PSY280 - 3. MIDDLE VISION Flashcards
neural convergence
126 million photoreceptors, but only 1 million ganglion cells
last station before retina
convergence: # of neurons will synapse into single neuron
10 photoreceptors converge on ganglion cells
rods have more convergence since there are way more of them
RODS
fact that rods converge more than cones explains why rods are more sensitive than cones
red dot - ganglion cell - needs 10 units to fire
dim light interact with rods to produce little bit of activation - 2 units because its low intensity
RODS
cones have less convergence, some cases, 1 to 1 connection
it needs 10 units, but under dim light, each cone is only activated to 2 units so it’s not over the threshold
rods are more sensitive than cones - dim light, no problem
dim light for cones - problem
neural convergence
diminishes acuity of rods
ganglion doesn’t know which rod/how many are activated
can’t tell the difference between 1 spot, 2 spots of light
Cones
better visual acuity because they have less convergence.
monogamous, can tell which cone is activated because they’re only connected to 1
better precision
Convergence cause the rods to be more sensitive than the cones
takes less light to generate a response from rods then from cones
One unit of light intensity causes the release of one unit of excitatory transmitter which causes one unit of excitation in the ganglion cell
For Cones ganglion cells the fire, he must increase the intensity to 10
Lack of convergence causes the cones to have better acuity then the rods
Cones a better visual acuity because they have less conversions
Visual acuity is highest in the fovea
High convergence results in high sensitivity but poor acuity
Retinal ganglion cells
can receive info from multiple photoreceptors
photoreceptors are at very back + converge at ganglion cells
there is always some level of convergence
first place where action potential occurs
Retinal ganglion cells
electrical activity of other neurons in the retina is transient, local, and graded.
affects levels of neurotransmitters, but doesn’t cause action potentials
receptive field
area on receptor surface that, when stimulated, affects the firing rate of that neuron
collecting out at the blind spot
receptive field is neurons that feed into ganglion cell
receptive field
light hits photoreceptors + produces electrical activity + converge into a ganglion cell
area on receptor field that affects a particular neuron
can have overlapping receptor fields
centre-surround receptive fields
depending on where light shines, there are diff effects
light on centre, increase response
light on the inhibitory surround, inhibition, less response than baseline
no response if you activate the centre + whole surround`
centre-surround receptive fields
Receptive field is called an excitatory – center, inhibitory – surround receptive field
Receptive field which responds with inhibition when the center stimulated + excitatory when this around stimulated is inhibitory
centre-surround receptive fields
each retinal ganglion cells fire betw 20 + 50 ties per sec in uniform white light/absence of life + any wave length as long as its totally uniform
ganglion celss are optimized for detecting diff in lighting
inhibitory-centre, excitatory- surround.
OFF center
spot in centre: less response
spot in surround: more response
Center–surround antagonism:
small spot of light presented to excitatory center causes small increase in rate of nerve firing + increases light side so that it covers entire center of the receptive field increases cells response
On + Off Bipolar Cells
Whether RGC has an excitatory or inhibitory centre depends on bipolar cells that is receives signals from:
link betw photoreceptors + ganglion
In the light
photoreceptors hyperpolarized & release less glutamate
ON bipolar cells spontaneously depolarize in absence of glutamate
light - less glutamate - on bipolar cells depolarize
In the light
OFF bipolar cells become hyperpolarized in the light, making RGCs less likely to fire.
less glutamate - hyperpolarize - doesn’t fire
in light on higher firing rate, off lower firing rate
horizontal cells + amacrine
between receptors
Signals sent horizontally by horizontal cells & amacrine cells
plays role in surround organization
horizontal: communication betw receptors
amacrine: communication betw bipolar + ganglion
ON ganglion cell
Signals produced by photoreceptors in surround are passed on to the horizontal cells, which inhibit photoreceptors linked to the centre responses.
center increases firing rate, but firing rate of on centre will be weaker if spot reaches inhibitory area
horizontal cells provide lateral inhibition
ON/OFF
ON/OFF ganglion cell will maximally fire to light spot that precisely fills its centre excitatory region in the presence of a dark surround. OFF/ON ganglion cell will maximally fire to a dark spot that fills its centre in the presence of a light surround.
ganglion interested in contrast
uniform illumination
retinal ganglion cells fire between 20 & 50 times per sec.
with same stimulation for centre + surround = no change in baseline
Lateral inhibition
antagonistic neural interactions betw adjacent regions of retina
some photoreceptors are excited, horizontal cells turn that into inhibition on adjacent cells
turns into an antagonistic relationship
Lateral inhibition
inhibition that is transmitted across the retina and can cause perceptual effects
inhibition that get transmitted across retina to illustrate contrast
lateral plexus
sheet of branching axons that transmit lateral inhibition
like horizontal cells
lateral plexus
when you shine light at other photoreceptors, the activity of A goes down
produces antagonistic effect on A
increasing intensity of light at B, decreases the firing
inhibition is proportional to excitation
Simultaneous contrast
perception of brightness affected by presence of an adjacent/surrounding area
Dark area surrounding square on the right causes receptors under area to fire less rapidly though they send less Inhibition to the neurons under the right Square
Because cells under the left square receive more inhibition, their response is to decrease
Lateral inhibition
just at 4, firing rate increases, betw 3-5 increases more, when you expand to inhibitory surround it results in less firing
Mach Bands
perceptual system translates it as to something diff
Illusory light + dark bands near a light dark border
Lateness high as we begin moving to the right across a later start but then near the border right be the lightness becomes even higher
Mach Bands
6 receptors in circuit send signals to bipolar cells + each bipolar cells sends lateral + addition to its neighbors on both sides
Receptors ANB are on the light side of the border so receive intense illumination, receptors C and D are on the darker side and received a illumination
Receptors X, a and B generate responses of 100 whereas C, D and Y generate responses of 20
Increase in lightness on the light side of the border at sea and a decrease in lightness on the Darkside at D
perception
not a step function
change in stimulus intensity is a step function.
perception of nonuniformity
uneven inhibitory signal
Lateral inhibition
signal is slightly higher response in activity
even more inhibition for dark side due to higher intensity light
slightly higher activation on 1 side translates into greater contrast
The Hermann grid: seeing spots at intersections
A is excitatory centre
C,D,E,B is all receiving same intensity so they give a LI of 10 units
D receives lateral inhibition signals from F, G, H, A,
F + H only send over 2 units of inhibition
so A is receiving more inhibition
simultaneous contrast
affected by surrounded area
physically exactly the same intensity
lateral inhibition
white surround send over more inhibition than dark surround
light surround lessens centre stimulation
Parasol RGCs
large RF - more coverage of peripheral retina
‣~10% of RGCs
‣stimulation: transient
receptive field reflected in dendretic field
increase in response occurs on changes, then goes back to normal
Midget RGCs
small RF - especially concentrated in the fovea
~90% of RGCs
stimulation: sustained
need a lot to cover whole retina
fires at same rate for as long as you leave the light on
thalamus
collection of nuclei that’s primary role is to relay sensory info from periphery to the cortex.
sensory gateway
on top of the brain stem - subcortical, last stop to cortex
superior colliculus
involved in reflexive orienting to visually salient events
reflexive orients to something unexpected
to inspect the event/object more
superior colliculus
About 10% of RGCs travel directly to superior colliculus via tectopulvinar pathway, mostly of the parasol type
90% go through geniculate to thalamus
mostly of parasol in periphery, highly sensitive, but not accurate, so superior colliculus orients you to there
optic chiasm
Half of the projections from each eye cross over to the opposite hemisphere through the optic chiasm
right side of each eyeball translates to right hemisphere = left visual field
left side of each eyeball translates to left hemisphere = right visual field
ipsilateral vs. contralateral
part of the retina closest to the temple receives information from the contralateral side of space.
contralateral organization is by visual field, not by eye
contralateral = opposite
by the time it reaches the V1, theres a contralateral representation
lateral geniculate nucleus
2 major purposes: regulation of information & organization of information.
tiny part of the thalamus
2 thalami + LGN
regulation
for every 10 impulses, it sends only 4 to the cortex
receives more info from cortex than it sends out
cortex tells LGN what it wants
top-down processing
Magnocellular
input from parasol cells - color-blind
‣large RFs
‣large,fast-moving objects
‣sensitive to light contrast
Magnocellular
geniculate = bent knee 6 layers
inner 2 - magnocellular layers from parasol RGCs, from rods - colour blind
like transient - respond to large contrast changes to movement
Parvocellular
input from midget cells - mostly foveal information
small RFs - details of stationary objects
sensitive to color contrast
Parvocellular
outer 4 layers mostly from midget
midget ganglion cells mostly from cones which have high degree of precision
prefer sustained activation which is needed to discover detail
LGN
layers alternate between contralateral & ipsilateral inputs (by eye)
left LGN + right LGN
from magnocellular - parvocellular: C I , I C I C
LGN
retinotopically organized: areas on retina adjacent to each other are represented in adjacent regions of LGN
same organization on LGN as retina
centre of LGN, get a column of cell that analyze same part of the retina
perpendicular electrode track
V1: Striate cortex
primary receiving area for vision
signals from retina first reach cortex after LGN
retinal ganglion cells + LGN cells have same centre surround organization
orientation tuning curve
neurons in V1 like bars of light
adjacent arrangement
determined by measuring responses of simple cortical cell to bars with different orientations
orientation tuning curve
Cell response with 25 nerve impulses per second to a vertically oriented bar and cells response decreases as the bar is tilted away from the vertical and begin stimulating inhibitory areas of the neurons receptive field
measured output of neuron depending on orientation
as you move bar of light in orientation - it increases firing as it aligns more with orientation of its receptive field
most firing at a perfectly align bar of light
RGCs
Circular RFs in LGN are transformed into elongated RFs in striate cortex via convergence.
V1 can see the lines
other cortexes can see more and more
temporal cortex can see the picture
Simple cortical cells
respond best to bars of light of particular orientation
cells with side-by-side receptive fields
Cells in striate cortex have excitatory + inhibitory areas
any bar that also stimulates inhibitory surround inhibits activation
Complex cortical cells
respond best to bars of light of particular orientation that are moving
Many complex cells respond best to particular direction of movement
End-stopped cells
respond best to moving bars of light of a particular length, or to moving corners or angles
Neurons respond selectively to oriented lines + stimuli with specific length
firing increases as length increases until preferred length
corners or angles - specific angle + specific movement
surround inhibitory field that slows firing rate
Feature detectors
simple, complex and end-stop sells fire in response to specific features of the stimulus
sensory code
very specific
Firing causes neurons to eventually become fatigued/adapt
Neurons firing rate decreases + neuron fires less when that stimulus is immediately presented again
Only neurons that were responding to verticals are near vertical adapt + neurons that we’re not firing do not adapt
sensory code
Method: psychophysical measurement of the effect of selective adaptation to orientation
Contact threshold: minimum intensity difference between two adjacent bars that can just be detected
And adapt the person to one orientation by having the person view a high contrast adapting stimulus for a minute or two
sensory code
Remeasure the contrast threshold for all to stimuli
One vertical feature detectors are adapted it is necessary to increase the difference between the black and white vertical bars in order to see them
Adaptation selectively affects only some orientations
Orientation detectors play a role in perception
prosopagnosia
Damage to the fusiform face area
Inferotemporel cortex in the Temporel lobe was right for study
Prosopagnosia: people with Temporel damage unable to recognize faces
Fusiform face area: area on the underside of the Temporel lobe of the human cortex
Specificity coding
proposes that a particular object is represented by the firing of a particular neuron.
Specificity coating propose that a particular object is represented by the firing of a neuron that responds only to that object and not the other objects
Specificity coding
There are neurons that are specifically to eat object in the environment
Grandmother cell: neuron that responds only to a specific stimulus
Distributed coding
proposes that a particular object is represented by the pattern of firing across a large number of neurons.
Representation of a particular object by the pattern of a firing of a large number of neurons
Large number of stimuli can be represented because groups of neurons can create a huge number of different patterns
sparse coding
occurs when a particular object is represented by a pattern of firing of only a small group of neurons
Particular object is represented by a pattern of firing of only a small group of neurons with majority of neurons remaining silent
Features are objects are represented by the pattern of firing of groups of neurons
Sometimes the groups are small sometimes large
Grandmother cell
there are elements of grandma cells that are being described in literature
patient getting surgery for epilepsy
neuron is firing preferentially for steve carrell
there are probably gonna find another preference of another cell
