The Scale Illusion Detection Task:

Objective Assessment of Binaural Fusion

Niclas A. Janßen1, Lars Bramsløw2, Søren Riis3, Jeremy Marozeau1 1) Hearing Systems Group, Technical University of Denmark, Lyngby, Denmark 2) Eriksholm Research Centre, Snekkersten, Denmark 3) Oticon Medical, Smørum, Denmark

www.hea-dtu.dk/research/ Correspondence: [email protected]

current-phd-projects/janen

Introduction The Scale Illusion Detection Task The normal auditory system can fuse sounds from a) Streams presented b) Streams Perceived c) Frequencies both ears into a single sound object (binaural fusi- Seq. Electrode No. Seq. Electrode No. Note Freq. / Hz on). This ability can be measured subjectively by asking whether listeners perceive one or two sounds Side Left Right Stream lower higher C4 261.63 or by the scale illusion percept found by Diana Deut- 1 C4 C5 1 C4 C5 D4 293.66 sch in 1975. This study demonstrates an objective 2 D4 B4 2 B4 D4 E4 329.63 task to measure binaural fusion. 3 A4 E4 3 A4 E4 F4 349.23 4 F4 G4 4 G4 F4 Aim: G4 392.00 5 G4 F4 1. Find a fast, objective measure for binaural fusion 5 F4 G4 6 A4 E4 A4 440.00 2. Compare monaural performance 6 A4 E4 7 B4 D4 B4 493.88 3. Using one ear alone shall not allow to reliably 7 D4 B4 8 C4 C5 8 C4 C5 C5 523.25 detect the change in one of the two streams 4. Test whether a severe difference in timbre leads Fig. 2: Melodic pattern and percept for the origi- Tab. 1: Pattern of the scale illusion paradigm, based on Deutsch 1975. to degraded performance. nal scale illusion (Deutsch 1975). a) The streams as they are presented to the left and right headphone channel. b) The streams as they are perceived by the participants. c) The frequencies for the notes used in the scale illusion paradigm.

Description of the Task • 14 young, normal hearing participants • Listening experiment in sound isolated booth, 0.75 h • Loudness balancing of all stimuli (per frequency) Stimuli: 1st stream: Ordered notes, contains a deviant 2nd stream: Randomized notes from another pitch range (cf. Fig. 4 to the right) Task: Detect the deviant note in one of the two streams, pre- Fig. 3: Pattern of the scale illusion detection task, based on Deutsch 1975. A random deviant occurs in the ordered stream. The other stre- sented in only one of 3 intervals (3-AFC). am consists of random sounds. There is always one higher and one lower stream and half of each stream is presented to the other ear.

Fig. 1: Structure of a single trial. The Target consists of two repetitions of the target stream alone (either the low or the Results high stream of notes). After that, the randomized stream joins it. The streaming can build up over 3 iterations of the Fig. 4: Results of the scale basic sequence, before finally, the deviant occurs in one of illusion detection experi- the three last repetitions, A, B or C. ment in percent correct for the three conditions: Conditions: C1: binaural streaming, 1. Monaural Streaming: C2: binaural control and Both streams presented to the same ear. C3: monaural streaming. 2. Binaural Streaming: Every second note from each stream is presented to the other ear. 3. Binaural control condition:

The timbre of the sounds of one ear is altered severely by Correct Percent manipulating the temporal envelope and harmonics. Every condition was repeated 24 times.

Condition

Analysis Details Conclusions

A binomial distribution underlies this task, • Binaural streaming and fusion ability of normal hearing participants demonstrated successfully since the answer is either correct or false. The- refore the significance level is given by: • Monaural performance equals binaural performance • Severe timbre difference across ears leads to performance breakdown Pr(X ≥ 13, n=24) ≈ 0.0284 (*) (≥ 54.17 %) • Binaural information no longer fused into a single segregated object Pr(X ≥ 15, n=24) ≈ 0.00323 (**) (≥ 62.50 %) • Single-ear listening insufficient in binaural condition Pr(X ≥ 16, n=24) ≈ 0.00860 (***) (≥ 66.67 %) • Task variants can be used to test fusion ability of CI users (bilateral and bimodal)

References Acknowledgments 1. Deutsch, D. (1974): An Auditory Illusion. Nature, Vol. 251, p 307-309 The authors would like to thank everybody who participated in the experiment for their help. We could not have done it 2. Deutsch, D. (1975): Two-Channel Listening to Musical Scales, Journal of the Acoustical Society of America, Vol. 57, No. 5, without you! p. 1156-1160, DOI: 10.1121/1.380573 3. Deutsch, D. (1999): Grouping Mechanisms in Music in The Psychology of Music (book), p. 299-348, Academic Press, ISBN: 9780123814609 4. Bregman, A. S.(1994): Auditory Scene Analysis. The Perceptual Organization of Sound (book), MIT Press, ISBN: 9780262521956