Visual Perception Flashcards
(27 cards)
What are the 2 visual receptor cells in the Retina?
Cones: colour, detail perception, located in fovea
Rods: vision in dim light, in periphery
What are the retinlal-geniculate-situate systems:
Pavovecellular (P) pathway: sensitive to colour + fine detail, most input from cones
Magnocellular (M) pathway: sensitive to motion, most input from Rods
V1-V5, what are they involved in?
V1 + V2: early processing stages
V3 + V3A: cells respond to form, not colour
V4: cells respond to colour + line orientation
V5: specialised for visual motion
What’s involves in shape processing?
V1, V2, V3, V4 all help process object shape + form
Neurons in IT (inferotemporal) cortex respond to:
- specific semantic categories
- 3D object shape
Anterior IT neurons respond to aspects to form/shape, rather than object category
Colour processing in V4:
Found small brain area close to V4 in nearly all cases of achromatopsia (no colour perception)
Using fMRI found more activation in V4 when watching colour movies
V4= important in colour processing
Motion processing in V5:
When V5 disrupted, observes ability to discriminate between diferent speeds= impaired
The binding problem:
- Binding by Synchrony
- features from single object dire in synchrony - Pattern of neural activity over time helps coordinate binding
Dorsal + Ventral streams:
- 2 streams of neural activity flow from V1 into parietal + temporal lobes
What does ventral stream do?
What stream> helps identity shapes and name objects (what are they?)
System used to decide what object it
What does dorsal stream do?
How stream» processes spatial + movement
Used for visually-guided action
Used when grabbing object - need to calculate position of object
Principle role of ventral stream?
Top-down processing
Info from visual + semantic memory provides perceptual representations (recognition)
Principle role of dorsal stream?
Bottom-up
Provide real time visual guidance of our movement
The ventral stream:
- vision-for-perception
- identifies objects
- labels objects with reference to self
- conscious
- ‘what’ stream
- takes P Pathway
- form + colour processing
- susceptible to illusions
The Dorsal stream:
- vision-for-action
- proceses spatial info to guide movement
- egocentric ( represents objects relevant to self)
- unconscious
- ‘how’ stream
- takes M Pathway
What happens in damage is dorsal stream?
- damage to posterior cortex (dorsal stream)
- patients= poor at making visually guided movements
- vision + ability to move arms = intact
- difficulty in rotating hands when reaching into large slot
- damage to dorsal stream IMPAIRS VISUALLY-GUIDED ACTIONS
What happens in damage to ventral strea,?
- show high levels of activation when grasping for objects
- but can not identify drawings of common objects but can reach out and grasp them §
What stream is more susceptible to illusion?
Ventral
Illusion size = bigger when verbalising illusion (ventral) than pointing it out (dorsal)
Hollow face illusion:
- Strong illusion when observed asked to draw target potion
- accurate preformance when asked to make flicking movement to target
Opponent-processing theory
It’s impossible to see blue+ hello together or red + green, but other colour combinations = possible
Dual-processing theory:
Signals by 3 cones sent to opponent cells
- Achromatic (non-colour) channel combines activity of medium + long wavelength cones
- Blue-yellow channel: represents different between sum of medium + long wavelength cones on one hand + short-wavelength cones of other
- Red-green channel: represented difference between activity levels in medium and long wavelength cones. Direction of difference determines red or green seen
Dual processing theory. Channels effects on colour reception?
Individuals have different proportions of different cone types
Little effect on colour preception
Trichromatic theory (3-coloured theory)
- retinal cones= specialised for colour vision
- cone receptors contain light sensitive photopigmentation- allows them to respond to light
- there are 3 receptor cell types:
- Sensitive to short-wavelength light > responds strongly to blue stimulus
- sensitive to medium wavelength light> responds strong to yellow-green stimulus
- Responses most to long-wavelength light> orange-red
Colour constancy
- tendency for surface/ object to appear to have some colour despite change in wavelengths contained in illumination
- indicates: colour vision does not solely depend on wavelengths of light reflected from object
What would happen if we lacked colour constancy?
Apparent colour of familiar objects would dramatically change when lightening conditions affected
Would make it hard to recognise objects fast + accurately
Depth perception
Depends on numerous visual +other cues
Depth cues provided by movement of observer or objects in visual environment
