|Year : 2017 | Volume
| Issue : 2 | Page : 83-88
Effect of inter-stimulus interval on the acoustic change complex elicited with tone-complex and speech stimuli
Mohan Kumar Kalaiah, Anjali Jude, Vinaya Palora Malayil
Department of Audiology and Speech Language Pathology, Kasturba Medical College, Manipal University, Mangalore, Karnataka, India
|Date of Web Publication||14-Jun-2017|
Mohan Kumar Kalaiah
Department of Audiology and Speech Language Pathology, Kasturba Medical College, Attavara, Mangalore - 575 001, Karnataka
Source of Support: None, Conflict of Interest: None
Background: Acoustic change complex (ACC) is an auditory-evoked cortical potential elicited for a change in the physical properties of the sound. It can be elicited for a change in frequency, amplitude, or periodicity in the ongoing speech and nonspeech stimuli. Earlier investigations have established a significant effect of stimulus-related factors on the cortical auditory-evoked potentials. However, investigations to understand the effects of stimulus factors on the ACC are rare. Purpose: This study aimed to investigate the effect of inter-stimulus interval (ISI) between the stimuli on the ACC elicited for speech and tone-complex stimuli. Research Design: This is a cross-sectional study. Method: A total of 12 young adults aged between 18 and 22 years with normal hearing in both ears participated in the study. The ACC was elicited using a consonant-vowel syllable/sa/and frequency-changing tone-complex. Tone-complex had an onset frequency of 1000 Hz and changed its frequency to 2000 Hz at 120 ms after the onset. Both stimuli had a total duration of 350 ms and changed their characteristics at 120 ms after the onset. The stimulus was presented monaurally with ISIs of 500, 1000, 1500, and 2000 ms, and the ACC was recorded from Cz to Fz. Results: The present study showed a significant effect of ISI on peak latencies and peak-to-peak amplitudes. Shorter ISI was found to elicit peaks with smaller amplitude and slightly longer latency, whereas longer ISI elicited peaks with larger amplitude and shorter latency. Further, the amplitude of response for tone-complex was larger than response for syllable/sa/. Conclusions: ISI is one of the stimulus-related factors which significantly affects the ACC. Longer ISIs (≥1500 ms) should be used to elicit the ACC.
Keywords: Acoustic change complex, auditory-evoked potentials, cortical auditory-evoked potentials, inter-stimulus interval, speech, tone-complex
|How to cite this article:|
Kalaiah MK, Jude A, Malayil VP. Effect of inter-stimulus interval on the acoustic change complex elicited with tone-complex and speech stimuli. Indian J Otol 2017;23:83-8
|How to cite this URL:|
Kalaiah MK, Jude A, Malayil VP. Effect of inter-stimulus interval on the acoustic change complex elicited with tone-complex and speech stimuli. Indian J Otol [serial online] 2017 [cited 2020 Aug 13];23:83-8. Available from: http://www.indianjotol.org/text.asp?2017/23/2/83/208034
| Introduction|| |
Acoustic change complex (ACC) is a cortical auditory-evoked potential (CAEP) elicited for a change in the physical properties of the sound. It is an obligatory P1-N1-P2 response elicited for a change in an ongoing stimulus. It can be elicited for a change in frequency,, amplitude, or periodicity  in the ongoing stimuli. Further, it can be reliably recorded in young children, individuals with hearing loss, and among individuals using hearing aids and cochlear implants. In addition, several investigations have found a good agreement between the ACC and behavioral thresholds of intensity discrimination, frequency discrimination, and temporal resolution. Recently, the ACC is gaining popularity among researchers, and results of several investigations suggest that it could serve as an objective tool for measuring speech discrimination capacity and supra-threshold auditory processing. Further, the ACC shows an excellent test-retest reliability within and between the sessions, at the level of individual participants, making it suitable for clinical use.
Stimulus-related factors such as stimulus type, frequency, intensity, stimulus duration, and inter-stimulus interval (ISI) have effect on the CAEPs. Several studies have examined the effect of these stimulus factors on onset P1-N1-P2 response, and their effects on the onset P1-N1-P2 response are well understood. In contrast, there are limited investigations to understand the effect of stimulus factors on the ACC. Investigations eliciting the ACC for frequency change, intensity change, and gaps in noise reveal the effects of magnitude of change on the ACC. These investigations have shown systematic changes for the latency and the amplitude of the ACC with the magnitude of change for frequency, intensity, and gaps in noise. The ACC is larger in amplitude and shorter in latency for greater change in the stimulus, while its amplitude decreases and latency slightly increases with decrease in the magnitude of change. Dimitrijevic et al. elicited the ACC for frequency changes using continuous tones of 250 and 4000 Hz, and found a differential effect of frequency on the latency and amplitude of the ACC. Similar results were found using tones of 500 and 3000 Hz. Results of the above investigations show a drastic reduction in the latency and increase in the amplitude of the ACC, for low-frequency tones compared to high-frequency tones. In contrast, frequency had no significant effect on the latency and amplitude of the ACC for intensity changes, for tones of 250, 1000, and 4000 Hz. Based on the above studies, it may be concluded that the magnitude of change and stimulus frequency have effect on the ACC.
ISI is another stimulus-related factor which has a significant effect on the CAEPs. It has a significant effect on the morphology of the onset P1-N1-P2 response in children. Among adults, the latency and amplitude of the onset P1-N1-P2 response change systematically with the ISI. Studies investigating the effect of ISI on the ACC are rare. Further, since the ACC is an obligatory response similar to the onset P1-N1-P2 response, it could be possible that the effect of ISI on the ACC is same as that on the onset P1-N1-P2 response. Martin et al., to investigate the efficiency of different stimulus presentation strategies elicited the ACC with ISIs of 1 s and 2 s. The results showed reduced amplitude of the ACC for shorter ISIs. This finding suggests that the ISI has an effect on the ACC. To conclude, studies examining the effects of ISI on the ACC are rare and its effects on the ACC are not well understood. Hence, the present study was planned to investigate the effects of ISI on the ACC.
Martin et al. investigated the effect of ISI on the ACC using synthetic vowel /u/, which had a frequency change of 1000 Hz for second formant frequency. This change in the stimulus resulted in change in the perception of vowel from /u/ to /i/. In contrast to the above investigation, we have used consonant-vowel syllable (which is more realistic signal in real-life situation) to investigate the effect of ISI on the ACC. In addition, we also investigated the effect of ISI on the ACC using nonspeech stimuli, pure-tone with a frequency change (termed as tone-complex). By using speech stimuli and tone-complex, we can examine the effects of ISI on stimulus complexity. Further, since the ACC is gaining popularity among the researchers, there is a need to understand the effects of various stimulus-related factors on the ACC. Thus, in the present study, we investigate the effects of ISI on the ACC using simple and complex stimuli.
| Methods|| |
A total of 14 young adults (5 males and 9 females) aged between 18 and 22 years (mean = 19.4 years) participated in the study. All the participants had hearing thresholds <15 dB HL in both ears at octave frequencies between 250 and 8000 Hz. None of them had history of otologic or neurologic problems, exposure to ototoxic medications or hazardous noise, and speech understanding problems. Ethical approval was obtained from the Institutional Ethics Committee to carry out the present study. Informed consent was obtained from all the participants, prior to their participation.
The ACC was elicited using a consonant-vowel syllable /sa/ and frequency-changing tone-complex. The syllable /sa/ used in the present study was the same signal used to elicit the ACC in an earlier investigation. The total duration of syllable /sa/ was 350 ms, with duration of fricative portion /s/ was 120 ms and duration of following vowel /a/ was 230 ms. Tone-complex was generated by concatenating two short duration pure-tones (1000 Hz pure-tone of 120 ms and 2000 Hz pure-tone of 230 ms). Onset and offset of the tone-complex were windowed (using 10 ms Hamming window) to reduce spectral splatter. [Figure 1] shows spectrogram of tone-complex and consonant-vowel syllable /sa/ used to elicit the ACC.
|Figure 1: Spectrogram of the stimuli used to elicit the acoustic change complex. Top panel shows spectrogram of tone-complex, and bottom panel shows spectrogram of consonant-vowel syllable /sa/|
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Acoustic change complex
The ACC was recorded using IHS Smart EP version 3.92 (Intelligent Hearing Systems, Florida, USA)-evoked potential system. When recording CAEPs, participants were seated on a reclining chair in comfortable position. Electroencephalogram (EEG) was differentially recorded from scalp using gold-plated disc electrodes. Noninverting electrodes were placed on vertex (Cz) and frontal lobe (Fz), inverting electrode was placed on test ear mastoid (A2), and ground electrode was placed on the nontest ear mastoid (A1). Electrode impedance was maintained below 5 kΩ for each electrode, and the inter-electrode impedance was <2 kΩ. The ACC was elicited by presenting the stimuli monaurally to the right ear of the participants at 80 dB SPL, using ER-3A insert phones. The stimulus was presented at repetition rates of 0.42/s, 0.54/s, 0.74/s, and 1.17/s, which result in ISIs of approximately 2000, 1500, 1000, and 500 ms, respectively. The ongoing EEG was amplified 50,000 times and filtered using a band-pass filter of 1–30 Hz. A total of 300 artifact-free sweeps were obtained and averaged to obtain averaged waveform, and whenever the amplitude of ongoing EEG was >50 μV, it was rejected from averaging. The ACC was recorded using an analysis window of 800 ms, with a prestimulus duration of 100 ms. To check reliability of the response, the ACC was obtained twice and only those peaks which are replicable were considered as response. All the recordings were carried out in a sound-attenuating, electrically shielded room. To minimize eye blinks and head movements, participants were made to watch a close-captioned video of their choice. Short interval of rest period was provided between the recordings (5–10 min) as required by the participants.
Grand-averaged waveforms were obtained by averaging waveforms of all the participants, separately for two stimuli. These waveforms showed that both stimuli elicited obligatory P1-N1-P2 complex for stimulus onset and the ACC for change in the stimulus, as expected. Based on the latency of various peaks of P1-N1-P2 complex and the ACC in the grand-averaged waveforms, peaks were identified in the waveforms of individual participants. Latency of peaks P1, N1, P2, P1', N1', and P2' and peak-to-peak amplitude for P1-N1, N1-P2, P1'-N1', and N1'-P2' were obtained at each ISI for each participant. Latency and amplitude data were subjected to repeated measure ANOVA to check for the presence of any significant difference between the measures across the conditions and stimuli. At 500 ms ISI condition, since the ACC was not elicited in all the participants, it was not considered for statistical analysis.
| Results|| |
The findings of the present study showed that the ACC was present in all the participants for both stimuli at longer ISIs, i.e., 2000, 1500, and 1000 ms. While at an ISI of 500 ms, it was elicited in 92.8% of the participants for tone-complex and in 92.8% and 85.7% of the participants for syllable /sa/ at ISIs of 1000 and 500 ms, respectively. Further, the morphology of the ACC was found to be better when elicited using longer ISIs, longer than or equal to 1000 ms. [Figure 2] shows grand-averaged waveforms of the ACC across ISIs for tone-complex and syllable /sa/. In the figure it can be noted that, tone-complex elicits larger amplitude response compared to syllable /sa/ across the ISIs. In addition, the waveforms show gradual decline in the amplitude of response with decrease in ISI for both the stimuli.
|Figure 2: Grand averaged waveforms (black) of the acoustic change complex for tone-complex (left panel) and syllable /sa/ (right panel). Gray lines represent averaged waveforms of individual participants. Waveforms on top row represent acoustic change complex elicited with inter-stimulus interval of 2000 ms, second row for 1500 ms, third row for 1000 ms and bottom row for 500 ms|
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[Figure 3] shows mean latency of peaks P1, N1, P2, P1', N1', and P2' for tone-complex and syllable /sa/ across the ISIs. In the figure it can be noted that, the latency of all the peaks decreases systematically with increase in the ISI, for both tone-complex and syllable /sa/, except for P1 of tone-complex. Latency is longer for 500 ms ISI condition, and slightly decreased with increase in the ISI. In addition, the latency of all the peaks, except P1, are slightly shorter for tone-complex compared to syllable /sa/. To investigate if the latency of peaks are significantly different across the stimuli and ISIs, repeated measure ANOVA was carried out with stimuli (tone-complex and syllable /sa/) and ISIs (1000, 1500, and 2000 ms) as repeated measures. Result showed, a significant effect of ISIs on the latency of P1 (F [3,24] = 5.365, P= 0.022), N1 (F (3,30) = 30.356, P< 0.001), P2 (F (3,27) = 27.359, P< 0.001), P1' (F [3,12] = 12.109, P= 0.015), and N1' (F [3,12] =15.225, P< 0.001). Post hoc analysis using Bonferroni test showed mean latency to be significantly different across the ISIs for peaks P1, N1, and P2, except for intervals between 500 and 1000 ms of P2. Mean latencies were not significantly different across the ISIs for P1', N1', and P2'.
|Figure 3: Left panel shows mean latency and standard deviation (±1 standard deviation) for peaks P1, N1and P2. Right panel shows mean latency and standard deviation (±1 standard deviation) for peaks P1', N1' and P2'. Filled symbols with solid lines represents latency of peaks for syllable /sa/, and unfilled symbols with dashed lines represent latency of peaks for tone-complex|
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[Figure 4] shows mean peak-to-peak amplitude of P1-N1, N1-P2, P1'-N1' and N1'-P2' for both stimuli across ISIs. In [Figure 4] it can be noted that, the peak-to-peak amplitude gradually reduced with decrease in ISI. To investigate whether the mean peak-to-peak amplitudes are significantly different across ISIs and stimuli, a repeated measure ANOVA was carried out with stimuli and ISI as repeated measures. It revealed a significant effect of ISI on the amplitude of P1-N1 (F [3,18] = 23.896, P< 0.001), N1-P2 (F [3,18] = 29.892, P< 0.001), P1'-N1' (F (3,30) = 61.392, P< 0.001) and N1'-P2' (F (3,33) = 14.256, P< 0.001). Stimuli had a significant effect on the amplitude of N1-P2 (F [1,6] = 9.312, P= 0.022), P1'-N1' (F [1,10] = 11.472, P< 0.007), and N1'-P2' (F [1,11] = 67.813, P< 0.001). In addition, there was a significant interaction between stimuli and ISI for amplitude of P1'-N1' (F (3,30) = 7.306, P= 0.001) and N1'-P2' (F (3,33) = 3.8, P= 0.019). Pairwise comparison across the ISIs revealed presence of significant difference, for P1-N1 and N1-P2 amplitude, across all the intervals except for ISIs between 2000 and 1500 ms, and 1000 and 500 ms. For P1'-N1' and N1'-P2' amplitude, since there was a significant interaction between stimuli and ISI, separate repeated measure ANOVA was carried out with ISI as repeated measures, for two stimuli. Result showed a significant effect of ISI on P1'-N1' and N1'-P2' amplitude for both stimuli. Pairwise comparison revealed significant difference for P1'-N1' and N1'-P2' amplitudes across all the ISIs for tone-complex, except for N1'-P2' amplitude between 1500 and 2000 ms; while for syllable /sa/ amplitudes were significantly different only for ISIs between 500 and 2000 ms, 1000 and 2000 ms, and 500 and 1500 ms.
|Figure 4: Mean amplitude and standard deviation (±1 standard deviation) for peak-to-peak amplitude of P1-N1 (top left panel) N1-P2, P1'-N1' and N1'-P2'. Filled symbols with solid lines and unfilled symbols with dashed lines represents amplitude for syllable /sa/ and tone-complex respectively|
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| Discussion|| |
The present study showed a significant increase in the amplitude of obligatory P1-N1-P2 response (P1-N1 and N1-P2) with an increase in the ISIs for both stimuli. In addition, ISI had a significant effect on the latencies of peaks (N1 and P2) for both stimuli. Here, although the latencies are significantly different statistically, clinically this difference may not be significant. These findings in the present study are in consonance with the findings of several investigators.,,,,, Reduced amplitude and prolonged latencies of peaks at short ISIs may be attributed to neural refractoriness ,, or habituation/inhibition,, which could decrease the response through negative feedback mechanisms, for a repeating stimulus. Longer ISIs leads to availability of longer recovery periods for neurons to overcome refractory effect, thus allowing the neurons to respond with higher discharge rates, resulting in larger amplitude response. Comparison of P1-N1 and N1-P2 amplitudes across the ISIs showed the amplitude growth to be faster for N1-P2, for both stimuli. In consonance to the findings of present study, earlier investigations have found greater amplitude growth for N1-P2 using tones , and speech stimuli. Further, results of the present study showed larger amplitude P1-N1-P2 response for tone-complex compared to syllable /sa/. This finding is comparable to the results of earlier investigation, which has also shown large amplitude response for tone-complex. But, in contrast to the findings of present study, several investigations have found larger amplitude response for speech stimuli compared to tones., Smaller amplitude response for syllable /sa/, in the present study, could be attributed to smaller amplitude of fricative portion /s/, which elicits onset P1-N1-P2 response. In addition, the amplitude differences could be attributed to differences in the onset portion of syllable /sa/ and tone-complex.,
Detectability of the ACC was found to decrease with reduction in the ISI between stimuli, for both tone-complex and syllable/sa/. Similar finding has been reported for N1-P2 complex of the obligatory P1-N1-P2 response by earlier investigations among children. Reduced detectability of response at shorter ISIs, among children, has been attributed to lack of complete maturation in the auditory system. Incomplete myelination and synaptogenesis in the immature central auditory system leads to reduced neural synchrony of the generators and longer refractory periods, resulting in reduced detectability of the response. In adults, the maturation of central auditory system would be complete, thus reduced detectability of response at shorter ISI could be attributed to refractoriness of the neurons. Further, the results showed a systematic effect of ISI on the amplitude of P1'-N1' and N1'-P2' of the ACC for both stimuli, while the latency of peaks was not significantly different. These findings in the present study for the ACC is similar to the findings obtained for the onset P1-N1-P2 response. Investigations eliciting the ACC using different ISIs have also found larger amplitude response for longer ISIs. Smaller amplitude and longer latency for the ACC at shorter ISI could be attributed to refractoriness of the neurons.
Davis et al. investigated the effect of ISI on the onset P1-N1-P2 response for 2.4 kHz tone pips. Results of their study found an increase in the N1-P2 amplitude with increasing ISI. Similar findings have been reported for pulsed pure-tones of 500, 1000, and 2000 Hz  and 1000 Hz tones. The amplitude of the onset P1-N1-P2 response was found to increase with increasing ISI up to 8 s. On the other hand, stimulus frequency has no effect of on the amplitude of P1-N1 and N1-P2, across the ISIs. Similar finding has been reported using speech syllable /uh/, N1-P2 amplitude of the onset P1-N1-P2 response increased with increasing ISIs. In consonance to the findings of above investigations, the present study found an increase in the amplitude of the P1-N1-P2 response for both stimuli. However, the amplitude of the response for tone-complex was found to increase rapidly compared to syllable /sa/.
| Conclusions|| |
The present study showed a significant effect of repetition rate on the latency and amplitude of ACC. For shorter ISI, latency of peaks was slightly prolonged, while amplitude of response was greatly reduced. In addition, detectability of the response was also affected by the ISI. ISI of 1000 ms was found to elicit the ACC in all the participants for tone-complex, while for speech stimuli longer interval (>1000 ms) was required. Based on these findings in the present study, it may be recommended to use a longer ISI (≥1500 ms) to elicit the ACC.
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