Optional-switch cognitive flexibility in primates: Chimpanzees' (Pan troglodytes) intermediate susceptibility to cognitive set.

Within human problem-solving, the propensity to use a familiar approach, rather than switch to a more efficient alternative is pervasive. This susceptibility to "cognitive set" prevents optimization by biasing response patterns toward known solutions. In a recent study, which used a nonverbal touch screen task, baboons exhibited a striking ability to deviate from their learned strategy to use a more efficient shortcut. Humans, on the other hand, displayed the opposite response pattern and almost exclusively used a less efficient, but familiar, response. In the current study, we sought to further explore variation in susceptibility to cognitive set within the primate lineage by conducting the Learned Strategy-Direct Strategy task with 10 chimpanzees (Pan troglodytes). Using multilevel multinomial modeling, we found that chimpanzees' shortcut use was intermediate to baboons' and humans'. However, unlike either baboons or humans, there was pronounced inter- and intraindividual variability in chimpanzees' shortcut use. Additionally, a subset of chimpanzees employed a unique solution, wherein they switched strategies midtrial. Further, we found that chimpanzees did not exhibit switch costs when switching between the learned strategy and the shortcut, but humans did. We propose that differences in abstract rule encoding may underlie differences in susceptibility to cognitive set on the Learned Strategy-Direct Strategy task within the primate lineage. (PsycINFO Database Record (c) 2020 APA, all rights reserved).

Age-related effects in the neocortical organization of chimpanzees: gray and white matter volume, cortical thickness, and gyrification.

Among primates, humans exhibit the most profound degree of age-related brain volumetric decline in particular regions, such as the hippocampus and the frontal lobe. Recent studies have shown that our closest living relatives, the chimpanzees, experience little to no volumetric decline in gray and white matter over the adult lifespan. However, these previous studies were limited with a small sample of chimpanzees of the most advanced ages. In the present study, we sought to further test for potential age-related decline in cortical organization in chimpanzees by expanding the sample size of aged chimpanzees. We used the BrainVisa software to measure total brain volume, gray and white matter volumes, gray matter thickness, and gyrification index in a cross-sectional sample of 219 captive chimpanzees (8-53 years old), with 38 subjects being 40 or more years of age. Mean depth and cortical fold opening of 11 major sulci of the chimpanzee brains were also measured. We found that chimpanzees showed increased gyrification with age and a cubic relationship between age and white matter volume. For the association between age and sulcus depth and width, the results were mostly non-significant with the exception of one negative correlation between age and the fronto-orbital sulcus. In short, results showed that chimpanzees exhibit few age-related changes in global cortical organization, sulcus folding and sulcus width. These findings support previous studies and the theory that the age-related changes in the human brain is due to an extended lifespan.