Quantitative Assessment of Cortical Auditory-tactile Processing in Children with Disabilities

Summary

Objective and easy measurement of sensory processing is extremely difficult in nonverbal or vulnerable pediatric patients. We developed a new methodology to quantitatively assess infants and children's cortical processing of light touch, speech sounds, and the multisensory processing of the 2 stimuli, without requiring active subject participation or causing discomfort in vulnerable patients.

Abstract

Objective and easy measurement of sensory processing is extremely difficult in nonverbal or vulnerable pediatric patients. We developed a new methodology to quantitatively assess children's cortical processing of light touch, speech sounds and the multisensory processing of the 2 stimuli, without requiring active subject participation or causing children discomfort. To accomplish this we developed a dual channel, time and strength calibrated air puff stimulator that allows both tactile stimulation and sham control. We combined this with the use of event-related potential methodology to allow for high temporal resolution of signals from the primary and secondary somatosensory cortices as well as higher order processing. This methodology also allowed us to measure a multisensory response to auditory-tactile stimulation.

Introduction

The study of developing cortical sensory processes is essential to understanding the basis for most higher order functions. Sensory experiences are responsible for much of the brain's organization through infancy and childhood, laying the foundation for complex processes such as cognition, communication, and motor development^1-3. Most pediatric studies of sensory processes focus on auditory and visual domains, mainly because these stimuli are easiest to develop, standardize, and test. However, tactile processing is of particular interest in infants and children as it is the first sense to develop in the fetus^4,5, and somatosensory information is integral to the function of other cortical systems (e.g. motor, memory, associative learning, limbic)⁶. Current methods assessing somatosensory processing are limited by the choice of tactile stimulus. A common choice is direct electrical median nerve stimulation^7,8, with the potential for discomfort. Other effective methods use active tasks such as discrimination, recognition, and localization of stimuli, requiring both attention and high levels of comprehension⁹. All of these methods are therefore limited in their use in young children and infants.

Therefore, our goal was to develop a tactile paradigm that addresses these limitations by being noninvasive and reducing the need for a subject's active participation. Additionally, it needed to have a standardized level of stimulation and a sham-control. For this we developed the "puffer" system, a dual-channel, timed, and calibrated air-puff delivery system, allowing us to measure the effects of light touch in infants and other vulnerable populations.

Functional MRI studies showed that stimulation by puffs of air activates sensory cortices, although the length and challenges of such studies, such as immobilization, lengthy sessions, and anxiety-provoking settings make them difficult to perform in young children. Therefore, we combined our novel delivery system with Event-Related Potential (ERP) methodology in order to provide temporal resolution of sensory processing of light touch in a brief, child-friendly testing session.

This new paradigm offers the needed flexibility to study sensory processing in diverse populations, ages and clinical settings. It also has the advantage of being compatible with auditory stimuli, allowing for multisensory assessments. Until now, accurate and reliable tactile assessment has not been possible in infants or in children who are unable to reliably respond due to intellectual/language disorders. This methodology aims to fill this gap in order to aid in early identification of sensory processing deficits and intervention during a period of maximal brain plasticity. Improvements in sensory processing in infancy may influence the cascade of neurodevelopmental

The following procedures are all included in Vanderbilt Institutional Review Board approved protocols.

Protocol

1. Assessment of Response to Light Touch

1. Place the electrode net (e.g. 128-channels geodesic sensor net) on child or infant's head. Adjust sensors for full contact using warm saline solution. If on a child, ensure child is sitting comfortably in parent or caregiver lap. If on an infant, ensure that infant is lightly swaddled and either held in caregiver's arms or in a supine position in an open crib.
2. Position a 1 mm nozzle 0.5 cm below the tip of the index finger of the tested hand. Place finger for a young child or palm for an infant in a mold holder and secure with Velcro tape proximal and distal to joint to ensure consistent distance from nozzle to the finger or hand. It is absolutely essential that the child maintains the proper finger position throughout the testing session. Ensure this by periodically assessing finger and hand placement and having child with caregiver if young. If testing an infant, stop protocol if infant cries and provide comfort before restarting. If testing young child, ask caregiver to provide comfort and reassurance throughout the short testing period.
3. Start air compressor at 40 psi through regulator to supply valve inputs for tactile stimuli.
4. Run stimulus delivery program.
   1. For the tested hand, present 60 puff stimuli randomly interspersed with 60 sham trials (an air puff delivered via a separate nozzle pointed away from the finger).
   2. Do not present more than two repetitions of a puff or sham in a row. Vary the inter-trial intervals randomly between 2,000-2,500 msec. The purpose of this is to reduce habituation, where a stimulus is no longer perceived. The total time for a sequence of 120 trials should be 4.5-5 min.
   3. Run the identical protocol again for the other hand if studying asymmetrical somatosensory disorders.
5. For protocols not requiring attention to stimulus no further set up is needed. This applies to infant testing. For enhancement of attention in young children (which results in larger specific ERP peaks in recording), provide a task.
   1. Task example for 5-year-olds: Describe air puffs as "bubbles" blown by "fish" in a "fish tank" (a decorated box concealing the puffer apparatus). Ask children to guess whether each "bubble" is delivered by a blue or a red "fish". Tell the child that they do not need to and should not say anything while they are performing this task (see set up with mock aquarium in Figure 1).

2. Assessment of Response to Multisensory Protocol (Auditory-tactile Simultaneous vs. Summed Individual Responses)

1. Run through steps 1.1-1.3 as described above. Stimuli are described in Table 1.
2. Run the stimulus delivery program (e.g. in E-Prime software). For the tested hand, an auditory-tactile paradigm can present the following 4 stimuli randomly, with 60 trials/ stimulus: puff , puff-/ga/, /ga/-sham, sham. Again, to limit the possibility of habituation, do not present more than two repetitions of a puff or sham in a row in any condition, and vary the inter-trial intervals randomly between 2,000-2,500 msec. Each sequence of 240 trials should take between 9-10 min.
3. Run identical protocol over for the other hand.
4. Provide a soundless age-appropriate cartoon at initiation of protocol and continue it throughout the procedure to prevent increase in motor artifacts from restlessness, and to decrease the background from large patient-generated delta waves when they are bored. For example, in 5-year olds, we used a loop of 20 min of a purchased video, played on mute and restarted before each subject was tested. No attention to stimulus is needed, therefore the looped cartoon provides a visual background disconnected from the stimuli.

3. Software and Equipment Set Up

1. To program the software, set up two serial commands sent by the stimulus control application. One identifies the puff, the other the sham. Have the stimulus control application send the commands to a microcontroller.
2. Have the microcontroller generate a TTL pulse (e.g. 20 msec duration) to the corresponding digital output channel. This output must be split into two lines, one for the digital input to the EEG recording system and one to the solenoid-gated air valves. Mark the opening of both valves in the EEG data stream.
3. Measure the pulse to puff latency for both real and sham conditions with an oscilloscope and a microphone. These should be uniform, and in the order of 10-15 msec. Adjust for the latency post-recording.
4. Calculate the force exerted at the nozzle in PSI using a manometer and by measuring the nozzle diameter. Use the formula F(N) = Pressure*Area. For example, the force exerted from a 1 mm radius nozzle at 6 psi yields F(N) = 0.03 lbs.
5. To modify the control application for the multisensory protocol, send two serial commands identifying a real puff or sham to the microcontroller as well as a recorded speech sound or silence. Note: In our paradigm we have used the computer generated, accent-neutral /ga/ sound, among others such as /da/, /du/, /bu/, etc.
6. Present auditory stimuli through a speaker placed at midline, 2 ft in front of the subject.
7. Align the sound onset timing to be simultaneous with the onset of the puff or with the delay measured in step 3.3, depending on which condition is desirable to the tester.

4. Data Acquisition and Preparation

1. Choose filters and references settings to sample data based on standard ERP methodologies. Here, use a 1,000 Hz with filters set to 0.1-400 Hz. During data collection, refer all electrodes to Cz and rereferenced them offline to an average reference.
2. To segment the data, filter the recorded data with desired filters and segmenting. For this study use a 0.3-40 Hz bandpass filter and segment the ongoing EEG based on the stimulus onset to include a 200 msec prestimulus baseline and a 500 msec post-stimulus interval.
3. Perform quality control of the data. Screen each segment for motor and ocular artifacts such as high frequency muscle activity, using computer algorithms included in the ERP software. Follow this screen by a manual review.
4. The automated screening criteria are set as follows in this protocol but can be modified: for eye channels, voltage >140 µV = eye blink and voltage >55 µV = eye movements.
5. Correct data from contaminated trials using an ocular artifact correction tool. Note: Any channel with voltage >200 µV is considered of poor quality. If >15 channels are of poor quality, we chose to discard the entire trial for reproducibility reasons.
6. Average ERPs. Rereference them to an average reference and then perform baseline-correction based on criteria chosen in step 4.2. Extract mean amplitude and peak latencies for various peaks, extrapolated from grand average waveforms of predefined populations. Note: In our case, we based the following on established literature of older children's response to median nerve stimulation^10-14. We used P50 (30-80 msec), N70 (50-100 msec), P100 (80-150 msec), N140 (130-230 msec), and P2 (250-350 msec) peaks.
7. Include only data from electrodes overlapping preset locations (Figure 2). Derive data for individual electrodes and average within each cluster.

Results

Assessment of light touch (Figure 3):

Characteristics of the cortical response to tactile stimulation using the Puffer system: The patterns of peaks in response to the puff are very similar to the cortical responses obtained using median nerve stimulation in normal adults^10,11. The early response (P50, N70, P100 peaks) primarily reflects activity in the primary sensory cortex¹² and does not require awareness of stimulation....

Discussion

This novel combination of air puff and ERP (referred to as the "Puffer system") to measure cortical processing of light touch and tactile-auditory responses is well tolerated by young children with disabilities and by infants. This holds true for unisensory and multisensory versions, and whether the attentional component is added or not in the case of young children. The reasons for the success of this methodology in assessing a young and vulnerable population are due to both the use of an innocuous tactile stimu...

Materials

Name	Company	Catalog Number	Comments
Geodesic sensor net	EGI, Inc., Eugene, OR	depends on size
Net Station EEG software v. 4.2	EGI, Inc., Eugene, OR	NA
E-Prime stimulus control application	PST, Inc. Pittsburgh, PA	NA
Manometer (model 6 in, 0-60 psi)	H. O. Trerice Co, Oak Park, MI
Custom Puffer setup	Nathalie Maitre