Abstract
A wealth of data demonstrates that nonhuman animals, infants, and adults represent number nonverbally as analog magnitudes (e.g., Brannon and Roitman, 2003). A recent study (Barth et al., 2003) suggests that the nonverbal number representations held by adults are modality independent, leading to the prediction that nonlinguistic organisms may also hold number representations that transcend the modality in which they were created. In our first experiment, we ask whether rhesus monkeys will preferentially attend to visual displays of vocalizing conspecifics that numerically match the number of calls they hear. Subjects viewed two side-by-side videos of vocalizing, unfamiliar monkey faces; one contained 2 monkeys and one contained 3. Synchronous with the videos, subjects heard 2 or 3 concurrent coo calls. Onset and offset of auditory and visual stimuli were equated to eliminate the possibility that monkeys could match based on synchrony. Each subject was tested with only a single trial and was not rewarded. This design eliminated the possibility of matching based on the relative intensity of auditory and visual stimuli. Monkeys directed 60% of their total looking time to the matching display, which differed significantly from chance [t(19)=3.00, p < 0.01]. A 2 (match vs. non-match) × 2 (2-sound vs. 3- sound) × 3 (stimulus set 1, 2, or 3) ANOVA revealed that monkeys looked longer at the numerically matching display than at the nonmatching display [F(1,15) = 7.5, p < .02] and no other main effects or interactions. Our second experiment demonstrated that monkeys did not look preferentially at the numerically matching display when arbitrarily related animated visual shapes and complex tones were used as stimuli. Results demonstrate that rhesus monkeys represent the equivalence between the number of voices they hear and the number of faces they see and suggest that monkeys share with adult humans nonverbal number representations unfettered by stimulus modality.