BN chapter 6. Vision

sensory receptor

a specialized neuron that detects a particular category of physical events

sensory transduction

the process by which sensory stimuli are transduced into slow, graded receptor potentials

receptor potential

a slow, graded electrical potential produced by a receptor cell in response to a physical stimulus

The stimulus

• no species can see in the dark, but some are capable of seeing when there is little light

• light: photons of energy, waves of electromagnetic radiation

• humans see light btwn 400-700 nanometers

light interacts w the environment

• absorption and reflection of visible light by objects determine the colors we see

• air and water refract, or change the direction, of traveling waves of light

protecting the eye

• located in bory orbit of the skull

• cushioned by fat

• blinking and tears - lacrimal gland - blink every 4-6 seconds

the functional anatomy of the eye

• sclera, cornea, anterior chamber (aqueous humor), pupis, iris, lens, vitreous chamber (vitreous humor), retina

accommodation

lens - focuses light on the retina

• ciliary muscles alter the shape of the lens

• looking near - ciliary muscles contract - reducing tension on ligaments - lens relaxes - refracting more light so close objects are sharper focus

• looking far - ciliary muscles relax - increasing tension on ligaments - lens flattens - refracting less light and far images are clearer

retina

the neural tissue and photoreceptive cells located on the inner surface of the posterior portion of the eye

rod

photoreceptor cells of the retina, sensitive to light of low intensity

cone

photoreceptor of the retina, maximally sensitive to one of three diff wavelengths of light hence encodes color vision

fovea

• area of retina that mediates the most acute vision

• contains only color-sensitive cones

optic disk

• Location on the retina where fibers of ganglion cells exit the eye

• responsible for the blind spot

the retina

• built inside out - light passes through several cell layers before reaching its receptors

• back of eye going toward front of eye

• receptors → horizontal cell layer → bipolar cells → amacrine cell layer → retinal ganglion cells

• lateral communication: horizontal cells, amacrine cells

layers of retina

• ganglion cell layer: ganglion cells

• inner plexiform layer: ganglion cells form connections w amacrine and bipolar cells

• inner nuclear layer: cell bodies of bipolar, amacrine, and horizontal cells

• outer plexiform layer: bipolar cells form connections w horizontal cells and photoreceptors

• outer nulcear layer: cell bodies of photoreceptors

fovea - in the macula

• at the center of the retina, high acuity

• axons thinnist over fovea

• reduced light distortion

the axons of retinal ganglion cells penetrate the retina - go out of eye

• exit at the optic disc - no receptors

• creates the blind spot

completion

Duplexity theory of vision

cones and rods mediate diff kinds of vision

cones

phototopic vision

• high acuity in good lighting

• less convergence at next layer of retina

rods

scotopic vision

• high sensitivity, allowing for low acuity vision in dim light

• high convergence

More convergence in rod system, increasing sensitivity while decreasing acuity

which receptors are found at the fovea?

• High Cone Density: The fovea contains an extremely high concentration of cones and virtually no rods.

• Color & Detail: Because cones require brighter light and are responsible for detecting color, the fovea provides our sharpest, highest-resolution central vision (visual acuity).

• Direct Connection: In the center of the fovea (the foveola), other retinal layers and blood vessels are pushed aside so light can hit the cones directly without obstruction.

which receptors are found in the periphery?

• High Density of Rods: While cones are concentrated in the center of the retina (the fovea), rods dominate the outer edges. They make up the vast majority of photoreceptors in the human eye (roughly 120 million rods compared to 6 million cones).

• Night and Dim-Light Vision: Rods are extremely sensitive to light, making them ideal for seeing in low-light conditions and at night (scotopic vision).

• Motion Detection: Peripheral vision is highly attuned to detecting movement and contrast across a wide field of view, though it provides lower detail and resolution compared to central vision.

• Low Cone Density: A small number of cone receptors are still scattered throughout the periphery, but their concentration drops off significantly as you move away from the central retina.

Saccades

we continually scan the world w small and quick eye movements, eyes shake

• these bits of info are then integrated

eye movement

visual system responds to change

• stabalize retinal image - see nothing

• Photoreceptors adapt to constant stimulation and stop firing. If a retinal image is stabilized on the exact same spot, vision completely fades; tiny involuntary eye movements (microsaccades) keep images moving to prevent this.

results in detailed color and wide angled vision

• despite small size of fovea

• The fovea provides sharp color vision but covers only 1–2 degrees of view. Rapid eye jumps (saccades) constantly point the fovea at different areas, blending detailed snapshots with peripheral vision to build a seamless, wide-angled image.

transduction

conversion of one form of energy to another

visual transduction

light energy to neural signals by visual receptors

pigments absorb light

• absorption spectrum of these light pigments determines spectral sensitivity

• rhodopsin - rods

lamella

• a layer of membrane containing photopigments

• found in rods and cones

opsin

• a class of protein that, together w retinal, constitutes the photopigments

retinal

• a chemical synthesized from vitamin a

rhodopsin

• a particular type of opsin found in rods

• a G protein-linked receptor that responds to light

processing by retinal interneurons

photoreceptors communicate w horizontal and bipolar cells

horizontal cells

• integrate info from photoreceptors close to one another

• communicate through graded potentials w bipolar cells

bipolar cells

two types: diffuse (peripheral) and midget (fovea)

• respond to light falling in receptive fields

• antagonist center - surround organization

• lateral inhibition - distinct edges

• communicate through graded potentials w amacrine and ganglion cells

receptive feilds of bipolar and ganglion cells

no illumination baseline: -70 mV baseline polarization (no light)

amacrine cells

• form connections btwn bipolar, ganglion, and other amacrine cells

• possible role in processing movement

ganglion cells

• recieve input from bipolar, amacrine cells

• produce action potentials

• axons leave the eye as the optic nerve

three types: M, P, and K cells

receptive fields recipricate the info passed to them by bipolar cells

• maximum response to dots of specific size

• sensitive to contrast

The retina-geniculate-striate pathway: from retina to cortex

• ~90% of axons of retinal ganglion cells

• info from the left visual field of each eye projects to the right lateral geniculate nucleus (LGN; thalamus) - vice versa for right

• most LGN neurons that project to primary visual cortex (striate cortex) terminate in the lower part of cortical layer IV

• visual cortex = striate cortex

the lateral geniculate nucleus (LGN)

• recieves almost 90% of optic tract axons

• six distinct stacked layers

• keeps input from each eye seperate

• modifies flow of info based on levels of alertness

the superior colliculus

• recieves about 10% of optic tract axons

• guides movements of the eyes and head toward a newly detected object

axons from non-image forming ganglion cells

• synapse in the suprachiasmatic nulceus of the hypothamalus

• help regulate daily rhythmus (light info)

cortical mapping

• correlate location of neural activity w the position of an object in the visual field

• cortical magnification of signals from fovea

cortical receptive fields

• simple cortical cells - which respond to specific line orientations in a fixed location

• complex cortical cells - which respond to oriented lines anywhere in their field, often detecting movement

David Hubel and Torsten Wiesel

• 1960’s study at harvard university on a cat

• discoevered that neurons in the visual cortex did not simply respond to light

• they selectively responded to specific features of the visual world

receptive fields in the striate cortex

neurons of lower layer IV of the striate cortex have rectangular receptive fields

most interneurons in IV layer of striate cortex are either:

• simple - respond to bar-shaped stimuli w a particular slant or orientation in a particular location on the retina - antagonist center surround

• complex - same as simple but without reference to the stimuli’s location on retina, as long as, it is in the receptive field - can be motion sensitive

simple receptive fields in striate cortex

• rectangular

• on and off regions

• orientation and location sensitive

• all are monocular

complex receptive fields in striate cortex

• rectangular

• larger receptive fields than simpler

• not location sensitive

• motion sensitive many are binocular

monocular cues

• perspective

• texture and shading

• comparison of size of familiar objects

retinal disparity

• diff btwn the images on the 2 retinas

• both are greater when objects are close - provides brain w distance info

binocular cells

• cells in cortex that fire most vigorously when both eyes are looking at the same thing

• cells also respond to the degree of retinal disparity

Anterior parietal lobe combines disparity w shape changes of the objevct due to movement

subtractive color mixing

• physical process of color mixing that happens within the stimulus

• primary colors: red, yellow, blue

• like mixing paints

additive color mixing

• mixing that happens when lights of diff wavlengths are percieved by the eye

• primary colors: red, green, blue

Trichromatic theory (component theory)

• proposed by Young (1802), refined by Helmboltz (1852)

• 3 types of receptors, each w diff spectral sensitivity

Blue - 419 nm

green - 531 nm

red - 559 nm

• # of cones that are active of a particular color give you a summed stimulus and therefore color differentiation

opponent process theory

• Hering (1878)

• 2 diff classes of cells encoding color, and another class encoding brightness

• each encodes 2 complementary color perceptions

• Red/green; blue/yellow; black/white

Both theories of seeing color are correct

• coding of color by cones - operate by trichromatic theory

• coding of color at ganglion cell and LGN - seems to operate by opponent processing

Dichromacy

two cone photopigments

• red-green is sex-linked and most common

• absence of blue photopigments is rare

Dichromats - no green

monochromacy

one/ no cone photopigments

anomalous trichromacy

• the cone photopigments have peak responses at abnormal wavelengths

color constancy

color perception is not altered by varying reflected wavelengths

retinex theory

color is determined by theg proportion of light of diff wavelengths that a surface reflects

relative wavelengths are constant, so perception is constant

• that is, the wavelength a surface reflects is the same relative to the wavelengths surfaces next to it reflect

primary visual cortex

posterior occipital lobe

prestriate cortex

a band of tissue surrounding primary visual cortex

• inferotemporal cortex

association cortex

various areas, largest single area in posterior parietal cortex

hyperopia

farsightedness

• difficulty focusing on nearby objects

myopia

nearsightedness

• difficulty focusing on distant objects

astigmatism

• corneal, lens, or eye defect

• causes some areas of vision to be out of focus

presbyopia

farsightedness caused by aging

• lens is less flexible

eyeball shape influences quality of vision

depth of eyeball

normal: 2020 vision

deep: myopia

shallow: hyperopia

scotoma

an area of blindness in our visual field resulting from damage to primary visual cortex

• blind in corresponding contralateral visual field in both eyes

Test for using a perimetry test

deficit may not be readily detected due to phenomenom of completion

Blindsight

the ability to respond to a visual stimulus even with no conscious awareness of the stimulus - due to scotoma

• may be that some connections still exist in primary visual cortex, allowing for reactions w/out awareness

• may be that message gets to the brain by connections that dont pass through the scotoma

• ie. LGN to secondary cortex

dorsal stream

where or spatial aspect of stimuli

• primary to dorsal prestriate cortex to posterior parietal

ventral stream

characteristics of stimuli, shape

• primary to ventral prestraite cortex to inferotemporal cortex

Agnosia

failure of recognition

visual agnosia

able to see, unable to recognize

prosopagnosia

an agnosia for faces

• unable to recognize faces

• they also unable to recognize other specifics (which cow…)

• a result of bilateral damage to the ventral “what” perception stream

can be unconscious recognition - supported by data - altered skin conductance responses to familiar vs unfamiliar faces

fusiform face area

involved in perception of faces and other complex objects that require expertise to recognize

• a region of the visual association cortex located in the inferior temporal

• areas in ventral stream may be specific to humans, cats, or horses

• but - more than one area responds to each class and there is great overlap