L4: Low-level Visual Processing Flashcards
Describe the process of ‘from eye to cortex’
Reception: photons of light hit your retina
Transduction: physical energy is turned into an electrochemical pattern in the neutrons
Coding: one to one mapping between physical stimulus (light) and the resultant pattern of neuronal activity
Define and outline accommodation
The variation in optical power produced by the thickening of the lens of the eye
- when receiving light from a distant object the lens is pulled out thin
- when receiving light from a close object the elastic lens is more convex
What are the names of the two types of receptors in the eye?
Rods and cones
Briefly outline cones
Used for colour vision and sharpness of vision. Contain light sensitive photopigment allowing them to respond to light
Briefly outline rods
Specialised vision in dim light and movement
Light reflected from each given stimulus has a certain value on 3 different dimensions. Name and outline them
Hue (H): what distinguishes red from yellow or blue
Brightness/value (v): the perceived intensity of light
Saturation (S): allows us to determine whether a colour is vivid or pale
Outline trichromatic theory
According to this theory there are 3 different types of cone receptors
- one that is especially sensitive to short wavelength light (blue)
- one medium wavelength light (yellow-green)
- another especially sensitive to long wavelength light (red)
According to this theory the colours we perceive is due to the relative stimulation of each cone type (normally light stimulates two or 3 cone types)
Name 5 visual pathways
1) retina-geniculate-striate pathway
2) ‘what’ pathway (dorsal/parietal pathway)
3) ‘where’ pathway (ventral/temporal pathway)
4) Parvocellular (P) pathway
5) magnocellular (m) pathway
Outline the retina-geniculate-striate pathway
-transmits info form the retina to V1, then to V2, via the lateral geniculate nuclei of the thalamus
Outline the lateral geniculate nucleus
- contains 6 layers, 3 represent each eye
- cells in the LGN don’t respond to changes in light per se, but instead respond to differences in light between the centre and surround of the receptive field
- maintains a retinotopic map
Outline the parvocellular and magnocellular pathways
Parvocellular: sensitive to colour and fine detail. Most of its input comes from cones
Magnocellular: most sensitive to motion/movement, most of its input comes from rods
Name and outline the 2 main properties of visual neurons
Receptive fields: region of the sensory space (i.e. Retina) in which light will trigger the firing of that neurone
Lateral inhibition: reduction in activity of one neurone caused by activity in a neighbouring neurone. Useful for enhancing contrast at edges of objects
Outline simple and complex cells
Simple cells: combine signals from cells in the LGN (combines signals from multiple visual fields). They really care about orientation
Complex cells: combine the responses of many simple cells (which prefer the same orientation) and have overlapping receptive fields), they have large receptive fields and respond more to contours
Outline primary visual cortex (V1)
- extracts basic info from the visual scene (edges, orientation, wavelength of light). This info is used in later stages of processing to ectract info about shape, colour, movement etc.
- made up of simple and complex cells
- retinotopically organised. Certain neurones care about certain areas of space, and other neurones care about other areas of space, arranged in a map like way
Outline blindsight/cortical blindness
- condition caused by damage to V1
- patients with this affliction are consciously unaware of what is happening in that part of the visual field but can still make rough guesses/judgements about it
Outline the ‘where/how’ pathway and the ‘what’ pathway
‘What’: temporal/ventral pathway. Concerned with colour and form processing
‘Where’: dorsal/parietal pathway. Concerned with movement processing
Outline functional specialisation theory (Zeki)
- based upon monkey studies
- this theory suggests that different parts of our visual cortex care about different concepts/specialised for different functions (Zeki compared the visual system to a team of workers with each their different role to play to solve a complex problem)
- assumes the organisation of the visual system into 4 main parts (V1 and 2 -> V3 and V3a -> V4 -> V5) not gonna delve into the functions of these areas on this card
Outline the V1 to 5
V1+2: involved in the early stages of visual processing. Contain different cells responsive to colour and form
V3+V3a: cells in this area are responsive to form (especially moving objects) but not colour
V4: cells are responsive to colour, some to orientation
V5: cells specialised for visual motion p. Not colour
Outline cortical achromotopsia
- caused by damage to the V4 area
- patients with this affliction can’t see colour. However they can guess colour via scheme (if they were presented with a banana they would say it is yellow cause they know they are)
Outline Akinetopsia
- caused by damage to V5
- patients with this affliction cannot perceive motion in their visual field
What is a problem with trichromatic theory?
Negative afterimage
Outline opponent process theory
-theory states there are three types of opponent processes
~red-green
~blue-yellow
~black-white
And we cannot perceive opposing colours at the same time
- some colour deficiencies behave in a opponent colour fashion.
- explains negative after images: prolonged viewing of a certain colour (e.g green) produces one extreme activity in the relevant opponent process. When attention is then directed towards a white surface the opponent process moves to its other extreme (e.g redd) due to basic adaption processes
Outline colour constancy
-tendency for a surface or object to be perceived as having the same colour when there is a change in lighting conditions
Outline chromatic adaption
Sensitivity to light of any given colour decreases over time
