Week 4 Flashcards
how many photoreceptors
125 million
how many retinal ganglion cells
1.5 million
how do photoreceptors work
Signal presence of light by generating electrical currents proportional to the amount of light absorbed by photopigment
How do other neural elements in retina help to condense image information while retaining important features e.g. presence of edges, color, motion?
Highlighting differences in light and dark
By summarising key information into digestible chunks for further processing
how do retinal ganglion cells work
Almost all do not absorb light but receive input from other retinal cells
Their output is in form of generating action potentials carried to the brain
Undergo single cell recording process
- There is a baseline level or spontaneous activity without any visual stimulus
- Then move a small spot around the screen to find region of stimulated retina where there is a spot that causes an increase in number of action potentials (ON)
- Turn light off, response rate returns to spontaneous activity level
4.Move position of stimulus to neighboring area of the retina
5.Turn light on, notice decrease in firing rate - In same position, tun light off, see an increase in spontaneous activity (OFF)
- Map out on and off regions for the neuron
what is the receptive field
The region of the visual field in which presentation of a stimulus evokes responses (change in firing rate) from a given sensory neuron
Size of RF determines size of the most effective stimulus
Orientation of RG does not show anything
what is lateral inhibition
Modulation of central signal by surround response, due to interaction between antagonistic regions of receptive field
What does it mean when a cell is called ‘linear’?
Cell’s response is determined by adding responses from excitatory and inhibitory influences within a cells receptive field
Spontaneous discharge = equal stimulation of excitatory and inhibitory regions
role of bipolar neuron
All photoreceptors hyperpolarise when photons are absorbed leading to decrease in neurotransmitter release
what are 2 different responses to decrease in neurotransmitter
on center BP cells
off center BP cells
how does light affect on center BP cells
Light on - increase in neurotransmitter (Glutamate)
Light off – decrease in neurotransmitter (Glutamate)
how does light affect off center BP cells
Light on – decrease in neurotransmitter (Glutamate)
Light off – increase in neurotransmitter (Glutamate)
What do horizontal cells do
Every photoreceptor has HC and BP
They spread over wide retinal areas
Provide information almost everywhere / sporadic
Gather info from cones and influence signals of BP cells by adding opponent surrounding signal to their receptive fields
Lateral inhibition
What are Amacrine cells
Outnumber both HC and GC
Influence the temporal responses of ganglion cells e.g. how vigorously they respond
Enhance responses to RF surround in primate
Convey rod signals to cone bipolars so they have a role in scotopic vision (low light)
process from light to create action potentials
- light ats first on photoreceptors
- signal progresses to ON (left) and OFF (right) bipolars in different layers
- delayed contribution of HC’s to OFF surround (left) and ON surround (right)
- Signals flow to ON (left) and OFF (right) centre ganglion cell responses
- Delayed OFF (left) and ON (right) surround responses follow
- Action potentials are generated by GCs to send information on to LGN
What is the Hermann Gridd
Illusory dark spots at intersections of white Horizonal and vertical stripes or vice versa
On response is dialed down due to the antagonistic response from the off response
What are mach bands
Look at environment from one shade to another – grades
We might see a ‘jump’
This is due to lateral inhibition
Fully on and fully off but once it hits slightly off it will move down in response but then increases to baseline when on is hit again
How does Human Visual Electrophysiology work
- Current flow into and out of the cell is proportional to the strength of the stimulus
- Activity of neurons gives rise to electrical slow waves at the surface of the body ERG and VEP
- Quantitative measurement of bio-electric signals evoked by a specific sensory stimulus
- Measured by a means of recording leads placed on surface of body/non-invasive
- Signals are low voltage
Types of ERGs
Flash ERG
Pattern ERG
Rod ERG
Cone ERG
multifocal ERG
What is Flash ERG
Retinal response to diffuse, even flash of light
Response obtained from whole retinal area
What is Pattern ERG
Retinal response to spatially structured stimulus
Response obtained from central retinal area
What is Rod ERG
Dark adapts patients for 20 minutes to enable recovery of rod sensitivity
Stimuli: blue or white flashes
Use stimulus intensity below activation threshold of cones
What is a Cone ERG
Adapt to white light for 10 mins of low intensity to saturate rods
Stimuli: bright red or white
Use brief flashes or 30Hz flickering flashes
