Cross-education of strength is a neural adaptation defined as the increase in strength of the untrained contralateral limb after unilateral training of the opposite homologous limb. The neural mechanisms of the effect have remained elusive, although it appears to be a motor learning adaptation. Despite cross-education of strength being an inter-limb effect, no previous study has determined the influence of handedness and the direction of transfer (dominant to non-dominant or the reverse). Arguably, this is partly responsible for massive variation in the literature regarding the magnitude of the effect. The primary purpose of this document is to attempt to determine the central and peripheral neuro-physiological mechanisms controlling cross-education of muscular strength. Prior to determining the mechanisms of the effect, the influence of handedness and the preferred direction of transfer for cross-education of strength must be addressed. The secondary purpose is to determine the preferential direction of transfer of cross-education of strength in order to isolate the circumstances in which the effect is more pronounced. Two experiments were necessary to meet these objectives. <p>Experiment 1: The purpose was to determine the effect of the direction of transfer on cross-education in right-handed individuals. Subjects were randomized into a left-hand training (LEFT), right-hand training (RIGHT), or non-training control (CON) group. Strength training was 6 weeks of maximal isometric ulnar deviation, 4 times per week. The change in strength in the untrained limb was greatest in the RIGHT group (39.2%; p<.01), whereas no significant changes in strength were observed for the untrained limb of the LEFT group (9.3%) or for either of the CON group limbs (10.4% and 12.2%). There were no changes in muscle thickness of untrained limbs compared to CON. Changes in untrained limb EMG were not different compared to CON. Cross-education with hand strength training occurs only in the right-to-left direction of transfer in right-handed individuals. Cross-education of arm muscular strength is most pronounced to the non-dominant arm. <p>Experiment 2: The purpose of this study was to determine the central and peripheral mechanisms of cross-education of strength after actual and imagery training. Subjects were randomized into an actual training, imagery training, or non-training control group. A sub-sample of 8 subjects (4 actual, 4 imagery training) had brain activity during exercise assessed with functional magnetic resonance imaging (fMRI). Strength training was 6 weeks of maximal isometric handgrip ulnar deviation (Biodex) of the right arm, 4 times per week. Actual training was highly effective for increasing strength in trained (45.3%; p<.01) and untrained (47.1%; p<.01) limbs. Imagery training and control groups had no increases in strength for either arm. Muscle thickness increased only in the trained arm of the actual group (8.4%; p<.001). After actual training, there was an increase in activation of contralateral sensorimotor cortex and left temporal lobe during actual contractions with the untrained left arm (p<.001). Actual training was associated with a significantly greater change in agonist muscle activation pooled over both limbs, compared to the imagery and control groups (p<.05). Cross-education of strength is only significant after actual training, indicating that peripheral feedback is necessary for the effect. Cross-education of strength is accompanied by changes in cortical activation indicative of motor learning and the retrieval of memory of movement acquired by the trained limb. <p>General Conclusion: The neuro-physiological mechanism of cross-education of strength is that changes in cortical activation indicative of motor learning occur in both brain hemispheres after unilateral training. Cross-education of strength is influenced by strength asymmetries related to handedness, and the preferential direction of transfer is from dominant to non-dominant limb. Cross-education is a motor learning adaptation also reliant on peripheral feedback during training.

Cross-education of strength is a neural adaptation defined as the increase in strength of the untrained contralateral limb after unilateral training of the opposite homologous limb. The neural mechanisms of the effect have remained elusive, although it appears to be a motor learning adaptation. Despite cross-education of strength being an inter-limb effect, no previous study has determined the influence of handedness and the direction of transfer (dominant to non-dominant or the reverse). Arguably, this is partly responsible for massive variation in the literature regarding the magnitude of the effect. The primary purpose of this document is to attempt to determine the central and peripheral neuro-physiological mechanisms controlling cross-education of muscular strength. Prior to determining the mechanisms of the effect, the influence of handedness and the preferred direction of transfer for cross-education of strength must be addressed. The secondary purpose is to determine the preferential direction of transfer of cross-education of strength in order to isolate the circumstances in which the effect is more pronounced. Two experiments were necessary to meet these objectives. <p>Experiment 1: The purpose was to determine the effect of the direction of transfer on cross-education in right-handed individuals. Subjects were randomized into a left-hand training (LEFT), right-hand training (RIGHT), or non-training control (CON) group. Strength training was 6 weeks of maximal isometric ulnar deviation, 4 times per week. The change in strength in the untrained limb was greatest in the RIGHT group (39.2%; p<.01), whereas no significant changes in strength were observed for the untrained limb of the LEFT group (9.3%) or for either of the CON group limbs (10.4% and 12.2%). There were no changes in muscle thickness of untrained limbs compared to CON. Changes in untrained limb EMG were not different compared to CON. Cross-education with hand strength training occurs only in the right-to-left direction of transfer in right-handed individuals. Cross-education of arm muscular strength is most pronounced to the non-dominant arm. <p>Experiment 2: The purpose of this study was to determine the central and peripheral mechanisms of cross-education of strength after actual and imagery training. Subjects were randomized into an actual training, imagery training, or non-training control group. A sub-sample of 8 subjects (4 actual, 4 imagery training) had brain activity during exercise assessed with functional magnetic resonance imaging (fMRI). Strength training was 6 weeks of maximal isometric handgrip ulnar deviation (Biodex) of the right arm, 4 times per week. Actual training was highly effective for increasing strength in trained (45.3%; p<.01) and untrained (47.1%; p<.01) limbs. Imagery training and control groups had no increases in strength for either arm. Muscle thickness increased only in the trained arm of the actual group (8.4%; p<.001). After actual training, there was an increase in activation of contralateral sensorimotor cortex and left temporal lobe during actual contractions with the untrained left arm (p<.001). Actual training was associated with a significantly greater change in agonist muscle activation pooled over both limbs, compared to the imagery and control groups (p<.05). Cross-education of strength is only significant after actual training, indicating that peripheral feedback is necessary for the effect. Cross-education of strength is accompanied by changes in cortical activation indicative of motor learning and the retrieval of memory of movement acquired by the trained limb. <p>General Conclusion: The neuro-physiological mechanism of cross-education of strength is that changes in cortical activation indicative of motor learning occur in both brain hemispheres after unilateral training. Cross-education of strength is influenced by strength asymmetries related to handedness, and the preferential direction of transfer is from dominant to non-dominant limb. Cross-education is a motor learning adaptation also reliant on peripheral feedback during training.

Training is associated with specific neural adaptations. Skill training has been associated with increases in corticospinal tract excitability, leading to long term adaptations within motor cortex. Neural adaptations associated with strength training are less well established. It is not known how they are affected by volume, intensity, rate of muscle activation, or rest period. We evaluated the acute neural adaptations to a single session of strength training with the goal of evaluating the influence of rate of muscle activation on neural adaptations at the cortical and spinal levels. Thirty subjects participated in a single session of maximal, isometric knee extensions with the right leg. The training consisted of 4 sets of 5 contractions, with sets separated by approximately 5 minutes. Subjects were randomized into a high rate of muscle-activation group (Ballistic), a low rate of muscle-activation group (Ramp), and a Control group that did all testing but no training. Cortical spinal tract excitability was assessed using transcranial magnetic stimulation, spinal excitability was assessed using peripheral nerve stimulation, and inhibition of motor cortex was assessed using short-interval intracortical inhibition. In addition, we measured changes in motor performance. These same measures were assessed 24 hours later. Results showed that an acute strength training session is associated with a depression in resting cortical spinal tract excitability, but no change in active excitability. This change was immediate, taking place after just 2 contractions. Training was also associated with an increase in the excitability of the monosynaptic reflex circuit within the spinal cord during muscle activation, but not at rest. After 24 hours, cortical motor tract excitability had returned to normal, but intracortical inhibition was decreased from the original measure. Subjects from all groups increased maximum rate of torque development from Day 1 to Day 2. These results indicate that strength-training is associated with neural adaptations, though the adaptations were different than those for skill training. Understanding these adaptations will allow coaches and clinicians to better design programs to optimize the strength potential of the nervous system along with that of the muscles.

abstract: Animals learn to choose a proper action among alternatives according to the circumstance. Through trial-and-error, animals improve their odds by making correct association between their behavioral choices and external stimuli. While there has been an extensive literature on the theory of learning, it is still unclear how individual neurons and a neural network adapt as learning progresses. In this dissertation, single units in the medial and lateral agranular (AGm and AGl) cortices were recorded as rats learned a directional choice task. The task required the rat to make a left/right side lever press if a light cue appeared on the left/right side of the interface panel. Behavior analysis showed that rat's movement parameters during performance of directional choices became stereotyped very quickly (2-3 days) while learning to solve the directional choice problem took weeks to occur. The entire learning process was further broken down to 3 stages, each having similar number of recording sessions (days). Single unit based firing rate analysis revealed that 1) directional rate modulation was observed in both cortices; 2) the averaged mean rate between left and right trials in the neural ensemble each day did not change significantly among the three learning stages; 3) the rate difference between left and right trials of the ensemble did not change significantly either. Besides, for either left or right trials, the trial-to-trial firing variability of single neurons did not change significantly over the three stages. To explore the spatiotemporal neural pattern of the recorded ensemble, support vector machines (SVMs) were constructed each day to decode the direction of choice in single trials. Improved classification accuracy indicated enhanced discriminability between neural patterns of left and right choices as learning progressed. When using a restricted Boltzmann machine (RBM) model to extract features from neural activity patterns, results further supported the idea that neural firing patterns adapted during the three learning stages to facilitate the neural codes of directional choices. Put together, these findings suggest a spatiotemporal neural coding scheme in a rat AGl and AGm neural ensemble that may be responsible for and contributing to learning the directional choice task. / Dissertation/Thesis / Ph.D. Electrical Engineering 2014

This study examined the effects of linguistic task demands on the neuroanatomical localization of the neural response related to automatic semantic processing of concrete German nouns combining the associative priming paradigm with functional magnetic resonance imaging (fMRI). To clarify the functional role of the inferior frontal gyrus (IFG) for semantic processing with respect to semantic decision making compared to semantic processing per se, we used a linguistic task that involved either a binary decision process (i.e., semantic categorization; Experiment 1) or not (i.e., silently thinking about a word's meaning; Experiment 2). We observed associative priming effects indicated as neural suppression in bilateral superior temporal gyri (STG), anterior cingulate cortex (ACC), occipito-temporal brain areas, and in medial frontal brain areas independently of the linguistic task. Inferior parietal brain areas were more active for silently thinking about a word's meaning compared to semantic categorization. A conjunction analysis of linguistic task revealed that both tasks activated the same left-lateralized occipito-temporo-frontal network including the IFG. Contrasting neural associative priming effects across linguistic task demands, we found a significant interaction in the right IFG. The present fMRI data give rise to the assumption that activation of the left inferior frontal gyrus (LIFG) in the semantic domain might be important for semantic processing in general and not only for semantic decision making. These findings contrast with a recent study regarding the role of the LIFG for binary decision making in the lexical domain (Wright et al. 2011).

This study examined the effects of linguistic task demands on the neuroanatomical localization of the neural response related to automatic semantic processing of concrete German nouns combining the associative priming paradigm with functional magnetic resonance imaging (fMRI). To clarify the functional role of the inferior frontal gyrus (IFG) for semantic processing with respect to semantic decision making compared to semantic processing per se, we used a linguistic task that involved either a binary decision process (i.e., semantic categorization; Experiment 1) or not (i.e., silently thinking about a word's meaning; Experiment 2). We observed associative priming effects indicated as neural suppression in bilateral superior temporal gyri (STG), anterior cingulate cortex (ACC), occipito-temporal brain areas, and in medial frontal brain areas independently of the linguistic task. Inferior parietal brain areas were more active for silently thinking about a word's meaning compared to semantic categorization. A conjunction analysis of linguistic task revealed that both tasks activated the same left-lateralized occipito-temporo-frontal network including the IFG. Contrasting neural associative priming effects across linguistic task demands, we found a significant interaction in the right IFG. The present fMRI data give rise to the assumption that activation of the left inferior frontal gyrus (LIFG) in the semantic domain might be important for semantic processing in general and not only for semantic decision making. These findings contrast with a recent study regarding the role of the LIFG for binary decision making in the lexical domain (Wright et al. 2011).