Enhanced auditory disembedding in an interleaved melody recognition test is associated with absolute pitch ability

Absolute pitch (AP) and autism have recently been associated with each other. Neurocognitive theories of autism could perhaps explain this co-occurrence. This study investigates whether AP musicians show an advantage in an interleaved melody recognition task (IMRT), an auditory version of an embedded figures test often investigated in autism with respect to the these theories. A total of N = 59 professional musicians (AP = 27) participated in the study. In each trial a probe melody was followed by an interleaved sequence. Participants had to indicate as to whether the probe melody was present in the interleaved sequence. Sensitivity index d′ and response bias c were calculated according to signal detection theory. Additionally, a pitch adjustment test measuring fine-graded differences in absolute pitch proficiency, the Autism-Spectrum-Quotient and a visual embedded figures test were conducted. AP outperformed relative pitch (RP) possessors on the overall IMRT and the fully interleaved condition. AP proficiency, visual disembedding and musicality predicted 39.2% of variance in the IMRT. No correlations were found between IMRT and autistic traits. Results are in line with a detailed-oriented cognitive style and enhanced perceptional functioning of AP musicians similar to that observed in autism.


Methods
This study is part of a larger project consisting of various experiments investigating cognitive performance in audition and vision of absolute vs. relative pitch possessors. Therefore parts of the methods (sample description, general procedure, description of absolute pitch assessment, covariate measurements) are by their nature similar to previous publications of our lab ( participants. Thirty-one AP musicians (16 female, mean age = 25.7 (SD = 9.7)) and 33 RP (relative pitch) musicians (15 female, mean age = 24.0 (SD = 7.13), see Table 1) -primarily students or professional musicians at the University for Music, Drama and Media, Hanover -were recruited via an online survey using UNIPARK software (https://www.unipark.com/). Recruitment was initially based on self-reports with respect to AP ability. Participants had the choice to select "I have absolute pitch", "I do not have absolute pitch" and "I do not know, whether I have absolute pitch". A first pitch identification test during the online survey (see general procedure below) was then used to evaluate the self-reports for group assignment. Four AP and two RP were amateur musicians. Non-native German speakers (4 AP) had the choice between a German and an English version of the experiments. All participants denied any history of psychiatric or neurological disorders, however, one AP reported taking Mirtazapine. The primary instruments played by the AP were piano (15), string instruments (9), woodwind instruments (3), voice (2), and brass instruments (2); for RP they were piano (13), string instruments (4), woodwind instruments (6), voice (3), brass instruments (3), bass (1), guitar (1), accordion (1), and drums (1). All AP but one were consistently right handed according to the Edinburgh Handedness Inventory 55 three RP were left-handed, two RP were ambidextrous. A total of five participants (4 AP, 1 RP; final sample: AP = 27, RP = 32) were excluded during analysis due to missing data (1 case), extreme values for reaction times (2 cases) or because they did not follow the instructions (2 cases). The study was approved according to Helsinki Declaration by the ethic committee of the Hanover Medical School (Approval no. 7372, committee's reference number: DE 9515). The methods were carried out in accordance with their guidelines and regulations. All participants gave written informed consent and received financial compensation for their participation.
General procedure. The overall project consisted of three parts: one online survey and two appointments in the lab of the Institute for Music Physiology and Musicians' Medicine, Hanover. The online survey was used for demographic questions, Autism Spectrum Quotient (AQ 56 , German version by C.M. Freiburg, available online: https://www.autismresearchcentre.com/arc_tests), Musical Sophistication Index (GOLD-MSI 57 (questionnaire)), an in-house questionnaire assessing musical education as well as total hours of musical training within the life span and a preliminary assessment of absolute pitch ability in order to allocate participants to groups (AP vs. RP) and evaluate their self-reports. The absolute pitch assessment was performed using an in-house pitch identification screening (PIS) consisting of 36 categorical (i.e. semitone distance between the tones based on equal-tempered tuning) sine waves in the range of three octaves between C4 (261.63 Hz) and B6 (1975.5 Hz). Participants exceeding 33% correct trials (>12/36 tones named correctly) were assigned to the AP group. The cutoff of 33% correct trials admittedly reflects a low threshold to select absolute pitch possessors compared to much more rigorous cutoffs (e.g. 50%, 80% or higher) in other studies 23,58,59 . However, it is still an ongoing debate, if and to what extend AP is distributed dichotomously or perhaps more gradually in professional musicians 23,58 . Furthermore, the use of relative strategies in pitch identification tests cannot be securely excluded 23 . Therefore we inspected scatterplots between PIS and Pitch adjustment test (PAT, see below) after the appointments in the lab and reassigned subjects that had differing performance on PIS and PAT based on their self-reports in the online survey. From our experience, professional musicians as investigated within this study usually know whether they themselves have absolute pitch. Due to this 2 RP's with initially high scores on the PIS (13 respectively 21 of 36 tones named correctly) were assigned to the RP group because they reported not to have AP and performed like RP possessors in the PAT. Likewise one AP possessor, who reported having technical issues with the presentation of the sine waves in the online study and therefore a low PIS Score, was reassigned to the AP group. The low cutoff of 33% therefore is caused by this issue and might most likely not apply to other samples investigating AP with pitch identification tests. Tests on general intelligence (Raven's Standard Progressive Matrices (SPM)) 60 , information processing speed ("Zahlenverbindungstest" (ZVT)) 61 63 and all experiments were conducted in the lab (see Table 1 for general group differences regarding covariates).

experiments and material. Pitch adjustment test (PAT). A pitch adjustment test (PAT) developed after
Dohn et al. 64 was used to quantify fine-graded differences in absolute pitch proficiency. The test consisted of 108 target notes, presented as letters on a PC screen in semi-random order in 3 Blocks of 36 notes each (3*12 different notes per block) with individual breaks between the blocks. Participants' task was to adjust the frequency of a sine wave with random start frequency (220-880 Hz, 1 Hz steps) and to try to hit the target note (letter presented centrally on PC screen, e.g. "F#/Gb") within at most 15 seconds. Participants were allowed to choose their octave of preference. The tones were presented through sound isolating Shure 2-Way-In-ear Stereo Earphones (Shure   Table 1. Participants' characteristics. Age, nonverbal IQ (SPM), information processing capacity (ZVT), musical training (total hours during life span on main instrument), musicality (AMMA; MSI) and online pitch identification screening (PIS) for each group; *Two RP reported not having absolute pitch but reached a screening score of 13 respectively 21. Because of this and their weak performance in the pitch adjustment test, the subjects were assigned to the RP group; Significant group differences (uncorrected) highlighted in bold. p < 0.10, *p < 0.05, **p < 0.01, ***p < 0.001 (Bonferroni-Holm corrected).
www.nature.com/scientificreports www.nature.com/scientificreports/ SE425-CL, Shure Distribution GmbH, Eppingen, Germany). Participants were explicitly asked to try to adjust each tone as precisely as possible without the use of any kind of reference and to confirm their answer with a button press on a Cedrus Response Pad (Response Pad RB-844, Cedrus Corporation, San Pedro, CA 90734, USA). If no button was hit, the final frequency after 15 seconds was taken and the experiment proceeded with the next trial. In both cases, the Inter Trial Interval (ITI) was set to 3000 ms. Online pitch modulation was performed by turning a USB-Controller (Griffin PowerMate NA16029, Griffin Technology, 6001 Oak Canyon, Irvine, CA, USA) and implemented in Python according to Dohn et al. 64 . Participants could choose between rough (10 cent, by scrolling the wheel) and fine tuning (1 cent, by pressing down and scrolling the wheel). The final or chosen frequencies of each participant were compared to the nearest target tone (<6 semitones/600cent). For each participant, mean absolute deviation (MAD 64 Eq. (1)) from target tone was calculated as the mean of the average absolute deviations c i of the final frequencies to the target tone (referenced to a 440 Hz equal tempered tuning). MAD reflects the pitch adjustment accuracy of the participants. The consistency of pitch adjustments (SDfoM, standard deviation from own mean), possibly reflecting the tuning of the pitch template 64 , was then estimated by taking the standard deviation of the absolute deviations (Eq. (2)).
For regression analyses (see below), we performed a z-standardization of the MAD (Z_MAD, Eq. (3)) and SDfoM (Z_SDfoM, Eq. (4)) values relative to the mean and standard deviation of the non-AP-group, as originally proposed by Dohn et al. 64 .
Interleaved Melody recognition test (IMRT). The experiment consisted of 16 training trials followed by 128 test trials presented in two blocks of 64 trials each with a break in between. Each trial was made up of a 6-tone probe melody (2400 ms, 400 ms per tone; ~74.5 bpm) followed by an interleaved sequence (see Fig. 1), in which the probe melody was embedded within a distractor melody (1 st and then every 2 nd tone belonging to the probe melody; 12 tones, 2400 ms, 200 ms per tone). In half of the trials, a modified version of the probe melody was embedded within the interleaved sequence. The general task for the participants was to press the right button on the Cedrus Response Pad (Response Pad RB-844, Cedrus Corporation, San Pedro, CA 90734, USA), if the probe melody was embedded inside the interleaved sequence, and the left button if not (required responses were counterbalanced; button colors (yellow and blue) were counterbalanced across participants). Each trial was preceded by the German or English word for "Attention!" lasting for 1000 ms, and probe melody and interleaved sequence were separated by an ISI (Inter-Stimulus-Interval) of 1000 ms. The ITI (Inter-Trial-Interval) was set to 2200 ms. Probe melodies, modified probe melodies and distractor melodies were constructed according to Bey & McAdams (2002 2,3 as follows: The mean frequency of each probe melody was within a range from −3 to +2 semitones (ST) around the equal-tempered A 4 (440 Hz), i.e. from F # 4 to B 4 . Modified melodies were then created by altering the 2 nd and 4 th or the 3 rd and 5 th note of the probe melody within a range of +−4 ST, which always also altered the melodic contour (see Appendix A of Bey & McAdams (2003) 3 ). All melodies ranged from 5 to 11 ST and intervals within the melodies were between 1 and 8 ST. The 36 probe melodies and 36 modified melodies comprised 46 diatonic and 26 non-diatonic melodies. For each of the 36 melody pairs (probe and modified version) one distractor melody was composed (see Appendix B of Bey & McAdams (2003) 3 ). All notes of distractor melodies were maximally 1-2 ST (alternating) above respectively below the two neighboring tones of the target melody to ensure, that the task could not be solved by the global contour of the sequence. In half of the melodies the first distractor note started below the target melody and the reverse was true for the other half. Finally, 8 interleaved sequences were built for each of the 36 melody pairs: distractor and either probe or modified melody were interleaved and -relative to the note closest to the mean frequency -separated by 0 (fully interleaved), 6 (separated by augmented fourth), 12 (one octave) or 24 ST (two octaves, see Fig. 2).
Two experimental versions (A, B) were created, with half of the participants receiving the trials related to probe melody 1-18 and the other half receiving the trails related to probe melodies 19-36. The 16 trials of the first two (Version A), respectively 19 th and 20 th target melodies (Version B) served as training trails. Order of trials was pseudo-randomized with the same probe melody never occurring twice in a row. The experiment was programmed and controlled via the Python toolbox PsychoPy 65,66 . Due to a technical error in the first version, one trial occasionally appeared twice during the experiment while another one was missed. The error occurred randomly with respect to probe vs. modified melody and semitone separation. To control for possible effects on (2019) 9:7838 | https://doi.org/10.1038/s41598-019-44297-x www.nature.com/scientificreports www.nature.com/scientificreports/ the statistical tests, an equal number of participants in each group performed this imperfect version (N = 12; final percentage after exclusions: RP: 12/31, AP: 9/27), and all other participants afterwards performed the correct version. Calculation of Hit rates and False alarm rates was adapted accordingly, as described below. The total experiment without instructions lasted approximately 21.6 minutes. 63 , a paradigm often used to investigate cognitive style in autism 38,46,47 , was chosen to additionally assess participants ability to find smaller shapes in drawings of a meaningful whole. The test is performed with pen and paper and consists of 18 trials in two blocks of 9. The participant has to find a geometric form in a large and more complex whole and mark it with a pencil. As we observed ceiling effects as to the total amount of correct trials in the GEFT during pilot experiments, we additionally measured the time participants needed to find each form so that we could better discriminate between participants' abilities at this task. Performance on the GEFT was therefore expressed as average time (over correct trials) needed to solve an item. As items increase in difficulty similarly to intelligence tests, a maximum time of 3 minutes per item was provided to avoid extreme reaction times on difficult items. Further, inspection of histograms was used to ensure that response times were not skewed within participants. Two participants have been excluded because they represented extreme values on individual or group level. statistical analysis. All statistical analyses were performed using the open-source statistical software package R (Version 3.5, https://www.r-project.org/). As melodies unfold over time, no reaction times were analyzed with respect to the IMRT. Instead, sensitivity index d′ and response bias (decision criterion) c were calculated according to signal detection theory 67,68 . Signal detection theory is a suitable method for analyzing decision processes in recognition or discrimination tasks like in the IMRT, in which only two responses are possible (here: whether or not the probe melody is embedded). Therefore, responses fall into one of four categories: hit (correct identification of probe melody in the interleaved sequence), miss (miss of the probe melody in the interleaved sequence), false alarm (false "yes" response to modified melody in the interleaved sequence) or correct rejection (correct "no" response to modified melody in the interleaved sequence). According to Macmillan 69 the difference of the z-scores of proportions of hits (H in %) and false alarms (F in %) gives the sensitivity index d', which is independent of the decision criterion c.

Group embedded figures task (GEFT). The group embedded figures test (GEFT)
The latter is a measure of the tendency to say "yes" or "no", i.e. the response bias, and is calculated as: Figure 1. Graphical and musical notation of experimental material within the IMRT (interleaved melody recognition test). Each trial begins with a probe melody (A, blue) followed by an interleaved sequence with either the same probe melody (B, blue) or a modified melody (C, blue, here 2 nd and 4 th note modified compared to probe melody) interleaved with a distractor melody (red). Graphical notation of musical melodies in this example shows the pitch height of each note in ST relative to the A 4 (440 Hz). The distractor notes always encompassed the neighboring melody notes in terms of the pitch height.
www.nature.com/scientificreports www.nature.com/scientificreports/ Therefore, positive values of c indicate a tendency towards "no" responses, while negative values indicate a tendency towards "yes" responses. As z-values for perfect proportions yielding H = 1 and F = 0 cannot be calculated (infinite values), Macmillan 69 recommends to reduce the proportion of N correct /N total by subtracting 0.5 point from the number of correct trials within the affected condition: H = (N correct − 0.5)/N total , respectively to increase the proportion of false alarms by adding 0.5 points: F = (N false + 0.5)/N total . The IMRT comprised (in the errorless version) 16 trials per condition (8 conditions: 4 separation conditions x 2 melody conditions (probe/modified)); with this number of trials, some perfect scores were likely. As 21 participants had unequal trial numbers per condition (see Fig. A1, Supplementary Material A) hit rates and false alarm rates were always calculated according to the true number of trials of each participant per condition. However, since different numbers of trials affect calculated proportions and z-values, we performed additional calculations (e.g. using the number of trials of the participant with the most trials per condition as N total ) of proportions for hit rates and false alarm rates. These alternative analyses did not affect the direction or effect sizes of the reported statistical tests. ethics approval and consent to participate. The study was approved by the ethic committee of the Hanover Medical School (Approval no. 7372, committee's reference number: DE 9515). All participants gave written consent.

Results
participants' characteristics. Multiple t-tests (Bonferroni-Holm-corrected) revealed no differences between AP and RP groups with respect to age, intelligence (SPM-IQ), information processing speed (ZVT-IQ), practice hours on instrument and musicality (AMMA total, Gold-MSI (see Table 1)). As expected, AP outperformed RP on both absolute pitch tests (PIS: t(43.1) = −16.25, ***p < 2.2e-16; MAD: t(44.4) = 14.890, ***p < 2.2e-16; SDfoM: t(41.0) = 11.675, ***p = 1.272e-14). Group differences for age of onset of musical training, the tonal subscale of AMMA and autistic traits (AQ) were only significant before Bonferroni-Holmcorrection (see Table 1).  Table B1 Supplementary Material B). Exploratory analyses revealed higher intelligence (SPM-IQ: t(47.2) = −2.621, p = 0.012), faster information processing speed (ZVT-IQ:: t(57) = −1.870, p = 0.067, marginally) and higher values on AMMA total score (t(57) = −2.128, p = 0.038) and subscales (tonal: t(57) = −2.194, p = 0.032; rhythmic: t(57) = −2.516, p = 0.015) for participants performing version B compared to version A of the test. In contrary, no group effects for version of IMRT were found for MSI, age, age of onset of musical training, hours of musical practice and performance on absolute pitch tests. Therefore, we argue that the effect of IMRT version on the performance of IMRT is most likely an artefact of accidental differences in intelligence and melody recognition (AMMA) between the participants that undertook the two versions and not due to differences in item difficulty. This is especially likely, as the musicality test AMMA is from its concept (items and recognition task) very similar to the IMRT experiment. A high correlation was therefore expected between them. Further, we have ensured that an equal number of AP and RP participants performed each version. No group differences of absolute pitch performance between the groups of the two versions confirms this procedure. Nevertheless, to correct for potential influences of version on regression models (see below), a dichotomous variable containing information of version performed per participant was included in the analyses as a covariate.
In contrast, different numbers of trials within frequency separation conditions (see Methods section) did not lead to significantly different results either in overall performance or for separate conditions (see Table A1, Supplementary Material A for further details), so number of trials was not included as a covariate.
Hence, IMRT was deemed to be an adequate test for investigating auditory disembedding. Therefore, a 2 × 4 repeated measurements ANOVA with separation (0, 6, 12, and 24) as within-subject factor and group (AP vs. RP) as between subject factor was run.   Table 2). We would also like to draw attention to a likely ceiling effect in condition 24-ST (see Figs 3b and 4e), the results of which should be interpreted with caution.

Linear models to predict perceptual sensitivity d'.
To investigate which musical and cognitive variables influence performance on IMRT, we performed multiple linear regressions separately for overall performance and separation conditions (0, 6, 12, 24 ST). In a first step, bivariate correlations between IMRT and variables of interest were calculated to get an overview of possibly predictive variables (see Table 3). As expected, we found a high correlation between both of our absolute pitch tests (r = −0.859, p = 2.2e-16). Interestingly, better absolute pitch performance in the labelling task was also associated with more autistic traits (PIS, AQ: r = 0.393, p = 0.003, see Table 1 and Fig. 5).
Apart from the already known correlation with GEFT (r = −0.406, p = 0.0014), IMRT (overall performance) was related to absolute pitch proficiency in both producing a tone (Z_MAD, standardized mean absolute deviation from target tone; r = −0.340, p = 0.0085) and naming a tone (PIS, Pitch identification screening; r = 0.283, p = 0.0364), age of onset of musical training (r = −0.289, p = 0.0264), information processing speed (r = 0.304, p = 0.0199) and marginally to musical sophistication index (MSI (total score); r = 0.238, p = 0.077). However, after Bonferroni-Holm-correction to adjust for multiple comparisons only the correlations with GEFT and Z_ MAD were (marginally for Z_MAD) significant. Because of this and additional multicollinearity reasons age of onset and PIS were not included into the regression models. Both showed high correlations with absolute pitch performance (Z-MAD: r = 0.392, p = 0.0021; PIS: r = −0.361, p = 0.00679) respectively pitch adjustment (Z_MAD: r = −0.859, p = 2.2e-16). Therefore, a total of 5 variables (Z_MAD, MSI, GEFT, ZVT and version) were included into the 5 resulting regression models. Afterwards, variables with non-significant beta-weights were removed from the model leading to 5 reduced models (see Table 4). Bonferroni-Holm-corrections were performed according to the overall number of computed models. In general, IMRT performance was highly predicted by absolute pitch ability (Z-MAD) and GEFT. Performance in the 0-ST condition was additionally predicted by musical sophistication index (MSI) and the version of IMRT (A vs. B). Models of the 24-ST-condition did not reach significance and must be interpreted with caution due to ceiling effects (see Figs 3b,c and 4e). From Fig. 4 it can easily be seen that the four IMRT separation conditions (0, 6, 12, 24) decrease in variance (y-axis), indicative of decreasing task difficulty. Because at six semitones and greater separations the MSI is no longer predictive of IMRT (d'), this effect likely is due to our highly over-trained sample of professional musicians.
In summary, 39.2% (R² adjusted ) of variance on overall performance on IMRT and 45.8% (R² adjusted ) of variance in the fully interleaved 0-ST-condition were explained by absolute pitch ability (Z-MAD), musicality (MSI) and disembedding in vision (GEFT) (with IMRT version as a covariate). Even though R² adjusted decreased for the easier conditions with a greater separation of probe and distractor melodies (6,12), absolute pitch performance remained significant in both cases, while the other variables lost more of their influence (see Table 4). Figure 4 shows regression models for overall performance (Fig. 4a) and separation conditions (Fig. 4b-e) with regression line corresponding to a simple linear regression of absolute pitch ability (Z_MAD) on d' for simplicity reasons. Color and shape scales correspond to values on GEFT and MSI, respectively.  Table 2. IMRT group differences (N = 59). Means, standard deviation (SD) and statistical comparison (post hoc t-test, two-tailed) of IMRTperformance (perceptual sensitivity index d') by group (absolute pitch, AP, vs. relative pitch, RP) and per separation condition (in semitones, ST). p<010, *p < 0.05, **p < 0.01 (after Bonferroni-Holm correction of separation conditions).

Discussion
For the first time we investigated auditory streaming ability 1 , an ability closely related to the embedded figures concept from many autism frameworks (e.g. WCC theory 41,42 ), in absolute and relative pitch professional musicians. By using an interleaved melody recognition test 2,3 , we aimed to explore, if and to what extend the auditory perception of absolute pitch possessors might have a similarly auditory weak central coherence compared to relative pitch possessors 30 . This could serve as a common framework to explain the higher proportion of absolute pitch ability in autism and autistic traits in absolute pitch possessors.
Interestingly, general performance on the IMRT correlated with a visual embedded figures test (GEFT 63 , mean time needed to solve an item), which confirmed our hypothesis that the interleaved melody recognition test serves as an auditory hidden/embedded figures test 6 . As embedded figures tests were traditionally used as evidence for a detail-oriented processing style in autism [36][37][38][39][40] and for the weak central coherence account (WCC) 41,42 , superior performance of AP possessors in our IMRT might indicates a more detail-oriented auditory processing of absolute compared to relative pitch possessors. This is especially interesting, as several studys including the present investigation have reported (slightely) increased autistic traits in AP musicians 31,32 . However, performance on IMRT was not correlated with autistic traits in our study. Further, it must be made clear that direct  Table 4 for statistical values). (2019) 9:7838 | https://doi.org/10.1038/s41598-019-44297-x www.nature.com/scientificreports www.nature.com/scientificreports/ measures of streaming ability using alternating high and low frequency tones 70 may not necessarily have anything to do with the embedded figures concept. In the following we therefore stick to the terms "interleaved melody recognition test" and our acronym (IMRT) which we interpret to be a paradigm for both auditory streaming and the auditory embedded figures test.
General performance (sensitivity index d' , signal detection theory) as well as performance at different levels of frequency separation between probe and distractor melodies exceeded chance level (d' = 0) in both groups. This effect can be explained by above average musical knowledge in this sample which could either result in increased availability of musical schemas (schema-based auditory streaming 2 ) or be based on heightened ability to recognize melodies per se. However, as we did not include a control condition in which no distractor tones were present in the second sequence of each trial, we cannot finally decide upon this issue. Ceiling effects obtained for the high separation condition (24-ST) are perfectly in line with previous reports of ceiling effects with respect to large distances between probe and distractor melodies 2 . Responses were only slightly biased towards "no" responses overall and this tendency was significantly stronger for the most difficult 0-ST condition compared to all other conditions. As RP possessors always exhibited a more pronounced response bias (however not significant) this was seen as an indicator of uncertainty of responses, especially when the task was most challenging. Failure to  Table 4. Comparison of models predicting IMRT. Parameters, significance (F-statistics) and comparison of different models for total performance on IMRT and performance on separation conditions (0, 6, 12, 24 semitones). Models are compared using R² and R² adjusted . Higher R² and more parsimonious models (fewer predictors) indicate superior models. Significance after Bonferroni-Holm correction: p < 0.10, *p < 0.05, **p < 0.01, ***p < 0.01. www.nature.com/scientificreports www.nature.com/scientificreports/ reach significance with respect to group comparisons on response bias might be due to inter-individual strategies to handle insecurity on melody judgements (tendency towards "no" vs. "yes" responses).
Furthermore, increasing the distance between probe and distractor melody (separation conditions) also increased performance of participants in both groups. Therefore and in accordance with other authors 2-4 pitch separation as a perceptual cue (i.e. separation of target and distractor melody by presentation on different average pitch helps to perceive two different melodies) seems to be of general importance for the processes of auditory streaming. However, in our study, we found an above chance level performance in the fully interleaved condition (0-ST). To our knowledge other authors have always found weak performance in healthy participants for this condition and have argued for the necessity of this pre-attentive pitch cue based mechanism to come into play. In their view, schema-based auditory streaming might only be possible in interaction with bottom-up sensory processes (such as using pitch cues) that structure the auditory signal [2][3][4]6 . In line with this, increased performance in our sample of professional musicians may have been caused by extensive musical experience which led to additional musical schema-based processes in the 0-ST condition for these participants. Musicians for example have daily exercise in extracting and focusing on melodies and fore-vs. background in orchestral or other ensemble music as well as in solo performances of harmonical instruments like piano, which can play several melodies at the same time. This explanation would be consistent with evidence that auditory experience, such as familiarity with particular voices 71 or words 72 , can influence the auditory streaming of speech. This hypothesis is further strengthened by the marginal correlation of Musical Sophistication Index (MSI) 57 with overall performance and its contribution to some of the regression models. MSI is a general measure of musical sophistication aimed at the investigation of non-musicians which might explain the only small degree of correlation. It is possible that musicality tests which might be more suitable for trained musicians would have yielded higher correlations. However, as the interleaved melody recognition paradigm exhibits high similarity to famous tests of musicality (e.g. AMMA, Advanced measures of music audiation 62 ) both in terms of stimuli and task (recognition of melodies) we did not include the AMMA into our regression analyses. The AMMA therefore only served to ensure equal musicality levels in both groups (AP and RP).
The even higher performance of absolute pitch possessors in the fully interleaved and all other separation conditions as well as in overall performance allows us to assume that additional processes related to the absolute perception and/or naming ability of pitches lead to an advantage of AP's in this test. We cannot rule out the possibility, that AP possessors might also use relative pitch strategies in this experiment. Despite we did not control for RP strategies, it can be assumed that AP possessors as well have a wide variety of relative pitch abilities 23 . The present group differences therefore indicate specific additional perceptual mechanisms of AP that enhance the performance of AP possessors. Perhaps stable pitch-label associations comprise an additional schema-based mechanism, by which probe melody and tones in the interleaved sequence can be compared and extracted. However, as AP's performance on IMRT also increases with the separation of probe and distractor melody, pitch-label associations might also increase auditory streaming ability due to an additional pre-attentive perceptual quality (pitch chroma) similar to e.g. tone-color perceptions of colored hearing synesthetes. Indeed, some authors have argued to explain absolute pitch ability as a form of synesthesia 12,73 , which also leads to enhanced low level perception 13,74 . Interestingly, enhanced low-level perceptional functioning respectively a tendency to focus on details are two main elements of famous theories to explain autistic symptoms 41,43,44 . Furthermore, nearly all special abilities up to savant skills in autism share certain features like enhanced low-level perception, focus on details and mapping of two cognitive or perceptual structures according to their inherent elemental structure (Theory of Veridical Mapping 12 ). The one-to-one mapping of pitches to pitch labels might therefore be similar to the one-to-one mappings of pitches to colors or letters to colors. This could lead to enhanced performances on experiments in the affected sensory modalities. This enhancement might in turn be caused by pre-attentive processes that make use of the additional sensory quality that other participants lack. www.nature.com/scientificreports www.nature.com/scientificreports/ On top of that, autistic people can perform above chance with fully interleaved sequences while faring relatively poorly with respect to separation conditions 30 . However, in our sample, none of the performances on IMRT was explained (even in part) by autistic traits. Despite that, we cannot rule out the possibility that the lack of correlation with autistic symptoms is due to very heterogeneous factors that play a role in the development of absolute pitch (e.g. heritability, onset of musical training, ethnicity). A tendency towards higher autistic traits in AP, as was also present in our sample might form only one out of various different influences on the acquisition of AP. Therefore the lack of a correlation might be in part because the relation of AP and autistic traits is not true for all AP's.
In general, performance on IMRT (sensitivity index d'), i.e. the ability to recognize a melody in an interleaved sequence was highly (R² = 15-39.2%, smaller R² for easier conditions) dependent on absolute pitch proficiency as measured by a pitch adjustment test (PAT, developed after Dohn et al. 64 ), visual disembedding (GEFT) 63 and in the more difficult conditions musical sophistication (MSI) 57 . We therefore conclude that the interleaved melody recognition test is an auditory streaming paradigm 5,6 which also serves as an auditory hidden figures test (as for the correlation with GEFT). Furthermore, performance on the IMRT is enhanced by musical training 2,3 and absolute pitch ability. The latter might be due to an additional sensory quality leading to enhanced low-level perceptual functioning in general. This study therefore to our knowledge is the first to show enhanced auditory disembedding in absolute pitch possessors. Nevertheless some constraints have to be mentioned: First, we did not include a control condition for melody recognition (see above) or a non-musical control group, second, due to a technical error the number of trials per condition and subject was not counterbalanced (see Methods section). However, additional statistical analyses indicated that results were not affected by this issue (see Methods section and Supplementary Material A). Third, with respect to the discussed similarities to autism, a subsample including autistic people (and a matched control group) would have been desirable.
To conclude we would like to draw attention to goals that future studies should address. Because of the partial, but nevertheless re-occurring, similarities between absolute pitch possessors and autism in terms of cognition, perception, personality traits and neurophysiology and anatomy it remains to be investigated whether and to what extend one is the cause or a side effect of the other, or which external factors lead to the coincidence. Such investigation would not only increase the knowledge about both phenomena but also help to understand fine-graded differences in human perception and its relation to other cognitive functions and personality traits.

Data Availability
The datasets generated and/or analysed during the current study are not publicly available due to specifications on data availablity within ethics approval. Data are however available from the corresponding author upon reasonable request and with permission of the ethics committee of the Hanover Medical School.