4. The CSF.pptx

The Contrast Sensitivity Function Introduction Objectives and Readings Sine waves (and Gabor patches) and definition of contrast The normal adult CSF The effects of disease on contrast sensitivity SFO1004 Dr Sarah J Waugh© Objectives and readings The student should be able to: • Draw a normal adu...

The Contrast Sensitivity Function Introduction Objectives and Readings Sine waves (and Gabor patches) and definition of contrast The normal adult CSF The effects of disease on contrast sensitivity SFO1004 Dr Sarah J Waugh© Objectives and readings The student should be able to: • Draw a normal adult contrast sensitivity function (CSF). • Understand why the CSF has its shape. • Define Michelson contrast and contrast sensitivity. • Define spatial frequency (SF) with units. • How does the CSF change with development/disease? (Give at least 2 examples). SFO1004 Optometry: Science, Techniques and clinical management. Rosenfield and Logan. • Chapter 3 (pages 41-72), Chapter 5 (pages 73-75), Chapter 12 (pages 179-180) Visual Perception: a clinical orientation. Schwartz. • Chapter 7 (Spatial Vision: pages 151-173) https://webvision.med.utah.edu/ • Part VII: Contrast Sensitivity. Dr Sarah J Waugh© The Contrast Sensitivity Function SFO1004 Dr Sarah J Waugh© Luminance gratings (stimuli for measurement of CSF) Orientation Spatial frequency Contrast Phase Gratings have four defining parameters: SF, Phase, Orientation, Contrast. Each affects their appearance. Why are gratings useful targets for assessment of vision? The Sine Wave 1 cycle L max amplitude L mean L min Contrast (Michelson) = Lmax – Lmin Lmax + Lmin Range: 0-1 SFO1004 Dr Sarah J Waugh© The Sine Wave Spatial Frequency 4 cycles/deg 1 cycle/deg Contrast High ContrastLow Contrast Image adapted from Snowden, Thompson, Troscianko: Basic Vision, 2006. OUP. Demonstration of Gabor Patches as Visual Stimuli. • A Gabor patch is a sine wave grating seen through a Gaussian window. • Gabor patches are useful in research as they have characteristics like those that match response characteristics of cortical neurons called Receptive Fields. • Here is a demonstration link to help you understand sine waves and Gabor patches. • Gabor Patch Activity SFO1004 Dr Sarah J Waugh© Linear Systems Analysis Fourier Analysis or Using Grating Targets to Investigate Vision • Originated in mathematical discovery by French physicist, Jean Fourier, in 1822, who demonstrated that a periodic waveform of any complexity could be broken down (analysed) into the linear sum of sine waves of specified spatial frequencies, amplitudes and phases • First became widely applied to understanding spatial vision in 1960’s. SFO1004 Dr Sarah J Waugh© All images can be made up from different combinations of sine waves. SFO1004 Dr Sarah J Waugh© The Spatial Contrast Sensitivity Function (CSF) shows human sensitivity to sine waves of different frequency. Spatial frequency (c/deg) increases from left to right, while contrast increases from top to bottom. Medium SFs are visible at the lowest contrast (higher up the image). Spatial sensitivity for stationary gratings is “band-pass” in shape. SFO1004 Dr Sarah J Waugh© log Contrast Sensitivity Spatial Contrast Sensitivity Function (CSF) Peak low SF rolloff High SF Cutoff log Spatial Frequency (c/deg) SFO1004 Dr Sarah J Waugh© The outer envelope • Is the CSF shape measured psychophysically (behaviourally) due to all contributing neurones having the same overall sensitivity profile? SFO1004 Dr Sarah J Waugh© Adaptation and Multiple Mechanisms Contrast Sensitivity • Can the shape of the CSF be changed psychophysically? YES Blakemore and Campbell (1969) measured the CSF, adapted to high contrast sine-wave grating (e.g. 8 c/deg), and then remeasured CSF and found a notch. Spatial Frequency (c/deg) From deValois, K Vision Research (1977) Is the CSF seen due to a large number of neurons all with the same tuning? • If this were true, after you adapt to high contrast grating of any particular single frequency, the whole curve should shift down. • Does this happen? NO. • The presence of notches (specific losses) demonstrates the presence of different populations of neurons tuned to different bands of SF. • Similar results were found when the adapting grating had other spatial frequencies. Image from deValois&deValois, Spatial Vision, 1990, OUP. SFO1004 Dr Sarah J Waugh© The Contrast Sensitivity Function • The function has a characteristic inverted U shape, with the peak somewhere around 4 c/deg • On either side of this peak there is a fall in sensitivity • The whole function spans about 7 octaves (0.5 c/deg to 60 c/deg). (An is SFO1004octave is a doubling, so 0.5-1 is an octave, Dr Sarah 1-2 J Waugh© The Contrast Sensitivity Function • The high frequency cut-off is at about 60 c/deg, though more often is lower at around 40 c/deg. This is equivalent to the resolution acuity limit of vision. • The peak sensitivity is around 200, i.e., the measured contrast threshold is about 0.5% (although viewing conditions are important). SFO1004 Dr Sarah J Waugh© Why does the Contrast Sensitivity Function have its characteristic shape? • Why is there visual system less sensitive to high sfs? – Optical factors: optics of eye tend to blur image according to optical point-spread function. At high SFs, peaks and troughs are blurred together so that the effective contrast of high SF gratings is reduced. – Gratings higher than 60 c/deg do not get through eye’s optics even at 100% contrast. SFO1004 Dr Sarah J Waugh© Why does the Contrast Sensitivity Function have its characteristic shape? • Why is the visual system less sensitive to high sfs? – Neural factors • It is the spacing of photoreceptors that imposes the upper limit on the resolvable SF of a grating at 100% contrast. • There needs to be a receptor under the trough and a receptor under the peak of the light distribution to represent the sine wave accurately. (2 cones/cycle) • So sampling density of retinal cone mosaic regulates the eye’s resolution limit and high SF cutoff. SFO1004 Dr Sarah J Waugh© Why does the Contrast Sensitivity Function have its characteristic shape? • Why is the visual system less sensitive to low sfs? – Neural factors • The eye’s optics do not contribute to this. • Essentially there are fewer neurons “tuned” to lower spatial frequencies. • Also, for neurons tuned to medium spatial frequencies, lower spatial frequency stimuli also lead to a reduction their response through a spatial interaction process within the “receptive field” called “lateral inhibition”. • The low frequency fall-off in sensitivity can only be explained by neural factors primarily at cortical level. SFO1004 Dr Sarah J Waugh© Contrast Sensitivity The CSF: Effect of Luminance 17 cd/m2 .0017 cd/m2 Spatial Frequency (c/deg) Spatial Frequency (c/deg) The CSF: Effect of Luminance 1) Overall human sensitivity decreases as light levels decrease. 2) Peak sensitivity shifts to left (towards lower SFs). 3) The high frequency cut-off shifts to left (towards lower SFs) (because at lower light levels, spatial integration areas are larger due to greater “pooling” of rod information per ganglion cell receptive field) 4) The low SF roll-off becomes less marked so shape changes from band-pass to low-pass shape (at very low light levels, inhibitory surround believed to contribute very little, or is switched off) Development of the CSF 3 month s 1 month Dr Sarah J Waugh, Reader, Anglia Ruskin University. Contrast Sensitivity Function in Disease A Normal B MS C Cataract D Refractive Blur or Amblyopia Image from Webvision SFO1004 Dr Sarah J Waugh©