Rhesus macaques (Macaca mulatta) exhibit the decoy effect in a perceptual discrimination task

Area of Science:

• Cognitive psychology
• Animal behavior
• Decision science

Background:

• The asymmetric dominance effect (decoy effect) is a bias in decision-making where a third option influences the choice between two alternatives.
• This effect is well-documented in human consumer behavior and animal preference studies.
• Previous research has not extensively explored decoy effects in non-human primate perceptual discrimination tasks.

Purpose of the Study:

• To investigate whether decoy stimuli influence size judgments in rhesus monkeys.
• To determine if the orientation of decoy stimuli affects choice behavior in a perceptual discrimination task.
• To extend the understanding of context-dependent choice biases to non-human primates.

Main Methods:

• Rhesus monkeys performed a computerized task to discriminate the larger of two rectangles.
• A third stimulus (decoy) was introduced in probe trials, varying in size and orientation relative to the target stimuli.
• The decoy's orientation was either congruent or incongruent with one of the target rectangles.

Main Results:

• Monkeys rarely selected the decoy stimulus.
• Performance (selecting the largest rectangle) improved when the decoy's orientation was congruent with the larger rectangle.
• Performance decreased when the decoy's orientation was incongruent with the larger rectangle.

Conclusions:

• Decoy stimuli can significantly impact perceptual choice behavior in rhesus monkeys, even when not viable options.
• The findings suggest that comparative evaluation mechanisms play a role in mediating decoy effects in non-human primates.
• This study provides evidence for context-dependent choice biases in a non-human primate perceptual task, extending previous findings.