object recognition Flashcards
what is perception
- Perception refers to our ability to extract meaning from sensory input
It includes:
- audition
- taste
- touch
- olfaction (smell)
but research is dominated by vision.
- Vision alone accounts for over 50% of all neurons in our cortex
- How many senses do we have? many types of senses[ eg sense of hunger sense of when you need the toilet] but limited amount of receptors
- Perception is a constructive process
the visual system
LVF processed by right hemisphere
RVF processed by left hemisphere
- corpus callosum acts as pathway between left and right hemisphere → study using ppl with epilepsy
Two processing streams: ventral, dorsal
dorsal
processsing stream.
where things are → in the posterior partietal cortex
ventral
processing stream
what you’re seeing → in the inferior temporal cortex
a misleading impression of simplicity regarding vision
- Studies like this (Tootell etal, 1982) support the notion that a near perfect representation of the external world is “projected”onto our primary visual cortex.
- The stimuli were displayed for 25-30 minutes and the monkeys were not conscious during the experiment…
- monkeys injected with radioactive isotope and became concentrated
a 3 stage model
- local features = edge detection / contrast etc [outlines]
- shape representation = gestalt principles / feature integration
- object representation = stored representation / knowledge
gestalt principle
- The whole visual percept is more than the sum of its parts.
- Our perceptual system constantly tries to impose organisation on its input
- Components of an image are grouped together on the basis of certain visual properties
- Laws of perceptual organisation e.g. “good continuation” and “closure” give rise to “illusory contours” → we use our previous experiences to create assumptions
shape perception
Primarily “bottom up” processes produce a “primal sketch” (Marr,1982)
This sketch contains “primitives”– edges / orientations / positions/ lengths / colours etc
“Top down” processes (such as Gestalt laws) are used to group collections of primitives together into “lines, curves, larger blobs, groups and small patches” - symbolic primitives.
object recognition- 3 models
- template matching [prototype theory]
- . feature analysis
- recognition by components
template matching
- A template is an internal representation
- A memory against which the visual input is matched.
- Computer based object recognition programmes use templates.
- Intuitively plausible – object recognition must involve some kind of contact with a “comparable internal form”
But what rules determine whether a match is made?
How many different templates are needed?
feature analysis
Assumes lower level features are analysed first
- The perceptual system searches for simple but characteristic features of an object
- Supported by neurological evidence (e.g.Orientation selective cells in visual cortex)* Most research focuses on letter / word recognition (used to read postcodes!)
- Letter A made up of / \ and –
- But what about V and X? Both made of /and \ … Spatial relationship important
- What about complex natural objects?
recognition by components
Recognition By Components = (Feature analysis in 3d)
- any specific view of an objectcan be represented as anarrangement of simple 3D shapes – geons (Biederman,1987)
- Geons are “viewpoint invariant”– easily “recoverable” from a 2D retinal image
- “Invariant properties” include cotermination & parallelism
- Object recognition is impaired when geons are made “non-recoverable” by removing termination points
but… Geons reappear with ‘splats’ - the ‘splats’ means your brain makes the assumption that its a 3D object - not easily explained by ‘recognition by components
- colour can air recognition too
