Afferent somatosensory inputs ascend from the periphery to the cortex carrying information about touch that is critical for planning motor responses. At the cortical level, this information is subject to modulation from its earliest arrival in somatosensory cortex where factors such as task-relevance begin to shape how the sensory signals are processed. The goal of such modulation is largely to facilitate the extraction of relevant sensory information (and suppression of irrelevant signals) early in the processing stream, and these functions are in part carried out by top-down influences from cortical and sub-cortical structures. Efforts to understand the mechanisms contributing to modulation of sensory-specific cortex have revealed that crossmodal signals (i.e. simultaneously presented stimuli from a different modality) can also influence early sensory processing, but the precise nature of this modulation and what may drive it is largely unknown. It is the purpose of this thesis to investigate the modulation of somatosensory cortex, specifically how task-relevant modulation of somatosensory cortex might be influenced by crossmodal (visual) stimuli, and whether specific task requirements have any bearing on SI excitability. The studies comprising this thesis aim to address these gaps in our mechanistic understanding of the networks involved in modulating somatosensory cortex. Studies 1 and 2 employed functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) to investigate how task-relevant visual and vibrotactile stimuli modulate somatosensory cortex and to probe the role of a frontoparietal network in mediating this modulation. Studies 3 and 4 also used EEG to determine how manipulating the relevance of the stimuli affects the modulation of somatosensory event-related potentials (ERPs), and to probe how task-specific sensory-motor requirements mediate excitability in somatosensory cortex as well as frontal and parietal regions. The results of this thesis provide insight into the factors that modulate somatosensory cortex and the role of a fronto-parietal network in subserving these modulations.

Afferent somatosensory inputs ascend from the periphery to the cortex carrying information about touch that is critical for planning motor responses. At the cortical level, this information is subject to modulation from its earliest arrival in somatosensory cortex where factors such as task-relevance begin to shape how the sensory signals are processed. The goal of such modulation is largely to facilitate the extraction of relevant sensory information (and suppression of irrelevant signals) early in the processing stream, and these functions are in part carried out by top-down influences from cortical and sub-cortical structures. Efforts to understand the mechanisms contributing to modulation of sensory-specific cortex have revealed that crossmodal signals (i.e. simultaneously presented stimuli from a different modality) can also influence early sensory processing, but the precise nature of this modulation and what may drive it is largely unknown. It is the purpose of this thesis to investigate the modulation of somatosensory cortex, specifically how task-relevant modulation of somatosensory cortex might be influenced by crossmodal (visual) stimuli, and whether specific task requirements have any bearing on SI excitability. The studies comprising this thesis aim to address these gaps in our mechanistic understanding of the networks involved in modulating somatosensory cortex. Studies 1 and 2 employed functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) to investigate how task-relevant visual and vibrotactile stimuli modulate somatosensory cortex and to probe the role of a frontoparietal network in mediating this modulation. Studies 3 and 4 also used EEG to determine how manipulating the relevance of the stimuli affects the modulation of somatosensory event-related potentials (ERPs), and to probe how task-specific sensory-motor requirements mediate excitability in somatosensory cortex as well as frontal and parietal regions. The results of this thesis provide insight into the factors that modulate somatosensory cortex and the role of a fronto-parietal network in subserving these modulations.

Successful interaction with the external world requires continual sensory detection, sensorimotor translations and goal-directed motor execution. Attention to task-relevant stimulation can facilitate sensory detection and improve behavioural performance. Crossmodal visual and somatosensory interaction within early sensory regions appears to further enhance processing, but required stimulus congruency for optimal sensorimotor communication is relatively unknown. This thesis first investigates the impact of visual-tactile temporal presentation on somatosensory activation within healthy young adults. As expected, findings revealed simultaneous crossmodal stimulation to maximally augment tactile event-related potentials (ERPs). These results were subsequently applied to determine the influence of attentional or low-level priming effects on motor performance within young and older adults. The bulk of this thesis assesses whether crossmodal interaction is similarly influential across age. Task-relevant visual-tactile stimulation was predicted to facilitate sensory regions and improve motor behaviour for both young and older subjects. Visual distraction was expected to limit tactile processing and impair performance only within older subjects. Tactile (P50, P100, N140, P230) and visual (N1) ERPs were recorded from 32 channels while healthy young and older subjects preformed a sensory integration task. Three conditions varying in modality of stimulation (tactile/visual) and task relevancy (relevant/irrelevant) required subjects to attend to stimuli and make an appropriately graded motor response. Blocked training prior to collection ensured stimulus-response associations and task demands were learned. Individual ERPs were time-locked to the onset of the first or second stimulus and quantified at CP3, CP4, FCZ, O1 and O2. Despite evidence of age-dependent effects in tactile processing, grand average waveforms suggest older adults maintain the ability to selectively attend to task-relevant information. Improved motor accuracy was not associated with crossmodal facilitation in either age group, however results indicate that performance of older adults declines with visual distraction. Differential N1 modulation across age suggests younger adults disengage from visual distraction after initial saliency (earlier latency with second distractor), while older adults may use a conscious strategy to shift attention away from distraction (latency unchanged but reduced amplitude with second distractor). Overall, these results follow previous studies and suggest older adults compensate for a general increase in processing background information by altering performance strategy. This vulnerability to distraction appears to negatively impact motor performance even within healthy aged adults.

Fine motor control of the hand relies on intact somatosensory integration and feedback. Impaired hand movements are observed in patient groups where touch perception and processing within the primary somatosensory cortex (SI) is abnormal. A repetitive transcranial magnetic stimulation paradigm called continuous theta-burst stimulation (cTBS) can be used to induce physiological changes to the underlying cortex. The effect of cTBS on tactile perception is unknown. This Master’s research thesis examined the effect of cTBS over SI on tactile discrimination on the hand in healthy humans. Specifically, the goal of this thesis was to reveal the modulatory effects of cTBS on tactile temporal and spatial psychophysical measures on the hand. In separate experiments, temporal discrimination threshold (TDT) and the spatial measure of amplitude discrimination were measured from the right hand before and for up to 35 minutes following cTBS over left SI. Compared to pre cTBS values, TDT was elevated immediately following cTBS (3-6 minutes) and at later intervals (11-18 minutes). Spatial tactile perception was also measured through amplitude discrimination over the same time course and compared to pre cTBS values thresholds were impaired for up to 18 minutes. These experiments reveal that cTBS over SI impairs tactile acuity on the contralateral hand. The effects last for up to 18 minutes and subsequent measures return to pre cTBS levels. This work is important in identifying means to modulate SI cortical excitability and has potential for clinical application in patient groups with altered somatosensory processing.

Successful interaction with the external world requires continual sensory detection, sensorimotor translations and goal-directed motor execution. Attention to task-relevant stimulation can facilitate sensory detection and improve behavioural performance. Crossmodal visual and somatosensory interaction within early sensory regions appears to further enhance processing, but required stimulus congruency for optimal sensorimotor communication is relatively unknown. This thesis first investigates the impact of visual-tactile temporal presentation on somatosensory activation within healthy young adults. As expected, findings revealed simultaneous crossmodal stimulation to maximally augment tactile event-related potentials (ERPs). These results were subsequently applied to determine the influence of attentional or low-level priming effects on motor performance within young and older adults. The bulk of this thesis assesses whether crossmodal interaction is similarly influential across age. Task-relevant visual-tactile stimulation was predicted to facilitate sensory regions and improve motor behaviour for both young and older subjects. Visual distraction was expected to limit tactile processing and impair performance only within older subjects. Tactile (P50, P100, N140, P230) and visual (N1) ERPs were recorded from 32 channels while healthy young and older subjects preformed a sensory integration task. Three conditions varying in modality of stimulation (tactile/visual) and task relevancy (relevant/irrelevant) required subjects to attend to stimuli and make an appropriately graded motor response. Blocked training prior to collection ensured stimulus-response associations and task demands were learned. Individual ERPs were time-locked to the onset of the first or second stimulus and quantified at CP3, CP4, FCZ, O1 and O2. Despite evidence of age-dependent effects in tactile processing, grand average waveforms suggest older adults maintain the ability to selectively attend to task-relevant information. Improved motor accuracy was not associated with crossmodal facilitation in either age group, however results indicate that performance of older adults declines with visual distraction. Differential N1 modulation across age suggests younger adults disengage from visual distraction after initial saliency (earlier latency with second distractor), while older adults may use a conscious strategy to shift attention away from distraction (latency unchanged but reduced amplitude with second distractor). Overall, these results follow previous studies and suggest older adults compensate for a general increase in processing background information by altering performance strategy. This vulnerability to distraction appears to negatively impact motor performance even within healthy aged adults.

This work characterized the ability of mice to respond for conditioned reinforcement, a phenomenon that can be used to investigate neural substrates of incentive learning. In both C57Bl/6 and CD1 mice, a reward-associated stimulus acted as a conditioned reinforcer (CR). Responding was stable over multiple test days, enhanced in CD1 mice by the dopamine transporter (DAT) blocker methylphenidate, and was extinguished when responding no longer produced the CR. However, transgenic C57Bl/6 mice overexpressing DAT, which decreased striatal dopamine by 40% responded normally for CR. Therefore, these results suggest that mice can be used to study brain mechanisms of incentive motivation. However, the choice of mouse strain in this paradigm is important as outbred CD1 mice appeared more susceptible to a DAT blocker compared to the inbred C57Bl/6 strain. These results also suggest that selective responding for a CR remains intact in a chronically hypodopaminergic state.

This work characterized the ability of mice to respond for conditioned reinforcement, a phenomenon that can be used to investigate neural substrates of incentive learning. In both C57Bl/6 and CD1 mice, a reward-associated stimulus acted as a conditioned reinforcer (CR). Responding was stable over multiple test days, enhanced in CD1 mice by the dopamine transporter (DAT) blocker methylphenidate, and was extinguished when responding no longer produced the CR. However, transgenic C57Bl/6 mice overexpressing DAT, which decreased striatal dopamine by 40% responded normally for CR. Therefore, these results suggest that mice can be used to study brain mechanisms of incentive motivation. However, the choice of mouse strain in this paradigm is important as outbred CD1 mice appeared more susceptible to a DAT blocker compared to the inbred C57Bl/6 strain. These results also suggest that selective responding for a CR remains intact in a chronically hypodopaminergic state.

Fine motor control of the hand relies on intact somatosensory integration and feedback. Impaired hand movements are observed in patient groups where touch perception and processing within the primary somatosensory cortex (SI) is abnormal. A repetitive transcranial magnetic stimulation paradigm called continuous theta-burst stimulation (cTBS) can be used to induce physiological changes to the underlying cortex. The effect of cTBS on tactile perception is unknown. This Master’s research thesis examined the effect of cTBS over SI on tactile discrimination on the hand in healthy humans. Specifically, the goal of this thesis was to reveal the modulatory effects of cTBS on tactile temporal and spatial psychophysical measures on the hand. In separate experiments, temporal discrimination threshold (TDT) and the spatial measure of amplitude discrimination were measured from the right hand before and for up to 35 minutes following cTBS over left SI. Compared to pre cTBS values, TDT was elevated immediately following cTBS (3-6 minutes) and at later intervals (11-18 minutes). Spatial tactile perception was also measured through amplitude discrimination over the same time course and compared to pre cTBS values thresholds were impaired for up to 18 minutes. These experiments reveal that cTBS over SI impairs tactile acuity on the contralateral hand. The effects last for up to 18 minutes and subsequent measures return to pre cTBS levels. This work is important in identifying means to modulate SI cortical excitability and has potential for clinical application in patient groups with altered somatosensory processing.

Repeated focal electrical stimulation of the brain results in kindling, the development of generalized seizures that progress in length and severity as more seizures are elicited. Barnes et al. (2001) paired one context (CS+) with kindling stimulation of the amygdala, and another context (CS-) with sham stimulation. They found conditioned anticipatory fear responses in the CS+, a conditioned place aversion to the CS+, and more intense convulsions in the CS+ than in the CS- in a probe trial. The present experiment was an attempt to replicate the findings, and to extend them by recording electroencephalographs (EEG). As well, I tested for conditioned effects on afterdischarge threshold (ADT). Rats received 45 pairings in each context before a conditioned place preference/aversion test, to determine whether the stimulation and seizures were rewarding or punishing. After more pairings, rats received suprathreshold stimulation in each context (switch test). Ictal measures in this test included afterdischarge duration, clonus duration, latency to clonus, class of convulsion, and falls. After more pairings, ADT was measured in each context. I partially replicated the findings of Barnes et al., in that conditioned anticipatory responses and conditioned place aversion were found. However, there were no conditioned effects on any ictal measures, including ADT. I conclude that conditioning is unlikely to play a major role in epileptogenesis.

Everyday experience suggests that certain people can find their way to a destination easily, while others have considerable difficulty. This dissertation focused on gaining a greater understanding of navigational strategies that can facilitate or hinder an individual’s wayfinding performance. The first study was conducted to gain a broad idea of various factors that may influence navigational performance. Participants were guided through a building and then asked to find their way to a destination. It was found that good navigators made fewer errors in traversing a learned route than did poor navigators. They were also better at recognizing landmarks they had seen along the route, recalling the appropriate directions to be turned at each landmark, and at drawing the correct pathways on a map drawing task. A discriminant analysis revealed that the best predictor of determining navigational performance was the ability to form spatial relationships between landmarks. Results from the first study demonstrated that good navigators were better at determining spatial relationships between landmarks, but it did not address whether this was due to spatial relationships between distances and/or angles. The focus of the second study was to gain a greater understanding of the degree to which distance and angular information are used by good and poor navigators in determining spatial relationships between landmarks. Results showed that neither a distance nor an angular strategy were preferred in either group of wayfinders. An analysis of navigators initial heading angle error to a target location suggested that good wayfinders may be more efficient at finding their way because they appear to plan routes prior to initiating self-locomotion. Such pre-planning was confirmed by the fact that good wayfinders’ initial heading direction error was significantly less than in poor wayfinders. Poor wayfinders appear to head in a random direction and then attempt to determine target locations. The use of landmark information may be useful in certain contexts, but this may not always be the most efficient strategy. The last experiment was aimed at determining whether good navigators adjust strategies used (landmark vs. street), depending on contextual factors. Differences in strategies used were not found, however the results suggest that good navigators appear to be more skilled at navigating in environments rich with streets compared to poor wayfinders. Good and poor navigators were equally skilled at navigating in environments rich in landmarks. It appears that the ability to determine spatial relationships between landmarks is the strongest predictor of navigational performance compared to a wide range of other navigational skills.