sensory contributions 3b Flashcards
vestibular system components
called end organs
otoliths
- saccule, utricle
semicircular canals
- superior, posterior, horizontal
vestibular system function
in inner ears
detects head acceleration
helps determine head position/motion and body orientation
detects acceleration when head moves
can detect head movement in all directions/rotations
vestibular apparatuses
on both sides of the head
work together to signal movement or orientation
- allows for greater signal to noise ratio and thus increases sensitivity to motion
otoliths
sense linear head acceleration (ie/ changing translational motion through environment) and changes in head orientation relative to gravity
saccule
detects acceleration in vertical plane
ultricle
detects acceleration in horizontal plane as well as head tilt
semicircular canals
sense angular head acceleration
- turning or tilting
- rotatory body movements
hair cells
stereocilia
located in vestibular apparatus get deflected by otolithic membrane (in otoliths) or endolymph (in semicircular canals)
depolarization ocurs because of an influx of potassium
resting discharge allows afferents to respond to bi-directional motion
positive mechanical deformation
towards the kinocilium which opens potassium channels in the stereocilia
calcium enters the cell allowing for vesicle fusion and the release of transmitter
tip links open the ion channels more
depolarixation - increased afferent discharge
negative mechanical deformation
away from the kinocilium causes potassium channels to close
tip links close the ion channels
hyperpolarization - decreased afferent discharge
tip links
connect individual hair cells together and are connected to spring-gated ion channels
slightly open the ion channel and allow a small amount of potassium inside
how do semicircular canals signal angular acceleration
canals are filled with viscious fluid which is rich in potassium called endolymph
because of its inertia, when the head rotates the endolymph displaces a gelatinous structure call the cupula which has hair cells embedded in it which are deflected
cupula
displaced by the flow of endolymph when the head moves
as a result the hair bundles are also displaced
endolymph flow
thick liquid when we move head it deflects cell to show which direction we are going
how do the otoliths signal head acceleration
linear (translational) head motion through environment or change in head orientation relative to gravity causes movement of the otolithic membrane
key is having an inertial substance (endolymph or otolithic membrane that is not rigidly attached to the rest of the body
when the body accelerates, inertia cause the substance to lag behind and hair cells detect this relative motion
otolithic membrane
contains otoconia stones
lags behind head motion
deflects hair cells (which project up into this membrane)
main function of vestibular system
postural stabilization
gaze stabilization (in conjunction with visual system)
perception of self motion
role in spatial navigation
postural stabilization in the vestibular system
maintenance of balance
- via vestibulospinal reflexes
- vestibular afferents project to vestibular nuclei in brainstem, which gives rise to descending tracts that activate muscles
helps keep head upright (and perception of spatial orientation when head is not upright)
gaze stabilization in the vestibular system
via vestibular-ocular reflect
- this reflex compensates for head movement - loops eyes in opposite direction ie/ when fixated
- when the head moves, the eyes rotate in orbits to maintain gaze fixation on target of interest
perception of self motion in vestibular system
head motion can tell CNS about you motion
vestibular system’s role in spatial navigation
linked with self motion as well as knowing orientation
GVS (galvanic vestibular stimulation)
used to study the vestibular contribution to balance
activates the vestibular afferents and and hair cells of the otoliths and semicircular canals causing illusory perception of head (and body) tilt and compensatory tilt in the opposite direction
gives the illusary perception of sway towards cafe and their is resulting compensatory sway towards anode
the eye
designed to focus the visual image on the retina with minimal optical distortion
light is focused by the cornea and lens onto photoreceptors in the retina
light rays must converge at the retina for light to be in focus
light is refracted when it passes through the cornea (2/3 of refection here) then at the lens (1/3 of total refraction here)
lens
can change its shape to alter the distance at which objects will be in focus
- known as accommodation
- due to the contraction/relaxation of the ciliary muscles
retina
contains numerous cells in multiple layers
has photoreceptors
fovea
retinal ganglion cells for the optic nerve and propagate the signal to visual areas in the brain
lies in front of pigment epithelium that lies in the back of the eye
photoreceptors
transduce light into electrical signals
fovea
contains only cones; rods dominate elsewhen in retina
part of the retina that allows for vision of fine details
cells in the pigment epithelium
filled with a black pigment, melanin, which absorbs any light not captured by the photoreceptors
this prevents light from being reflected off the back of the eye, which would degrade the visual image
light must travel throguh the layers of other retinal neurons before striking the photoreceptors
cell bodies of the proximal retinal neurons in the fovea are shifted to the side, enabling the photoreceptors to recieve the visual image in its least distorted form
visual field
the region of space where the eyes sees
shifts with eye movement
central vision
deals with identifying details
central ~5 degrees of visual field
predominently contains cones
peripheral vision
deals with where things are
info regarding environmental context and moving limb
contains mostly rods and sparse cones
binocular retinal disparity
refers to the difference in image location of an object seen by the left and right eyes, resulting from the eyes horizontal seperation (parallax)
can be used to process object motion
if object moves and the eyes remain fixed the image activates the reitina progressively more laterally
depth perception
facilitated by the different in image location of an object as viewed by the left and right eyes
primary visual cortex
about 2mm thick and has 6 layers
- neurons carrying visual input from LGN enter layer 4
also known as visual striate cortex
striate means striped
- this area has a prominent layer 4 that gives rise to a striped appearence in V1 cross sections
contralateral visual field representation is preserved in V1
has cells with many different visual receptive fields like layer 4c, simple, and complex cells
retinotopic representation
light (images) from specific areas of visual field hits a specific part of retina; each specific part of the retina is mapped onto the visual cortex, adjacent points in sensory space are represented at adjacent points in the brain
retinal ganglion and lateral geniculate nuclus (LGN) neurons
have receptive fields such that each neuron responds to a tiny spot of light (like a tiny region of the visual field)
layer 4c neurons in V1 (onto which LGN neurons synapse ) are similar
simple cells in V1
have elongated receptive fields
sense lines/edges of a particular location
respond to bars of light and borders between light and dark
can detect an edge of an object
converge info into complex cells
complex cells in V1
have larger receptive fields
sense lines/edges of a particular orientation anywhere within the receptive fields
many also are sensitive to moving lines/edges
- respond to moving bars of light
hierarchial organization in neurons in visual pathway
receptive fields get bigger as you move from LGN to visual cortex (from layer 4c to simple to complex cells)
at each step of the hierarchy, when receptive fields get more complex, a receptive field is the sum of multiple, smaller receptive fields from “upstream” neurons (those that synapse onto the “downstream” neuron
retinal ganglion to LGN cells to layer 4c - simple V1 - complex V1
fundamental principle of how info is integrated (and expanded) from one level to another within the nervous system
the idea applies to many brain areas and many different senses
tuning curve
shows what orientation the neuron responds best to by measuring its discharge frequency
extrastriate
refers to all cortical areas outside of V1
cortical regions important for processing visual motion (optic flow)
middle temporal (MT) region [also called area V5]
medial superior temporal (MST) region
middle temporal region
neurons have large receptive fields
neurons have preference for a certain direction of motion
neuronal activity is sensitive to speed of motion
project to MST region- medial superior temporal area
medial superior temporal region
divided into two sub-regions that process object-motion or self-motion
neurons have larger receptive fields (larger than MT)
optic flow
the continuous change over time of the spatial pattern of light (variations in intensity and wavelength composition) reaching a point as it moves through its surroundings
whole field motion
motion of an image sweeping across the eye when an eye rotates in its socket with the head stationary (from the image moving relative to the retina)
like looking left to right without moving head
optic flow components
whole field motion
image flow caused by the eye moving through the environment as a person moves and the eye turning in its socket simultaneously
- translational flow and rotational flow
translational flow
referred to as radial outflow
motion due to movement of eye through environment
rotational flow
motion due to the eye turning within the environment
what are the two factors important for generating optic flow
- the speed and direction of the eye’s movement through the surrounding
- the distances from the eye to the points in the surroundings that hit the retina. images of closer objects flow across the eye faster when the eye moves through space
what does optic flow provide info on
stability and balance
velocity and direction of movement
movement of objects in the environment
time-to-contact
several ways to calculate TTC
optic flow
- calculate ratio of the object’s image size to the rate of its radial expansion on the retina; a variable called tau
binocular retinal disparity
oculomotor vergence feedback
- feedback from muscle spindles in eyes as the eyes rotate inward or outward to track an object
ventral visual stream
primary visual cortex to temporal lobe
vision-for-perception system
ventral visual stream
responsible for fine analysis of the visual scene (form, colour) and object recognition
uses object-centred frame of reference
- ie/ the laptop is X distance to the coffee mug
aka the what stream
dorsal visual stream
primary visual cortex to posterior parietal cortex and beyond
vision-for-action system
dorsal visual stream
for guiding movement and spatial characteristics of the environment
uses various egocentric frames of reference
how or where stream
same-different discrimination task
two shapes presented together, and participant asked to determine visually if they are the same or different
object manipulation task
grasp shape appropriately
grasp requires placing thumb and index finger at stable points
damage to dorsal stream
results in an improper grip of object even through able to visually discriminate
damage to ventral stream
results in an inability to discriminate between shapes but proper grasping
vision contributes to locomotion
implementing avoidance stategies
accommodating different terrain
navigation and determining the direction of walking
planning and controlling precise foot placement
- people tend to fixate where they eventually step, look-ahead distance depends on the complexity of the terrain (distance decreases with more complex terrain
vision is used to monitor lower limb trajectory when stepping over obstacles
vision is used both to plan upcoming movement and as sensory feedback to correct the movement on-line
vision contributes to reaching
vision provides extrinsic, world-based coordinate information
- used to plan spatial features of movements toward visual targets
vision also provides information regarding initial limb (hand/arm) configuration
absense of vision of hand/arm during reaching results in changes in movement kinematics and decreased accuracy
research suggests that we use visual feedback continuously during the movement or after the initial (very early) phase of the movement
