Perceptual category learning of photographic and painterly stimuli in rhesus macaques (Macaca mulatta) and humans

Abstract

Humans are highly adept at categorizing visual stimuli, but studies of human categorization are typically validated by verbal reports. This makes it difficult to perform comparative studies of categorization using non-human animals. Interpretation of comparative studies is further complicated by the possibility that animal performance may merely reflect reinforcement learning, whereby discrete features act as discriminative cues for categorization. To assess and compare how humans and monkeys classified visual stimuli, we trained 7 rhesus macaques and 41 human volunteers to respond, in a specific order, to four simultaneously presented stimuli at a time, each belonging to a different perceptual category. These exemplars were drawn at random from large banks of images, such that the stimuli presented changed on every trial. Subjects nevertheless identified and ordered these changing stimuli correctly. Three monkeys learned to order naturalistic photographs; four others, close-up sections of paintings with distinctive styles. Humans learned to order both types of stimuli. All subjects classified stimuli at levels substantially greater than that predicted by chance or by feature-driven learning alone, even when stimuli changed one every trial. However, humans more closely resembled monkeys when classifying the more abstract painting stimuli than the photographic stimuli. This points to a common classification strategy in both species, on which humans can rely on in the absence of linguistic labels for categories.

Pupillary response to representations of light in paintings

“…This might be the result of several factors, like differences in stimulus size and relative difference between luminance of stimuli and background. Our stimuli are also much more complex and may require higher cognitive load (Altschul et al, 2017;Tatler & Melcher, 2007), which is known to cause pupil dilation (Beatty, 1982;Hess & Polt, 1964;Just & Carpenter, 1993).…”

“…In the present study, we measured the pupil response to artistic paintings representing scenes with a visible sun, a visible moon, or the presence of diffused light to address the effect of cognitive interpretation of very complex stimuli. In fact, paintings render a scene through the artist's mind, requiring an even higher level of interpretation compared with photographs or artificial stimuli (Altschul, Jensen, & Terrace, 2017;Tatler & Melcher, 2007). In addition to the effect of image content, we also investigated the effect of contextual information, such as color and global layout.…”

Inferential Learning of Serial Order of Perceptual Categories by Rhesus Monkeys (Macaca mulatta)

“…It was therefore highly unlikely that subjects would see the same image more than once within an interval of several hundred trials. Stimuli from the first four categories were previously used by Altschul et al (2016) with a different set of subjects.…”

“…Can animals, for example, treat categories as though they were informative stimuli, signaling appropriate behavior in a cognitive task? Altschul et al (2016) demonstrated that rhesus macaques cannot only distinguish between four simultaneously presented categories of stimuli, but that they can also learn their serial order using a variant of the simultaneous chain task (Terrace, 1984(Terrace, , 2005. This suggests that animals cannot only learn to identify categories but they can also process categories in the same way they can process single stimuli.…”

“…However, we make no claims about how categorization is performed, or whether it is innate or acquired. Past studies of animal categorization suggest that animals exhibit serial learning with abstract artificial stimuli (Altschul et al, 2016), and that they can categorize visually degraded photographic stimuli (Basile and Hampton, 2013) and artificial stimuli (Matsukawa et al, 2004). Although our own use of photographic stimuli may introduce an ecological confound, an ample literature suggests that subjects should be able to learn to categorize and serially order stimuli beyond those that are "ecologically relevant.…”