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Stimulus discrimination by horses under scotopic conditions
Evelyn B Hanggi1, Jerry F Ingersoll
1Equine Research Foundation, Aptos, CA 95001, USA. EquiResF@aol.com
Behavioural Processes
|April 25, 2009
Summary
Horses possess remarkable scotopic (low-light) vision, successfully performing visual tasks even in near-total darkness. This study confirms their superior night vision capabilities, supporting their rod-dominated visual system.
Area of Science:
- Veterinary Ophthalmology
- Animal Behavior
- Equine Vision Science
Background:
- Scotopic vision, or the ability to see in low light, is crucial for nocturnal and crepuscular animals.
- Previous research suggested horses have a rod-dominated visual system, implying good night vision, but direct behavioral evidence was lacking.
Purpose of the Study:
- To behaviorally investigate and quantify scotopic vision in horses (Equus caballus) for the first time.
- To determine the minimum light levels at which horses can perform visual discrimination tasks.
- To assess horses' ability to navigate in extremely dim light conditions.
Main Methods:
- Four horses were trained to discriminate between geometric shapes (circles and triangles) under controlled, progressively dimmer light conditions.
- Brightness levels were measured using a Sky Quality Meter, simulating outdoor twilight to dark, moonless night conditions.
- Navigation around obstacles in dim light was also observed.
Main Results:
- Horses successfully completed visual discrimination tasks at brightness levels as low as 23.77 magnitudes per square arcsecond (mag/arcsec(2)).
- They navigated obstacles in light levels exceeding 23.50 mag/arcsec(2), conditions too dark for human observers.
- Performance was maintained across a wide range of low light intensities.
Conclusions:
- Behavioral evidence strongly supports a rod-dominated visual system in horses, confirming excellent scotopic vision.
- Horses' ability to perform complex visual tasks and navigate in very dim light has significant implications for understanding their ecology and behavior.
- These findings validate previous physiological data and anecdotal observations of equine nocturnal activity.
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