Name: psychophysically-anchored, robust thresholding in studying pain-related lateralization of oscillatory prestimulus activity
Uploaded: 2017-01-21T14:00:00
Description: Watch this Scientific Journal Video about Psychophysically-anchored, Robust Thresholding in Studying Pain-related Lateralization of Oscillatory Prestimulus Activity at JoVE.com

This work has been funded by the Transregional Collaborative Research Centre TRR169 &#34;Crossmodal Learning: Adaptivity, Prediction and Interaction&#34; / German Research Foundation (DFG). The authors thank Stephanie Shields for the helpful comments on the manuscript.

The experiment has been approved by the ethics commission of the Hamburg medical association (PV4509).
1. Participant Selection
<ol>
	<li>Beyond standard selection criteria, such as fitness for pain stimulation, head implants or pre-existing neurological conditions, make sure the participants are not suffering from acute or chronic pain, are not taking any pain medication, and have no known history of substance abuse. Participants should also not have taken part in any pharmacological studie...

<ol>
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Here we used the well theoretically founded QUEST method to efficiently estimate a robust psychophysical threshold between non-pain and pain perception. Using constant stimulation at this threshold enables an analysis of perceptual decisions independent of changes in stimulus magnitude. While we examined threshold intensity at the transition point between innocuous and noxious sensation domains, other points along the pain scale (e.g.,&#160;50 on a 100-point pain scale) can also be anchored with the here present...

When conducting behavioral experiments involving human participants, it is important to be able to closely control the intensities of presented stimuli. Using stimuli of equal intensity for all participants, however, will in some settings introduce the bias of subjective perception. For some perceptual qualities such as pain, there are high inter- and intra-individual variations in perceived intensity at a constant stimulus level1,2. For experiments that assume equal subjective percepts, it is thus a necessity to match the subjectively perceived intensity across participants. This is also important when examining perception at threshold level, e.g., between painful and nonpainful stimulation. Psychophysics research has addressed these kind of problems for decades, and today there are sophisticated but easy-to-use methods available to achieve robust psychophysical anchoring.
A simple, classical method of mapping the intensity of a stimulus to an individual sensation magnitude is the staircase method3. Hereby, the intensity of successive stimuli is increased or decreased, until there is a change in the participant&#39;s response relating to the desired threshold or position on the subjective sensation scale. Repeating this process&#160;number of times, yields a plausible estimate of the reversal point. Classical methods, however, fail to make use of all the information contained in each rating trial. This leads to an unnecessarily high number of trials required to reach convergence. Methods such as (linear) regression or function fitting might fail, if the assumptions for the relationship between stimulus intensity and sensation magnitude are wrong or do not hold for the tested stimulus range. The adaptive procedures not only yield a robust point estimate for a certain subjective intensity, but do so more efficiently. Especially for longer experiments, which heavily rely on accurate estimation of a threshold or sensation magnitude, it is necessary for the psychophysical method to be both robust and at the same time efficient with respect to the number of required trials. This is especially important in fields such as pain research, where the total exposure to painful stimulation should be kept as low as possible for the participants&#39; benefit.
Although the classical staircase methods are still widely used, e.g. in quantitative sensory testing, the use of more advanced estimation methods that make better use of the acquired information across trials is steadily increasing. In the case of the maximum likelihood estimation method QUEST4,5 used here, this is probably due to the readily available implementation in the popular Matlab PsychToolbox6 suite. The modern, revised version of this procedure is superior to classical estimation methods both in robustness and the low number of trials required to arrive at a sufficient estimate, if used with the right settings7.
The rationale behind the QUEST procedure is to fit a Weibull function to the incoming data to model the psychophysical transformation between stimulus intensity and sensation magnitude. The parameters for the psychophysical Weibull function are in part given by the experimenter, e.g. the steepness of the function or the offset due to the false positive rate and responder inconsistency. The positioning of the parameter of interest along the intensity dimension is approximated by the procedure using Bayesian maximum likelihood estimation. Hereby, a probability distribution is assumed over the location of the target parameter, i.e. the threshold intensity. Given a sensible prior assumption for such a distribution, the algorithm will determine the most informative intensity that the participant should respond to. For the current implementation of the procedure, this is the mean of the prior probability distribution8. For each successive trial, the prior probability distribution is in essence multiplied with the likelihood of the participant&#39;s given response at the tested stimulation level, as characterized by the Weibull function. Every response will be used to continuously update the probability distribution estimate for the threshold parameter. This procedure is repeated until a satisfying estimate is produced. The procedure is more efficient than a simple regression because it makes immediate use of the collected responses to determine which stimulation intensity to test next. Also, the procedure will specifically probe around the point of interest, e.g. a threshold or certain sensation intensity. Using only testing data from such a limited range in regression would lead to an unstable estimate, making adaptive procedures more robust in settings where only relatively low numbers of trials are feasible.
Such robust psychophysical anchoring can be used to measure changes in pain sensitivity over time, modulatory effects in hyperalgesia/allodynia research or analgesic effects in pharmacological interventions, amongst other settings. Another interesting prospect of being able to anchor stimuli to the intensity just at the threshold between two sensory continua is to examine subjective perception across the transition from non-painful to painful sensation9,10,11. This scenario is very interesting because if the pain threshold has been robustly estimated, pain and no-pain conditions can be contrasted in electroencephalographic (EEG) activity, for example, without changing the physical stimulus intensity12. This allows for the observation of pain-specific perceptual processes under constant stimulus conditions by examining the difference in brain activity between trials rated as painful and non-painful.
We will demonstrate how to use the readily available implementation of adaptive estimation in PsychToolbox to robustly determine the individual pain threshold in an EEG experiment where the contrast between pain and no-pain activity is examined for lateralization effects, depending on the stimulation site. Since the stimulation intensity can be kept constant after the thresholding procedure, it is not necessary to account for EEG activity co-varying with stimulus intensity in the subsequent analysis.

<table border="0" cellpadding="0" cellspacing="0" width="835">
	<tbody>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">EasyCap electrode cap</td>
			<td style="width:241px;">EasyCap, Woerthsee-Etterschlag, Germany</td>
			<td style="width:211px;">CUCHW-58</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">actiCap active Ag/Cl EEG electrode set</td>
			<td style="width:241px;">BrainProducts GmbH, Gliching, Germany</td>
			<td style="width:211px;">-</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">SuperVisc EEG eletrode gel</td>
			<td style="width:241px;">EasyCap, Woerthsee-Etterschlag, Germany</td>
			<td style="width:211px;">V16</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">BrainAmp EEG amplifier</td>
			<td style="width:241px;">BrainProducts GmbH, Gliching, Germany</td>
			<td style="width:211px;">BrainAmp Standard</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">PsychToolbox-3</td>
			<td style="width:241px;">Mario Kleiner / Open Source</td>
			<td style="width:211px;">-</td>
			<td>Available at http://psychtoolbox.org/</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Matlab</td>
			<td style="width:241px;">MathWorks, Natick, MA</td>
			<td style="width:211px;">Matlab R2015b</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">DigiTimer DS7A constant current electrical stimulator</td>
			<td style="width:241px;">DigiTimer Ltd., Hertfordshire, United Kingdom</td>
			<td style="width:211px;">DS7A</td>
			<td></td>
		</tr>
	</tbody>
</table>

psychophysically-anchored, robust thresholding in studying pain-related lateralization of oscillatory prestimulus activity

In perceptual studies, it is often important to objectively assess the equality of delivered stimulation across participants or to quantify the intra-individual sensation magnitude that is evoked by stimulation over multiple trials. This requires a robust mapping of stimulus magnitude to perceived intensity and is commonly achieved by psychophysical estimation methods such as the staircase procedure. Newer, more efficient procedures like the QUEST algorithm fit a psychophysical function to the data in real time while at the same time maximizing the efficiency of data collection. A robust estimate of the threshold intensity between painful and nonpainful perceptions can then be used to reduce the influence of variations in sensory input in subsequent analyses of oscillatory brain activity. By stimulating at a constant threshold intensity determined by an adaptive estimation procedure, the variance in the ratings can be directly attributed to perceptual processes. Oscillatory activity can then be contrasted between &#34;pain&#34; and &#34;no-pain&#34; trials directly, yielding activity that closely relates to perceptual classification processes in nociception.

Using a rating scale split into one half for nonpainful and one half for painful sensations (Figure 1a), constant stimulation can be applied over many trials while still yielding ratings across the scale midpoint (Figure 1b). This way, changes in sensory input can be avoided, and the rating outcome can be directly related to intrinsic perceptual classification processes related to pain.
<img alt...

Watch this Scientific Journal Video about Psychophysically-anchored, Robust Thresholding in Studying Pain-related Lateralization of Oscillatory Prestimulus Activity at JoVE.com

Psychophysically-anchored, Robust Thresholding in Studying Pain-related Lateralization of Oscillatory Prestimulus Activity

Psychophysical methods such as the QUEST estimation procedure can efficiently yield robust estimates of the stimulation intensity at which nonpainful sensations transition into painful sensations. By stimulating repeatedly at the threshold intensity, the variability in rating responses can directly be attributed to perceptual classifications in subsequent analyses.

In perceptual studies, it is often important to objectively assess the equality of delivered stimulation across participants or to quantify the ...

The overall goal of this procedure, is to obtain a robust estimate of individual pain thresholds, using a well-established psychophysical estimation method. This method can help answer key questions in the field of pain research, such as identifying oscillatory neural activity that is connected to the subjective perception of pain, and of constant stimulation. The main advantage of this technique is that it yields very robust estimates of the stimulation intensity necessary to elicit a certain subjective sensation.Begin by selecting the appropriate cap size for the participant. And prepare the electroencephalography, or EEG electrodes based on the system's instruction manual. Then, set the sample rate and high-low cut off, as well as the impedance limits of the recording equipment.Run the EEG system on a battery, to ensure the stimulation device and the EEG are not electrically coupled. Then, make sure that any link between the EEG system and the computer controlling the electrical stimulation device, is potential-free. Next, set the stimulator to a single mono-phasic stimulation pulse, with one millisecond duration and 400 volt maximum voltage.Make sure that the electrical stimulator is switched on, but the output to the electrode is switched off. Then ask the participant to lay their hand on a flat surface, with all digits stretched out, and opposed. Identify the stimulation site by bisecting the distance between the first knuckles of the thumb, and the index finger.Clean the skin by applying electrode preparation gel. Then attach the stimulation electrode, and fasten it in place with textile tape. Next, ask the participant to place their hand comfortably, so that it will be still for the duration of the experiment.Place a soft tissue under the hand, to absorb any humidity. Finally, flip the output switch to the upward position. Begin by instructing the participant on how to operate the rating scale on the screen, using the mouse.Inform them, the left half represents the non-painful sensations, and the right half corresponds to the standard pain VAS scale. Point out to the participant that the absolute center point of the scale cannot be selected, and provide standardized instructions about the anchor points. Next, give the participant the opportunity to get comfortable with the rating process by applying stimuli of varying intensity, and recording the responses.Pick the intensities randomly, exploring the most extreme ends, and repeating some to evoke responses around the scale center. Ask the participant to provide verbal feedback on the intensities. Use the information gathered during this phase to get an estimate for two intensities that consistently evoke strong but non-painful sensations, a low point, and moderately painful sensations, a high point.After the low and high point starting intensities have been obtained, inform the participant that the first part of the experiment is about to start, and that he or she could keep on rating as practice, while random stimulus intensities are presented. To determine the threshold estimate, use the prior high and low starting intensities as a baseline. Then, as the participant is presented with stimulation, update the distribution of the threshold estimate, depending on the likelihood of participant's response.Finally, compute a new estimate by combining the prior likelihood distributions and use the resulting estimate as the new prior baseline estimate in the new iteration of the procedure. Begin by informing the participant that for the remaining part of the experiment, more blocks with random stimulation will be following, and they should keep rating as they did before. Start the EEG recording.Set the electrical stimulator to the previously determined mean threshold estimate, and keep the setting constant throughout the rest of the experiment. Next, start a rating block and observe the data quality of the EEG recording. Depending on the EEG data quality, run four to five rating blocks, and allow the participant to take short breaks in between the blocks.After the experiment is complete, stop the EEG recording, switch the stimulator output to off, and remove the stimulation electrode. Finally, remove the EEG cap and debrief the participant. Results indicate that taking the mean of the high and low intensity starting point estimates yields the final threshold estimate.Furthermore, the subjective stimulation intensity evoked by repeated stimulation at the estimate threshold is stable across multiple blocks of 30 trials, each within one experimental session. Here, you can see the sum of the lateralized differences between painful and non-painful stimulation in the theta-band EEG frequency range of four to seven Hertz. Once mastered, this procedure can be completed within 40 minutes, if it it done properly.While attempting this procedure, it's important to remember to explore a wide range of stimulus intensities, so the participants will give reliable pain ratings. Using this procedure, it's also possible to conduct experiments with other subjective pain intensities, such as levels well above the pain threshold. This method and its derivatives will enable pain researchers to not just control the intensity of the presented stimulus, but to also take into account the subjective sensation that is perceived by the participant.After watching this video, you should have a good understanding of how to perform psychophysical estimation of individual pain threshold, and how to use this technique in your own experiments. Don't forget that when working with painful stimulation, care has to be taken to ensure the safety of the participant at all times, and also the experiment has to be approved by the responsible ethics committee.

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