Chapter 7 - Vision: From Eye to Brain

simple cortical cell

A cell in the primary visual cortex that responds best to an edge or a bar that has a particular width, as well as a particular orientation and location in the receptive field. as we get closer to preferred stimulus, increased firing rate of action potentials.

complex cortical cell

A cell in the visual cortex that responds best to a bar of a particular movement and direction anywhere within a particular area of the receptive field. as we get closer to preferred stimulus, increased firing rate of action potentials. receive info from lots of other cells.

trichromatic hypothesis

A hypothesis of color perception stating that there are three different types of cones, each excited by a different region of the spectrum and each having a separate pathway to the brain.

spatial-frequency model

A model of vision that emphasizes the analysis of different spatial frequencies, of various orientations and in various parts of the visual field, as the basis of visual perception of form.

photoreceptors

A neural cell in the retina that responds to light.

rods

A photoreceptor cell in the retina that is most active at low levels of light bc very light-sensitive, critical at night. these do not see color and have low acuity. one primary type rather than many subtypes. don't respond to different wavelengths of light. about 120M total. there are no rods in fovea, instead concentrated at periphery of retina.

scotoma

A region of blindness within the visual fields, caused by injury to the visual pathway or brain.

on-center bipolar cell

A retinal bipolar cell that is excited by light in the center of its receptive field. on-center bipolar cells inhibited by glutamate but not receiving much when photoreceptors hit with light, so they avoid inhibition and become depolarized. on center bipolar cells prefer when receptors in middle of receptive field exposed to light and periphery receptors to dark.

off-center bipolar cell

A retinal bipolar cell that is inhibited by light in the center of its receptive field. off-center bipolar cells excited by glutamate but not receiving much when photoreceptors hit with light, so not excited and instead hyperpolarized. prefer when receptors in middle of receptive field exposed to dark and periphery receptors to light.

on-center ganglion cell

A retinal ganglion cell that is activated when light is presented to the center, rather than the periphery, of the cell's receptive field.

off-center ganglion cell

A retinal ganglion cell that is activated when light is presented to the periphery, rather than the center, of the cell's receptive field.

lens

A structure in the eye that helps focus an image on the retina. transparent series of tissues that light travels through. refracts light further after cornea does.

photopic system

A system in the retina that operates at high levels of light, shows sensitivity to color, and involves the cones. See Table 7.1. Compare SCOTOPIC SYSTEM.

scotopic system

A system in the retina that operates at low levels of light and involves the rods. See Table 7.1. Compare PHOTOPIC SYSTEM.

primary visual cortex (V1)

Also called STRIATE CORTEX or AREA 17. The region of the occipital cortex where most visual information first arrives. See Figures 7.10, 7.11, 7.20.

occipital cortex

Also called VISUAL CORTEX. Cortex of the occipital lobe of the brain, corresponding to the primary visual area of the cortex. See Figure 7.10.

bipolar cells

An interneuron in the retina that receives information from rods and cones and passes the information to retinal ganglion cells. usually 1:1 ration b/w bipolar cells and cones, so only one receptive field; however, larger ratio of rods to bipolar cells, so multiple receptive fields.

ganglion cells

Any of a class of cells in the retina whose axons form the optic nerve. only 1 million of these to carry action potentials through optic nerve to brain.

cones

Any of several classes of photoreceptor cells in the retina that are responsible for color vision. different subtypes of these are responsible for different wavelengths of light/colors. help us to see finer detail; high acuity. about 6M total. concentrated in fovea and around it. mostly useless at night; better for seeing during bright daylight.

optic nerve

Cranial nerve II; the collection of ganglion cell axons that extend from the retina to the brain.

myopia

Nearsightedness; the inability to focus the retinal image of objects that are far away.

extraocular muscle

One of the muscles attached to the eyeball that control its position and movements.

ciliary muscles

One of the muscles that control the shape of the lens inside the eye, focusing an image on the retina. pull light in. pull or loosen lens to allow us to focus better. changes in contraction of these muscles leads to process of accommodation.

brightness

One of three basic dimensions of light perception, varying from dark to light.

saturation

One of three basic dimensions of light perception, varying from rich to pale. Compare BRIGHTNESS and HUE.

hue

One of three basic dimensions of light perception, varying through the spectrum from blue to red. Compare BRIGHTNESS and SATURATION.

amblyopia

Reduced visual acuity that is not caused by optical or retinal impairments.

on-center/off-surround

Referring to a concentric receptive field in which stimulation of the center excites the cell of interest while stimulation of the surround inhibits it. prefers when receptors in middle of receptive field exposed to light and periphery receptors to dark.

off-center/on-surround

Referring to a concentric receptive field in which stimulation of the center inhibits the cell of interest while stimulation of the surround excites it. prefer when receptors in middle of receptive field exposed to dark and periphery receptors to light.

visual acuity

Sharpness of vision.

optic ataxia

Spatial disorientation in which the patient is unable to accurately reach for objects using visual guidance.

optic tract

The axons of retinal ganglion cells after they have passed the optic chiasm. Most of these axons terminate in the lateral geniculate nucleus. See Figure 7.10.

refraction

The bending of light rays by a change in the density of a medium, such as the cornea and the lens of the eyes.

fovea

The central portion of the retina at back of eye, which is packed with the highest density of photoreceptors and is the center of our gaze. only cones here, not rods. cells look pulled away from fovea bc light doesn't have to diffuse through cells here as much. less useful at night bc the cones aren't very good at seeing at night. high acuity here because light reaches cones directly rather than being obscured by other cellular layers and blood vessels. looks dark in diagrams bc it absorbs light.

iris

The circular structure of the eye that provides an opening to form the pupil. pigmented muscle.

transduction

The conversion of one form of energy to another, as converting light into neuronal activity. occurs at photoreceptors, based on actions of photopigment molecules. rods and cones have tons of photopigments on them. photoreceptors respond to dark; are inhibited by the light. release most amount of glutamate in dark and less in light. in dark, Na+ channels open, causes depolarization, Ca2+ channels open, glutamate released. in light, Na+ channels closed, causes hyperpolarization, Ca2+ channels closed, less glutamate or none released.

wavelength

The length between two peaks in a repeated stimulus such as a wave, light, or sound. lower side of spectrum is blue, higher side is red.

pupil

The opening, formed by the iris, that allows light to enter the eye. allows light to go through center of eye, iris muscle compresses or opens it up more. reflects a balance of influences; dilation is controlled by sympathetic nervous system, constriction controlled by parasympathetic nervous system. in bright light, it contracts quickly to admit only about 1/16 as much light as when illumination is dim.

blindsight

The paradoxical phenomenon whereby, within a scotoma, a person cannot CONSCIOUSLY perceive visual cues but may still be able to make some visual discrimination.

lateral geniculate nucleus (LGN)

The part of the thalamus that receives information from the optic tract and sends it to visual areas in the occipital cortex. visual part of the thalamus. axons of neurons from here form the optic radiations and terminate in the primary visual cortex.

lateral inhibition

The phenomenon by which interconnected neurons inhibit their neighbors, producing contrast at the edges of regions. what we see is defined by edges. bipolar cells that prefer dark being inhibited by their neighbors that prefer light, so dark cell on other side of first dark cell exaggerates contrast. most basic type of illusion.

convergence

The phenomenon of neural connections in which many cells send signals to a single cell.

rhodopsin

The photopigment in rods, light-sensitive

optic chiasm

The point at which parts of the two optic nerves cross the midline. axons from have of retina toward nose cross over to opposite side of brain. after axons cross here, they are known collectively as optic tract.

blind spot

The portion of the visual field from which light falls on the optic disc. looks light because reflecting light because no receptors to absorb it. brain doesn't like inconsistencies; fills in this portion of visual field w/ what it predicts to be true.

accommodation

The process by which the ciliary muscles adjust the lens to focus a sharp image on the retina. helps us distinguish light.

retina

The receptive surface inside the eye that contains photoreceptors and other neurons. focuses vision coming in, responds to light. contains many different cells. rods and cones are furthest back, respond to light, but light has to pass through other layers first, then info passes from photoreceptors to bipolar cells to ganglion cells and optic nerve.

optic disc

The region of the retina that is devoid of receptor cells because ganglion cell axons and blood vessels exit the eyeball there. at this spot, light can't be detected and we can't see anything.

receptive field

The stimulus region and features that affect the activity of a cell in a sensory system. for vision, everything in space that one cell can see; each cell can detect one part of visual field. collectively, receptive fields of cells make up whole visual field.

adaptation

The tendency of rods and cones to adjust their light sensitivity to match current levels of illumination. when photopigments detect light and break apart, must regenerate after being hyperpolarized.

cornea

The transparent outer layer of the eye, whose curvature is fixed. The cornea bends light rays and is primarily responsible for forming the image on the retina. light goes here before iris. uses refraction to form image. only a fraction of the light that hits here ever makes it to retina.

visual field

The whole area that you can see without moving your head or eyes; everything in space your eyes can see

extrastriate cortex

Visual cortex outside of the primary visual (striate) cortex, contains v2-v5.

patient DF

after experience w/ carbon monoxide poisoning, DF lost ability to identify things she was viewing. she can say what objects are made of, and she has ability to use them if given to her, but she can't name them. DF's ventral vision stream had been damaged but not the dorsal stream. her intact dorsal pathway not only tells her where objects are, but also guides her movements to use these objects properly.

topographic projection

aka retinotopic mapping. A mapping that preserves the point-to-point correspondence between neighboring parts of space. For example, the retina extends a topographic projection onto the cortex.

visual pathways in human brain

dorsal stream on top is called the where pathway; cells in this area help us figure out where objects are in space, their motion, and guides our movement toward them. ventral stream on bottom is the what pathway, cares about color of objects and their form (damage here can lead to facial blindness). both originate from common source in visual cortex. pathways not completely separate bc many cross sections b/w them.

color perception

each cone has one of three classes of pigments. no receptors only sensitive to narrow parts of the visual spectrum; instead, the pigments exhibit broad sensitivities that substantially overlap even though they have different peaks of sensitivity (420, 530, and 560 nm). almost every stimulus uses two types of cones. acuity much lower for short wavelengths. some women have 4 types of cones.

visual signal

photoreceptors first detect light, send info to bipolar cells, then ganglion cells, whose axons form the optic nerve, which crosses the optic chiasm. photoreceptors and bipolar cells carry graded signal, but ganglion cells send action potentials. visual info from right visual field is processed first in left occipital lobe. after optic chiasm, the same ganglion cell axons are renamed the optic tract, which projects to the LGN in the thalamus, which then projects to the striate cortex/primary visual cortex (V1). cells in PVC send to secondary visual cortex/extrastriate cortex (V2-V5). some axons cross over from side of eyes closest to nose.

increase in myopia

this is the most common vision problem in young people. it develops if the eyeball is too long, causing the cornea and lens to focus images in front of the retina rather than on the retina. vision can be properly restored by glasses that correct refraction of the visual image onto the retina. causes blurred vision, and may be caused by indoor lighting and the lack of outdoor lighting.

hyperpolarization in response to light

visual system responds to changes in light. photoreceptors respond to dark not light because darkness is the preferred stimulus, so they become hyperpolarized by light. off-center bipolar cells excited by glutamate but not receiving much, so not excited and instead hyperpolarized. on-center bipolar cells inhibited by glutamate but not receiving much, so they avoid inhibition and become depolarized. when bipolar cells hit with preferred stimulus, become depolarized; when hit w/ not preferred stimulus, become hyperpolarized.

context effects on perception of brightness

when moving from bright sunlight to dim room, briefly can't see anything, rods more sensitive to light and become overwhelmed and nonfunctional, then recover gradually. when moving from dim room to bright sunlight, cones recover faster than rods because less light-sensitive.