Neural Processing and Perception Flashcards
How many Ganglion cells are there for every photoreceptor?
1:126
What do ganglion cells do?
- Ganglion cells must condense raw information from the photoreceptors
- Aim to extract important information from retinal image
What is used to study ganglion cells?
Single cell recordings - a physiological approach
How does single cell recording with a ganglion cell work?
- Electrode inserted in ganglion cell of animal
- Stimulus presented to animal
- Activity of ganglion cell recorded
- There is baseline activity from the ganglion cell
- Experimenters try to find a stimulus that changes the activity of that ganglion cell
What is a cell’s receptive field and what two regions does it have?
- the area on the retina which, when stimulated by light, elicits a change in the firing rate of the cell:
- Excitatory region: excitatory response (increase in cell’s response rate)
- Inhibitory are: inhibitory response (decrease in cell’s response rate)
Why are ganglion cells influenced by a region on the retina?
- Ganglion cells are influenced by a region on the retina because of convergence
- Light falling on any of the photoreceptors that lead to the ganglion will lead to the receptive fields
What is lateral inhibition?
- Inhibition that is transmitted across the retina by horizontal and amacrine cells
- Amacrine and horizontal cells send their signals horizontally: they will send inhibition across retina: this is the reason we can get inhibitory and excitatory regions
- Some photoreceptors send signals directly to the ganglion but others send it through other neurons which has an inhibitory effect on the ganglion cell.
What is Centre-surround antagonism?
- They way we describe the receptive field of the ganglion cell
- Centre photoreceptors have an excitatory effect on the ganglion cells
- Surround photoreceptors have an inhibitory effect on the ganglion cells.
- These are ON-centre OFF-surround receptive fields
- However other ganglion cells have the opposite effect: Off-centre On-Surround receptive fields:
Centre has an inhibitory effect
Outside has an excitatory effect
Describe what happens with On-centre off-surround antagonism if you fill different parts of the receptive field with light
- If there is no light then the ganglion cell would respond at a baseline level
- If you fill the receptive centre with light you will have an excitatory effect with the ganglion cells
- If you fill the surrounding cell will light you will have an inhibitory effect
- If everywhere is filled with light you will be at a baseline level
- If only a small amount of the central excitatory area is filled with light you would get a weaker excitatory response
- If the whole of the centre and a little bit of the inhibitory centre was filled with light you would get a weakly excitatory response
Describe what happens with Off-centre on-surround antagonism if you fill different parts of the receptive field with light
- No light: baseline firing
- Stimulate surround: strongly excitatory response
- Stimulate centre of field: Inhibitory effect
- Fill everywhere with light: baseline firing
Describe receptive fields in terms of photoreceptors and ganglion cells
- Each photoreceptor is part of the receptive field of more than one ganglion cell
- Receptive fields of neighbouring ganglion cells overlap
- Receptive fields of all ganglion cells together cover the whole visual field
Why do Ganglion cells have receptive fields?
- Ganglion cells respond to changes in light falling within receptive fields
- They are ideal for detecting spots of light, lines or edges: give a response where there is some area of light and some area of dark
- They are not able to detect orientation of lines
- There don’t change their response when there is a change in overall level of illumination
What do ganglion cells do?
- Respond to changes in pattern of light
- Ganglion cells reduce the amount of information in a stimulus by finding the contours and boundaries between fields of light and dark
Why do ganglion cells respond to changes in light?
because changes carry the most important information
How do receptive fields explain the Hermann Grid illusion?
- Two on-centre cells centres on light regions of grid
- When RF at intersection – more light falls on the surround (OFF) region) so received more inhibition and cell fires less
- Less firing interpreted as less bright so we perceive a dark spot
