06-07. circuits & receptive fields

Question 1

What are neural circuits?

Answer 1

Connected cells with combinations of excitatory and inhibitory inputs that detect patterns of external stimulation

Question 2

What is lateral inhibition?

Answer 2

when the output of some receptors has the effect of reducing the signals from neighboring receptors

Question 3

3 perceptual consequences of lat. inhibition?

Answer 3

1. Simultaneous Contrast
2. Hermann Grid
3. Mach Bands

Question 4

Simultaneous contrast example

Answer 4

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

Question 5

Hermann grid - what is it, what happens?

Answer 5

• Grid of black spots with white lines
• you see a grey spot in the intersections you’re not focusing on

Question 6

Hermann grid - why does it happen?

Answer 6

• 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

Question 7

Mach bands - what is it, what happens?

Answer 7

• you see a gradient (of color/brightness) in a constant/uniform band
• “change in perceived brightness is much bigger than change in physical brightness” !!

Question 8

Mach bands - why does it happen? Why is it good for us perceptually?

Answer 8

• percept. system amplifies __slight__ differences in brightness (due to lat. inhibition)
• helps with contour of shape & identifying edges relative to background

Question 9

What does the lat. inhibition neural circuit look like for mach bands?

Answer 9

Look at ppt 06 slides 29&30

(compared to slides 27&28 without lat. inhibition)

Question 10

How to calculate output of a neural circuit? (equation)

Answer 10

Output = input X gain

Excitatory: G>0
Inhibitory: G<0

The problem will give you all info you need

Question 11

receptive field definition?

Answer 11

specific region/area on the retina that, when stimulates, changes the activity of the ganglion cell/neuron

Question 12

Where are receptive fields smaller?

Answer 12

Near the fovea

Question 13

On center cell explanation?

Answer 13

• excitatory when light strikes center
• inhibitory when light strikes surround

Question 14

Off center cell explanation?

Answer 14

• opposite of on cell
• inhibitory when light strikes center
• excitatory when light strikes surround

Question 15

Receptive fields of Lateral Geniculate Nucleus cells?

Answer 15

• 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

Question 16

2 main types of cortical cells (& their receptive fields)

Answer 16

1. simple cells
2. complex cells

both much more specialized

Question 17

Simple cells? Respond best to…?

Answer 17

• Have on off areas too, but not center surround
• respond ONLY to BARS of light
• have a PREFFERED ORIENTATION
• neural circuit is made up of several LGN cells oriented in a line (bar)
  -^^ see ppt 7a slide 21-22

Question 18

Complex cells – 3 differences from simple cells?

Answer 18

1. C much more numerous
2. C responds to both light and dark stim (S prefers one or other)
3. C responds to stimuli regardless of where they are in receptive field
   –> S has on/off areas

Question 19

Complex cells? Respond best to…?

Answer 19

4 types
1. motion (orientation, direction and speed)
2. color
3. size
4. length (end stopped)

Question 20

Feature detectors def & examples?

Answer 20

• They are groups of specialized cells
• they respond only to specific features of visual stimulation
• Ex.
  –> simple cells
  –> complex cells

Question 21

Do feature detectors play a role in perception? how do we know?

Answer 21

• yes
• selective adaptation
• see ppt 7b slide 6