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How do we define a receptive field?

A region of the visual world through which the firing pattern of a cell can be altered.


What are the 2 functionally distinct regions of each receptive field? What does the typical receptive field look like?

ON region - that increases the cells firing
OFF region - that decreases the cells firing
A typical receptive field is circular with a small central region and a larger surround, both of which can be either ON or OFF


What visual feature do spatially opponent cells primarily detect?

local contras - Edges - tend to correspond with important things


What is the effect of total light intensity on the activity of ganglionic cells?

No effect. These cells detect relative ratios of light intensity falling on their centre or surroundings.


What happens to the ON-centre cell activity in following situations:
- entire receptive field in light
- small part of the receptive shield(only periphery) in shadow
- edge of light and shadow goes through the middle of the receptive field?
- large part of the receptive field (peripery and centre) in the shadow
- entire field in shadow

- no activity
- increased activity
- no activity
- decreased activity
- no activity


What does lateral inhibition mean?

It is the antagonism of effects of the centre and surround on activity of the cell.


What is Hermann/Herring grid? How does the illusion work?

Black & White, Manhattan-like grid. An on-centra receptive field located at an intersection has much more light in its inhibitory surround than the receptive field located anywhere else along a street. More light in the inhibitory surround means that there is more lateral inhibition at the interception. This reduces the firing rate of the ganglion cell, while a neighbouring ganglion cell down the street with less inhibition will be firing faster. The reduced firing of ganglion cells cetntred on the intersection is interpreted by higher visual areas as lower luminescence at intersection - dots. If we focus on 1 intersection we dont see a dot because receptive fields in fovea are smaller - too small to span the width of an intersection.


What are the counter arguments to Hermann grid explanation theory?

1) Illusion greatly diminished if grid is rotated 45 degrees
2) Scintillating (wavy) grid - illusion disappears
More complex processing responsible for that


What is Trexler fading?

2 blue crosses in the centre of the grey background and blurry black and white dots on both sides of the upper cross. If you fixate focus on the upper cross the blurry dots disappear. If you fixate on the lower cross, illusory black and white dots appear analogously to the upper ones.


How Trexler fading can be explained?

2 types of cells:
- sustained response - light on its field increases firing, which continues until the light stops.
- transient response - bursts of firing upon turning on or off stimulus. no activity in between.
In Traxler fading, dots disappear because transient cells report no change in the image and the sustained cells have given up reporting the presence of the blurry blobs.


What are simultaneous contrast effects?

Circles of the same shade of grey in a row against a background gradually changing from light to dark grey. Circles on the darker side appear lighter grey and vice versa. This is because there is more contrast in the lateral circles, which is registered by the receptive fields of ganglionic cells and interpreted as difference in the shade of grey.


What is a COC illusion?

2 grey rectangles, 1 lighter fading to lighter grey near the border and other fading from darker near the border to the same colour as the other one. The one with a darker edge appear slightly darker, even though it is the same colour.