Audio-spinal reflex response in human limb musclesLuxon, S., Blinch, J., Rurak, C., Blouin, J.-S., Chua, R., Inglis, J.T.

School of Kinesiology, University of British Columbia, Vancouver

Introduction

PurposeThe purpose of this study is to investigate whether repeated auditory stimuli less than 124 dB can induce EMG responses in upper and lower limb human musculature among participants in the supine position.

Method

Eleven healthy subjects (8M, 3F) age 20-29 lay supine with eyes closed while making low level contractions (~10-15% MVC) of right first dorsal interosseus, bilateral biceps, and bilateral soleus

Both elbows were placed at a 45o angle to the horizontal, both index fingers were pointed straight with a 20 g weight suspended via an elastic loop, and both feet were pushing flat against a wooden board such that the ankle angle was 90o

Auditory stimuli (1000 Hz, 40 ms duration, 3-5 s inter-stimulus interval) were presented through earphones

AnalysisEMG data collected from all muscles were rectified and averaged for a mean at 115 dB and 95 dB for each subject. Trials exhibiting SCM activity above baseline were used as an indicator for the startle response, and were subsequently removed to prevent a potential confounding effect. The first trial was removed in each block due to insufficient pre-stimulus data. A response was considered significant if it was greater than two standard deviations from the pre-stimulus baseline.

Results: Proximal to distal modulation

Summary and Conclusion

Results: Response in upper and lower limbs

EMG recordings of the SCM, right biceps, right FDI, and right soleus for a single participant in each experimental condition (115 and 95 dB) are shown below. Figures (a) (b) and (c) show the rectified average of 5, 50, and 99 trials, respectively. The rows alternate 115 and 95 dB for each muscle listed above. Note the minimal response elicited at 95 dB when compared to the response at 115 dB. For this reason further analysis was conducted on only the responses elicited at 115 dB.

EMG modulation with one or more significant peaks greater than two standard deviations from the pre-stimulus baseline in both biceps and soleus was observed in 7 of 11 participants. The surface EMG recordings for each participant are shown below. The top and bottom rows represent right biceps and right soleus respectively, averaged over 99 trials (the first block with the first trial removed) at 115 dB.

S1 S2 S3 S4 S7 S9 S10 Biceps

Soleus

Biceps

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Time (ms)

Surface EMG recordings of the right biceps, right FDI, and right soleus demonstrate a proximal to distal muscular modulation in 7 of 11 subjects as quantified by the time to the first significant peak. Results indicate an increase in time to peak from biceps to FDI, and biceps to soleus, by an average of 19.8 ms (SD= ± 15.6 ms) and 21.6 ms (SD= ± 17 ms), respectively. Our results of a disproportionately long time to peak for FDI are in agreement with results of Brown et al. (1991).

The Natural Sciences and Engineering Research Council of Canada supported this study.

1Brown et al., J Neurol Neurosurg Psychiatry (1991) 114, 1891-1902

Task

Surface EMG was collected from sternocleidomastoid (right; SCM), first dorsal interosseus (right; FDI), bilateral biceps , and bilateral soleus

EMG signals were amplified 20 000 x and bandpass filtered: 30 – 1000 Hz (Grass P511), sent to Micro 1401 (CED) and sampled at 5208 Hz

Collected with Spike 2 software, analyzed in Matlab 2007

A single trial of 124 dB was presented before and after six blocks of 100 trials

Two blocks of 95 dB and two blocks of 115 dB were counterbalanced with 1-2 minute rest intervals between each block

Experimental Conditions

Measurement

1. A muscular response persists with repeated auditory stimuli consistently at 115 dB.

2. The mean peak onset of EMG modulation is observed earlier in the biceps than in the soleus at stimulus intensities of 115 dB. This demonstrates a proximal to distal pattern of activation.

3. Auditory stimuli can elicit both upper and lower limb EMG reflex responses in habituated participants. Furthermore, these auditory-evoked lower limb EMG responses were identified in supine participants and may provide a new tool to clinically assess spinal cord integrity.

Summary

Conclusion

Time (ms)

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Results for time to the first significant peak at an intensity of 115 dB for a single participant are shown here. The rectified average of 99 trials is shown for the right biceps, FDI, and right soleus, with corresponding time to peak latencies of 62.6, 75.3, and 77.4 ms respectively.

The startle response associated with brief (40ms) but loud (124 decibels, 1000 Hz) auditory stimuli is known to evoke electromyographic (EMG) activity in human axial and lower limb musculature.1 The induced muscular response has been shown to rapidly attenuate with repeated exposure, termed habituation, and is characterized by a decreased muscle response with each subsequent stimulus.1 Many studies have investigated the startle response using an acoustic stimulus of 124 decibels (dB) in various paradigms, but very little is known about acoustic stimuli less than 124 dB that do not induce a stereotypic startle response. To date, it is unknown if an audio-spinal response will exist at acoustic stimuli less than 124 dB in minimally activated human musculature. Furthermore, if an audio-spinal response does exist, it is unknown whether this response will also habituate with repeated auditory stimuli.

(d) Trials 101 - 121 of 700 (e) Trials 673 - 693 of 700

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(a) 5 trials (b) 50 trials (c) 99 trials

Surface EMG recordings of the right biceps for a single participant are shown below. A total of 700 trials (7 blocks of 100 trials each) at 115 dB were completed. Figures (d) and (e) show the rectified average of the first and last 20 trials of block 2 and 7 respectively, excluding the first trial of each block.

Biceps

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0 100 200 Time (ms)

Results: Persistent response at 115 dB

Results: Amplitude is consistent