VF - Spatial Vision - Week 2 Flashcards
What contrast level is a snellen chart capable of measuring? What can be used to measure the other contrast (high/low)?
Snellen charts measure high contrasts, gratings can be used to measure low contrasts.
What kind of functions can a grating follow, and what are the visual differences between them? Which one is preferred, and why? Describe the equation of this function.
They can be square wave, or a sine wave. Square waves have sharp edges, while sine waves are faded. Sine waves are preferred because Fourier analysis can be applied, and the wave profile is preserved. The equation is y = a (sin x) + c x = spatial position (degree phase) a = scaling factor (contrast) c = constant
In a sine wave contrast function, what is ∆L, and Lav?
∆L is the change of luminance from the average, which is midway between maximum and minimum amplitutdes, Lav.
Define Fourier analysis and Fourier synthesis.
Any 2-dimensional visual image can be represented as the sum of a set of sinusoidal gratings that differ in spatial frequency, amplitude of contrast modulation, orientation and phase. By combining this set of sinusoidal gratings, one can recreate the original image exactly (Fourier synthesis).
Describe a square wave function in terms of sine waves.
Square waves can be obtained with the following function:
(4/π) x [sin (F) + 1/3 sin (3F) + 1/5 Sin (5F) + 1/7 sin (7F) + …]
The more sine functions used, the more square-like the wave becomes.
What happens to an image if low frequencies are removed?
What about if high frequencies are removed?
Low - loss of fine details, image becomes blurred
High - fine details are accentuated
On a human Spatial Contrast Sensitivity Function, what is the curve like, and what corresponds with what is visible/invisible?
It is a negative hyperbolic-like curve, with contrasts above the line being invisible, and contrasts below being visible.
Is a human Spatial Contrast Sensitivity Function generated as one, or via many smaller channels?
The curve is actually composed of many smaller channels.
How was it thought that the visual system and auditory system related with sensing frequencies? What test was done, and what were the results?
It was thought they were similar, cochlear has regions specifically for low and high frequencies. A human SCS function was generated in a study, and subjects were then adapted to a specific frequency. The channels did not add up to a standard SCS function. This suggested channels were present to generate a standard SCS function.
How can a subject be adapted to a frequency range?
Four squares with certain ranges are used, on the left are two frequencies with a large difference, on the right are two very similar, and near indistinguishable.
In between the two boxes on the left is a rectangle which the subject looks to the left and right of continuously.
After over a minute of this, they look at a square between the boxes on the right. The difference in the frequencies is accentuated greatly, despite being a very small difference.
Describe the two kinds of retinal ganglion cell receptive fields.
On-centre off-surround - the middle of the receptive field has a high fire-rate, the periphery is low.
Off-centre on-surround - the middle of the receptive field has a low fire-rate, the periphery is high.
Describe the four kinds of visual cortical cell receptive fields.
Light line detector - middle line has high fire-rate, two lines on either side are low.
Dark line detector - middle line has low fire-rate, two lines on either side have high.
Dark-to-light edge detector - left line has low fire-rate, right is high fire-rate.
Light-to-dark edge detector - left line has high fire-rate, right line is low fire-rate.
Why do cats act strangely in the dark sometimes?
They can see frequencies much lower than humans, and in the dark, can see things humans cant.
Vice versa, humans can see things at higher frequencies that cats cant.
Do cortical cells respond well to light itself? How is this expained by the difference of their receptive fields vs retinal ganglion cells?
No, they are very selective, and respind only to certain orientations of frequencies.
Retinal ganglion cells have centre-surround receptive fields, while cortical cells have elongated bars instead, hence the sensitivity to specific orientation only
How do different retinal ganglion cells respond to different frequencies?
They are only senstive to frequencies that are in line with their receptive fields, on/off centre/surround. Light must fall on the on regions, and no light on the off regions.
What are the requirements to resolving the finest details of an image?
- The detector must be small enough, the centre size of the RGC receptive field determines the high frequency cutoff for that cell.
- The array of detectors must be fine enough, and determines whether there will be aliasing or not.
