Samenvatting
Summary Topics discussed during weekly quizzes
This document contains the topics that are discussed during the weekly quizzes.
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Voorbeeld van de inhoud
Quiz topicsThis document contains all the topics discussed in the quizzes.
Module 1
Saccadic eye movements cover 35° of visual angle to left or right. To go further, we have to
move our head. If you detect something in the corner of your eye, how much do you have to
move if the eyes move about 35°? We see +- 105° to the left, so if saccadic eye movement is
35°, then 105-35 = 70 degrees.
A neuron in visual cortex responds to stimuli in far left of visual field. The neuron most likely
resides in right hemisphere and has a relatively large receptive field.
How cortical magnification is related to receptive field size, and is related to spatial
convergence: Spatial convergence means that neurons combine the signals from multiple
receptors, starting in the retina. This convergence is stronger for peripheral vision than for
the fovea, at the expense of resolution (but also with the benefit of higher sensitivity). This
pooling of signals means that there are fewer cells collecting peripheral signals than there
are cells collecting foveal signals. It also means that these cells will have larger receptive
fields, as the cover a larger chunk of space. Cortical magnification refers to the fact that far
more cortical areas are devoted to central, foveal vision, than to peripheral vision. This is
a direct consequence of the spatial convergence: Given that signals in the periphery are
being pooled already at the retinal level, fewer cortical cells are necessary to process
peripheral information. Another way of saying: the cells for the periphery have larger
receptive fields so fewer cells are necessary to cover the same amount of space as
compared to more central cells (but at the expense of resolution).
The image below shows a line segment, with at the bottom a receptive field. It has spatial
filtering properties that makes it sensitive to orientations (white = ON, black = OFF). There
are also cells sensitive to end of a line segment, called end-stopped cells. End-stopped cells
have receptive fields that look like this. The inhibitory (OFF) surround extends beyond the
line. So, the cell responds when the line ends, but is being suppressed when the line does
not end.
Imagine you conduct a crowding experiment. The target is placed at 9 degrees eccentricity.
We vary distance between target and flanker until flanker no longer affects performance.
According to Bouma’s law, when should flankers stop having an influence? At 4.5 degrees:
Bouma’s law: critical distance = 0.5 * eccentricity.
When recruiting new soldiers from the army, you want to determine level of stimulus
contrast a recruit can still read. Explain how to use staircase procedure to find out. You
present text on a screen in a particular contrast, and ask the recruit to read. Whenever
s/he reads correctly, you reduce the contrast by one step. Whenever s/he reads
incorrectly, you increase the contrast. You repeat this procedure long enough so that the
contrast starts to stabilizes around a certain value. That value is your threshold.
Term used for time it takes for an eye movement to be triggered towards a stimulus?
Saccadic latency. (= time between stimulus onset and initiation of saccade).
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