Vision

Vision
Fall 2002

Every object absorbs and reflects light. Your eye is able to detect reflected light from objects (e.g., leaves absorb most light & reflect green light). Light is carried by particles that have no mass called photons. To see something, a photon from the environment must hit a sensory receptor in the eye. The eye can only detect a limited range of these photons. That is why you can not see ultraviolet light (Figure 9.2). Other animals, like bees can detect ultraviolet light.

Parts of the eye (Figure 9.6):

cornea--protective covering over the eyes (lots of pain fibers). The cornea also bends light (and holds contact lenses). pupil--the hole through which photons enter the eye. The iris is a muscle that determines how much light enters the pupil. The iris regulates the amount of light entering the eye.
lens--focuses the incoming image on the retina
retina--location of sensory receptors and other neurons There are 5 types of neuron in the retina (Figure 9.11 & 9.12). photoreceptor--detects photons. Located in back of retina.
bipolar cell-transmits message from photoreceptor to ganglion cell
ganglion cell--projects to brain. The axons of these cells form the optic nerve (Cranial nerve #2). The place where these axons leave the retina has no photoreceptors and creates a blind spot (Box 9.1).
horizontal cell-run horizontally at level of photoreceptors & bipolar cells
amacrine cell--run horizontally at level of bipolar & ganglion cells Dark current--photoreceptors are odd in that they have a resting potential of -30 mV (vs. -70 mV in most other neurons). This resting potential is maintained by a constant inward Na+ current. Light striking photopigments in the photoreceptor activates second messengers that close these Na+ channels. This causes the photoreceptor to be hyperpolarized. That is, photoreceptors are active in the dark and inactive in the light. Because glutamate released by the photoreceptors hyperpolarizes some bipolar cells, inhibition of glutamate causes bipolar cells to become active.

Each of the neurons in the retina contributes to processing information in the visual field. An important concept in understanding this processing is receptive field. Receptive field is defined as the stimulus that a neuron responds to. That is, what stimulus in the environment causes a change in the neuron?

The receptive field for a photoreceptor is a photon coming from one part of the visual field.

Bipolar and ganglion cells have center-surround receptive fields. That is, light in one spot will activate the neuron and light in the surround will inhibit the neuron. This lateral inhibition is from horizontal cells carrying information from the surrounding area (Figure 9.22, 9.23, & 9.24). These horizontal and amacrine cells are very important in that they enhance contrast between ganglion cells and this helps you see contrasts (called lateral inhibition; Figure 9.25).

There are several differences between the center and peripheral retina (Figure 9.14):

Retina location	Photoreceptor	Function	Relationship to Ganglion cells	Visual field
Fovea (center)	Cones	Color vision Acuity	One to one	Front
Peripheral retina	Rods	Night vision Motion detection	Many to one	Periphery

Color vision

There are two theories of color vision:

1) Trichromatic theory (Figure 9.20)--colors are produced by the relative activity in 3 types of photoreceptors: blue (420 nm); green (531 nm); & red (558 nm). This is an example of a pattern code (Figure 9.21).

2) Opponent Process Theory (Figure 9.27)--colors are produced by excitatory and inhibitory input to ganglion cells. Ganglion cells show two types of opponent processes: Red/green and blue/yellow.

Visual pathways in the brain (Chapter 10)

The 2nd cranial nerve (optic nerve) takes visual information from the retina to the brain. The optic nerve changes name to optic tract as it enters the brain. The optic tract synapses in the lateral geniculate nucleus (the LGN is in the thalamus). Input to the LGN is segregated into six layers. LGN neurons synapse in the V1 region of the occipital lobe.

There are actually three distinct systems (motion, shape, and color) that follow this pathway from the retina to V1 cortex.

The human brain divides world into right and left visual fields (Figures 10.3 & 10.4).

Everything in right part of world goes to left hemisphere.
Everything in left part of world goes to right hemisphere. Light goes into both eyes so the input to each eye is split. Input to the lateral retina (by the temple) goes to the LGN & cortex on the same side.
Input to the medial retina (by the nose) crosses to the LGN & cortex on the opposite side.
This crossing occurs at the optic chiasm.
What visual field deficits do you have following lesions to the: Left optic nerve?
Left optic tract?
Optic chiasm?
The primary visual cortex (V1) distributes information to cortical regions called V3, V4, &V5

Cortex	Function	Deficit with lesion
V1	Relays input	Blind
V2	Relays input	?
V3	Form	Canít copy drawings
V4	Color	Only see grays
V5	Motion	Canít detect motion

Receptive fields in the cortex become increasingly complex.

Ganglion cells and LGN neurons have center-surround receptive fields.

Multiple LGN neurons project to a single neuron in V1 cortex. Combining three center-surround receptive fields creates a line. Thus, a line is the best stimulus to activate V1 neurons.

Increasingly complex shapes are coded by adding inputs from multiple V1 neurons.

Although the optic nerve carries all visual input to the brain. Four other cranial nerves contribute to vision by controlling the muscles that move the eyes. These cranial nerves are: #3 Oculomotor
#4 Trochlear
#6 Abducens
#11 Spinal accessory: coordinates eyes with movements Return to Psych 372 Syllabus