Chapter 6 Vision

Sensation and Perception:Important Vocabulary Terms

• Sensation is the process of receiving, transducing, and coding stimulus energy in the world.– Stimulus energy is

physical energy, such as light, sound, heat

– Sense organs such as eyes, ears, skin receive energy

Sensation and Perception:Important Vocabulary Terms

– Reception: the absorption of physical energy by receptors

– Transduction: the conversion of physical energy into electrochemical energy

Sensation and Perception:Important Vocabulary Terms

– Perception: the brain’s process of organizing and interpreting sensory information to give it meaning

– Coding: a one-to-one communication between an aspect of a physical stimulus and an aspect of nervous system activity

Neuroanatomy Handout #4: The Visual System

• Sclera (A):– Outermost layer of

eye; white, fibrous, protective globe

• Cornea (A1): Clear disk at front of eye; focuses light rays onto the receptor cells at back of eye (retina)

• Aqueous humor (B): Fluid in the very front of the eyeball; refracts light rays onto retina

Neuroanatomy Handout #4: The Visual System

• Iris (C): Colored part in center of eye; ring of muscles that controls amount of light that gets into eye

• Pupil (D): Opening in center of eye (appears black); constricts or dilates with movement of iris

Neuroanatomy Handout #4: The Visual System

• Lens (E): Along with cornea, focuses light onto receptor cells

• Vitreous humor (F): Fluid filling majority of eyeball; refracts lightwaves

• Retina (G): lined with visual receptor cells, rods and cones

Neuroanatomy Handout #4: The Visual System

• Rods (I) - most abundant in the periphery of the eye

– 120 million per retina

– respond to faint light

• Cones (J) - most abundant in and around the fovea

– 6 million per retina

– essential for color vision, which requires bright light

• Photopigments: chemicals released by rods and cones when struck by light

Neuroanatomy Handout #4: The Visual System

• The fovea (G2): central portion of the human retina which allows for acute and detailed vision.– Packed tight

with receptor cells

Neuroanatomy Handout #4: The Visual System

• Rods and cones are located in the outmost layer of the eye.

• They communicate their messages to neurons called bipolar cells (K) and horizontal cells, which are located closer to the center of the eye.

• Bipolar cells send messages to ganglion cells (L) and amacrine cells that are even closer to the center of the eye.

Neuroanatomy Handout #4: The Visual System

• Axons of ganglion cells (L1) join one another to form the optic nerve.

• The optic nerve (L2) exits through the back of the eye and travels to the brain.

Neuroanatomy Handout #4: The Visual System

• Blind spot (G1): The point at which the optic nerve leaves the back of the eye– it contains no

receptor cells– It does contain

retinal veins and arteries (H)

Visual Coding and the Retinal Receptors• Perception of color is dependent upon the

wavelength of the light.

Visual Coding and the Retinal Receptors• “Visible” wavelengths depend upon species’

receptors.• Human range: 350 nanometers (violet, short

wavelength) to 700 nanometers (red, long wavelength).– Some species can detect ultraviolet light

Visual Coding and the Retinal Receptors

• Discrimination among colors depends upon the combination of responses by different neurons.

• Two major interpretations of color vision:

1. Trichromatic theory/Young-Helmholtz theory

2. Opponent-process theory

Visual Coding and the Retinal Receptors• Trichromatic theory -

Color perception occurs through the ratio of activity across the three types of cones :– short wavelength– medium-wavelength– long-wavelength

Visual Coding and the Retinal Receptors

• More intense light increases the brightness of the color but does not change the ratio and thus does not change the perception of the color itself.

Visual Coding and the Retinal Receptors• The opponent-process theory

(Ewald Hering) suggests that we perceive color in terms of paired opposites.– white/black– red/green– yellow/blue

• A possible mechanism for the theory is that bipolar cells are excited by one set of wavelengths and inhibited by another.

Negative color afterimage

Visual Coding and the Retinal ReceptorsThe opponent-process and trichromatic theories

can’t explain:• Color constancy, the ability to recognize color

despite changes in lighting.• Retinex theory suggests the cortex compares

information from various parts of the retina to determine the brightness and color for each area.

Visual Coding and the Retinal Receptors

• Color vision deficiency: impairment in perceiving color differences

• X-linked trait• Causes:

– lack of a type of cone– cone has abnormal

properties• Most common form:

difficulty distinguishing between red and green

The Neural Basis of Visual Perception

• Ganglion cell axons form the optic nerve.

• Optic chiasm: place where the two optic nerves meet.

• In humans, half of the axons from each eye cross to the other side of the brain.

• Most axons go to the lateral geniculate nucleus, a smaller amount to the superior colliculus and fewer go to other areas.

• Visual field: the whole area of the world that you can see at a given time

• Receptive field: the portion of the visual field to which any one neuron responds

“Eyes Right”by Oliver Sacks

From The Man Who Mistook His Wife for a Hat

The Neural Basis of Visual Perception

• Some people with damage to the primary visual cortex (V1) show blindsight, an ability to respond to visual stimuli that they report not seeing.

The Neural Basis of Visual Perception

• Visual agnosia is the inability to recognize objects despite satisfactory vision.– Caused by damage to the

pattern pathway usually in the temporal cortex.

• Prosopagnosia is the inability to recognize faces.– Occurs after damage to

the fusiform gyrus of the inferior temporal cortex.