- Created by: becky_99
- Created on: 20-12-19 21:18
The bipolar cells
There are different varieties of bipolar cells; they all follow the same structure but differ in axon thickness or how many dendrites they have.
Bipolar cells are specialised by the type of excitatory and inhibitory signals they send and receive as well as the types of light frequency.
- Hyper-polarity - polarity drops
- De-polarity - polarity increases
It is the inhibitory because the rods send signals to turn the bipolar cells off when there is not light.
When light is received by the rods, they stop sending signals which turn the bipolar cells on.
Photoreceptors are connected to more than one bipolar cell. They are usually in the form of "off" and "on".
The neurons help a receptive field to detect a contrast via a process called lateral inhibition.
The bipolar cells
The light hits the back of the fovea which hits the photoreceptors first, the bipolar cells second, and the ganglion cells third.
The difference between the two receptive fields is to do with the number of photoreceptors in the network.
The ganglion cells
Ganglion cells form the "highway" of the visual pathways.
They have long axons that allow for signals to travel efficiently to the brain via the optic nerve.
- Parvocellular neuron
- Magnocellular neuron
These 2 types of neurons mainly differ in the size of their receptive fields.
Magnocellular ganglion cells have thicker axons and are often more myelinated than parvocellular neurons.
Parvocellular ganglion cells have thinner axons, less myelin and smaller receptive fields.
The visual field
The visual field is the view seen by the two eyes without movement of the head.
The left visual field projects onto the right half of each retina.
The right visual field projects onto the left half of each retina.
The visual field is represented in topographical order in the primary visual cortex. However, this topographical map is distorted; not all areas of the retina are represented equally.
The fovea has a much larger representation in the cortex.
About half of the primary visual cortex represents the foveal region of the retina.
The peripheral portions of the visual field are much less represented.
Two essential pathways to perception
Upwards into the parietal lobe - dorsal pathway.
Down into the temporal lobe - ventral pathway.
Step 1 - object segregation; i.e. figuring out what, in a complex scene, are the individual objects.
Step 2 - object recognition; mapping the object to something that is already known.
Primary visual cortex analyses object edges
The primary visual cortex cells register contours or edges of visual objects in short lines of various orientation. It ignores areas that are homogenous (no edges or other changes).
Contours are extremely important but are not our only cue about objects.
Gestalt principles of perceptual organisation
Law of similarity.
Law of proximity.
Law of good continuity.
Law of common fate.
When you see edges, how do you know which part is the object and which is the background?
We can deduce the missing contours, using Gestalt grouping principles.
We're not conscious of these processes, and they occur largely automatically.
- Contours and grouping cues
- Occurs largely automatically
Once you have performed object segregation and can see the objects as distinct from the background and other objects, does that mean you have recognised it?
- Agnosia = "without knowledge"
- Failure to recognise objects from visual information (can describe the object; can tell objects apart; cannot recognise what they are)
Prosopagnosia = fast agnosia:
- Failure to recognise faces
- Areas of the brain that processes the features of the object are intact
- Areas of the brain that store the representation of the object are intact
- The damage is in the pathway from the visual areas to the recognition areas
Damage can also occur in areas of the temporal lobe. Representations stored in those areas will be lost. That results in complete agnosia (no form of sensory input will allow you to recognise it).
If damage occurs in the face processing area, you will not be able to recognise faces.
- Other areas of the temporal lobe hold other types of representations:
Brain areas involved in recognition?
Visual cortex: segregates objects and processes colour, shape, motion.
Pathway: visual cortex --> temporal lobe.
Temporal lobe: stores representations that permit recognition.