Auditory sequence perception in common marmosets (Callithrix jacchus)Behav Processes. 2019 May;162:55-63. doi: 10.1016/j.beproc.2019.01.014.

Two marmosets (Cj190 and Cj195) were trained to discriminate auditory patterns to test whether they are sensitive to the regularity of auditory patterns. For this purpose, three experiments were conducted using simple Morse-code like patterns. In Experiment 1, stimuli were consisted of the combination of two properties (i.e., frequency and inter-tone interval). Both marmosets could choose S+ out of four sound trains as indicated by the increment of HIT responses (‘1’ in the S+ trials) and decrement of FA response (‘1’ in the S- trials). Thus, the marmosets’ ability to recognize physical properties of sounds was showed. In Experiment 2, stimulus patterns were ABAB and AABB patterns. However, the marmosets could not discriminate these patterns although the results of Experiment 1 showed that the marmosets can notice the difference between A and B elements. Such results were indicated by the comparable HIT and FA ratios across sessions. Thus, the results of Experiment 2 implied that marmosets are not sensitive to the ordering regularity of sounds. In the phase 1 of Experiment 3, ABAB and AABB patterns in which either frequency or tone duration properties were shared were presented. Consequently, the marmoset learned to avoid two S- stimuli that did not share the common physical properties with S+ as indicated by the marked decrement of FA ratios to two of three S- stimuli. However, their discrimination performance did not rely on the ordering features. In the phase 2 of Experiment 3, training was continued using S+ and S- that the marmosets could not inhibit responding. Notably, FA to such S- was not inhibited. Consequently, they could not discriminate ABAB and AABB patterns. In conclusion, the current experiments imply that the marmosets can recognize physical features of sound. However, they cannot process the order information of auditory signals.