Visual Perception Flashcards
What are the key parts for vision and the brain?
Eye
Lateral Geniculate Nucleus (LGN)
Visual cortex - V1, V2, V3, V4, V5 (MT = medial temporal lobe)
What in the eye helps with vision?
Two visual receptor cells in the retina
- cones
- rods
Retinal ganglion cells receive input from a few cones or hundreds of rods
What do cones do?
Colour and detail perception
Mostly located in the fovea
What do rods do?
Vision in dim light
Located in the periphery
What is LGN?
Lateral Geniculate Nucleus
What is the retina geniculate-striate system
Parvocellular (P) pathway
- sensitive to colour and fine detail
- most input from cones
Magnocellular (M) pathway
- most sensitive to motion
- most input from rods
What are V1 and V2?
Primary visual cortex (V1)
Secondary visual cortex (V2)
Receptive field
- Area sensitive to a visual stimulus
Lateral inhibition
- Increases contrast at edges
What is form (shape) processing?
V1, V2, V3, V4 likely all process object shape and form
Neurons in inferotemporal cortex respond to specific semantic categories (e.g., animals, body parts).
And form processing: Yamane et al., 2008 neurons within inferotemporal cortex responded to 3D object shape.
Baldassi et al. (2013) anterior inferotemporal neurons responded on the basis of aspects of form or shape rather than object category.
Colour processing in V4?
Bouvier and Engel (2006) achromatopsia cases (no colour perception): found small brain area close to V4 was damaged in nearly all cases.
Goddard et al. (2011). fMRI more activation in V4 with full-colour movie clips (compared to Black & White).
Banissy et al. (2012) found TMS reduced colour processing performance in V4.
V4 important in colour processing network, but not the “colour centre”
Motion processing in V5? (MT)
V5 (also known MT: medial temporal cortex) is heavily involved in motion processing.
fMRI show associations.
McKeefry et al. (2008) used transcranial magnetic stimulation (TMS) to disrupt V5/MT, it produced a subjective slowing of stimulus speed and impaired observers’ ability to discriminate between different speeds.
Whats the binding problem?
How are features combined and integrated?
1. Binding visual features together?
2. Binding information across multiple eye movements?
Hard to solve
Feature integration theory: selective attention plays a role (Treisman & Gelade, 1980)
What are the suggestions for the binding problems?
Suggestion 1: binding-by-synchrony
Features from single object fire in synchrony (e.g., Singer & Gray, 1995).
Suggestion 2: Guttman et al. (2007) suggested patterns of neural activity over time help coordinate binding.
What’s the dorsal and ventral visual streams?
These streams of neuron activity flow from V1 and into the parietal and temporal lobe,
One helps identify shapes and names the objects
The other processes spatial and movement
What are the two visual streams?
Ventral stream
Dorsal stream
What’s the ventral stream?
Vision-for-perception
Identifies objects
Allocentric (labelling of objects without reference to self)
Sustained representation
Usually conscious
Input from the fovea
Whats the dorsal stream?
Vision-for-action
Processes spatial information to guide movement
Egocentric (represents object in space relative to self)
Short-lived representations
Usually unconscious
What does dorsal stream damage cause?
Poor guided movement
Is there a connection between dorsal and ventral networks?
Had damage in the ventral stream and visual form agnosia
However, also made more errors than average on dorsal stream test
How can visual illusions help?
Visual illusions can be used to understand visual perception
Including whether they support the dorsal vs ventral streams theory
E.g.,
Bruno et al. (2008) found when pointing (using vision-for-action system), the illusion size was 5.5%
When verbalising a response (using vision-for-perception system), the illusion size was 22.4%
Evaluation of dorsal and ventral systems?
Independence of the dorsal and ventral systems is not absolute
The dorsal stream is appears to be only required for rudimentary actions
Double dissociation between optic ataxia and visual form agnosia is not clear cut
Whats the trichromatic theory?
Three types of colour receptors or cones identified by microspectrophotometry
Whats opponent process theory?
But what about negative afterimages, that are not well explained by trichomatic theory?
- Green - Red
- Yellow - Blue
- White - black
There is strong physiological evidence for the existence of opponent cells (Shevell & Martin, 2017).
the theory claims it is impossible to see blue and yellow together or red and green, but the other colour combinations can be seen. Abramov and Gordon (1994) found evidence for this.
What’s the dual process theory?
Combination of trichromatic and opponent process theory
Whats colour constancy?
The tendency for a surface or object to appear to have the same colour despite a change in the wavelengths contained in the illuminant
What’s size constancy?
The tendency for objects to appear the same size whether their size in the retinal image is large or small
Size perception of objects depends on memory of their familiar size rather than on perceptual information
What does perception without awareness mean?
Responding appropriately to visual stimuli in the absence of conscious visual experience
- Associated with severe brain damage to V1
- Impairments largely due to knock-on effects due to V1 damage (Silvanto, 2008)
Intact abilities rely upon a neural tract linking lateral geniculate nucleus to the human visual motion area (V5/MT) that bypasses V1
