University of Twente Student Theses


Frequency tagging of electrocutaneous stimuli for observation of cortical nociceptive processing

Nijhof, S.F.J. (2016) Frequency tagging of electrocutaneous stimuli for observation of cortical nociceptive processing.

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Abstract:Research on nociceptive (pain related) processing in the human nervous system is required to improve treatment for chronic pain patients. A key characteristic of chronic pain is development of maladaptive nociceptive processing. If these changes could be detected in an early stage, more accurate treatment could be given and would yield better results and less clinical effort per patient. Diagnostic methods are useful for characterizing the processing of nociceptive information. Observation methods have been developed already to measure responses to phasic stimuli applied to the peripheral nervous system. In combination with EEG measurements, more objective information can be obtained. Previous methods show good results in observation of neural responses to stimuli. This would be improved by an possibility to measure neural measurements of longer durations. New investigations are carried out aimed to evaluate responses of longer lasting tonic stimuli. ’Frequency tagging’ is a method that could be used for this. With frequency tagging, a pulse train of several seconds with pulses in the millisecond range is modulated on and off with a certain specified frequency. The goal of applying stimuli with this pattern is to see this same modulation frequency back in measurements of specific activated places in the bran corresponding to nociceptive processing. In this assignment, frequency tagging is implemented on a setup that was able to send phasic stimuli only. Key factor in the design of this setup is the strict timing requirement for generation of accurate frequencies. Properties related with timing are first evaluated, thereafter a validation experiment of the entire setup was performed. This validation experiment was done on a human subject. A relative nociceptive threshold was determined by applying pulse trains with increasing amplitude while measuring reactions. After the threshold was determined, the pulse train amplitude was set to twice this value to generate a definite pain sensation. Pulse trains with modulation frequencies of 13, 20, 33 and 43 Hz were applied and EEG was measured. Phase locked and non phase locked analysis in time and frequency domain was performed to interpret the data. Results show that frequency content corresponding to the input signal can be found back in the EEG measurements. Specific sharp peaks are observed in the frequency-magnitude plot of EEG channel derivations. Frequencies at which these peaks occurred where higher harmonics and combinations of the modulation frequency and frequency corresponding to single pulse timing. Phasic responses were found at the onset of pulse trains. Frequency content at the modulation frequency was only found for 33 Hz. For all other modulation frequencies, the fundamental frequency could not be observed clearly. Evaluating at 50 ms time around stimulus onset specifically, it was found that frequency content corresponding to the input signal was measured in EEG signals before human responses were possible. This is an indication of stimulus artifacts in the measurements. Due to the presence of stimulus artifacts in measurement data, the findings cannot yet contribute to characterizations of nociceptive processing. The setup is able to generate and measure accurate frequency components, however due to the presence of stimulus artifacts these frequencies cannot be clearly separated as measurements of nociceptive processing solely. The cause of stimulus artifacts is not yet fully understood. This could be dependent on voltage glitches in the output of the stimulator.
Item Type:Essay (Bachelor)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Subject:50 technical science in general
Programme:Electrical Engineering BSc (56953)
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