Visual Cortex and Perception Quiz

Vision Overview

• Vision (sight) is the perception of objects in the environment through emitted or reflected light.
• More than half of the sensory receptors in the human body are located in the eyes.
• A large part of the cerebral cortex is dedicated to processing visual information.

Light

• Light is a form of visible electromagnetic radiation.
• Human vision wavelengths range from 400 to 700 nm (visible light).
• Ozone, carbon dioxide, and water vapor in the atmosphere filter out most solar radiation outside the visible light range.
• The speed of light in a vacuum is 300,000,000 m/sec.

Light Refraction

• Refraction is the bending of light as it passes between different media (e.g., air to water).
• The amount of bending depends on the angle of incidence and the difference in refractive index between the two media.

Refractive Index

• The refractive index of a medium is the ratio of the speed of light in a vacuum to its speed in that medium.
• The refractive index of vacuum/air is arbitrarily set at 1.
• The refractive index of water is 1.33.

Lenses

• Lenses are transparent materials with curved surfaces that refract light.
• Convex lenses converge light rays, while concave lenses diverge them.
• The focal length of a lens is the distance between its optical center and its focal point.

Dioptric Power

• The dioptric power of a lens is the reciprocal of its focal length in meters.
• It is measured in diopters (D).
• The power of a lens is determined by its shape and material.

Eye Structure

• The eyeball is a sphere approximately 24 mm in diameter.
• The eye has numerous layers and components that contribute to light refraction and focusing.
• The major components of the eye (in sequence) are the cornea, aqueous humor, lens, and vitreous body.

Optical Components of the Eye

• The eye’s optical components (cornea, aqueous humor, lens, and vitreous humor) are transparent for light transmission and refraction.
• They bend light to create an image on the retina.
• The optical system of the eye creates images on the retina.
• Refraction occurs from cornea and the lens.
• Accommodation of the lens – change by ciliary muscles – adjusts lens shape.
• Pupil size changes, controlled by the iris, to adjust light entering the eye.

Reduced Eye

• The reduced eye simplifies the eye to a single refracting surface.
• Its central point is 17 mm in front of the retina.
• The total refractive power of the reduced eye is 59 diopters at a distance.

Refractive Power of the Eye

• The cornea and the lens contribute to the eye’s refractive power.
• The cornea constitutes about two-thirds (40 diopters) of the total refractive power.
• The lens constitutes about one-third (20 diopters) of the total refractive power.

Errors of Refraction

• Errors in the shape or curvature of the eye can lead to common vision problems:
• Emmetropia (normal vision), Hyperopia (farsightedness), and Myopia (nearsightedness).

Astigmatism

• Astigmatism is an eye condition where the cornea or lens has an irregular shape.
• This causes blurred vision due to the uneven refraction of light.

Accommodation

• Accommodation is the process of adjusting the eye's focus for near and far vision.
• It involves changes in the shape of the lens, controlled by the ciliary muscles.
• The near response involves accommodation, pupil constriction, and convergence.

Accommodation for Near Vision

• When focusing on a near object:
• Ciliary muscles contract.
• Lens gets thicker (increasing refractive power).
• The pupil constricts.
• The eyes converge.

Accommodation for Far Vision

• Focusing on distant objects is achieved by relaxation of the ciliary muscles, creating a thinner and flatter lens.

Amplitude of Accommodation

• Accommodation ability decreases with age.
• Young children have the highest amplitude (up to 14 diopters).
• Older adults lose the ability to accommodate (presbyopia).

Regulation of Pupil Size

• Pupil size is regulated by the iris.
• The iris contains radial and circular muscles.

Pupil Changes

• Pupil size changes in response to various factors including light intensity, emotions, and shifts in focus.
• Pupil constriction (myosis) occurs in response to bright light, and dilation (mydriasis) to dim light.
• There are two types of pupil reflexes: direct and consensual.

Argyll Robertson Pupil

• Argyll Robertson pupil is a neurological sign characterized by pupils that don't react to light but do react to accommodation.
• It is often caused by neurosyphilis in the past.

Near-Vision Triad

• Accommodation, pupil constriction, and convergence of eyes.

Fluid System of the Eye

• The aqueous humor maintains intraocular pressure.
• The vitreous humor maintains the eye’s spherical shape.

Retina

• The retina is the light-sensitive tissue lining the inner layer of the eye.
• The retina contains photoreceptors (cones and rods), bipolar cells, amacrine cells, and ganglion cells.

Photoreceptors

• Rods detect dim light (scotopic vision).
• Cones detect bright light and colors (photopic vision).

Fovea

• Fovea is the central part of the retina that has the highest visual acuity.
• Cones are highly concentrated in the fovea.

Photoreceptor Distribution/Retina

• The distribution of photoreceptors differs across the retina: higher density of cones in the center (fovea) and a greater concentration of rods in the periphery.

Phototransduction

• Phototransduction is the process by which light energy is converted into a receptor potential in the photoreceptors.

Rods and Cones

• Rods are more sensitive to light and are used for vision in low light conditions.
• Cones have a higher density in the fovea and provide color vision and better visual acuity in bright light.
• Rods are about 30 to 300 times more sensitive to light than cones.

Color Vision

• Color vision is attributed to three types of cones: short (blue), medium (green), and long (red) wavelength-sensitive cones.
• The perception of color depends on the balanced stimulation of these cones.

Retinal Neurons

• Horizontal cells provide lateral inhibition.
• Amacrine cells act as part of direct pathways for rod signals, provide offset signals, and signal illumination changes.
• Ganglion cells transmit visual signals to the brain.

Ganglion Cells

• Ganglion cells in cats and humans show differences in size and response/receptive field characteristics.
• Categories include "P" (parvocellular), and "M" (magnocellular).

Melanopsin-Containing Ganglion Cells

• These cells detect light, and they send signals, mainly to areas of the brain which are not visual.

Convergence in Retina

• Rods converge more than cones for better sensitivity in dim light.
• Cones have less convergence in the fovea for higher visual acuity.

Scotopic and Photopic Vision

• Scotopic vision is vision in low-light conditions (rods).
• Photopic vision is vision in bright light conditions (cones). These differ in their functions and pathways in the retina.

Visual Pathways

• The visual pathway is complex, involving multiple brain areas and relay stations from the retina to visual cortex.
• The visual cortex in the occipital lobe is involved in further processing of visual information.

Visual Cortical Cells

• Simple cells respond to lines of a specific orientation.
• Complex cells respond to moving lines and edges.
• Hypercomplex cells respond to lines of a particular length (and sometimes corners).

Visual Adaptation

• Light adaptation: Pupil constricts; reduces receptor potential amplitude and sensitivity to light.
• Dark adaptation: Pupil dilation; increases retinal sensitivity with photosensitive chemicals and receptors.

Summary Tables

• Includes tables comparing rods and cones, and scotopic/photopic vision in the retina.