# Luminance Contrast Perception in Killer Whales (Orcinus orca)

## TL;DR

Killer whales can perceive contrast better with gray backgrounds, which may help them recognize objects underwater.

## Contribution

This study experimentally demonstrates luminance contrast perception in killer whales using a novel visual discrimination task.

## Key findings

- Killer whales showed higher correct response rates with gray backgrounds in luminance discrimination tasks.
- Contrast perception in killer whales resembles the luminance contrast illusion observed in humans.
- Contrast may serve as a key cue for object recognition in killer whales despite limited visual acuity and color vision.

## Abstract

The underwater environment is very different from the terrestrial environment, in which most mammals live. The cognitive abilities of cetaceans (dolphins, porpoises, and whales) have been greatly affected by the characteristics of the underwater environment. While their auditory abilities have been studied extensively, there has not been enough research focused on their visual abilities. This study focused on their visual ability to perceive contrast and aimed to experimentally investigate whether the enhancement of contrast can be observed in killer whales. Luminance discrimination tasks were performed on two captive killer whales, which were required to compare the luminance of two targets presented in monitors through an underwater window and to choose the brighter one. Although there were some individual differences, both individuals showed higher correct response rates when the two targets were surrounded by gray backgrounds than when they were surrounded by black or white backgrounds. The results suggest that contrast was perceived as enhanced in killer whales, as in humans. For killer whales with neither high visual acuity nor color vision, contrast may be an important cue for visual object recognition and may help them to extract the contours of objects even in the underwater environment.

Cetaceans are highly adapted to the underwater environment, which is very different from the terrestrial environment. For cetaceans with neither high visual acuity nor color vision, contrast may be an important cue for visual object recognition, even in the underwater environment. Contrast is defined as the difference in luminance between an object and its background and is known to be perceived as enhanced by the luminance contrast illusion in humans. The aim of this study was to experimentally investigate whether the enhancement of contrast by the luminance contrast illusion could be observed in killer whales. Luminance discrimination tasks were performed on two captive killer whales, which were required to compare the luminance of two targets presented in monitors through an underwater window and to choose the brighter one. After baseline training, in which the target areas were surrounded by black or white inducer areas, the test condition of gray inducer areas was added. Although there were some individual differences, both individuals showed higher correct response rates for gray inducer conditions than for black and white. The results suggest that contrast was perceived as enhanced by the illusion also in killer whales and may help them to extract the contours of objects.