Participant

The participant was a female chimpanzee (Pan troglodytes) named Mizuki. She lived with five conspecific group members at the Hayashibara Great Ape Research Institute, Okayama, Japan, in an enriched environment consisting of a 7400 m2 outdoor enclosure and several indoor areas. She was raised by human caregivers since shortly after birth. Since her arrival at the Great Ape Research Institute when she was 2 years and 1 month old, she has spent her time with other chimpanzees. At the time of the present experiments, Mizuki was 11 years old and had undergone other behavioral cognitive experiments24,25, as well as earlier ERP experiments15,16,17,26. This research was conducted in accordance with the Guide for the Care and Use of Laboratory Animals of Hayashibara Biochemical Laboratories, Inc. and the Weatherall report, The use of non-human primates in research. The research protocol was approved by the Animal Welfare and Animal Care Committee of Hayashibara Great Ape Research Institute (GARI-051101).

Apparatus and stimuli

The experimental room was 2 m wide, 2.9 m long and 2.5 m high and was surrounded by concrete walls and metal bars with a metallic mesh door. The room was moderately lighted during the experiment. The chimpanzee, Mizuki, sat on a wooden draining board covered with an electromagnetic shielding sheet on a concrete platform. An experimenter stood in front of her to keep her still and facing the display. Because of her close relationship of mutual trust with the human experimenters, Mizuki was totally cooperative during the recording sessions. Prior to testing, we took approximately 6 months to gradually habituate her to the experimental device and procedure and throughout this habituation period, she exhibited no negative responses such as grimacing or screaming15.

A 17-inch CRT display (IIyama LA702U, 1024 × 768 pixels) was set up in front of Mizuki, approximately 40 cm away and at the horizontal level of her head. An infrared video camera was fixed on top of the CRT display to monitor the subject from a frontal view. We used this camera to check if the subject's gaze was directed to the stimulus display.

Experimental stimuli were color pictures of wild chimpanzees unfamiliar to Mizuki (Figure 2). These included three affective pictures of chimpanzees with affective expressions and 12 neutral pictures of chimpanzees without affective expressions. This set of stimuli was a subset of 80 pictures used in another study with a different chimpanzee, who had shown better performance in memory tasks for affective pictures than for neutral pictures12. Those pictures were still color images of wild chimpanzees from video recorded in Bossou, Guinea, West Africa, for research purposes27. The size of the pictures was 360 × 240 pixels. The average luminance of the pictures was set as equal. The pictures were displayed on a black background on the display.

Figure 2 Affective and neutral pictures used for the experiment. Full size image

Task procedure

An experimental session consisted of four or five blocks and each block consisted of either 60–75 trials. The experimenter chose whether or not to use blocks that contained attention-getting stimuli (short animations 800 ms or 1600 ms in duration that were not related to the test pictures) in between trials, depending on behavioral signs of concentration to the monitor by the chimpanzee. As a result, attention-getting stimuli were absent from the first 3 blocks but present in all of the remaining 16 blocks. On each trial, one of a total of 15 stimulus pictures was presented for 800 ms in a semi-randomized order, in which an affective picture was followed by at least two different neutral pictures. The same picture was not repeated in two or more consecutive trials. Each picture presentation, as well as attention-getting stimulus, was followed by a 800 ms inter-stimulus interval consisting of an empty black screen.

Between blocks in each session, Mizuki was given a rest of 1 min, during which she was allowed to make substantial body movements and to receive fruit rewards. One session was conducted in a day and four sessions were conducted on four different days. In total, Mizuki received 1201 trials in which affective or neutral pictures were presented (approximately 80 presentations for each picture; see Table 1), as well as 262 presentations of attention-getting stimuli of 144 different types, which were inserted between those test trials.

Table 1 The number of epochs used for analysis. The numbering of Picture IDs corresponds to the alignment of pictures of Figure 2: from left to right of the first row of Figure 2 (Affective 1, 2, 3), from left to right of the second row (Neutral 1, 2 and 3) and so forth Full size table

During the recordings, Mizuki's gaze occasionally appeared to avert from the monitor. When this occurred, another experimenter, who was monitoring the subject's gaze direction filmed on-line through the infrared camera, manually added a marker in the EEG data via a keyboard connected to the measurement computer.

ERP recording and analysis

The EEG was recorded from Ag/AgCl electrodes attached to five scalp positions (Fz, Cz, Pz, T5 and T6), according to the International 10–20 system for humans (Figure 3). The signals were referenced to the forehead midline (FPz). A ground electrode was positioned at the left earlobe. The electrodes were filled with Quick GEL and impedances were kept below 6 kΩ. Signals were amplified by NuAmp-40 and processed by Acquire 4.3 software (NeuroScan Inc.) with a 1000 Hz sampling rate. A 0.1–30 Hz band-pass filter (24 dB/oct) was applied in the offline analysis. All data were segmented into 900-ms epochs, including a 100-ms pre-stimulus baseline period, based on time markers of the stimulus onset. These epochs were baseline-corrected with respect to the mean amplitude over the 100-ms pre-stimulus period. Epochs where maximal difference of values exceeded 100 μV were excluded from analysis. Epochs that contained the “non-looking” markers described above were also excluded from analysis. The numbers of epochs accepted for analysis are shown in Table 1.

Figure 3 Chimpanzee participant, Mizuki, wearing electrodes. Only four of the seven electrodes are visible (Fpz, Fz, Cz and earlobe as a ground); the others are not visible (Pz, T5 and T6) because they are placed behind the vertex. Full size image

Independent t-tests were run on the data to compare the mean amplitudes for each time window of each channel separately. Time windows for analysis were chosen based on the averaged waveforms obtained, as described in the Results section. The t-tests assumed equality of variances, as homogeneity of variance was confirmed by Levene's Tests for Equality of Variance (F(2, 297) < 3.22, p > 0.07, for comparison of all pairs of groups). We initially chose an alpha level of 0.05, which was adjusted by use of the Bonferroni correction, yielding an adjusted threshold of significance of 0.001.

The main focus of the study was to compare ERPs to affective versus neutral pictures. However, it could be argued that the difference in ERPs may be attributable simply to a response to deviance, as the task could be regarded as an oddball task with a smaller number of affective pictures and a larger number of neutral pictures. It could also be the case that the results reflected other properties inherent in the stimuli set, such as the presence of faces or depictions of social interactions, rather than their affective properties. We therefore conducted the following additional comparisons. The first compared the picture without a visible chimpanzee face (Neutral 1, see Table 1 and Figure 1) to the pictures with chimpanzee faces (all of the other pictures). The second compared the picture depicting a grooming social interaction (Neutral 5, see Table 1 and Figure 1) to the pictures depicting no such social interaction (all of the other pictures).