Receptive Fields Flashcards
Which parts of the eye are responsible for focusing light on the retina?
Cornea and Lens
How do the cornea and lens help to focus light on the retina?
Bending the light so that it falls on the retina
Why does visual perception rely on rods at night?
Cones don’t work at night due to lower sensitivity
We only rely on the rods because it has very high sensitivity
Why do cones enable higher acuity vision?
Because there is less neural convergence and is able to pick up on the fine details
What is the ratio between retinal ganglion cells and photoreceptors?
1:126 (1 million : 126 million)
Are there more or fewer ganglion cells than photoreceptors?
Fewer
Because there are fewer ganglion cells, what do they do?
Ganglion cells must condense raw information from the photoreceptors
They aim to extract only important information from the retinal image
What happens in single-cell recording?
- Physiological approach
- An electrode is inserted into a neuron and it measures the electrical activity
- It measures the activity of a SINGLE neuron
How do neurons communicate with one another?
By sending electrical impulses that travel down the axon
What does single-cell recording measure?
It measures the activity of a SINGLE neuron
(change in voltage as the action potential/nerve impulse passes by)
As action potential passes down the neuron, there is a spike in voltage
True or False?
As action potential passes down the neuron, there is a drop in voltage
False
As action potential passes down the neuron, there is a spike (sudden increase) in voltage
The more excited the neuron is, what happens to the action potential?
The more the action potential will fire
What does the rate of firing neurons indicate?
How active that neuron is
True or False?
If you excite a neuron more, it increases the size of the action potential
False
If you excite a neuron more, it DOES NOT increase the size of the action potential
Instead, it fires more action potential (rate of firing AP increases)
Increased rate of action potentials indicates ….
Increased activity of the neuron
Often single cell recording from ganglion cells is performed on cats. Describe how the activity of the cells is measured
1) Electrode is inserted into the retina to measure the ganglion cell
2) Experimenters present different stimuli to the cat
3) Experimenters observe what effect each stimulus has on the activity of the ganglion cell
Before you present any stimuli, the ganglion cell is already active. What term is used to describe this cell activity?
Baseline activity from the cell
Through single-cell recording and presenting different stimuli, what do experimenters aim to measure?
Experimenters try to find a stimulus that changes the activity of that ganglion cell
How is ganglion cell response measured? List 9 steps
1) Present spots of light onto the retina
2) Observe whether the ganglion cells respond (through the rate of firing AP)
3) Find the spot of light that increases the activity of the ganglion cells
4) Map out an area on the retina where any light within that area causes an increase in the activity of the ganglion cell
5) Fill that area with light (this will increase AP)
6) Observe what happens when you place the light just outside that area
7) If the light is just outside, the ganglion cell activity will decrease in response relative to the baseline level
8) Map out a 2nd (larger) area that surrounds the 1st area where light within that outer region causes an inhibitory response
9) If you present a bigger spot of light in the 2nd area, you’ll get an even bigger decrease in response
What is an inhibitory response?
Hyperpolarisation
When the ganglion cell is responding less than it would be if there was nothing presented to the retina
When the ganglion cell is responding less than it would be if there was nothing presented to the retina
This is known as…?
Inhibitory response
The area on the retina which, when stimulated by light, elicits a change in the firing rate of the cell
This is known as…?
Receptive field of a cell
Define receptive field
The area on the retina which, when stimulated by light, elicits a change in the firing rate of the cell
When there is an increase in the ganglion cell’s response rate, this is known as…?
Excitatory response (depolarisation)
Higher rate of firing action potential per/sec
When there is a decrease in the ganglion cell’s response rate, this is known as…?
Inhibitory response (hyperpolarisation)
Lower rate of firing action potential per/sec
There are 2 types of regions for measuring ganglion cell activity. What are they?
1) Excitatory response region
2) Inhibitory response region
Why are ganglion cells influenced by a region on the retina?
Convergence
On the retina, there is a group of photoreceptors whose responses converge to a single ganglion cell
How can convergence create 2 types of region (excitatory and inhibitory)?
Lateral inhibition
Inhibition that is transmitted across the retina by horizontal and amacrine cells is known as…?
Lateral inhibition
What is lateral inhibition?
Inhibition that is transmitted across the retina by horizontal and amacrine cells
What do amacrine and horizontal cells do in lateral inhibition?
These cells send messages across the retina
They transmit inhibition across the retina which causes inhibitory areas
What are the 2 types of cells involved in lateral inhibition?
1) Amacrine cells
2) Horizontal cells
What happens when photoreceptors send signals directly through the ganglion cell?
Excitatory response (increase the firing rate of AP)
What happens when photoreceptors send signals through intermediate neurons (horizontal or amacrine cells) which then send them to the ganglion cell?
Inhibitory response (decrease the firing rate of AP)
Why does lateral inhibition lead to an inhibitory response?
Because when photoreceptors send signals to intermediate neurons first, they will excite horizontal and/or amacrine cells (which are inhibitory cells)
This causes the (inhibitory) horizontal and/or amacrine cells to fire which causes a decrease in response/ rate of firing AP
What is Centre-surround antagonism?
Areas of excitation and inhibition due to intermediate neurons
Areas of excitation and inhibition due to intermediate neurons
This is known as…?
Centre-surround antagonism
What happens in a centre-surround antagonism?
Area in the little circle has the opposite charge/effect and work antagonistically to the area outside of the little circle (but within the bigger circle)
e.g. Inner area = excitatory (+)
Outer area = inhibitory (-)
What is ON-centre OFF-surround?
Inner area = Excitatory (+)
Outer area = Inhibitory (-)
What is OFF-centre ON-surround
Inner area = Inhibitory (-)
Outer area = Excitatory (+)
Inner area = Inhibitory (-)
Outer area = Excitatory (+)
This is known as…?
OFF-centre ON-surround (antagonism)
Inner area = Excitatory (+)
Outer area = Inhibitory (-)
This is known as…?
ON-centre OFF-surround (antagonism)
A region of the retina that affects the response of the ganglion cell is known as…?
Receptive field
Stimulation of the receptive field can have (…………) effect on ganglion cell response
Inhibitory or excitatory
What is a receptive field?
A region of the retina that affects the response of the ganglion cell
In ON centre–OFF surround (antagonism), what happens when there’s no spot of light presented?
Ganglion cells respond at the baseline level
In ON centre–OFF surround (antagonism), what happens when there’s a spot of light presented only on the inner area (centre receptors)?
Ganglions produce an excitatory response
In ON centre–OFF surround (antagonism), what happens when there’s a spot of light presented only on the outer area (outer receptors)?
Ganglions produce an inhibitory response
In ON centre–OFF surround (antagonism), what happens when there’s a spot of light presented on all areas (all receptors)?
Ganglions produce an inhibitory and excitatory response which cancel each other out
Thus, the cells respond at the baseline level
In ON centre–OFF surround (antagonism), what happens when there’s a small spot of light presented on the inner area (centre receptors)?
Ganglions produce a WEAK excitatory response
In ON centre–OFF surround (antagonism), what happens when there’s a large spot of light presented on the inner area (centre receptors)?
Ganglions produce a STRONG excitatory response
In ON centre–OFF surround (antagonism), what happens when there’s a large spot of light presented on the inner area (centre receptors) but the light also seeps through onto a small area of the outer receptors?
Ganglions produce a WEAK excitatory response
In OFF centre–ON surround (antagonism), what happens when there’s a spot of light presented on the inner area (centre receptors)?
Ganglions produce an inhibitory response
In OFF centre–ON surround (antagonism), what happens when there’s a spot of light presented on the outer area (outer receptors)?
Ganglions produce an excitatory response
In OFF centre–ON surround (antagonism), what happens when there’s a spot of light presented on all areas (all receptors)?
Ganglions produce an inhibitory and excitatory response which cancel each other out
Thus, the cells respond at the baseline level
In OFF centre–ON surround (antagonism), what happens when there’s no spot of light presented on all areas (all receptors)?
Ganglions respond at the baseline level
In OFF centre–ON surround (antagonism), what happens when there’s a large spot of light presented on the inner area (centre receptors) but the light also seeps through onto a small area of the outer receptors?
Ganglions produce a WEAK inhibitory response
In OFF centre–ON surround (antagonism), what happens when there’s a large spot of light presented on the inner area (centre receptors)?
Ganglions produce a STRONG inhibitory response
In OFF centre–ON surround (antagonism), what happens when there’s a small spot of light presented on the inner area (centre receptors)?
Ganglions produce a WEAK inhibitory response
In OFF centre–ON surround (antagonism), what happens when there’s a small spot of light presented on the outer area (outer receptors)?
Ganglions produce a WEAK excitatory response
In OFF centre–ON surround (antagonism), what happens when there’s a large spot of light presented on the outer area (outer receptors)?
Ganglions produce a STRONG excitatory response
True or False?
Each photoreceptor is part of the receptive field of only one ganglion cell
False
Each photoreceptor is part of the receptive field of more than one ganglion cell
Receptive fields of neighbouring ganglion cells overlap
Why?
Because they share photoreceptors
True or False?
Receptive fields of neighbouring ganglion cells overlap
True
True or False?
Receptive fields of all ganglion cells together cover the whole visual field
True
If light falling on a region of the retina causes an increase in the response of a ganglion cell – what type of region is this?
Excitatory region
What term describes the organisation of the receptive field of a ganglion cell?
Centre surround antagonism
What pattern of activity do you get if the whole receptive field is illuminated?
Baseline level reponse
How is it possible for a single ganglion cell to be influenced by a large number of photoreceptors?
Convergence
Lots of photoreceptors converge on a single ganglion cell
True or False?
Ganglion cells respond to changes in light falling within the receptive field
True
Why do ganglion cells have their particular organisation?
- Ideal for detecting spots of light & edges
- NOT able to detect the orientation of bars
Why are ganglion cell organisations ideal for detecting spots of light & edges?
Because when there is light on the lines and edges, there will always be some areas of light and some areas of dark within the receptive field
True or False?
Ganglion cells only detect which area in the receptive field is illuminated, regardless of orientation
True
Ganglion cells have no response to changes in the overall level of illumination
What does this tell us?
Ganglion cells indicate that there are boundaries in the receptive field but do not indicate (overall) how much light is falling within the receptive field
If you make the light darker, what happens to the ganglion cell response?
Gives the same response
If you make the light brighter, what happens to the ganglion cell response?
Gives the same response
What do ganglion cells do?
Respond to changes in the pattern of light within the receptive field
(and respond to some areas of light and dark)
Respond to changes in the pattern of light within the receptive field
(and respond to some areas of light and dark)
What cell does this?
Ganglion cells
Ganglion cells respond to changes in the pattern of light within the receptive field
(and respond to some areas of light and dark)
Why?
Because ganglion cells have to condense down the amount of information received from photoreceptors
The changes in the pattern of light carry the most important information
So ganglion cells only take the most important info by having boundaries that indicate light and dark
True or False?
Ganglion cells increase the amount of information in a stimulus by finding contours and boundaries
False
Ganglion cells reduce the amount of information in a stimulus by finding contours and boundaries
Why are line drawings effective (based on what we know of ganglion cells)?
The importance of boundaries between areas of light and dark explains why line drawings are so effective
Hermann Grid can be explained by…?
Lateral inhibition
What is the Hermann grid?
A grid with white lines and black squares
But the illusion is = you can see a grey dot in the intersections of white lines
How can receptive fields explain the Hermann Grid illusion?
Because there are 2 ON-centre-OFF-surround cells centred on light regions of the grid
Because the inner area already has a lot more light than the outer area, it produces an excitatory response
At the intersection, more light falls on the surround area (OFF region) so it receives more inhibition and the cell fires less action potential
Overall, this produces a weak excitatory response and less firing of AP is interpreted as less bright so we perceive a dark grey spot
Why do grey illusory spots reduce/disappear when we look directly at (fixate) them in the Hermann grid?
Neural convergence and receptive field sizes relative to eccentricity
120 rods send signals to 1 ganglion cell
6 cones send signals to 1 ganglion cell
When we look directly at the intersection between white lines on the Hermann grid, the image is projected onto our fovea (centre of eye)
Fovea contains high concentrations of cones (which has less convergence) which produces higher acuity vision. Fovea also has a small field of perception
In the periphery of the retina, acuity is poorer (because there are more rods in the periphery areas of the retina and rods have more convergence) so the illusion is stronger
As retinal eccentricity increases (going from the fovea to peripheral areas of the retina), what happens to the receptive field size?
The more eccentric, the larger the receptive field
SImply = Fovea has a small receptive field and peripheral areas of the retina have a large receptive field
True or False?
According to our centre-surround antagonism & receptive field explanation, we should get the same illusion in grids with wave lines as well. And we do.
False
According to our centre-surround antagonism & receptive field explanation, we should get the same illusion in grids with wave lines as well. But we don’t.
The Hermann Grid illusion can be explained in terms of …? (List 2 ways)
- Centre-surround antagonism
- Varying receptive field sizes (and the illusion reduces when you look directly at it)
What is simultaneous contrast illusion?
When you are presented with the same square of the same shade of colour but each square is placed inside a bigger square with different shades of a colour (different background)
Simply = There are 4 small squares of the same shade of grey but we think they look different (because of their background)
Why is an inner square with a light background perceived as darker than an inner square with a darker background?
The brighter outer square causes a lot of inhibition around the edge of the inner square (because there’s less darkness falling within the receptive fields)
So the ganglion cells fire less AP and the inner square appears darker
Why is an inner square with a darker background perceived as lighter than an inner square with a lighter background?
The darker outer square causes weak inhibition around the edge of the inner square (because there’s more darkness falling within the receptive fields)
So ganglion cells fire more AP and the inner square appears brighter
