06-07. circuits & receptive fields Flashcards
What are neural circuits?
Connected cells with combinations of excitatory and inhibitory inputs that detect patterns of external stimulation
What is lateral inhibition?
when the output of some receptors has the effect of reducing the signals from neighboring receptors
3 perceptual consequences of lat. inhibition?
- Simultaneous Contrast
- Hermann Grid
- Mach Bands
Simultaneous contrast example
2 squares of medium grey are surrounded by light grey and black
- square surrounded by light grey looks darker –> light grey is bright stimulus, and produces lots of lateral inhibition
- square surrounded by the black looks lighter –> black is dark stimulus, no lateral inhibition
Hermann grid - what is it, what happens?
- Grid of black spots with white lines
- you see a grey spot in the intersections you’re not focusing on
Hermann grid - why does it happen?
- areas in intersection are surrounded by 4 sides of white (bright, intense stimulus)
–> produces lateral inhibition, and makes the intersection look darker (grey) - areas in between boxes only have white on 2 sides. on the other 2 sides they have black (dim, not intense stimulus)
–> no/little lateral inhibition, you see bright/normal white
Mach bands - what is it, what happens?
- you see a gradient (of color/brightness) in a constant/uniform band
- “change in perceived brightness is much bigger than change in physical brightness” !!
Mach bands - why does it happen? Why is it good for us perceptually?
- percept. system amplifies __slight__ differences in brightness (due to lat. inhibition)
- helps with contour of shape & identifying edges relative to background
What does the lat. inhibition neural circuit look like for mach bands?
Look at ppt 06 slides 29&30
(compared to slides 27&28 without lat. inhibition)
How to calculate output of a neural circuit? (equation)
Output = input X gain
Excitatory: G>0
Inhibitory: G<0
The problem will give you all info you need
receptive field definition?
specific region/area on the retina that, when stimulates, changes the activity of the ganglion cell/neuron
Where are receptive fields smaller?
Near the fovea
On center cell explanation?
- excitatory when light strikes center
- inhibitory when light strikes surround
Off center cell explanation?
- opposite of on cell
- inhibitory when light strikes center
- excitatory when light strikes surround
Receptive fields of Lateral Geniculate Nucleus cells?
- NOT very picky/SPECIALIZED
- they respond BEST to small spots of light
- but they also respond to bars of light and stimuli that move across the field in ANY direction