Spatial Vision Flashcards
What is visual system’s main task?
To extract information from the pattern of light delivered to the eye that is used to understand and interact with the environment.
What is a ‘pixel by pixel’ measurement of luminance inefficient?
We would need finer resolution, more pixels and a much larger optic never to get a good representation.
When might only coarse spatial information be necessary?
To get the ‘gist’ of a scene.
Give the name for patches of sinusoids.
Luminance gratings.
Give the equation associated with sinusoids.
Luminance = sin(x).
Give the most effective formula for frequency.
Frequency = cycles/degree (cpd).
Give the formula for amplitude.
Amplitude = max - min.
Give the formula for contrast.
Contrast = max - min/max + min.
Which sinusoid characteristic always has a value between 0 and 1?
Contrast.
What are the 4 sinusoid characteristics?
Spatial frequency, amplitude (contrast), orientation and phase.
Coarse and fine scales are processed by (the same/different) populations of neurons.
Different.
Give 2 reasons sinusoids are used to describe and study the visual system?
Images are decomposed into patterns of sinusoids, allowing it to be described using a mathematical ‘language’ and measuring their responses allow prediction of other responses.
Give the formula for contrast sensitivity.
Sensitivity = 1/contrast threshold.
Why is the c.s.f. an inverted U shape?
Processing of high spatial frequencies is limited by the eye’s optics, as very high spatial frequencies are beyond the resolution of the eye.
Give 2 advantages of acuity tests.
Quick to administer and most deficits are optical.
Give 4 advantages of CSF tests.
Predict object visibility in any scene, some diseases show deficits in specific CSFs, measures acuity, and allows us to predict how the brain responds to different spatial frequencies.
How do retinal ganglion cells respond when light is shone?
Their response can increase or decrease, depending on the specific location the light shines on.
Why is a weak retinal ganglion cell response evoked by a uniform field of light?
The excitatory and inhibitory regions affect a cell approximately equally.
How are edges detected by retinal ganglion cells?
Changes in luminance create a strong response, and contrast between borders is represented.
What do ‘simple cells’ in V1 respond maximally to?
High contrast border of bars of the correct orientation, in specific locations.
Give a 3rd reason sinusoids are used to describe and study the visual system, accounting for receptive fields.
Neurons with different sized receptive fields process the scene at different scales, so computation is akin to sinusoidal decomposition.
What does the low spatial frequency limit of receptive fields inform us about?
The largest receptive fields.
Spatial frequency channels are sensitive to a (narrow/wide) range of spatial frequencies.
Narrow.
Why does a ‘notch’ in c.s.f. show after prolonged exposure to one stimuli?
Only one channel has adapted to it, due to independence of channels.
How do we represent the whole range of spatial frequencies?
Ratios of activity are compared and if one changes, the visual system measures this as a change in spatial frequency of the stimulus.
Why after adaptation do we perceive two things that are the same to be different?
Because the ratio of activity has changed and so it is interpreted as a different stimulus.
