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Vision

Vision

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.

Visual System

Visual System

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.
Once through the pupil, the light passes through the lens, a...

Anatomy of the Eyeball

Anatomy of the Eyeball

The eye is a spherical, hollow structure composed of three tissue layers. The outer layer — the fibrous tunic, comprises the sclera — a white structure — and the cornea, which is transparent. The sclera encompasses some of the ocular surface, most of which is not visible. However, the 'white of the eye' is distinctively visible in humans compared to other species. The cornea, a clear covering at the front of the eye, enables light penetration. The eye's middle layer, the vascular tunic,...

Photoreceptors and Visual Pathways

Photoreceptors and Visual Pathways

At the molecular level, visual signals trigger transformations in photopigment molecules, resulting in changes in the photoreceptor cell's membrane potential. The photon's energy level is denoted by its wavelength, with each specific wavelength of visible light associated with a distinct color. The spectral range of visible light, classified as electromagnetic radiation, spans from 380 to 720 nm. Electromagnetic radiation wavelengths exceeding 720 nm fall under the infrared category, whereas...

The Retina

The Retina

The retina is a layer of nervous tissue at the back of the eye that transduces light into neural signals. This process, called phototransduction, is carried out by rod and cone photoreceptor cells in the back of the retina.

Color Vision

Color Vision

Color perception begins in the retina, the light-sensitive layer at the back of the eye. Two main theories explain how colors are seen: the trichromatic theory and the opponent-process theory. The trichromatic theory, proposed by Thomas Young in 1802 and extended by Hermann von Helmholtz in 1852, suggests that color vision is based on three types of cone receptors in the retina. These cones are sensitive to different but overlapping ranges of wavelengths corresponding to red, blue, and green.

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Related Experiment Video

Updated: Jul 20, 2026

4-Dimensional Imaging of Zebrafish Optic Cup Morphogenesis

4-Dimensional Imaging of Zebrafish Optic Cup Morphogenesis

Published on: May 26, 2021

Origin of the vertebrate visual cycle.

Noriko Takimoto¹, Takehiro Kusakabe, Motoyuki Tsuda

¹Department of Life Science, Graduate School of Life Science, University of Hyogo, Hyogo, Japan.

Photochemistry and Photobiology

|August 26, 2006

Summary

The ascidian visual cycle involves distinct proteins in larval and adult stages. Larval stages use Ci-opsin3, while adults rely on RPE65, suggesting a shift in visual cycle mechanisms.

Area of Science:

Marine Biology
Molecular Biology
Evolutionary Biology

Background:

The visual cycle is crucial for light detection in vertebrates and cephalopods, involving chromophore isomerization and regeneration.
Vertebrate visual cycles occur in photoreceptor and retinal pigment epithelial (RPE) cells, while cephalopod cycles are completed within photoreceptor cells.
Ascidians, as primitive chordates, offer a unique model to study the evolution of the visual cycle.

Purpose of the Study:

To investigate the presence and localization of visual cycle genes and proteins in the ascidian Ciona intestinalis.
To determine the specific roles of identified visual cycle proteins in larval versus adult ascidians.
To elucidate the cellular mechanisms of the visual cycle in ascidians and compare them to other animal groups.

Main Methods:

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Last Updated: Jul 20, 2026

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Published on: May 26, 2021

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Published on: February 11, 2022

In situ hybridization was used to determine the localization of ascidian visual cycle genes.
Whole-mount immunohistochemistry was employed to map the distribution of visual cycle proteins.
Comparative analysis of gene and protein localization between larval and adult ascidian stages was performed.

Main Results:

Four putative visual cycle genes were identified: Ci-opsin3, Ci-CRALBP, Ci-BCO, and Ci-RPE65.
Ci-BCO was localized in larval ocellus photoreceptor cells, while Ci-RPE65 was not significantly expressed in larval ocelli or brain vesicles.
Ci-RPE65, Ci-opsin3, and Ci-CRALBP were expressed in the adult neural complex, with proteins localized to photoreceptor cells.

Conclusions:

The larval ascidian visual cycle appears to utilize Ci-opsin3 as a photoisomerase.
The adult ascidian visual cycle is dependent on Ci-RPE65, indicating a developmental shift in mechanism.
Colocalization of visual cycle proteins within photoreceptor cells suggests the ascidian visual cycle occurs intracellularly, similar to cephalopods.