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Humans see __ and __ at a wide variety of __ __. One index of scale is in terms of what?
- bars and edges
- different scales
- one index of scale - Spatial frequency in grating patterns
How is spatial frequency measured?
- refers to the number of Cycles Per Degree (CPD) of visual angle
- (where luminance is varied cyclically along one direction either sinusoidally or as square-wave function)
We are more __ to some spatial frequencies than others. When __ _ __ is mapped against spatial frequency, this yields the what?
- sensitivity to contrast
- contrast sensitivity function (C.S.F)
- As we can see from the graph, after a certain point, there are many spatial frequencies that are too high for us to see in the world. (visual acuity is the limit)
What is the study about the nature of how we process spatial frequencies?
- Campbell & Robson (1968)
- Adapted observers to specific spatial frequencies
- Rather than decreased sensitivity for just the adapted spatial frequency, they found that sensitivity was reduced to a range of spatial frequencies around that to which observers had adapted.
- Therefore, limited number of spatial frequency 'channels' that are tuned to small range of frequencies.
- Other channels remain unaffected.
What is the neurological evidence for these spatial frequency channels?
- Visual cortical neurones are tuned to spatial frequencies in
- cats (Blakemore & Campbell, 1969) (respond optimally to certain sinusoidal S.F gratings)
- monkey (De Valois et al, 1983)
The spatial frequency approach was built upon what?
- Fourier analysis - any repeated pattern can be constructed from a series of sinusoidal wave functions of different frequencies
- It may be possible that early vision employs Fourier analysis to decompose scene into constituent sinusoidal wave functions.
Give 3 depth cues.
- 1. Binocular disparity
- 2. Motion of image
- 3. Static, monocular cues (based on heuristics)
Explain how binocular disparity works.
- If we fixate an object, it will be imaged on fovea of each eye
- Each fovea is at the same point on the two etes, and therefore images falling on fovea have no retinal disparity.
- Objects that are same distance away all appear at corresponding points in the two retinas. The imaginary ellipse formed by all these locations at same distance as fixation is termed horopter.
- Further objects - positive/uncrossed disparity
- Closer objects - negative/crossed disparity
- Also, greater disparity, the closer or further something is.
To use binocular disparity as depth cue, the images from two eyes must be __. This happens only for images near fixated distance around the __. The locations in which object images are fused is termed __ __. Outside this area, we see double or __ images.
- Panum's area
What is motion parallax?
A monocular depth cue in which we view objects that are closer to us as moving faster than objects that are further away from us.
Describe a simple model of motion detection employed by the retina.
- Reichardt (1969) (studied eyes of flies)
- Use of delay in one of the inputs so that when the object moves across one receptor then another, the delay will allow the two EPSPs (excitatory postsynaptic potential) to occur simultaneously and thus created a summed action potential.
- This mechanism allows for detection of movement in only one way
- aka. delay-and-compare detector
Individual motion mechanisms can be __. Describe a phenomenon showing this.
- Motion aftereffect (aka 'waterfall illusion' - first described by Aristotle)
- When we observe stimulus moving in one direction for a time, and then focus on a static stimulus, the latter appears to move in opposite direction to that of first stimulus.
What is the explanation behind the motion aftereffect? Who suggested it?
- Sutherland (1961)
- spatial pattern coding of motion
- cells with receptive fields at same location code many different directions of motion
- When something is static, motion detectors are constantly active but does so equally in all directions.
- However, when one direction of motion is adapted, leads to net bias in motion-responsive cells in the opposite direction.
Motion is often ambiguous. It thus uses heuristics to disambiguate motion. Give 2 examples related to motion.
- 1. Intertia: motion is assumed to continue in the same direction (Ramachandran & Antis, 1986)
- 2. Rigidity: points moving relative to one another are often assumed to remain same distance in sapce from one another (Johannson, 1950)
Heristics in motion - good example of study that others won't have!
- Tse & Cavanagh (2000)
- Perception of motion sequence over successive strokes of a Chinese character (for Chinese people and Americans)
- Chinese people, who were familiar with how it is written by hand, will perceive top-down cues even though the bottom-up cues suggest the opposite direction of motion.
- Americans, who do not have this knowledge/heuristic, are driven by the bottom-up cues.
- Show how important knowledge/heuristic is in something as fundamental as motion perception.
Other factors can override heuristic as well. What is a good paper for how different factors can affect motion perception?
- Sekuler, Sekuler, Lau (1997)
- Sound alters visual motion perception
- Computer screen with two objects moving towards each other and they can be seen to be either moving past each other or bouncing off. When sound played just when the two come into contact makes people perceive the objects to be bouncing off of each other.