Categorical face perception in fish: How a fish brain warps reality to dissociate "same" from "different"

Area of Science:

• Neuroscience
• Cognitive Science
• Animal Behavior

Background:

• Categorical perception (CP) transforms continuous stimuli into discrete categories, a phenomenon influenced by learning.
• While studied in humans and some animals, CP in visual discrimination, especially facial patterns, remains less understood in non-primates.
• Previous research suggests CP is not limited to innate mechanisms and can be shaped by experience.

Purpose of the Study:

• To investigate learned categorical perception (CP) in a lower vertebrate, the damselfish (Pomacentrus amboinensis).
• To determine if damselfish discriminate complex facial patterns based on categorical rather than metric differences.
• To explore the influence of top-down processing on perception in species lacking a visual cortex.

Main Methods:

• Damselfish were trained to discriminate between sets of two facial patterns.
• Stimuli were morphed to create a continuum, and just noticeable differences (JNDs) were determined.
• Discrimination performance was assessed for stimuli across the morph continuum to identify category boundary effects.

Main Results:

• Damselfish demonstrated significant discrimination for facial pattern pairs straddling a learned category boundary.
• Discrimination performance dropped to chance levels for equivalent stimulus pairs within categories.
• This indicates a classic "category boundary" effect, characteristic of CP.

Conclusions:

• The findings reveal that damselfish exhibit learned categorical perception in facial pattern discrimination.
• This suggests that top-down perceptual influences can shape category boundaries even without a visual cortex.
• The study provides evidence for the flexibility of perceptual systems in non-primate vertebrates.