute to those omissions: the visual input itself and an extra-retinal mechanism informing the brain of the impending interruption that would affect information processing around saccades and blinks. In a first series of experiments we studied the characteristics of masking of the saccadic smear, the extent to which clear and still pre- and post-saccadic images are responsible for the perceptual omission of saccadic smear. In particular, we designed an objective method to measure smear masking and studied its spatial extent and whether it is of peripheral or central origin. We replicated previous results of saccadic masking with this new method and found that smear masking seems to take place after the site of binocular interaction and survives separations between smear and mask as much as 6 deg. In a second study we compared sensitivity to low-frequency gratings around saccades and in fixation when the visual input simulates the visual consequences of saccades. Moreover we tried to determine whether the greater decrease in sensitivity around real, as compared to simulated, saccades that we found could be accounted for by the cinematic properties of the eye movement. The goal of the third study was to determine if masking was sufficient to explain the lack of perceived motion during saccades. To do that we presented, during fixation, a natural scene-like stimulus moving at saccadic speeds that could be preceded and followed by the initial or final static image. Results indicate that the amplitude of perceived motion considerably decreased in the presence of pre- and post-masks, even though motion was still perceived for long mask durations. In a final series of studies, we probed duration perception around blinks. In a first experiment we quantified the contribution of the duration of a blink to a longer period of darkness and in a second experiment we tested the perceived duration of an object interrupted or not by a blink. Results suggest the involvement of an extra-retinal mechanism that suppresses the perceived duration of the darkness caused by the blink, but not the duration of visual objects that straddle the blink. Taken together these results refine our understanding of the relative contributions of retinal and extra-retinal mechanisms to saccadic and blink omission.