6. afferent processes

afferent vs efferent neurons

• afferent neurons carry sensory msgs to brain

• efferent neurons carry motor msgs to the muscles

stimulus

any energy capable of exciting a receptor

3 kinds of stimulus-dependent(?) receptors

• mechanoreceptors - touch, pressure, vestibular, audition

• chemicoreceptors - taste, olfaction

• photoreceptors - light

4 things required to perceive a sensation

• a stimulus sufficient to initiate a response in the NS must be present

• a receptor must convert the stimulus to a nerve impulse

• the conduction of the nerve impulse must be transmitted from the receptor to the brain

• interpretation of the impulse must occur in a specific portion of the brain

2 things all receptors do

• absorb physical energy from the env

• convert (transduce) the energy into a neural impulse

rate law

the lvl of response is determined by the intensity of the stimulation

2 types of receptors

• tonic receptors

• phasic receptors

tonic receptors

continue to fire at a relatively constant rate as long as the stimulus is maintained, slow to adapt

phasic receptors

respond w a burst of APs when the stimulus is first applied, but quickly reduce their rate of firing when the stimulus is maintained

sensory receptors + what it does

specialized nerve cells that transduce energy into neural signals, lack axons but form synapses w dendrites of other sensory neurons

• CANNOT PRODUCE APs — only inc or dec NT released in response to detection of stimunli

law of specific nerve energies

sensory messages are carried via separate pathways to different areas of the brain, but all use the same neural impulse

the range of energy lvls which eyes, ears, and taste buds detect

• eye: 380-760 nM (wavelength)

• ear: 20-20,000 Hz (frequency)

• taste buds: specific chemicals

function of visual system + 3 functions of vision

detect electromagnetic radiation (EMR) emitted by objects

discriminate figure from background (food or rock?), detect mvmt (predator/prey?), detect color (is fruit ripe?)

wavelength

measured in nM, related to perceived color/hue

intensity

amplitude of radiation, related to brightness

an eye consists of 3 things:

• aperture (pupil) to admit light

• lens that focuses light

• photoreceptive elements (retina) that transduce the light stimulus

describe the path of light in the eye

cornea → aqueous humor → pupil (opening in iris) → lens (rounded for nearby objects; flattened for distant objects) → vitreous humor → retina (image is upside-down and reversed)

blind spot + where is it closest to

bundle of axons running back to the brain, has no photoreceptors, closest to the nose

how is color vision in peripheral vision?

poor

floaters

dead photoreceptors

3 types of eye mvmts

• vergence mvmts

• pursuit mvmts

• saccadic mvmts

vergence mvmts

the eyes rotate to keep an object on corresponding parts of the retina

pursuit mvmts

following an object to keep it on the same part of the retina (the fovea, to maximize detail and color appreciation)

saccadic mvmts

eyes fixate on objects during “smooth pursuit”

• eyes also “jump back and forth” — they are not stationary?

light passes through the … and is focused by the … onto the … at the back of the eye

pupil, lens, retina

3 layers of cells of the retina

• photoreceptor layer (back of eye) - transduce light into electrochemical impulses (eventually, APs), FARTHEST FROM LIGHT

• bipolar layer

• ganglion cell layer, CLOSEST TO LIGHT

2 types of photoreceptors + traits

• rods - light sensitive (not color), found in periphery of retina, low activation threshold

• cones - color sensitive, found mostly in fovea (center of retina)

the outer segments of a rod or cone contain different … that react to light

photopigments

what does each photopigment consist of? what do they do?

an opsin (a protein) and a retinal (a lipid)

• in the dark, membrane Na+ and Ca2+ channels are kept open by cGMP → NT (glutamate) is released which hyperpolarizes the bipolar cell membrane

• light splits the photopigment — opsin and retinal — apart

  • activates G protein (transducin)

  • activates phosphodiesterase → destroys cGMP

  • reduced cGMP → reduced NT release (closes presynaptic Na+ and Ca2+ channels)

net effect of light is to … the photoreceptors and … the release of NT

hyperpolarize, reduce

“on” vs “off” cells/pathways

• on - detects a stimulus and ganglia cells turn on (this is what we’re focusing on)

• off - detects a stimulus and turns off

“bleached” photoreceptor

occurs when light splits the photopigment, causing it to become white, inhibits the bipolar cells less

signals from the ganglion cells of the retina are sent to the … via the …

thalamus, optic nerve/tract

6 layers of the lateral geniculate nucleus (LGN) + functions

• magnocellular layers - the inner 2 layers that contain large cells

  • form, mvmt, depth, brightness

• parvocellular layers - the outer 4 layers that contain small cells

  • color and fine detail discrimination

neurons of the LGN project thru the optic radiations to a region of occipital cortex termed …

primary visual cortex (striate)

vision researchers say we have 4 retina. how?

we have 2 on each eye

• 2 retinas on the outside of the eyeball are TEMPORAL RETINAS

• 2 retinas close to the inside are NASAL retinas

light from LVF and RVF strike what side of both eyes?

light from LVF strikes RIGHT side of the eyes

light from RVF strikes LEFT side of the eyes

what part of the LGN do nasal and temporal retinas go?

nasal retinas transmit info to the CONTRALATERAL LGN of the thalamus

temporal retinas transmit info to the IPSILATERAL LGN of the thalamus

optic radiation

transfers visual info back to the brain

Meyer’s loop

outer areas of the optic radiation, located LOW going into the temporal lobes and processes info that’s HIGH (superior vision)

medial areas of the optic radiation

location is HIGH and goes up to the parietal lobes but processes info that’s LOW (inferior vision)

lesion to right eye

everything being transduced in right eye is lost → CAN’T SEE FROM RIGHT EYE

lesion to optic chiasm + potential cause

inputs from the nasal retina are cut, so you’d lose PERIPHERAL VISION on both sides. can be caused by a pituitary tumor

lesion to Region 3

loss of left hemifield (left nasal retina, right temporal retina). both eyes will be blind to anything on the left side of the world

anopsia

inability to see

left hemianopsia

lost the left side of the world

lesion to region 4

right Meyer’s loop has been cut, so vision will be lost in the upper visual world but only in the left hemifield (so upper left)

• left superior quadrantanopsia

lesion to region 5

parietal portion of the optic radiations were cut, so the lower visual world would be affected on one side

• left inferior quadrantanopsia

lesion in region 6 (primary visual cortex)

at first, appears to be a straightforward loss of one hemifield, but vision at the fovea is spared (foveal sparing), perhaps bc there’s such a large representation of fovea in the cortex

what happens if we lesion the parvocellular layers of the LGN in the right hemisphere?

we lose color vision in the LVF

ganglion cells in the retinal periphery vs ganglion cells in the fovea

ganglion cells in the retinal periphery receive input from many photoreceptors

ganglion cells in the fovea receive input from one photoreceptor → much higher resolution at fovea, higher sensitivity

why are cones better at detecting small details?

they’re highly dense in the fovea and ganglion cells hook up 1:1 for every high resolution (tho there’s a higher threshold necessary compared to a rod for a cone to activate)

why are rods better for night vision?

they’re extremely sensitive (only need 1 rod to activate the ganglion cell bc many of them are attached to one ganglion cell)

• to see better in the dark, use your periphery vision instead of the fovea

horizontal cells functions

• perform “lateral inhibition” — inhibit cells directly adjacent to them

• at the edge of light activation, bipolar cells at the edge that are in darkness are still inhibited

• allow us to see the edges of light very clearly (Mach Bands)

2 color vision theories

• trichromatic theory

• opponent theory

trichromatic theory

argued there are 3 different receptors in the eye, w each sensitive to a single hue

• blue, red, green

• any color could be accounted for by mixing 3 lights in various proportions

opponent theory

notes that ppl perceive 4 primary colors: yellow, green, blue, red

• negative color afterimages suggest that red and green are complementary colors as are blue and yellow

photoreceptors in primate retina and how it supports trichromatic theory

3 types, and each cone uses a different opsin which is sensitive to a particular wavelength (blue, red, green), supporting trichromatic theory

at the ganglion cell lvl, how does the system respond?

in an opponent-process fashion (yellow on, blue off. blue on, yellow off. red on, green off. green on, red off.)

• firing rate increases to green/yellow, decreases for red/blue

rebound effect + what does it cause?

when ganglion cells fire faster or slower than normal due to being excited or inhibited for a prolonged period of time

negative afterimages

protonopia

an inherited form of defective color vision in which red and green hues are confused; “red” cones are filled w “green” cone opsin (can’t see the color red)

• see the world in shades of yellow and blue; both red and green look yellowish to them

deuteranopia

an inherited form of defective color vision in which red and green hues are confused; “green” cones are filled with “red” cone opsin (can’t see the color green)

• see the world in shades of yellow and blue; both red and green look yellowish to them

tritanopia

an inherited form of defective color vision in which hues w short wavelengths are confused; “blue” cones are either lacking or faulty

• have trouble seeing yellows and blues

• see the world in greens and reds

• blue looks green and yellow looks pink

• rare

microelectrodes

used to record the firing activity of a single sensory neuron (impulses/sec)

primary visual cortex aka … aka …

striate cortex, V1

orientation sensitivity

some cells fire best to a stimulus of a particular orientation and fire less when orientation is shifted (striate cortex)

spatial frequency (low vs high)

cells vary firing rate according to the sine wave frequency of the stimulus (V2)

low frequency = large areas of light/dark

high frequency = fine details

V3 of brain is involved in

gestalt processing

V4 of brain

only in humans, part of learning process

• have categories of neurons (e.g., 9, dogs, horses, etc.)

• for symbolic, visual association, learning

list out the basic functions of V1 to V4

V1 - orientation

V2 - shape/object

V2-V4 temporal: what

V2-V4 parietal: where

fusiform face area (FFA) (location, what determines the degree to which we respond, its role in autism)

• at base of brain, grouping of neurons that responds to faces of all kinds

• degree to which we respond to activation related to lvl of expertise

• ASD (autism) tends to have reduced activation of FFA; they avoid faces, can’t understand facial expressions well

  • but their FFA lights up for their fixations → indicates expertise

dorsal stream

dorsal → parietal lobe: where/how stream; where is an object in space and how to interact with it?

ventral stream

ventral → temporal lobe: what stream; what is an object? shape, object recognition

2 clinical symptoms of lesions to temporal and parietal lobes

temporal

• trouble identifying objects by sight (touch should help)

• trouble w “boston naming test”

parietal

• trouble w location/mvmt of objects

• grasping problems

anomia

may be indicated by failure in boston naming test (BNT), is the inability to name objects due to a lesion in the Broca’s Area (indicated by if giving a phoneme cue helps them name it)

prosopagnosia

inability to recognize faces, even close friends and family

• area just below FFA affected: anterior inferior temporal region (Alzheimer’s degradation)