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24 Cards in this Set

  • Front
  • Back
Top-Down Processing
- Uses prior knowledge
- Overview of a system is broken down into smaller parts
Proximal Stimulus
- A near stimulus that acts directly on an aspect of the nervous system
E.g. - Light waves on retina of eye
Distal Stimulus
- Objects and events out in the world about you that can be perceived
Marr's Approach
- Bottom-Up approach to perception
- Starts with input from retinal image
- Each stage of processing uses input from previous stage
- Computational
Primal Sketch
- 2D description of edges, light and dark areas, and contours
- Involved in visual processing
- Marr
2 1/2 - D Sketch
- Makes use of information from shading texture, binocular disparity etc. to provide description of depth
- How surfaces relate to one another
- Involved in visual processing
- Marr
Gestalt Approach
- The whole is greater than the sum of its parts
- Top-Down approach (and bottom-up)
- Interested in principles of perception such as: segregation and grouping of visual objects
Laws of Perceptual Organisation
Gestalt laws of grouping:
- Closure
- Good continuation
- Similarity
- Proximity
- Law of Pragnaz
Law of Pragnaz
- Gestalt principle
- Perceive the simplest and/or most stable interpretation
Invariants
- Unambiguous information about environment
- Directly perceived
- e.g. texture gradients, horizon ratio relation
- Gibson
Gibsons Ecological Theory of Perception
- Bottom-up approach
- Perception is direct
- Complex cognitive processes unnecessary
- Study in natural environments not lab
Ambient Optic Array
- Structured arrangement of light with respect to a point of observation
- Gibson ecological theory
Affordance
- Opportunites for action provided by an object
- Gibson ecological theory
Grey-level Description
- Intensity of light measured on retina
- Most basic level of visual processing
- Marr
Constructivist Approach
- Retinal image not enough
- Perception depends on stored knowledge
- Top-down and bottom-up processing
- Too vague
Unconscious Inference
- Human vision incomplete so involuntary assumptions are made
- Constructivist
- Helmholtz
Receptive Field
- Part of visual field (or retina) where stimulus causes change in firing rate of cell
- Effect of stimulus can be excitatory or inhibitory
- Increase in complexity (from retina to cortex)
- On centre - off surround
Hubel and Wiezel (1977)
- Found oriented bar detectors in V1 (primary visual cortex) of cats
Tilt After-Effect
1. Orientation sensitive neurons respond best to preferred orientation, but respond to similar
2. Perceived orientation determined by distribution of responses across cells (distribution peak)
3. Adaptation (decreasing sensitivity).

SO, after adaptation cells are desensitised and so distribution peak is shifted slightly (asymmetrical)
Size After-Effect
- Similar to tilt after-effect
- Evidence for size tuned cells in humans
Spatial Frequency
- Number of bars per unit distance
- High = Thin bars (fine details)
- Low = Fat bars (course information)
- Cycles per degree
- Retinal size vs. real size
Spatial Contrast Sensitivity Function
- Higher contrast needed to see bars in low and high spatial frequency
- Middle spatial frequencies can be perceived at low contrasts (higher sensitivity)
Spatial Frequency Channels
- Collections of neurons tuned to same spatial frequencies
- Respond to same range of spatial frequencies
Resolution Limit
- Point at which higher spatial frequencies cannot be perceived
- Beyond contrast sensitivity threshold