Pitch and rhythmic pattern discrimination of percussion instruments for cochlear implant users

Transcription

1 PROCEEDINGS of the 22 nd International Congress on Acoustics Psychological and Physiological Acoustics (others): Paper ICA Pitch and rhythmic pattern discrimination of percussion instruments for cochlear implant users Federico Nahuel Cacavelos (a), Ricardo L. Marengo (b) Shin-ichi Sato (c) Florent Masson (d) (a) Universidad Nacional de Tres de Febrero, Argentina/Universidad de San Buenaventura Medellín, Colombia, (b) Grupo CIAC, Argentina, (c) Universidad Nacional de Tres de Febrero, Argentina, (d) Universidad Nacional de Tres de Febrero, Argentina, Abstract Cochlear implants are mostly designed for speech understanding of people with hearing impairments. The appreciation of music by cochlear implant (CI) users is still under study. This work investigates the capability of CI users to recognize impulsive pitch sounds. It focuses on the discrimination of rhythmic patterns using percussion instruments samples from a kick, a snare and a ride cymbal. These signals were tuned, without changing the natural sound, into an easily recognizable pitch for CI users and were presented with different rhythmic patterns. An ABX test was carried out with two groups: 30 normal hearing (NH) subjects and 7 people with CI. The test was divided into two sessions. First, the same instrument was used for the whole pattern of each stimulus and the three instruments were compared to each other. Second, the samples of the three instruments were combined in each stimulus and the instrument of only one sample was varied and compared. Results were compared with previous studies which used the continuous tones as test signals. They showed that both groups can distinguish rhythmic differences. However NH subjects can easily recognize patterns with different sample while the CI users have much more difficulties than NH users to discriminate pitch difference in combined samples composed by different percussion instruments sounds. Keywords: Cochlear implant, subjective test, pitch, impulsive, discrimination.

2 Pitch and rhythmic pattern discrimination of percussion instruments for cochlear implant users 1 Introduction Music perception of Cochlear Implant users (CI) has been under study for more than 25 years. Most of the studies investigated the ability of melodic contour identification and timbre recognition, as well as emotional communication, rhythm and meter recognition [1]. The central auditory processing in human being has two possible mechanism to perceive pitch. The temporal theory considers that the perception of time periods smaller than 1 ms enabling people to perceive frequencies up to 1 khz. This is essential to recognize the fine structure of signals. For CI users this mechanism can be hampered by the low stimulation rates used in some coding strategies. The other mechanism used by the central auditory processing is the place theory. It describes how resonances of the basilar membrane produce a spatial arrangement of frequencies, commonly called the tonotopic distribution [2]. The CI cannot stimulate two sectors of the cochlea simultaneously due to electrical interference between electrodes. Therefore the codification strategies keep just the signal s peaks of a period, discarding the weak intensity. This is why CI users have difficulty to detect fine structure of pitch information and impulsive sounds. Cognitive factors may also disturb the identification process. Disturbance Auditory Processing is a hearing impairment in analyzing and/or interpreting sound patterns. In this sense it is necessary to attend the central auditory processing [2, 3]. Several studies clarified that people with hearing impairments, including CI users, perceive rhythm approximately as well as those with normal hearing [1, 4-6]. Gfeller and Lansing performed a test called PMMA (Primary Measures of Music Audition developed by Gordon) to 18 postlingual deafened CI subjects [8]. Mean identification scores on the rhythm subtest (88%) were higher than on the tonal subtest (78%). Leal et al. conducted rhythm discrimination tasks with twenty-nine postlingual deafened adults and showed their score was 95% [9]. Kong et al. performed an identification task with four tempo conditions (60, 80, 100, and 120 beats per minute) and found no significant difference between normal hearing subjects and cochlear implant users [10]. Their participants were asked to read and chose the musical notation displayed on the screen that corresponded to the rhythmic pattern presented. There are few works addressing the perception of impulsive signal sounds and possible identification of CI users. Therefore this study investigates the identification of impulsive signals by using percussion instruments that were tuned in easily distinguishable tones for CI users. The hypothesis is that CI users should perform worse pitch discrimination of impulsive signals than tone signals due to their codification strategies. After describing the methods and procedures for this research, the results of the tests are analyzed and compared with previous studies. 2

3 2 Procedure 2.1 Stimuli preparation This work combines the methodologies of previous studies on pitch and rhythm discriminations [5, 11]. Source signal consist of rhythmic patterns made with three percussion instruments as the kick, snare and ride cymbal. In order to get clear pitch identification, each percussion instrument was carefully processed and transposed to have a 6 semitone distances between each instrument. The kick sound was adjusted to a F3 (170 Hz), the snare to a C4 (261 Hz) and the ride cymbal to a G4 (390 Hz). This distance was based on the minimum change condition found by Kim et al. [6]. Their spectrum is shown in Figure 1. The three instrument signals were normalized in peak amplitude because of peak detection method used in the common coding strategies. These signals were then used to create different rhythmic patterns as test stimuli. Figure 1: The three pitched instrument spectrums used in the patterns. The test was divided in two different sessions. In Session 1, the three instruments are compared to each other using the same pattern. This attempts to discern if CI users really perceive the difference in impulsive pitch. An example of the MIDI sheets to the first session is shown in Figure 2a. In this figure each frame corresponds to a sixteenth note. In Session 2, the three instruments were combined using just a single pattern (Figure 2b). Between two different stimuli just one note for one instrument was changed. This aims to analyze the discrimination in a combined pattern of different instruments, providing more complex information for the recognition. In both sessions, the number of notes for all rhythmic configurations was fixed to six, showing good appreciation by Kim et al. [6] after testing with different quantities of tone intervals. 3

4 (a) (b) Figure 2: Rhythmic patterns consisting of the three instruments for Session 1 (a) and Session 2 (b). All sounds were made in Albeton Live 7.0 using an Electric Drum Roland DR-909 and DR-707, and transpose of drum rack of native software. The tempo condition was fixed to 120 beats per minute (bpm) following the results of Kong [10] since he did not find significant differences in discrimination skills when the bpm was changed. 2.2 Subjective Test An ABX test was conducted to examine the discrimination of the stimuli. In order to increase the number of trials and improve the confidence interval of the responses, Session 1 was repeated twice while Session 2 was repeated three times since more variation was expected in the results of Session 2. Thus, in total 15 combinations were used in the ABX test. A self-developed algorithm was used in order to present the stimuli randomly. For each repetition the sequence of the stimuli was changed to avoid the subject to remember the judgments of the previous pattern. The subjects were asked to identify if the last signal (X signal) is equal to A (X = A) or B (X = B). Each ABX combination was played only once. The test was carried out with two groups: 30 normal hearing (NH) subjects of years old and 7 postlingual CI user of years old (more than 2 years of experience with CI). Implants with electroacoustic stimulation (EAS) were not considered in this study. Test environments were different for both groups. The NH subjects conducted the tests using a headphone Audio Technica ATH-D49 in a quiet room (NC 25) with acoustic treatment For CI users the line out of the processor was used to avoid the effects of the acoustic field. In all cases a Notebook computer with M-Audio Fast Track was used to perform the test. In both cases, the stimuli were presented to only one ear. The subjects were asked to adjust their own conformable sound level. Total duration of the test was of approximately 5 minutes for each subject depending on the time spent for the judgments. 4

5 3 Results The percentage of correct answers in the ABX test was calculated for all subjects of both groups. Figure 3a shows the results of Session 1 where the different instruments using the same pattern are compared. Figure 3b refers to Session 2 where patterns are composed by different instrument combinations. By the assumption that the statically analysis of an ABX test follows a binomial distribution, [12]. It was able to obtain a 95% of confident interval of each result. (a) (b) Figure 3. Percentage of correct answers for Normal Hearing subjects (NH) and Cochlear Implant users (CI) for Session 1 (a) and Session 2 (b). The error bars show a 95% confident interval. 4 Discussion The percentage of correct answers for NH subjects was more than 89 % in the combined patterns (Session 2) while almost 100% for the patterns with the same instrument (Session 1). As shown in Table 1, these results are consistent according to the previous studies where rhythm discrimination was performed. Although the numbers of responses by CI users were not sufficient to permit a reliable data analysis, it is possible to see that CI user performed worse than NH users. The present results for the CI users revealed lower performance than the previous studies for a rhythm discrimination. 5

6 Table 1: Comparison of the percentage of correct answers with previous studies Present study (Impulsive sounds) Previous studies (Continuous notes) Session 1 Session 2 Sucher and McDermott [5] Gfeller and Lansing [8] Schulz and Kerber [11] NH subjects 95% 89% 89% 95% 95% CI users 86% 45% 60% 78% 68-84% Both groups were able to distinguish pitch differences in accordance with previous studies. However CI users have much more difficulties in discriminate pitch in rhythms composed by different percussion instruments sounds than NH subjects. This result is consistent with the frequency tuning distance of Kim et al. [6]. It is interesting to point out that the previous studies used a continuous note while this study used impulsive sounds. As expected, impulsive sounds composed by different instrument combinations leaded to lower performance in rhythm discrimination for IC users. In the ABX test, some CI users showed a difficulty in the discrimination tasks, and the forced choice obliged them to respond randomly. Furthermore, the subjects expressed that they were not able to identify certain sounds but recognized the difference by the loudness rather than the pitch components. One of the reasons for this is because CI users may have different performance of their calibration. Sounds coincide with specific channels having different impedance and thus generate loudness differences, easily recognizable by the CI users. Some parameters could not be controlled in both groups. The most important factor is the degree of subjects attention during the test because loss of attention may cause a decrease of the test performance. This can be due to limited memory capacities and slower processing speed since IC users normally presents disorders of the central auditory processing according to Oscar Cañete [4]. Furthermore different models, settings and codification strategy of the CI devices may add wide variation to the identification process of the subject. Several authors also pointed out that there is a great variation between the perceptual skills of users because of the resilience of the subjects to electrical stimulation of their particular physiological characteristics [1, 6]. 5 Conclusions As expected in the hypothesis, the CI users have much more difficulties to discriminate the impulsive pitch rhythms than the NH subjects. Particularly, it is much more difficult for them when the signal is composed by a combination of different impulsive instruments. Comparison with previous studies show that for the CI users, discrimination of impulsive pitch is more complex than that of continuous tones. This effect has not been observed for the NH subjects. A further development of this study could reveal valuable data for understanding the phenomenon involving a substantial advance of the hearing aid mechanisms. To achieve more significant results the number of CI users will be increased. Possible future studies can include others factors by using more frequencies in order to achieve greater accuracy in the cochlear implant identification although the previous studies using continuous tone showed no variation in frequency scale. Some cochlear implant users can identify a difference between sounds but it does not mean that they can perceive the sounds correctly. For this reason, future works may consider more numbers of instrument sounds with different harmonic compositions. 6

7 Acknowledgments The authors would like to acknowledge the Board of the International Commission for Acoustics and ICA- ASA Young Scientist Conference Attendance Grants Programme for their financial support to present this work at the congress. The authors also express special gratitude to Nilda Vechiatti for her support in the application for this grant. The deepest gratitude is delivered to the CIAC Argentina institution to provide test environment and the participants for the test as well as the assistance and dedication of ST. Mastroianni S. Also a special thanks to the people who participated in the test without financial remuneration. References [1] Papadogianni-Kouranti, M. Auditive and audiotactile music perception of cochlear implant users, A Thesis presented for the degree of Master of Science, Technical University of Berlin, 2014, pp [2] Valeriewei, L. Music perception of cochlear users, Doctorate Thesis of Department of Otolaryngology, University of Melbourne, 2006, pp [3] Restrepo, I; Medina, J.R. Desórdenes del procesamiento auditivo, IATREIA, 19 (4), 2006, pp [4] Oscar Cañete, S. Central auditory processing disorder, Revista Otorrinolaringológica Cabeza Cuello, (66), 2006, pp [5] Sucher, C.M.; McDermott, H.J. Pitch ranking of complex tones by normally hearing subjects and cochlear implant users, Hear Res 230 (1-2), 2007, pp [6] Kim, E.N.; Lee, H.J.; Kim, H.J. Music perception ability of Korean adult cochlear implant listeners, Clinical and Experimental Otorhinolaryngology, 5 (1), 2012, pp S53-S58. [7] Donnelly, P.; Limb, C. Music Perception in Cochlear Implant Users. The Johns Hopkins University, Baltimore (USA), [8] Gfeller, K.; Lansing, C.R. Melodic, Rhythmic and timbral perception of adult cochlear implant users, Journal of Speech, Language and Hearing Research, 34, 1991, pp [9] Leal, M.C.; Shin, Y.J.; Laborde, M.-L.; Verges, S.; Lugardon, S.; Andrieu, S.; Deguine, O.; Fraysse, B. Music perception in adult cochlear implant recipients, Acta Oto-Laryngologica, 123 (7), 2003, pp [10] Kong, Y.Y.; Cruz, R.; Jones, J.A.; Zeng, F.G. Music perception with temporal cues in acoustic and electric hearing, Ear Hear 25 (2), 2004, pp [11] Schultz, E; Kerber, M., Music perception with the MED-EL implants, Advances in cochlear implants, 1994, pp [12] Boley, J.; Lester, M. Statistical analysis of ABX results using signal detection theory. Proc. Audio Engineering Society Convention 127, October 2009, [13] Fujita, S.; Ito, J. Ability of nucleus cochlear implant to recognize music, Annals Otol Rhinology Laryngology, 108 (7), 1999, pp [14] Phillips-Silver, J.; Toiviainenc, P.; Gosselina, N.; Turgeonb, C.; Leporeb, F.; Peretz, I. Cochlear implant users move in time to the beat of drum music, Hearing Research, 321, 2015, pp [15] Gfeller, K.; Turner, C.; Mehr, M.; Woodworth, G.; Fearn, R.; Knutson, J.F.; Witt, S.; Stordahl, J. Recognition of familiar melodies by adult cochlear implant recipients and normal-hearing adults. Cochlear Implants Int. 3(1), 2002, pp