Measuring the brain’s response to temporally modulated sound stimuli Chloe Rose Institute of Digital Healthcare, WMG, University of Warwick, INTRODUCTION METHODS AND MATERIALS RESULTS REFERENCES ABSTRACT CONTACT Figure 2:ASSR recording apparatus An EEG recording taken via electrodes fixed onto scalp by an electrode cap. Subject asked to detect amplitude modulation and respond via the keyboard. Chloe Rose, Institute of Digital Healthcare, WMG, University of Warwick, Coventry, CV4 7AL, United Kingdom. An objective predictive measure of how well an individual perceives certain features of sound would be a great asset to the field of audiometry. In this experiment, the idea behind this predictive measure was explored. The overall aim was to identify on an EEG when a subject has correctly detected an amplitude modulated noise and when they haven’t. Nine subjects were asked to take part in an amplitude modulation detection task.First of all their threshold values were detected which was fixed as the modulation depth and then the percentage correctly detected of the amplitude modulated noises(modulated at 40Hz)was measured whilst an EEG recording was taken. Sensitivity limitations of the statistical tests used meant we were unable to identify a subjects perception of the modulated noise. However a significant peak at 40Hz on the EEG was detected in response to the amplitude modulated noise corresponding to auditory nerve fibers firing at the modulated frequency. Table 1 The threshold values vary from person to person due to differences in auditory systems and therefore sensitivity of amplitude modulation. Percentage correct varied between subjects, our aim was to obtain 75% correct. Reasons why this was not achieved may be due to the long duration of the experiment (1 hour 40 minutes excluding preparation) The subject’s concentration levels may have fallen throughout the experiment or the pace that the subject learnt how to perform the task may have varied. To eliminate these variables we could reduce the time course of each test by increasing the amount of tests and reducing the duration of each test in order to reduce falling concentration levels but still obtain the required information needed to analyse the electrophysiology data. Also, we could include a practice run before the experiment begins to allow the subject time to become familiar with the task. Figure 2 Discriminating the ASSR to an AM noise from the brains response to noise alone in the time domain(fig 2a) was difficult, so we looked at the data in the frequency domain(fig 2b). A significant peak at 40Hz(labelled with a red cross in fig 2b)to the AM noise demonstrates that the neurons in the auditory nervous system have increased their rate of discharge at the modulation rate of the stimulus. A 40Hz peak is absent in the noise alone data. A peak at 50Hz can be seen in the AM noise spectrum, this can be ignored as it corresponds to line noise. Conclusion We’ve demonstrated through an electrophysiological study that the brain responds to an amplitude modulation rated noise at 40Hz, but we haven’t established a link between a subject’s perception of the noise and the EEG recording,as the statistical test used wasn’t sensitive enough. Further research may include exploring other parameters such as stimulus levels which would increase the ASSR amplitude by increasing the firing of auditory nerve fibres and reducing noise levels which in turn could improve the significance of the 40Hz peak. Threshold detection – Threshold value for each subject identified Stimuli: For every test, two narrowband noises of length 2 seconds played having these parameters;stimulus intensity 70dB, carrier frequency 2-4kHz.One noise modulated at 40Hz with a modulation depth of the subjects threshold,the other noise left unmodulated. One experiment included 20 tests, each test comprising of 50 modulated and unmodulated noises. Perception task: To identify the modulated from the unmodulated noises via the pressing 1 or 2 of the keyboard corresponding to which of the two sounds appeared rougher( the rougher sound being the modulated of the two.) Sound presented through ear inserts. ASSR recording: EEG activity recorded by detecting the difference between the reference and the mastoid, the ground placed on the forehead. An inter-electrode impedance kept below 5kΩ. The ability to detect fluctuations in amplitude of sound appear to play a great part in how well we perceive speech [1].We can measure an individuals sensitivity to these fluctuations, using a perception task that determines their Amplitude modulation(AM) threshold. The threshold can be defined as the smallest amplitude modulation that we can impose on a narrowband carrier and still hear and detect when we compare with an unmodulated noise., figure 1 presents an image of both a modulated and unmodulated sound. However in infants under 5 or 6 months of age, behavioral hearing tests can’t reliably judge hearing threshold. Therefore objective measures are recommended such as the auditory steady state response(ASSR). [2] An ASSR is an auditory evoked potential that is elicited with modulated tones that follows the envelope of the stimulus. [3] Evoked potentials(EP) are small electrical changes which can be recorded via an EEG from the scalp using electrodes following a sound. Recording EP’s can help us understand auditory perception and evaluate hearing disorders in patients. [4] The area linking electrophysiological and behavioral studies of the auditory system has not been extensively researched and therefore was a matter that we aimed to explore. [1]Fitzgerald et al. (2011) Acoustical Society of America; 129(2): [2]Rodrigues et al. (2010) International Journal of pediatric Otorhinolaryngology(2010) [3] 3.aspx [4] Terence W.Picton. Human Auditory Evoked Potentials (2011) p1 [5] Figure 2: Electrophysiology recording in Time domain(figure 2a) and in frequency domain(figure 2b) An average of 1000 tests within 20 experiments for one subject. Figure 2a- Red box presents the ASSR when presented with an amplitude modulated noise(blue) and noise alone(green) To the left of red box shows the onset and to the right the offset of the noise stimuli. Figure 2b- Spectrum of brain’s response to an AM noise(top panel) and unmodulated noise(bottom panel). Table 1: Perception based task summary Threshold value of each subject( middle panel) and when combined with an EEG, their percentage correct (right panel) from the amplitude modulation detection task using their threshold value as a fixed modulation depth DISCUSSION Figure 1: Amplitude modulation detection task [5] Subject was asked to identify the amplitude modulated sound (b)(c) from an unmodulated one.(a) a b c 2a 2b