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