Dissertation (Ph.D.) --University of Texas Graduate School of Biomedical Sciences at San Antonio, 2008. / Vita. Includes bibliographical references.

Ideomotor theory holds that motor plans producing action and the sensory effects of the actions are cognitively represented in a functionally similar way. The response-effect (R-E) association is considered bidirectional and automatic in nature. The current research project was designed to test the hypothesized bidirectional nature of R-E associations by determining if motor codes were activated following perception of an effect. The automaticity of motor code activation was investigated via TMS–induced motor evoked potentials (MEPs) following the presentation an after-effect. To this end, participants completed a training phase in which they learned a specific R-E association. During the testing phase, the effects were presented prior to the imperative and TMS stimuli. Behavioural results replicated previous research; participants preferred to execute the response associated with the presented effect. MEP data, however, did not support the initial hypothesis. These results are discussed with relation to ideomotor theory and experimental design.

Ideomotor theory holds that motor plans producing action and the sensory effects of the actions are cognitively represented in a functionally similar way. The response-effect (R-E) association is considered bidirectional and automatic in nature. The current research project was designed to test the hypothesized bidirectional nature of R-E associations by determining if motor codes were activated following perception of an effect. The automaticity of motor code activation was investigated via TMS–induced motor evoked potentials (MEPs) following the presentation an after-effect. To this end, participants completed a training phase in which they learned a specific R-E association. During the testing phase, the effects were presented prior to the imperative and TMS stimuli. Behavioural results replicated previous research; participants preferred to execute the response associated with the presented effect. MEP data, however, did not support the initial hypothesis. These results are discussed with relation to ideomotor theory and experimental design.

No description available.

Spreading depression (SD) is a slow propagating wave of depolarization that can spread throughout the cortex in the event of brain injury or any general energy failure of the brain. Massive cellular depolarization causes enormous ionic and water shifts and silences synaptic transmission in the affected tissue. Large amounts of energy are required to restore ionic gradients and are not always met. When these energetic demands are not met, brain tissue damage can occur. The exact mechanism behind initiation and propagation of SD are unknown, but a general model is known. It may be possible to prevent or delay the onset of SD using non-invasive electromagnetic techniques. Transcranial magnetic stimulation (TMS), electrical stimulation (ES), and transcranial direct coupled stimulation (tDCS) could be used to decrease neuronal excitability in different ways. In theory, any technique that can reduce cortical excitability could suppress SD initiating or propagating.

Previous research and current models have proposed that the right temporoparietal junction (rTPJ) is crucially involved in the control and distinction of shared representations of action. Hitherto, this assumption has mainly been based on neuroimaging work ( (Spengler, von Cramon, & Brass, 2009); (Spengler, von Cramon, & Brass, 2010)) We tested this hypothesis, that the rTPJ is causally involved in managing shared representations by using repetitive transcranial magnetic stimulation in an offline paradigm to disrupt neural activity in this region. Using a simple imitation-inhibition task we showed that stimulation of the rTPJ led to increased reaction times when participants had to control automatic imitation of a perceived hand movement, as they had to concurrently plan and execute an opposite movement. Our study provides the first empirical evidence that the rTPJ is necessary for managing and navigating within a shared representational system. These results may also have important implications for future theorizing about the role of the TPJ region in controlling shared representations also in other domains, such as somatosensation or emotional experiences.

The technique of transcranial Doppler ultrasound (TCD) was used to assess cerebral blood flow (CBF) in humans. Studies were performed at rest and during dynamic submaximal exercise. In the resting experiments, TCD was combined with the technique of dynamic end-tidal forcing to study the dynamics of the CBF response to step changes in end-tidal (i.e. arterial) PC02 and PO2 In the resting and exercise experiments, the degree of consistency was examined between three indices of CBF that can be extracted from the TCD spectrum. Finally, the ventilatory and the CBF responses to acute isocapnic hypoxia were examined to try to quantify the possible reduction in ventilation that could be attributed to changes in CBF with hypoxia. In the studies performed at rest, during either hypoxia and/or hypercapnia (Chapter 2), the three indices of CBF extracted from the TCD spectrum were all consistent. However, during submaximal exercise (Chapter 5), the indices were less consistent and results suggest that the increase in CBF with exercise that has been reported with TCD needs to be treated with caution. The dynamic studies of the CBF response to step changes in end-tidal PC02 and PO2 in humans revealed that the CBF response to hypercapnia (Chapter 3) is characterised by a significant asymmetry, with a slower on-transient than off-transient, and also by a degree of undershoot following the relief of hypercapnia. The CBF response to hypocapnia (Chapter 4) is also characterised by a significant asymmetry, with a faster on-transient than off-transient. Furthermore, there is a slow progressive adaptation throughout the hypocapnic period. These studies show that the CBF responses to hypercapnia and hypocapnia are much faster than previously been thought. Finally, the work described in Chapter 6 attempts to quantify the possible reduction in ventilation that could be attributed to changes in CBF with hypoxia to determine whether it could be of sufficient magnitude to underlie hypoxic ventilatory decline (HVD). The results suggest that, in awake humans, changes in CBF during acute isocapnic hypoxia are quantitatively insufficient to underlie HVD.

The thesis designs, constructs, and tests an electrically small dipole antenna probe for the measurement of electric field distributions induced by a transcranial magnetic stimulation (TMS) coil. Its unique features include high spatial resolution, large frequency band from 100 Hz to 300 kHz, efficient feedline isolation via a printed Dyson balun, and accurate mitigation of noise. Prior work in this area is thoroughly reviewed. The proposed probe design is realized in hardware; implementation details and design tradeoffs are described. Test data is presented for the measurement of a CW capacitor electric field, demonstrating the probe’s ability to properly measure conservative electric fields caused by a charge distribution. Test data is also presented for the measurement of a CW solenoidal electric field, demonstrating the probe’s ability to measure non-conservative solenoidal electric fields caused by Faraday’s law of induction. Those are the primary fields for the transcranial magnetic stimulation. Advantages and disadvantages of this probing system versus those of prior works are discussed. Further refinement steps necessary for the development of this probe as a valuable TMS instrument are discussed.

The use of transcranial Doppler (TCD) ultrasound for the assessment of cerebral blood flow velocity (CBFV) provides an indication of cerebral blood flow assuming the diameter of the insonated vessel remains constant. Studies using TCD have traditionally described cerebrovascular hemodynamics with respect to CBFV and cerebrovascular resistance (CVRi); however, a more complete assessment of the cerebral circulation can be gleaned from the analysis of within beat characteristic of the TCD velocity waveform for the determination of cerebrovascular tone. Therefore, the general purpose of the presented studies was to assess CBFV responses and within beat characteristic for the description of cerebrovascular hemodynamics after long duration spaceflight, with sustained orthostasis, in response to changes in the partial pressure of end tidal carbon dioxide (PETCO2), and with NG stimulation. After long duration spaceflight, cerebrovascular autoregulation was found to be impaired along with a reduction in cerebrovascular CO2 reactivity (Study 1). Additionally, critical closing pressure (CrCP) was found to be increased suggesting potential remodelling of the cerebrovasculature contributing to an increase in cerebrovascular tone (Study 2). With sustained orthostasis, CBFV was found to progressively decrease and to be related to reductions in PETCO2 and increases in CrCP suggesting the contribution of changes in cerebrovascular tone leading to the development of syncope (Study 4). The CBFV reduction with the progression towards syncope was also associated with changes in waveform morphology such that the dicrotic notch point was less than the end diastolic value (Study 3). Mathematical modelling (RCKL) was used to further assess changes in cerebrovascular hemodynamics for physiological interpretation of changes in CBFV waveform morphology and found that the amplitude of the dicrotic notch and the calculation of the augmentation index were both significantly related to vascular compliance before and after stimulation with NG (Study 5). The use of quantitative assessments of common carotid artery (CCA) blood flow as an indicator of cerebral blood flow suggested the dilation of the middle cerebral artery (MCA) with NG (Study 5 and 6) and changes in MCA diameter with acute alterations in PETCO2 (Study 6). CCA and MCA velocity wave morphology were assessed showing that with changes in PETCO2, changes in CBFV velocity wave were not reflected in the CCA trace (Study 7). In addition, further assessment of the CBFV velocity trace and the calculation of CrCP and the augmentation index suggested that with changes in PETCO2 cerebrovascular compliance and cerebrovascular tension, both thought to be components of cerebrovascular tone, change independently (Study 7). Combined, the results of the presented studies suggest that changes in cerebrovascular hemodynamics can be determined from alterations in the CBFV velocity waveform morphology. However, further work is required to determine how these variations relate to specific components of cerebrovascular tone, including alterations in cerebrovascular compliance and vascular tension, and how these variables change with acute and chronic alterations in cerebrovascular hemodynamics.

Individuals with severe physical impairments have a reduced ability to communicate through movement and speech. We investigated transcranial Doppler ultrasound as a potential measurement modality for a novel brain-computer interface. It was hypothesized that cognitive activity would result in detectable changes in cerebral blood flow velocity within the middle cerebral arteries. Nine able-bodied participants alternated between rest and two different mental activities - silent word generation and mental rotation. Two analyses were performed to assess the feasibility and practicality of a TCD-based brain-computer interface. Both mental activities were independently differentiated from rest with high accuracy. Intuitive time-domain features were sufficient for classification. Data transmission rate was quadrupled by differentiating all three classes simultaneously using shorter state durations. Transcranial Doppler ultrasound can be used to automatically detect cognitive activity and may be useful as the basis of a brain-computer interface.