Vision

Basis of vision

• most comprehensively studied sensroy system in the human body

• estimated that 70% of out environment is experienced through vision

• requires the detection of individual particles of light (photons)

Retina

layer of neurons in the back of the eye

where transduction occurs

Similarities with camera (structures)

• cornea: bend slight entering eye

• lens: changes shape to focus light on retina

Refraction

bending of light rays

Ciliary muscles

adjust focus by changing shape of the lens

contraction causes nearer or farther images to come into focus, process of accommodation

Retina cell types

• Photoreceptors

• Bipolar cells

• ganglion cells

• Horizontal cells

• amacrine cells

Photoreceptors

sensory neurons that detect light

• Rods: Night vision (low illumination)

• Cones: Color vision

Bipolar cells

receive input from photoreceptors and synapse on ganglion cells, whose axons form the optic nerve, which carries information to the brain

Ganglion cells

conduct action potential out of the eye and towards the brain

Horizontal cells

contact photoreceptors and bipolar cells

Amacrine cells

contact bipolar and ganglion cells

Potentials

All cell types except ganglion cells generate only graded potentials

• ex: more light = greater response

Rods system

Scotopic system

works in dim light, sensitive to color

convergence in the scotopic system: information from many rods converges onto each ganglion cell

Cones system

Photopic system

requires more light, sensitive to different wavelengths, enabling color vision

Structure of rods and cones

Stacked-disc strcuture contributes to sensitivity

photons strike the discs and are captured by photopigment receptors

Photopigments

Rods: rhodopsin

Cones: photopsin

Photoreceptors and light

• In the dark: continually release neurotransmitter

• Light: triggers a hyperpolarization of the cell, causing it to release less neurotransmitter

Size of hyperpolarizing signal (brightness of light) determines how much neurotransmitter will not be released (graded potential)

Visual acuity

measure of how much detail we see, sharpest in the center of the visual field

Fovea

center region of the retina, has a high density of smaller, lightly-packed cones with high acuity

Rod properties

more numerous in the perophery, absent in the fovea

high sensitivity to dim light but low acuity

Visual acuity in the fovea comes from

1. high concentration of cones distributed closer to the fovea

2. ability for light to pass through unobstructed directly to the fovea to activate photoreceptors

Optic disc

where blood vessels and ganglion cell axons leave the eye

Blind spot

region on the retina that is sightless due to lack of photoreceptors in the optic disc

Missing information

brain systems fill in missing imformation so that we perceive an uninterrupted scene

pathway form eye to the brain

• ganglion cells axons (optic nerve) conduct action potentials, sending information to the brain

• in vertebrates, some or all of each optic nerve crosses the midline at the optic chiasm; after crossing, it is known as the optic tarct

• in humans, only axons from the medial half of the retina cross to the opposite cerebral hemisphere

• axons from the lateral half of the retina project to the same side of the brain

Lateral geniculate nucleus (LGN)

Most axons of the optic tract terminate on cells in the lateral geniculate nucleus, the visual part of the thalamus

Optic radiations

Axons of the LGN neurons form the optic radiarions which terminate in the primary visual cortex (V1) of the occipital cortex

V1

primary visual cortex

also called sriate cortex due to its striped appearance in cross sections

Extrastriate cortex (secondary visual cortex

visual cortex areas outside of V1

processes other aspects of visual perception such as form, color, location, and movement

Small proportion of the retina is porcessed by a proportionately larger area of V1 (cortical magnification)

this allows for greater processing of details of information arriving at the fovea (in addition to the high concentration of cones in the fovea)

Blindsight

describes the inability to consciously perceive visual cues, yet be able to discriminate based on them

Photoreceptors at rest

steadily release the neurotransmitter glutamate onto bipolar cells

Glutamate

hyperpolarizes one group of bipolar cells and depolarizes another

On-center bipolar cells

turning on light excites this cell because it receives less glutamate. Glutamate inhibits this type of bipolar cell

• turning on a light depolarizes this cell

Off-center bipolar cells

turning off light excites this cell because glutamate is excitatory and leads to depolarization

• turning on light hyperpolarizes this cell

Ganglion cells

bipolar cells synapse onto ganlgion cells

bipolar cells also release glutamate which alwasy depolarizes ganglion cells

on-center ganglion cells

on-center bipolar cells excite on-center ganglion cells when depolarized

• excited by light

Off-center ganglion cells

off-center bipolar cells excite off-center ganglion cells when depolarized

• excited by dark

Receptive fields

the receptive fields of retinal bipolar/ganglion cells are concentric—a cricular central area with a ring around it, called the surround

two teffects)ypes of receptive fields of the retina

• On-center/off-surround

• Off-center/on-surround

center and its surround are always antagonistic (photoreceptors with opposite effects)

Lateral inhibition

exciting the surround cells lead to a response from horizontal and amacrine cells that creates an opposing effect

Simple cortical cells

also called bar detectors or edge detectors

respond to an edge or bar of a particular width, orientation, and location in the visual field

Complex cortical cells

respond best to a bar of a particular width and orientation that is in motion anywhere in the visual field

complex cortical cells in V1 will respond o bars of light moving across the retina in a particular direction

Hierarchical model

suggest that simple stimuli is processed by lower-level regions (e.g. LGN) and that multiple cells send information to one cell at upper-level regions (e.g. simple cortical cell)

Hierarchical model example

one LGN cell processes one spot of light

A row of LGN cells sends this information to a simple cortical cell which processes it as a line of light in a particular orientation

Beyond V1

projections from the eye terminate at V1

only a small portion of cortex devoted to vision is contained in V1

extend to other cortical areas

V2, V4, and the inferior temporal lobe

V2 cells

can respond to boundaries and borders

V4

respond to color

V5

respond to motion

Color

created by visual system as we detect differences in the wavelike movement of photons through space

differences in wavelenths allow us to perceive different colors (within a certain range)

3 stages of color perception

1. cones receive visual information about color

2. neurons in the local retinal circuit are excited (or inhibited) by certain wavelengths of light, signals sent to retinal ganglion cells

3. information travels to V1 and other visual cortical areas for complex color perception (area V4)

Objects color

depends on which wavelengths are reflected and thus perceived

the other colors are absorbed

trichromatic model of color perception

• three different types of cones

• each responds to a specific, different path on the spectrum

• each has a separate pathway to the brain

• color recognized based on which receptors are activated

Each cone

contains one of three pigments which have overlapping but different peaks of sensitivity to different wavelengths

Color information

under most conditions, most objects stimulate at least two types of cones

brain compares the degree of activation and extracts color information

Color-specific cells

certain ganglion cells and higher level cells are color-specific—they excite when particular colors are detected

Cone names

• Short (S)—420 nm

• Medium (M)—530 nm

• Long (L)—560 nm

named for peak area of wavelength sensitivity

Pattern of activation

all perceived colors are a result of a pattern of activation across three types of cones

Primate ancestors

May have had only one photopigment gene on chromosome 7 (short wave), and one on the X chromosome (long wave; which was then duplicated and mutated into medium)

Color blindess is more common in males

genes encoding photopigments are carried on the X chromosome—in females a normal copy can compensate for a defective gene

Women

some women have four different kinds of cones (two different long wave cones)—they are very good at discriminating colors

• may be sensitive to clashing colors

opponent-process model of color perception

after different wavelengths are detected by 3 types of cones, these cones create opposing responses to pairs of colors in downstream cells

• three opposed pairs of colors

  • blue vs yellow (blueness and yellowness)

  • green vs red (greenness and redness)

  • black vs white (brightness)

Spectrally opponent cells (color-opponent)

have opposite firing responses to different regions of the color spectrum

Spectrally opponent ganglion cells

receive input from two or three different types of cones through bipolar cells

• excitatory connections from at least one input

• inhibitory connections from at least one input

Third stage of color vision system

perceptually opponent cells in cortex respond to differences in colors

V4 is rich in color-sensitive cells

V4 cells respond best if color outside receptove field is different from color preferred inside the receptive field