S&P 070307 - • First component of frequency is...

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CONTRAST SENSITIVITY How well can humans see contrast at different spatial frequencies? Is there some difference between the light and dark areas, or is it homogenous? What stimuli to use? Why Sine Gratings? Easy manipulation of scale and orientation Fourier’s Theorem : every image can be constructed by combining all sine gratings; it does not matter what the image is (with appropriately chosen frequencies, amplitudes, orientations, and phases) 1. Spatial Frequency – how many cycles per unit distance (1 degree of visual angle) (high=crowded, low=sparse), closer = more sparse. 2. Amplitude/Contrast – difference between the lightest and darkest bars (high=darker, low=brighter) (0-100%) 3. Orientation – what angle (-45 through 90 degrees) 4. Phase – relative position in referenced to a fixed landmark; determines continuity of structure (where it “starts”) Fourier analysis
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Unformatted text preview: • First component of frequency is ‘fundamental’, the rest are ‘higher harmonics’ Contrast Sensitivity Function (CSF) • Contrast threshold: the minimum amount of contrast (on a sine grating) that is visible • Pick a frequency, “ f” , and measure the contrast threshold for that specific frequency • High threshold means low sensitivity, and visa-versa • Repeat procedure for many different frequencies • Highest sensitivity in the middle range (gets worse both for higher and lower ends of scale) • Cut-off frequency = 35 cycles per degree • CSF determines our window of visibility • Can use CSF to compare vision across different visual conditions. Selective Adaptation • Measure CSF of subject • Adapt the observer to a certain high-contrast grating of a given frequency, “ f ” • Measure CSF again...
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