The present thesis set out to investigate how sensory modality and spatial presentation influence visual and auditory duration judgments in the millisecond range. The effects of modality and spatial location were explored by considering right and left side presentations of mixed or blocked visual and auditory stimuli. Several studies have shown that perceived duration of a stimulus can be affected by various extra-temporal factors such as modality and spatial position. Auditory stimuli lead to more precise duration judgments than visual stimuli and often last subjectively longer than visual stimuli of equal duration. The circumstances under which these modality differences occur are not clear yet. Recent studies indicated an interaction between temporal and spatial processing. Overestimation of durations was associated with right side presentation of visual stimuli, underestimation with left side presentation. However, the effect of spatial presentation has not been explored in the auditory temporal judgments. Furthermore, there is a debate concerning the mechanisms underlying processing of visual and auditory intervals with some researchers supporting the view that there is a central, amodal temporal mechanism and others arguing in favour of distinct, modality specific temporal mechanisms. The above issues were examined in a series of experiments using the duration discrimination paradigm. Processing demands where varied between experiments by varying the number of stimuli positions and the way that different modality trials were presented (mixed or blocked). Across all experiments we found no effect of location either in visual or auditory domain. However, in experiments in which different modality trials were intermixed, participants in the visual versions of the task tended to overestimate durations of comparison stimuli that were presented at different locations to the standard stimuli. In such conditions, visual stimuli were also judged to be longer than the auditory. However, when the location of the comparison stimulus was at the same side as the standard a reverse effect was observed. These findings call into question an influence of the position per se on temporal judgments as the visual duration judgments were affected rather by the change of the location. Auditory judgments were not affected by location manipulations, suggesting that different mechanisms might underlie visual and auditory temporal processing. Based on these results, we propose the existence of an error-correction mechanism, according to which a specific duration is added in order to compensate for the loss of time caused by spatial attention shifts. This mechanism is revealed under some circumstances (such as mixed modality) where it is over-activated, resulting into a systematic bias. This work has important implications for the contemporary research in time perception as it is shedding new light on the possible ways that a unified experience of timing arises from modally and spatially specific temporal mechanisms.

The purpose of the present thesis was two-fold. First, to evaluate the impact of cognitive demand on postural control in young adults and second, to examine the responsiveness of postural control to cognitive tasks presented in varying modalities. Seventeen young adults stood on a force platform while simultaneously performing cognitive tasks of varying difficulty (easy, moderate and difficult), each presented auditorily and visually. Performing the moderate and difficult tasks precipitated a greater reduction in area of 95% confidence ellipse and medio-lateral (ML) sway variability compared to the easy tasks. Presenting the tasks visually produced lower ML sway variability than presenting the tasks auditorily. Of secondary interest of this thesis was to determine if the duration of inter-stimulus intervals could modify the effectiveness of a cognitive task on postural control. Participants stood on a force platform while simultaneously performing cognitive tasks with five-second inter-stimulus intervals (i.e. discrete) and two-second inter-stimulus intervals (i.e. continuous), each presented auditorily and visually. Results revealed higher anterior-posterior (AP) mean power frequency (MPF) when performing the continuous tasks. In addition, presented the tasks visually resulted in a greater reduction in area of 95% confidence ellipse, AP and ML sway variability.

Our senses, while limited, shape our perception of the world and contribute to the functional architecture of the brain. This dissertation investigates the role of sensory modality and task demands in the cortical organization of healthy human adults using functional magnetic resonance imaging (fMRI). This research provides evidence for sensory modality bias in frontal cortical regions by directly contrasting auditory and visual sustained attention. This contrast revealed two distinct visual-biased regions in lateral frontal cortex - superior and inferior precentral sulcus (sPCS, iPCS) - anatomically interleaved with two auditory-biased regions - transverse gyrus intersecting precentral sulcus (tgPCS) and caudal inferior frontal sulcus (cIFS). Intrinsic (resting-state) functional connectivity analysis demonstrated that sPCS and iPCS fall within a broad visual-attention network, while tgPCS and cIFS fall within a broad auditory-attention network. Unisensory (auditory or visual) short-term memory (STM) tasks assessed the flexible recruitment of these sensory-biased cortical regions by varying information domain demands (e.g., spatial, temporal). While both modalities provide spatial and temporal information, vision has greater spatial resolution than audition, and audition has excellent temporal precision relative to vision. A visual temporal, but not a spatial, STM task flexibly recruited frontal auditory-biased regions; conversely, an auditory spatial task more strongly recruited frontal visual-biased regions compared to an auditory temporal task. This flexible recruitment extended to an auditory-biased superior temporal lobe region and to a subset of visual-biased parietal regions. A demanding auditory spatial STM task recruited anterior/superior visuotopic maps (IPS2-4, SPL1) along the intraparietal sulcus, but neither spatial nor temporal auditory tasks recruited posterior/interior maps. Finally, a comparison of visual spatial attention and STM under varied cognitive load demands attempted to further elucidate the organization of posterior parietal cortex. Parietal visuotopic maps were recruited for both visual spatial attention and working memory but demonstrated a graded response to task demands. Posterior/inferior maps (IPS0-1) demonstrated a linear relationship with the number of items attended to or remembered in the visual spatial tasks. Anterior/superior maps (IPS2-4, SPL1) demonstrated a general recruitment in visual spatial cognitive tasks, with a stronger response for visual spatial attention compared to STM.

The processes of pattern separation and pattern completion are very important in the correct discrimination of similar memories. Much research has been conducted on these processes, but there are some gaps that need to be addressed. First, there is some debate as to whether false alarms to lure items come about because of a failure to accurately encode a memory or a failure to retrieve a memory. Second, much of the research on pattern separation and pattern completion in humans is done with visual stimuli and contributions of stimulus modality to these processes are not well understood as a result.Study 1 consisted of three experiments conducted using a combination of eye tracking and functional magnetic resonance imaging (fMRI) methods. Analyses of eye tracking data in the experiments examined the contribution of fixation counts at encoding and retrieval, as well as target-lure similarity level, to accuracy on lure trials. Task designs were altered across studies to attempt to replicate specific research previously conducted with a specific answer period, as well as generalize the findings to a broader body of research that allows participants to answer while the stimulus is presented. The three experiments showed mixed support for the contribution of fixation counts at encoding and retrieval to the accurate discrimination of similar lures. Target- lure similarity, however, was a robust predictor of accuracy for all three experiments.Prior research examining activity in the hippocampus demonstrates a reduction of fMRI activity to repetitions of a stimulus. Greater activity is also observed in the dentate gyrus/CA3 (DG/CA3) subregions for correct rejections of lure items compared to lure false alarms. There should be a greater reduction in the DG/CA3 as a function of encoding for lure false alarms than for lure correct rejections if memory encoding drives the activity differences between these outcomes. The fMRI data showed a marked reduction of activity in the left hippocampus to repetition trials as a function of encoding trial fixation count. There was no significant difference between activity as a function of encoding fixation count in the DG/CA3 for lure correct rejections and lure false alarms. There was also no difference in activity for the CA1 either. Overall, the results of the eye tracking and fMRI data give support for the contribution of pattern completion to false alarms to lure stimuli rather than poor encoding.Study 2 examined the contribution of sensory modality to accurate discriminations of lure stimuli. A behavioral task was developed to directly compare discrimination of similar lures on visual and auditory stimuli. Participants were significantly more accurate and more confident of their responses when discriminating visual stimuli as compared to discriminating auditory stimuli.

Human-computer interaction design principles largely focus on static representations and have yet to fully incorporate theories of perception appropriate for the dynamic multimodal interactions inherent to virtual environment (VE) interaction. Theories of direct perception, in particular affordance theory, may prove particularly relevant to enhancing VE interaction design. The present research constructs a conceptual model of how affordances are realized in the natural world and how lack of sensory stimuli may lead to realization failures in virtual environments. Implications of the model were empirically investigated by examining three affordances: passability, catchability, and flyability. The experimental design involved four factors for each of the three affordances and was implemented as a 2 [subscript IV] [superscript 4-1] fractional factorial design. The results demonstrated that providing affording cues led to behavior closely in-line with real-world behavior. More specifically, when given affording cues participants tended to rotate their virtual bodies when entering narrow passageways, accurately judge balls as catchable, and fly when conditions warranted it. The results support the conceptual model and demonstrate 1) that substituting designed cues via sensory stimuli in available sensory modalities for absent or impoverished modalities may enable the perception of affordances in VEs; 2) that sensory stimuli substitutions provide potential approaches for enabling the perception of affordances in a VE which in the real world are cross-modal; and 3) that affordances relating to specific action capabilities may be enabled by designed sensory stimuli. This research lays an empirical foundation for a science of VE design based on choosing and implementing design properties so as to evoke targeted user behavior

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