The relation between motor imagery abilities, memory and plasticity in healthy adults

Motor imagery (MI) is defined as the ability to mentally rehearse a movement within working memory using a first-person perspective and without performing the action. The functional equivalence hypothesis sustains that MI has similar behavioural and neurophysiological characteristics compared to motor execution. These features have encouraged researchers and practitioners to deepen both the basic operations that underlie this mental process and the outcomes of its possible application in motor learning and rehabilitation. In this thesis we investigated the relation between the ability to generate and maintain/manipulate a motor image in mind (measured by the Movement Imagery Questionnaire-3 (MIQ-3) and the mental chronometry score, respectively), memory processes and plasticity phenomena. With this general goal, we conducted three studies on healthy young adults. In study 1 we had two aims: to (1) explore the link between the cognitive processes that occur during kinesthetic imagery and working memory; (2) and to understand how kinesthetic imagery skills influence the pattern of EEG changes during deliberate MI, within those frequency bands associated to working memory processes (Theta, Alpha and Gamma). Thus, in 19 participants we preliminary evaluated kinesthetic imagery (MIQ-3 and the mental chronometry score) and working memory abilities (neuropsychological tests). Then, we measured the EEG correlates of the kinesthetic imagery using the same items of the MIQ-3 as stimuli. We found that high MIQ-3 scores of kinesthetic imagery were associated with a widespread increase in Alpha power that could underlie participants’ ability of maintaining a mental representation in mind thus blocking the retrieval of irrelevant information, as well as long range correlated firing related to maintenance of the relevant one. This block could reflect not only a simple exclusion but also an active inhibition of the areas not involved in the task. On the other hand, the mental chronometry score, that is the discrepancy between the actual and the imagined movement timings (a low score is interpreted as a higher ability) was negatively correlated with the central executive system performance measured by means of the Dual N-Back task. This finding was strengthened by the fact that the changes in Theta power in temporal regions, which are related to the performance of the central executive system, are correlated with the chronometry scores. Participants with a good skill decreased Theta power in these areas, where this rhythm has been implicated in the temporal sequencing of the information retrieved from memory. Moreover, the mental chronometry score was positively related to the Gamma power of the medial regions of the brain that are linked to the ability of maintaining/manipulating the information in mind. In study 2 our aims were: (1) to describe whether an acute immobilization of the dominant upper limb (~30 minutes) produces negative behavioural outcomes; (2) to study whether a mental MI practice performed 15 minutes right before the cast removal could prevent the behavioural negative outcomes that follows the acute immobilization; (3) to investigate whether the effects of the mental MI practice are correlated to the ability to generate a motor image. Thus, 48 participants were preliminarily evaluated both by means of the MIQ-3 and a choice reaction time task where they had to answer as fast and accurate as possible with the left/right hand to left/right affordances. Then, they were assigned to one of three groups: Control, Cast and Cast-MI. In the Cast and Cast-MI the right arm was immobilized. A cast adaptation period of 15 minutes was followed by a training session that the three groups performed in a different manner. The Cast performed the training responding only with the left hand to left affordances; the Cast-MI besides actually answering with the left hand to left affordances, imagined to answer with the right hand to right affordances; the Control performed the training with both hands as the pre-evaluation. After the training, the cast was removed and participants were evaluated again with the same reaction time task performed at the baseline. The results highlighted that a brief immobilization induced a negative effect on the right immobilized arm. Contrarily to our expectations, the mental MI training not only did not prevent these negative outcomes but was also detrimental for the performance of the left (not immobilized) hand, likely due to divided attention between the two hands, with a stronger focus on the immobilized right hand that could have disadvantaged the left one. Moreover, the higher the participants’ ability in generating a visual internal motor image in mind the lower the improvement of the left-hand after the MI training, possibly attributable to a better inhibition of the right motor areas non-involved in the imagery task. Finally, in study 3 we had three aims: (1) to study whether AO could induce a perceptual priming effect in the motor related areas, leading to the appearance of the spontaneous MI; (2) to study whether the occurrence of spontaneous MI during AO modifies the recall of an observed movement sequence as well as the cortical excitability of the primary motor cortex in the region that controls the observed action; (3) to study the correlation between the spontaneous and deliberate MI ability scores. We recruited 21 participants that had to lean a sequence of ball pinches that an actor performed in a video that was shown six times. During AO we collected participants’ eye position and the motor evoked potentials in the muscles both involved (APB and FDI) and not involved (ADM) in the different phases of the observed movement stimulating their primary motor cortex by means of the TMS. After the AO trials, we asked participants to recall the observed motor sequence, to report whether they engaged in MI during AO and to give a rate of this experience using the Likert-type scale of the MIQ-3. Subsequently, participants had to deliberately imagine the movement previously observed giving again a rate by means of the same Likert-type scale used for the spontaneous MI. This protocol was repeated in two conditions: with and without the application of an attentional constraint during the AO trials that directed participants’ attention on the target of the movement (the ball). We found that AO exerted a sort of perceptual priming on the motor regions inducing spontaneous MI in both conditions. When we did not place the attentional constraint, participants not only reported higher score of spontaneous MI quality compared to the other condition but they also had small motor evoked potentials. In contrast, the attentional constraint enhanced the corticospinal excitability of spontaneous motor imagers. It is likely that the visual guide organized in time and space the mental activity during AO and attenuated the quality of spontaneous MI that, in this condition, did not appear to be related to the homologous score of deliberate MI.The evidences that emerged from these studies have relevant implications both in research and applied field, like motor learning and rehabilitation. The neurocognitive characteristics (EEG and Dual N-Back scores) that underlie the MI process highlighted in study 1, could be useful both in the assessment and in the mental training of MI skills. The brief immobilization paradigm that we tested in study 2, could be a fast and easy research method to study the behavioural outcomes of cortical plasticity phenomena linked to limb immobilization. In the same study, we discouraged the concurrent use of a mental and a physical practice that involves the different hands. Also, we underlined the scarce effects of the former in counteracting the negative behavioural outcomes of a brief immobilization. Finally, in study 3 we suggested a simple method that professionals could use to control the possible detrimental effects of spontaneous MI during AO, that is the application of an attentional constraint that guides participants AO experience.