Prior physical exertion modulates allocentric distance perception: a demonstration of task-irrelevant cross-modal transfer.

Physical exertion has been previously shown to influence distance perception in the egocentric framework. In this study, we show that physical exertion influences allocentric distance perception. Twenty healthy volunteers made allocentric line length estimates following varying levels of physical exertion. Each participant was presented with 30 different line lengths ranging from 1 to 12 cm, and each length was presented three times. Each line presentation was preceded by the participant exerting one of the following three levels of their maximal voluntary force (MVF): 20, 50, or 80 % MVF using their hand in the pinch force task. Psychometric curves were obtained for the lines perceived as 'long' following each of the three force levels. Lines that were perceived as 'short' following 20 and 50 % MVF were perceived as 'long' following 80 % MVF; that is, there was a significant leftward shift in the psychometric curve following 80 % MVF when compared to 20 and 50 % MVF. Here, we demonstrate that physical exertion influences perception of distances in the allocentric framework. We discuss our findings with respect to cross-modal interactions, fatigue physiology, peri- and extra-personal space interactions.

Post-stroke fatigue: a deficit in corticomotor excitability?

The pathophysiology of post-stroke fatigue is poorly understood although it is thought to be a consequence of central nervous system pathophysiology. In this study we investigate the relationship between corticomotor excitability and self-reported non-exercise related fatigue in chronic stroke population. Seventy first-time non-depressed stroke survivors (60.36 ± 12.4 years, 20 females, 56.81 ± 63 months post-stroke) with minimal motor and cognitive impairment were included in the cross-sectional observational study. Fatigue was measured using two validated questionnaires: Fatigue Severity Scale 7 and Neurological Fatigue Index - Stroke. Perception of effort was measured using a 0-10 numerical rating scale in an isometric biceps hold-task and was used as a secondary measure of fatigue. Neurophysiological measures of corticomotor excitability were performed using transcranial magnetic stimulation. Corticospinal excitability was quantified using resting and active motor thresholds and stimulus-response curves of the first dorsal interosseous muscle. Intracortical M1 excitability was measured using paired pulse paradigms: short and long interval intracortical inhibition in the same hand muscle as above. Excitability of cortical and subcortical inputs that drive M1 output was measured in the biceps muscle using a modified twitch interpolation technique to provide an index of central activation failure. Stepwise regression was performed to determine the explanatory variables that significantly accounted for variance in the fatigue and perception scores. Resting motor threshold (R = 0.384; 95% confidence interval = 0.071; P = 0.036) accounted for 14.7% (R(2)) of the variation in Fatigue Severity Scale 7. Central activation failure (R = 0.416; 95% confidence interval = -1.618; P = 0.003) accounted for 17.3% (R(2)) of the variation in perceived effort score. Thus chronic stroke survivors with high fatigue exhibit high motor thresholds and those who perceive high effort have low excitability of inputs that drive motor cortex output. We suggest that low excitability of both corticospinal output and its facilitatory synaptic inputs from cortical and sub-cortical sites contribute to high levels of fatigue after stroke.

Beta oscillations reflect changes in motor cortex inhibition in healthy ageing.

Beta oscillations are involved in movement and have previously been linked to levels of the inhibitory neurotransmitter GABA. We examined changes in beta oscillations during rest and movement in primary motor cortex (M1). Amplitude and frequency of beta power at rest and movement-related beta desynchronization (MRBD) were measured during a simple unimanual grip task and their relationship with age was explored in a group of healthy participants. We were able to show that at rest, increasing age was associated with greater baseline beta power in M1 contralateral to the active hand, with a similar (non-significant) trend in ipsilateral M1. During movement, increasing age was associated with increased MRBD amplitude in ipsilateral M1 and reduced frequency (in contralateral and ipsilateral M1). These findings would be consistent with greater GABAergic inhibitory activity within motor cortices of older subjects. These oscillatory parameters have the potential to reveal changes in the excitatory-inhibitory balance in M1 which in turn may be a useful marker of plasticity in the brain, both in healthy ageing and disease.