Subjects

We tested three chimpanzees (Ai: a 36-year-old female, Ayumu: a 12-year-old male and Cleo: a 12-year-old female), who live in a social group of 14 individuals in an environmentally enriched outdoor compound connected to the experimental room by a tunnel21. The care and use of the chimpanzees adhered to the 2002 version of the Guide for the Care and Use of Laboratory Primates published by the KUPRI. The experimental protocol was approved by the Animal Welfare and Animal Care Committee of the KUPRI and the Animal Research Committee of Kyoto University. All procedures adhered to the Japanese “Act on Welfare and Management of Animals”.

Apparatus and Stimuli: In order to get the chimpanzees to tap repeatedly, we introduced an electric keyboard (Casio LK208). The keyboard had a function to illuminate keys in a certain midi channel, so we made an auditory sequence to be tapped described below in a midi file and transformed it into a midi channel (i.e., “midi channel 1”) and other auditory sound such as the positive feedback sound in another midi channel (i.e., “midi channel 2”). If an illuminated key was tapped, the illumination immediately switched to the next key, so it is unlikely that the rhythm of tapping was generated by the illumination. The outputs of the keyboard were sent to a desktop computer through a USB cable and recorded in real time in Cubase 6 (Available from http://www.steinberg.net/en/products/cubase/) at a sampling frequency of 44.1 kHz. Cubase is a computer program for music recording and production and also have been used in human tapping study. The distractor sound was also made and played using the Cubase and the signal was sent from the same desktop computer to speakers (Inspire T10). In the distractor stimulus, each auditory pulse was presented for 200 ms.

Training using an electric keyboard: At the start of a training trial a “G” key was lit as a start button. When the chimpanzees tapped “G”, training started and a “C4” key was lit. If the chimpanzees tapped “C4”, the illumination immediately switched to a “C5” key (see Figure 2 and Supplementary video_S1). If they followed the illumination and tapped “C4” and “C5” alternately for certain times, a chime (positive sound feedback) was played and they were given a reward. We started training with tapping 4 times and if the chimpanzees passed criterion on two consecutive trials the number of required taps was increased until they were able to tap 30 times in total (criteria: no mistake for 4 taps and 8 taps training, two mistakes for 16 taps and 24 taps, three mistakes for 30 taps, on two consecutive trials). When the chimpanzees passed training for 30 taps, they proceeded to a test phase.

Test phase 1 (Conditions: No stimulus, 400 ms-ISI, I500 ms-ISI and 600 ms-ISI): We first conducted a synchronized tapping experiment for No stimulus, ISI-400 ms, ISI-500 ms and ISI-600 ms conditions. In the test phase, one session consisted of one training trial followed by three test trials. The procedure for a training trial was exactly the same as the one for tapping 30 times in training phase. The three test trials were also exactly the same as a training trial except that while the chimpanzees were tapping the keyboard, an auditory distractor for one of the four conditions (No stimulus, 400 ms-ISI, 500 ms-ISI and 600 ms-ISI) was played. The chimpanzees were rewarded whenever they completed 30 taps following the lit keys regardless of the distractor sound. The same distractor sound was played during one session. We conducted four sessions in one day for four conditions and the order of the four conditions was randomized every day. We tested for six days, so we collected six sets of 3 trials per tempo for each condition (thus, 540 taps in total for each condition).

Test phase 2 (condition: random ISI): After the first experiment we conducted a second experiment using the random ISI condition. The procedure was the same as in the first experiment but only one session was conducted per day. In the random ISI stimuli, seven ISI stimuli (i.e., 200 ms, 300 ms, 400 ms, 500 ms, 600 ms, 700 ms and 800 ms) were randomly sequenced. We also tested for six days and collected 540 taps.

Analysis

The data were imported into MATLAB using the MIDI toolbox and only the onset tapping times were analyzed. We used MATLAB with Circstat Toolbox for circular analysis and SPSS 13.0 for other statistical analysis such as t-test. For circular analysis, we transformed all ITIs (Inter-Tap-Interval) from all data onto a circular scale (in degrees: −180° to +180°; in radians:−π to π) with all stimulus beats aligned at 0° (Figure 3).

Monte Carlo simulation: Because we conducted 6 trials for each condition, we randomly selected one trial sequence and re-paired the 600 ms-ISI distractor sequence. A Monte Carlo test with 10,000 such simulated experiments resulted in a distribution of the number of “accurate” taps within before and after 60 ms from the onset of the sound and computed the P value of Ai's actual data with 600 ms-ISI stimuli. We expected that the number of “accurate” taps in 600 ms-ISI stimuli would be higher than in the Random condition. However, the p-value (i.e., P = 0.033, one-tailed), did not survive under Dunn-Sidák adjustment of significance.