Measuring Visual Acuity Flashcards
(24 cards)
What is the visual angle?
- The angle that an image subtends at the eye (α)
- The image size depends on the viewing distance so if you move closer the visual anfle is larger
What is 20:20 vision?
When α is 5 mins of arc at a viewing distance of 20 ft
What would limit the resolving power of the eye?
The spacing of the photoreceptors
What is spatial frequency?
The number of cycles per degree
- More cycles per degree means higher spatial frequency and so the detail is finer
What is the relationship between high spatial frequency and resolving power?
Higher the spatial frequency, the higher the resolving power
What do we use for high resolution vision?
Combination of red and green cones in fovea
What is contrast?
The difference between the maximum luminance and the minimum luminance
What is luminance?
The amount of light coming off of an image and this is low from black lines and high from white lines
the luminance graph
A cycle= going from black to white or visa versa
The more cycles per degree, the higher the spatial frequency, i.e. the finer the detail, higher the
resolving power
what is contrast
Contrast: difference between min and max luminance
what is The ultimate upper limit of the resolving power of the eye and spatial frequency,
assuming the optics
are perfect (the eye is correctly focusing the image as well as it can), is the spacing of the
photoreceptors
what is the cone spacing in the central fovea
- Cone spacing in central fovea: 160k per mm2
What is the formula to work out contrast?
lm-ln / lm+ln, where lm is the maximum intensity and ln is the minimum intensity
What is contrast threshold?
The minimum contrast required to detect a target
What is contrast sensitivity?
The reciprocal of contrast threshold
What is the contrast sensitivity function?
- All visible contrasts and frequencies are inside the curve
- Sensitivity is reasonably constant for intermediate frequencies
- Highest spatial frequencies require high contrast
what are orientation columns
- It’s the main property of neurones in the primary visual cortex
- They’re tuned to particular orientations
- are excited by visual line stimuli of varying angles
- span multiple cortical layers
What do orientation sensitive neurons sensitive to?
- Orientation but not just a single orientation, can respond to a range
What is population coding?
Idea that from a single neuron, information can be ambiguous
What can a reduction in the signal from a neuron be due to?
- Bars orientation stayed the same but changed in contrast
- Bars tilted but contrast stayed the same
what is the tilt-after effect
Stare at bar for long enough, then look at bars that are vertically orientated, there will be an illusion that
the bars are orientated in the opposite direction
* You adapt to that orientation but not to others so that when you’re presented with a new orientation,
the decoding mechanism is biased because of the previous adaptation
Explain the tilt after effect
- System does not need an orientation-sensitive cell for every possible orientation
- Orientation can be estimated by averaging cells w similar preferences
- Adaptation of cells occurs w prolonged stimulation
- If a cell is adapted to one orientation, then the response will be decreased to the same or a similar orientation immediately afterwards
- It will introduce error in the estimation of orientation
- This predicts that the effect will only be elicited by adapting to an orientation close to the test orientation so the neural tuning curves overlap
what is orientation tuning
Orientation-sensitive neurons do not respond to just a single orientation, they respond to a rangethey’re broadly tuned
* E.g. have a max response a 45 degrees but still respond at 35 degrees
what is the tuning curve
- Tuning curve: describes the range and selectivity with which a neuron responds to orientations
