<p>As an object approaches an observer’s eye, tau, defined as the inverse relative expansion rate of the object’s image on the retina (Lee, 1976) approximates time-to-collision (TTC). Many studies have suggested that human observers use TTC information, but evidence for the use of tau remains inconclusive. Here we present two studies that investigated the use of tau in object-motion and observer-motion situations. In Study I, we dissociated several monocular variables that potentially contributed to TTC perception, and found that participants were most sensitive to TTC information when performing a relative TTC estimation task; and less sensitive to non-time variables such as distance-to-collision, speed and object size. Additionally, when we manipulated sources of information to specify conflicting time-of-arrivals, TTC specified by tau was weighted more than TTC derived from distance and speed. Thus, we concluded that even in the presence of monocular depth information, observers strongly utilize tau when estimating TTC.</p> <p>In Study II, observers estimated TTC of a looming target in the presence or absence of background expansion. Results demonstrated that participants overestimated TTC in situations where the surroundings of the target’s contours expanded at a reduced rate. Moreover, simulated self-motion was unnecessary to induce this bias, as results were comparable in situations where this relative expansion was limited to the target’s immediate surroundings. Therefore, we also concluded that a relative tau variable, based on the relative rate of expansion, is utilized whenever expansion beyond the object’s immediate boundaries is less than the target’s absolute rate of expansion.</p> / Master of Science (MSc)

<p>Based on evidence from studies involving animal single cell recording, animal brain lesion, and human brain damage, researchers have suggested that there may be differential visual representations for objects in near (peripersonal, within arm’s reach) and far (extrapersonal, beyond arm’s reach) space in the human visual system. The findings in Chapter 2 and Chapter 3 of the present thesis provide the first behavioural evidence suggesting that healthy human observers prefer to rely on different visual mechanisms in processing information in near and far spaces. The different performance in detecting visual targets presented in near and far space indicates that the brain can actively modulate the information processing either in parvocellular and magnocellular pathways or in ventral and dorsal streams. <br /> To determine the loci of the neural modulation regarding near and far viewing, visual identification tasks were employed in Chapter 4. In four experiments, visual stimuli were presented in either isoluminant green or achromatic white in order to decouple the neural processing in parvocellular and magnocellular pathways. The different patterns of the visual performance in the four experiments suggest that the change to near or far viewing distance results in altered information transmission in parvocellular and magnocellular pathways. Thus, the data in the present thesis provide the first behavioural evidence indicating that the LGN serves as a gatekeeper for regulating and redistributing visual information for later cortical analysis.</p> / Doctor of Science (PhD)

<p>Previous human behavioral research has provided support for the existence of different frames of reference utilized during spatial processing that can be dependent or independent of the observer. These are known respectively as egocentric and allocentric frames of reference. However, it has been difficult to dissociate these two different processes under realistic conditions. Importantly, how these frames of reference are influenced by the visual and non-visual information is not well understood. Therefore, the studies of this thesis evaluated spatial processing utilizing realistic and ecologically valid stimuli in environments of different scales, while systematically manipulating the visual and non-visual information available during learning. We demonstrated that non-visual information generated by actively walking through an environment leads to more egocentric processing, whereas the same visual motion information presented passively via a video leads to more allocentric processing (Chapter 2). Further, characteristics of the visual scene can also influence how it is processed, dependent on the strength of the verbal identity of the features in the environment (Chapter 3). Specifically, in a small room environment subject’s representations of corners-to-corners (corners do not have an obvious verbal component) were not as strongly encoded relative to each other in comparison to objects-to-objects (objects with an obvious verbal identity ). Finally, we demonstrated differential influences of non-visual information dependent on whether the features in the visual scene were more allocentrically processed or egocentrically processed (Chapter Four). Specifically, when different features of layouts are made distinguishable by their identity, this lead to more allocentric processing whereas when different features are made distinguishable by their relative position, this lead to more egocentric processing. Further, non-visual information made available during spatial updating when the observer is changing viewpoints benefitted tasks focused on differentiating changes to objects’ identity and less so for differentiating changes in relative object position.</p> / Doctor of Philosophy (PhD)

<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)

<p>Recent efforts to explain the underlying mechanisms of action observation have resulted in several theoretical frameworks. The Common Coding framework proposes that the perception and action areas of the brain share representations of visual and motor feedback such that areas are activated concurrently, there is a benefit to an individual’s perceptual ability and confidence. The MOSAIC framework, on the other hand, proposes that these benefits derive from self generations of motor commands during voluntary movement. This study aimed to observe the effects of concurrent motor activity on perceptual ability and judgments of performance. Participants observed an animated avatar performing pairs of symbol tracings on a screen. Participants were also placed into one of three concurrent movement groups: voluntary concurrent symbol tracing, non-voluntary concurrent symbol tracing or observation (no concurrent symbol tracing). It was expected that the group with voluntary concurrent movement would exhibit a higher perceptual accuracy and greater ability to judge performance than both the non-voluntary concurrent movement group and the observation group. However, all participants demonstrated the same level of perceptual accuracy, regardless of the level of concurrent movement. The decreased ability for voluntary movement participants was likely due to significant movement initiation time delays associated with cognitive processing of visual stimuli. As such, voluntary participants did not move in synchrony with the display. Nevertheless, both movement groups had a greater ability of judging performance, suggesting that judgments of performance may be obtained using different pathways than those involved in perceptual accuracy. Findings support both the Common Coding and MOSAIC frameworks, suggesting that a perceptual benefit can only be obtained when movement is in synchrony with perception.</p> / Master of Science (MSc)

<p>Older adults tend to show overall recognition deficits and qualitatively different patterns in the particular expressions that are most difficult to identify (Ruffman et al., 2008). In the current study, 23 younger (18-33 years old) and 23 older (60-80 years old) adults performed a 4AFC (angry, fearful, happy, sad) facial expression categorization task varying orientation (upright/inverted) and stimulus duration (100, 500, 1000 ms). For both groups, happiness was the easiest expression to identify and fear and sadness were the most difficult. Compared to younger adults, older adults were more affected by stimulus orientation, and generally benefit less from increased stimulus duration. For upright faces, there was no age difference in response accuracy but response latency was longer in older subjects. For inverted faces, older adults showed lower accuracy and longer latencies for expressions of anger, fear, and sadness. Recognition of inverted happy faces was spared in older adults for accuracy, but not response latency. These findings could not be explained by impaired detection sensitivity, as no systematic age differences were found for perceived intensity ratings. Finally, the expressions that were most to least difficult to identify was the same in each age group at both orientations. Overall, these results suggest that older individuals process expressive faces in a qualitatively similar way to their younger counterparts, but are less efficient at extracting the diagnostic information.</p> / Master of Science (MSc)

<p>This dissertation examined perturbation effects during complex tactile information transmission. The four experiments provide evidence regarding sensory and information processing demands in early stages of complex tactile learning.</p> <p>Experiment 1 established complex tactile learning behavioural performance. Vibrotactile stimuli representing Morse code letters were communicated to participants with or without induced perturbation to the finger of letter reception. Response performance was measured and augmented feedback was provided retroactively. Perturbation conditions lead to poor performance during tactile acquisition, but improved performance during application of knowledge.</p> <p>Experiment 2 determined if the experiment 1 results demonstrated masking or response competition paradigms. Target “masking” is the reduced ability to detect or interpret a stimuli pattern by presentation of other information (Craig, 1985; Verrillo, 1985). Response competition is the competition or distraction from target response generation by secondary stimuli (Craig, 2000; Bolanowski et al., 2000). Experiment 2 tested response competition by spatially separating the perturbation and tactile information delivery sites.</p> <p>Experiments 3 and 4 served to replicate behavioural acquisition data from experiments 1 and 2. They also extended the findings of the first two experiments by introducing neurophysiological measurement to reflect the changes associated with the two perturbation conditions. The study discerned whether the masking and response competition paradigms from experiments 1 and 2 were predominantly impacting the peripheral or central information processing.</p> <p>Results from the four studies collectively demonstrate that increased demands are placed on the sensory system during early stages of complex tactile learning when perturbation is spatially congruent with tactile information delivery. Experiments 1 and 2 revealed that attention does not supersede spatial location of perturbation, and perturbation location is paramount to yield sufficient interference to impede acquisition yet lead to enhanced knowledge retention and transfer. Experiments 3 and 4 determined that cortical information processing associated with complex tactile information acquisition are neurophysiologically differentiated when relative locations of meaningful and perturbation stimuli are congruent or spatially separated. The findings from this dissertation serve as an advancement of our understanding of masking and response competition phenomenon as they pertain to complex tactile learning.</p> / Doctor of Philosophy (PhD)

<p>Cognitive psychologists have long been interested in the nature of the representations that underlie human behaviours. In the domain of human memory, two main accounts have been forwarded to explain how memory is represented. The multiple memory systems account posits that different phenomenological experiences of memory are the result of separate and distinct representations. In contrast, the processing account posits that there is one memory system, and that different phenomenological experiences of memory are the result of the processes engaged in when probing memory. The explanatory power of these two accounts has been evaluated in a number of domains, where it has been shown that a processing view can accommodate many of the key findings that previously led researchers to propose a separate systems framework. The goal of this thesis was to extend this work by assessing the nature of the representations that underlie implicit sequence learning. The empirical portions of this thesis provide preliminary evidence suggesting that contextual features are incorporated into implicit sequence learning. Overall, the results reported in the empirical chapters of this thesis are consistent with a processing account, as they indicate that the episodic representations that are involved in explicit remembering also support performance in implicit memory tasks.</p> / Doctor of Philosophy (PhD)

<p>Trajectory formation is an important quality of human movement wherein the analysis of its kinematic structure often permits inferences regarding the underlying sensorimotor organization. The studies reported herein aimed to reveal how task-specific encoding of saccades contribute to two movement models. We first examined saccades for changes in the evidence for online spatial corrections in response to different instances of visual tracking. Results from our first study indicated the sensorimotor operations associated with making timed series responses permitted unhindered operation of online corrections in predictive saccades. These saccades imposed a demand on spatial working memory without suffering diminished corrections typical of single delayed memory-guided saccades. Next, we queried the role of ventral stream visuomotor processing, as well as the contribution of basic stimulus features to explaining known modulations of online saccadic corrections. Our results are consistent with the explanation that ventral stream contribution to target metrics influence trajectory kinematics, but did not induce diminished online corrections if vision of that illusory target was available at saccade onset. Using another movement task, we examined how different saccadic tracking behaviours dissociated the contribution of various sensorimotor operations involved in oculomotor preparation and execution to eye-hand coupling. By observing changes in manual trajectory in an oculo-manual task, we contrasted the effects of saccadic tracking in predictive and reactive modes, overt and covert, horizontal and vertical, spatially congruent or perpendicular axes of primary motion relative to eye-hand coupling. From these data we concluded that saccadic encoding induced execution-dependent coupling, and optionally motor planning-dependent coupling when task constraints specified temporal synchrony between eye and hand movements. Moreover, eventual motor execution was a prerequisite for the emergence of oculomotor planning-dependent coupling effects. This implied no apparent contribution to the coupling dynamic as an exclusive function of shifts in the spatial allocation of attention without oculomotor output.</p> / Doctor of Philosophy (PhD)

Younger adults’ perception of and attention to facial stimuli are enhanced by positive and negative emotional expressions, with negativity leading to a greater benefit than positivity. Conversely, older adults demonstrate a positivity bias, devoting more attention to positive stimuli and less to negative. It is unclear if age differences in these attentional preferences emerge due to differences in how their perceptual systems respond to positive and negative stimuli. Emotional facial expressions elicit enhanced P1 and N170 components of visually-evoked event-related potentials (ERP) over posterior scalp regions associated with vision. The current study examined the extent to which angry and happy facial expressions evoked differential patterns of P1 and N170 enhancements in younger (n = 21, ages 18-30) and older (n = 20, ages 60-76) adults. Participants were presented with happy, angry, and neutral faces under four instructional conditions: passively view, passively view but consider emotion, categorize emotion, and categorize gender. ERPs were recorded from the posterior scalp electrodes of a 128- channel high density electrode array and were time-locked to the onset of facial stimuli. The recordings were segmented and averaged based on the instructional condition and emotional expression of the stimulus. Analyses of the average P1 and N170 latencies revealed no age differences. Overall, participants displayed larger amplitude P1 and N170 to all stimuli when asked to categorize gender or emotionality. Contrary to expectations, both younger and older adults displayed larger N170 amplitudes for angry and happy expressions relative to neutral ones. Although older adults display a positivity bias in allocating attention to emotional stimuli, in the current study, younger and older adults both displayed an enhanced N170 for emotional faces relative to neutral faces, suggesting that the perceptual systems of younger and older adults are similarly engaged in processing positive and negative facial expressions at early time points.