chapter 5 - vision

law of specific nerve energies

nerve activity always conveys a specific type of information to the brain

ex. optic nerve stimulation is perceived as light

visual field

the entire area that a person can see while looking in a particular direction

retina

light-sensitive layer of tissue at the back of the eye that contains visual receptors and other neurons

pupil

opening of the center of the iris where light enters the eye

optic nerve

bundle of ganglion axons that carry visual information from the retina toward the brain

blind spot

location where the optic nerve exits the retina

• contains no rods or cones and therefore cannot detect light

fovea

small central area of the retina responsible for most sharp and detailed vision

midget ganglion cells

around 10% of ganglion cells

• receive information from relatively few receptors and are especially important for detailed vision and color information

• recognizes change in time (temporal resolution → motion)

• you don’t see motion, you only infer that something moves because snapshots change

bipolar cells

retinal neurons that transmit information from the rods and cones to ganglion cells

ganglion cells

retinal neurons whose axons form the optic nerve and carry visual information toward the brain

• only cells that fire a full action potential

rods

sensitive to dim light and important for night vision

• no color vision

• releases glutamate after photon enters the rod

cones

function best in brighter light and are responsible for color vision and fine visual detail

amacrine cells

most varied photoreceptor

• can hook up to another amacrine cell, bipolar cell, and a ganglion cell

• last cell that filters

• helps decipher contrasting lights and detect movement from different points

photopigments

light-sensitive chemicals within rods and cones that change their activity when they absorb light

trichromatic theory (young helmholtz theory)

color vision depends on three types of cones

• short, medium, and long wavelength

opponent process theory

visual system processes colors through opposing pairs (red vs green; black vs white)

negative color afterimage

staring at one color causes you to see its opposing color after looking away

retinex theory

color perception depends on comparing information across the retina

• brain can judge an object’s color under different lighting conditions

color constancy

ability to perceive an object’s color as relatively consistent even when the lighting conditions change

color vision deficiency

someone has difficulty distinguishing certain colors because of differences in cone functioning or visual pathways

lateral geniculate nucleus (LGN)

in thalamus → receives visual information from the retina and sends it to the primary visual cortex

horizontal cells

suppress certain rods and cones to prevent overload

• horizontal cell to cone = 1:1

• horizontal cell to rod = 1:5 → less detailed, which is why you can’t see in dark

primary visual cortex

brain region in occipital lobe that receives and processes visual information from the LGN

• LGN: in thalamus that receives visual information from the retina → visual info sent here (primary visual cortex)

V1

another name for the primary visual cortex

• first major cortical processing area for visual information

secondary visual cortex

areas of visual cortex that receive information from V1

• also do visual processing

koniocellular neurons

neurons in LGN that transmit certain types of visual information (aspects of color vision)

magnocellular neurons

• movement

• rapid changes

• coarse visual patterns

parvocellular neurons

color and fine patterns

receptive field

area of the visual field where stimulation influences the activity of a particular neuron

lateral inhibition

activated neurons reduce the activity of neighboring neurons, helping the brain detect contrast and boundaries

• caused by horizontal cells

feature detectors

neurons that respond especially strongly to particular visual characteristics, like lines, edges, angles, movement

simple cells

visual cortex neurons that respond best to lines or edges at a particular orientation and location within their receptive fields

complex cells

visual cortex neurons that respond to lines or edges at particular orientations, regardless of their exact location within the receptive field

end-stopped cells (hypercomplex cells)

visual cortex neurons that respond to lines or edges of a particular orientation and length

• respond less when the line extends beyond a certain point

sensitive period

developmental period during which the brain is especially responsive to particular types of experience

strabismus

condition where the eyes are misaligned and do not consistently point in the same direction

retinal disparity

difference between images received by the two eyes because they view the world from slightly different positions

blindsight

ability of someone with damage to the primary visual cortex to respond to visual stimuli without consciously experiencing visual awareness of them

aphantasia

inability or difficulty in voluntarily creating mental visual images

hyperphantasia

ability to create unusually vivid and detailed mental visual images

astigmatism

condition caused by irregular curvature of cornea or lens that results in distorted or blurred vision

dorsal stream

visual processing pathway that travels toward the parietal cortex

• helps determine an object’s location and how to interact with it

• “where/how” pathway

ventral stream

visual processing pathway that travels toward the temporal cortex

• helps identify objects, shapes, faces

• “what” pathway

inferior temporal cortex

region in the temporal lobe involved in recognizing and identifying complex visual objects

visual agnosia

inability to recognize or identify objects through vision despite having relatively intact basic visual abilities

fusiform gyrus

region of temporal cortex that plays important role in recognizing faces and other complex visual patterns

prosopagnosia

difficulty recognizing familiar faces, even though basic vision may remain normal

middle temporal cortex (MT or V5)

brain region specialized in processing visual movement, including the direction and speed of moving objects

medial superior temporal area (MST)

processes complex visual motion and patterns associated with movement through the environment

motion blindness

inability to perceive movement normally, caused by damage to brain regions responsible for processing visual motion

saccade

rapid movement of the eyes that shifts the point of fixation from one location to the other

p ganglion cells

70% of all ganglion cells

• go to thalamus

• help with spatial resolution and contrast

k ganglion cells

• detect changes in shading (increasing or fading of light)

• send their signal to suprachaismatic nuclei which sends signal to the p ganglion cells to produce melatonin

  • melatonin sends neurotransmitter called melanopsin to increase or decrease melatonin based on this signal from k ganglion