2.04 spatial vision Flashcards
what is spatiotemporal vision
Spatiotemporal vision is about how we perceive patterns defined by luminance variation over space (spatial variation) and time (temporal variation)
What is the Michelson equation for contrast?
contrast = lmax - lmin / lmax + lmin
Greater contrast means….
bigger increment and decrement about the mean luminance
what does one cycle of a sine wave grating consist of
one dark stripe and one light stripe
What is the frequency of a grating
the number of cycles that can fit within a degree of visual angle
What do the bars look like when there is low spatial frequency vs high spatial frequency
low spatial frequency = bars are wider
high spatial frequency = bars are narrower
How do we measure the quality of an optical system
image contrast/ object contrast
perfect system = ratio should be 1 as no loss of contrast
How do you work out sensitivity
1 / threshold
What is the peak sensitivity
around 3-10 cycles per degree
sensitivity falls at low and high frequencies
what does low and high frequency roll off mean
the loss in contrast sensitivities for low and high spatial frequencies
what is the max value for contrast threshold
100%
what is the minimum contrast sensitivity
1
it is also known as acuity limit or high frequency cut off which is 50CPD and any spatial frequency above this will not be detected.
what are the differences in the different letter acuity charts
visual acuity chart - contrast is high, letter size is varied
Pelli-robinson chart - contrast varied, letter size is fixed
low contrast bailey-lovie - contrast is low and fixed, letter size is varied
How does retinal illuminance affect contrast sensitivity
calculated by doing luminance of light falling on cornea x area of pupil
As retinal illuminance changes:
- spatial freq of peak contrast sensitivity shifts to lower spatial frequency
- the high freq cut off shifts to low spatial freq
- the low freq roll of disappears when retinal illuminance is very low
How does retinal eccentricity affect contrast sensitivity
As it increases the CSF shifts leftwards ( towards lower spatial freq and peak is reduced)
Is the CSF improved when using both eyes or one
CSF is improved if we use both eyes
binocular view improves what 2 things and by how much
improves VA by 10%
improves contrast sensitivity by 40%
what is a receptive field
- a region within visual field in which stimulation can affect the neurons response
- it can range from few minutes of arc to tens of degrees
- RF increases with processing stage
- Receptive fields also increases with retinal eccentricity
- fovea has the smallest receptive field
what the differences between having a small or large receptive field
Large receptive fields have poor spatial resolution. Ability to resolve fine details is reduced
On centre ganglion cells
- This is excited by light falling on its centre
- increases firing rate when light falls on centre
- decreases firing rate when light falls on surround
off centre ganglion cell
- this is inhibited by light falling on its centre
- increases firing rate when light falls on surround
- decreases firing rate when light falls on centre
which kind of spatial frequencies are most informative for identification of faces and general objects
intermediate spatial frequencies
A ganglion cell responds best to gratings with a spatial frequency that…
…matches the size of its receptive field
i.e bright bar falls on centre
dark bar falls on surround
how to work out the logmar visual acuity when given cycles per degree
(0.5/cycles) x 60
thats your MAR
then do log mar
