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.

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.

Over two decades of patient and neuroimaging data have provided increasing support for a role of the posterior cerebellum in cognition, particularly attention. Contralateral connections between the prefrontal cortex and the cerebellum are a probable basis for this effect. It is the purpose of this thesis to understand the contribution of the fronto-cerebellar system to cognitive and attentional processes. The first aim of this thesis was to localize areas of the cerebellum that participate in non-motor behaviour. After transient disruption of cerebellar activity using continuous theta burst stimulation (cTBS), a form of transcranial magnetic stimulation, Study 1 and 2 identified the right posterior-lateral cerebellum as a contributor to a network involved in two non-motor tasks; word generation and the attentional blink. The aim of Study 3 was to investigate if manipulating task demands increased fronto-cerebellar recruitment. The final study of this thesis employed electroencephalography (EEG) and cTBS to probe the neural events disrupted during the attentional blink task when the left frontal- right cerebellar system was transiently disrupted. Understanding the manner in which these neural events are affected by transient perturbation is integral to the understanding of the fronto-cerebellar contribution to cognitive and attentional processes. Together these studies help elucidate the role of the fronto-cerebellar system in non-motor functions.

Over two decades of patient and neuroimaging data have provided increasing support for a role of the posterior cerebellum in cognition, particularly attention. Contralateral connections between the prefrontal cortex and the cerebellum are a probable basis for this effect. It is the purpose of this thesis to understand the contribution of the fronto-cerebellar system to cognitive and attentional processes. The first aim of this thesis was to localize areas of the cerebellum that participate in non-motor behaviour. After transient disruption of cerebellar activity using continuous theta burst stimulation (cTBS), a form of transcranial magnetic stimulation, Study 1 and 2 identified the right posterior-lateral cerebellum as a contributor to a network involved in two non-motor tasks; word generation and the attentional blink. The aim of Study 3 was to investigate if manipulating task demands increased fronto-cerebellar recruitment. The final study of this thesis employed electroencephalography (EEG) and cTBS to probe the neural events disrupted during the attentional blink task when the left frontal- right cerebellar system was transiently disrupted. Understanding the manner in which these neural events are affected by transient perturbation is integral to the understanding of the fronto-cerebellar contribution to cognitive and attentional processes. Together these studies help elucidate the role of the fronto-cerebellar system in non-motor functions.

We continually encounter different ground terrain such as slippery, compliant, uneven, rocky, and irregular terrain when walking, yet we know very little about how individuals safely negotiate this type of complex environment. Furthermore, we know little about how aging affects stability in these situations despite the increased risk of falls and fall-related injuries among older adults. Paramount to our comprehension of how individuals safely traverse challenging ground terrain is to understand how visual information is utilized as vision is the first line of defense for preparing for and/or avoiding potentially hazardous terrain or obstacles. Thus, the objective of this thesis was to provide a better understanding towards how individuals negotiate different ground terrain in the environment to maintain dynamic stability and prevent the occurrence of a fall. In particular, the role of vision and the effects of aging were investigated. Three studies focused on the role of vision while negotiating varying ground terrain while two studies examined stability across these surfaces. Two main conclusions can be drawn from the results of the three studies on the role of vision. First, regardless of age individuals fixate on highly task-relevant areas (i.e. surfaces eventually stepped on) in an on-line manner and by fixating approximately two steps ahead. Second, visual information from the lower visual field is important for negotiating varying ground terrain. This latter finding has implications for older adults who wear multi-focal glasses and suggests that these individuals should be cautious when wearing these glasses in complex environments. In terms of stability, the results suggest that young and older adults demonstrate greater instability when walking across varying unstable ground terrain compared to solid level ground. Older adults are particularly more unstable in the medial-lateral direction when negotiating the challenging terrain, which may explain the frequency of laterally directed falls and increased hip-fracture risk with advancing age. Interestingly, older adults appear more stable in the anterior-posterior direction; although, this can largely be explained by the cautious gait strategy (i.e. slower walking speed and shorter steps) adopted by these individuals. The results of the studies of my thesis provide valuable insight into how individuals safely negotiate different types of challenging ground terrain when walking. Importantly, this knowledge can serve as an initial step in attempting to reduce falling among those at risk.

The neural correlates that underpin fine motor control of the hand and their connections with the primary motor cortex (M1) require further investigation. Brodmann’s area 5 located in the superior parietal lobule (SPL) is suggested to be an important cortical area involved in the processing of somatosensory input important for precision movements. Area 5 is present in monkey species capable of opposable thumb movements and it is proposed that this area evolved with the ability to execute manual behaviours such as pinch grip. Further, area 5 is dominated by the representation of the hand and forelimb, and has direct connectivity with M1 implicating its role in the control of hand movements. Few studies have investigated the function of area 5 in humans and none have examined the connectivity between area 5 and ipsilateral M1. This thesis presents a novel approach to study the influence of area 5 on M1 output in healthy and awake humans during the processing of somatosensory inputs and during performance of motor tasks involving the hand. Using paired pulse transcranial magnetic stimulation over left area 5 and ipsilateral M1, the connections between the two cortical loci was probed. It was hypothesized that area 5 would facilitate M1 output at short and long latencies during the processing of tactile inputs and during the performance of motor tasks compared to rest. The current results demonstrate that changes in M1 output are task and temporally specific. Facilitation of the motor evoked potential (MEP) was present at short latency of 6 ms during the processing of somatosensory input whereas inhibition was present during conditions where the hand was performing a task with the thumb and index finger. Further, an inhibitory effect was seen at 40 ms during cutaneous stimulation. In experiments 1 and 2, there was no net influence of area 5 on M1 output observed at rest. The findings presented may have revealed a novel path with which to alter the motor output, and possibly movement of hand muscles.

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.

Functional recovery of the upper limb after stroke is influenced by many factors, one being amount of affected arm and hand use following stroke. In the healthy population, amount of hand use is influenced by degree of hand dominance. Depending on side of stroke and previous hand dominance, these preferences may be altered, consequently affecting the amount of upper limb use. Determining hand preference in patients after stroke, when measured, is commonly assessed with questionnaires; however, these reports are subjective and patients may have difficulty recalling from memory which hand they use for the numerous activities on the questionnaire. A preferential reaching task has been shown to correlate with the degree of hand dominance as determined by the Waterloo Handedness Questionnaire, providing an objective performance-based method to assess the continuum of hand dominance in healthy subjects. A modified version of this preferential reaching task, with varying degrees of proximal to distal control, was used to investigate the influence of impairment, pre-stroke dominance and task difficulty on affected arm reach percentage. Results of the study revealed that it is feasible to administer a modified preferential reaching task in the stroke population, as the test could be completed in less than 10 minutes with no adverse effects reported from the patients. Heterogeneity made it difficult to detect statistical effects of task difficulty and pre-stroke dominance on post-stroke preference; however, there were trends observed indicating that patients with their dominant arm affected may have greater preference for the affected arm compared to those with their non-dominant arm affected. This was despite similar impairment levels between these patient groups. Preference for the dominant arm (whether affected or unaffected) was stronger when the task was at midline or in contralateral space, and when tasks required the greatest degree of distal control. In future, the degree of hand preference measured with this tool will have important implications for identifying areas in therapy requiring greater focus as well as identifying individuals who would most benefit from therapies that promote affected arm use, such as constraint induced movement therapy