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Vision

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Vision is the result of light being detected and transduced into neural signals by the retina of the eye. This information is then further analyzed and interpreted by the brain. First, light enters the front of the eye and is focused by the cornea and lens onto the retina—a thin sheet of neural tissue lining the back of the eye. Because of refraction through the convex lens of the eye, images are projected onto the retina upside-down and reversed.

Anatomy of the Eyeball

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Visual System

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Light enters the eye through the cornea, a transparent, dome-shaped surface covering the surface of the eyeball that helps to direct and focus incoming light. This light is then channeled toward the pupil, an adjustable opening whose size is controlled by the iris. The iris, a pigmented muscle, regulates the amount of light entering the eye by contracting or dilating the pupil, thereby ensuring optimal light levels for clear vision.
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Related Experiment Video

Updated: Jul 8, 2026

A Simple Behavioral Assay for Testing Visual Function in Xenopus laevis

A Simple Behavioral Assay for Testing Visual Function in Xenopus laevis

Published on: June 12, 2014

Nonvisual photoreception in the chick iris.

Daniel C Tu¹, Matthew L Batten, Krzysztof Palczewski

¹Department of Ophthalmology and Visual Sciences, Washington University Medical School, St. Louis, MO 63110 USA.

Science (New York, N.Y.)

|October 2, 2004

Summary

This summary is machine-generated.

Chicken irises constrict to light using a non-visual pathway. This study reveals that cryptochrome, not opsins, mediates this light response in the embryonic chick eye, suggesting a conserved role for cryptochromes in vertebrate vision.

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Area of Science:

Ophthalmology
Molecular Biology
Developmental Biology

Background:

The embryonic chicken iris exhibits light-induced constriction.
This response's characteristics are inconsistent with typical visual phototransduction mechanisms.

Purpose of the Study:

To investigate the photoreception mechanism underlying light-induced iris constriction in embryonic chickens.
To determine whether opsin or non-opsin pigments mediate this response.

Main Methods:

Action spectrum analysis of iris constriction.
Assessment of pupillary response to saturating light exposure.
Effect of retinoid depletion and visual phototransduction inhibitors.
Impact of cryptochrome and melanopsin gene knockdown on photosensitivity.

Main Results:

Iris photosensitivity peaked in short-wavelength light, aligning with cryptochrome's absorption spectrum.
Pupillary responses showed potentiation, not attenuation, after intense light exposure.
Photosensitivity was independent of retinoids and visual phototransduction pathways.
Cryptochrome knockdown significantly reduced iris photosensitivity, while melanopsin knockdown had no effect.

Conclusions:

The embryonic chicken iris utilizes a non-opsin photoreception mechanism for light-induced constriction.
Cryptochrome appears to be the primary photoreceptor mediating this response.
This suggests a conserved role for cryptochromes in vertebrate ocular light sensing beyond traditional vision.