Visual nesting of stimuli affects rhesus monkeys' (Macaca mulatta) quantity judgments in a bisection task

Area of Science:

• Cognitive Science
• Comparative Psychology
• Neuroscience

Background:

• Nonhuman animals possess a remarkable ability to estimate relative quantities across various sensory modalities.
• Performance in quantity judgments often adheres to Weber's law, consistent with the approximate number system (ANS) hypothesis.
• While generally accurate, animal quantity perception can be influenced by stimulus organization, mirroring human perceptual biases.

Purpose of the Study:

• To investigate whether rhesus monkeys exhibit a perceptual bias related to the illusion of nested sets during quantity estimation.
• To compare the quantity estimation biases of rhesus monkeys with those previously observed in adult humans.
• To determine if macaques' perception of quantity is influenced by the spatial arrangement of stimuli, specifically nesting.

Main Methods:

• Rhesus monkeys were presented with arrays of circles on a computer screen.
• Monkeys were tasked with classifying arrays as larger or smaller than a central reference value.
• Stimulus arrays incorporated nested circle arrangements to test for perceptual biases.

Main Results:

• Monkeys demonstrated a consistent underestimation of quantity when circles were nested within each other.
• This finding parallels previous observations of a similar bias in adult humans.
• While some monkeys adapted and overcame the bias, the majority continued to underestimate nested quantities.

Conclusions:

• Rhesus monkeys, similar to humans, exhibit perceptual biases in quantity estimation, specifically underestimating nested sets.
• The results support the ANS hypothesis and suggest shared mechanisms for numerical cognition and perceptual biases between macaques and humans.
• The study indicates that stimulus organization, like nesting, can significantly impact quantity judgments in nonhuman primates, even when individual elements are discernible.