Lecture 5 - Spatial Recognition Flashcards
What are receptive fields?
Area on retina when stimulated by light elicits change in firing rate of cell
Can be excitatory/inhibitory
What is a single cell recording?
Electrode inserted into neuron + measures electrical activit
How do receptive fields increase in complexity?
Photoreceptors – ganglion cells – LGN cells – simple cells – complex cells – hypercomplex cells
What did Hubel/Wiesel find?
V1 simple cells respond to oriented bars and edges
Oriented bar detectors in V1 of cats using single cell recording but can’t do same to humans bc ethics
What is the tilt aftereffect?
Adapt to tilted right lines, then see straight lines and perceive it as tilted left
3 components:
- Orientation turned neurons respond best to preferred orientation but also other similar orientations
- Perceived orientation determined by distribution of responses across cells
- Adaptation – cell’s response decreases following prolonged activity
Size of after-effect depends on difference between adapt + test
What does the tilt aftereffect provide evidence for?
Orientation tuned cells in human visual system
What is the size aftereffect?
Adapt to thicker/thinner bars, then see middle size and perceive it as smaller/bigger
Before adaptation size perceived vertically
Adapt to fatter bars: lines look thinner after due to asymmetrical response distribution (more response to thinner bars)
What does the size aftereffect provide evidence for?
Size-tuned cells in human visual system
Size + orientation fundamental features of parts of visual scene + brain has cells tuned to these features
What is spatial frequency?
Number of bars per unit distance (cycles per degree)
Big bars = low spatial frequency, skinny bars = high spatial frequency
Natural images contain info at many spatial frequencies
High freq = fine details, low freq – course/coarse info
What are diffs between high/low contrast?
Difference in luminance between light/dark area in boundary
We have diff levels of sensitivities to diff spatial frequencies
We have greater sensitivity to intermediate spatial frequencies, can be perceived at low contrast, lower sensitivity to high/low spatial frequencies – need higher contrast to perceive
What does spatial frequency tell us?
Tells us about size on retina, doesn’t indicate real size in world since projected size depends on distance
What is orientation/size constancy?
Perceive object’s real orientation/size in world regardless of orientation on retina
