Attractive serial dependence in numerosity perception in dogs (Canis familiaris).

Recent literature in humans has described a perceptual phenomenon called serial dependence, by which the current stimulus appears similar to the previous one. It has been interpreted as an active stabilization process, integrating stimulus features over time for a stable and seamless conscious experience. Despite the growing number of studies investigating serial dependence across several visual domains in humans, it is not known if the phenomenon extends also to other species. In the current study we aim to fill this gap of knowledge by exploring the behavioral signature of serial dependence in numerosity perception in dogs. We enrolled 5 dogs, who were trained on quantity discrimination prior to being presented with a set of test trials. Simultaneously presented test stimuli included a variable probe (4-16 dots) and a reference (8 dots), which was preceded by a task-irrelevant inducer stimulus (4 or 16) in the identical location. We hypothesized that if dogs are susceptible to the serial dependence, the reference would be perceived either smaller or larger depending on the inducer numerosity. For each dog we fitted a psychometric curve based on the probability of choosing the reference and defined the point of subjective equality (PSE) for the reference preceded by either of the inducer, (PSE(4)=7.10; PSE(16)=10.07). To compare the perceptual effect of the two inducers, we performed an ANOVA, which revealed a significant effect of the inducer (F(2)=23.87; p<0.001), with both of theconditions being significantly different from the control condition (p(4)=0.35; p(16)=0.02, sequential Bonferroni corrections applied). This result suggests an attractive bias in consecutively presented stimuli in dogs, providing the first evidence that the phenomenon of serial dependence extends also to non-human animals. Moreover, the finding expands the similarities between humans and dogs in processing visual stimuli.