The Contrast Sensitivity Function (CSF)
USD Internet Psychology Laboratory
The spatial resolution of the visual system is usually assessed using a simple measure of static visual acuity. A typical visual acuity test consists of a number of high contrast, black-on-white targets of progressively smaller size. The smallest target that one can successfully read denotes one's visual acuity. For example, if the smallest letters that you can read upon a Snellen Eye Chart subtend 5 minutes of arc (minarc) in height, you are said to have 20/20 (or "normal") acuity. That is, the smallest letter that your visual system can successfully resolve is 5 minarc in height (with a "critical detail" subtending 1 minarc).
Acuity is a common measure of visual status because: (1) it is easy to measure and (2) small amounts of refractive error in the eye yield marked declines in acuity test performance. Fortunately, most sources of refractive error are correctable via glasses or contact lenses.
However, contemporary research has demonstrated that visual spatial processing is organized as a series of parallel - but independent - channels in the nervous system; each "tuned" to targets of a different size. As a result of this parallel organization of the visual nervous system, visual acuity measurements no longer appear to adequately describe the spatial visual abilities of a given individual. Vision research has clearly demonstrated that the capacity to detect and identify spatial form varies widely as a function of target size, contrast, and spatial orientation (see Braddick, Campbell & Atkinson, 1978 or Olzak & Thomas, 1985). As a consequence of the above, a simple assessment of visual acuity often does not predict an individual's ability to detect objects of larger size (Ginsburg, Evans, Sekuler & Harp, 1982; Watson, Barlow & Robson, 1983).
Contrast sensitivity testing complements and extends the assessment of visual function provided by simple acuity tests. At the cost of more complex and time-consuming procedures, contrast sensitivity measurements yield information about an individual's ability to see low-contrast targets over an extended range of target size (and orientation).
Contrast sensitivity tests use sine-wave gratings as targets instead of the letter optotypes typically used in a tests of acuity. Sine-wave gratings possess useful mathematical properties and researchers have discovered that early stages of visual processing are optimally "tuned" to such targets (Maffei, 1978; Watson, et al., 1983).
A contrast sensitivity assessment procedure consists of presenting the observer with a sine-wave grating target of a given spatial frequency (i.e., the number of sinusoidal luminance cycles per degree of visual angle). The contrast of the target grating is then varied while the observer's contrast detection threshold is determined. Typically, contrast thresholds of this sort are collected using vertically oriented sine-wave gratings varying in spatial frequency from 0.5 (very wide) to 32 (very narrow) cycles per degree of visual angle.
Contrast Sensitivity and the Contrast Sensitivity Function
Because high levels of visual sensitivity for spatial form are associated with low contrast thresholds, a reciprocal measure (1/threshold) termed the contrast sensitivity score is computed. The contrast sensitivity scores obtained for each of the sine-wave gratings examined are then plotted as a function of target spatial frequency yielding the contrast sensitivity function (CSF). Some typical CSF's are depicted in Figure 1 below. Note the characteristic inverted-U shape of the CSF and its logarithmic axes.
This experiment uses a 2-interval forced choice modified staircase procedure (2-down/1-up rule) to measure your contrast threshold for sine-wave grating targets varying in spatial frequency..
On each trial, two stimulus presentation frames will be presented consecutively. Only one of these frames will contain a sine-wave grating target stimulus. Your objective is to determine which interval, the 1st or the 2nd frame, contained this sine wave grating target..
Explanation of Stimulus Screens:
The first screen will provide you with general instructions for this
To begin the experiment, click the "Present Stimuli" button.
· When the experiment is in progress, two frames will be presented in the center of the screen in a consecutive manner.
· A sine-wave grating target will appear within only one of these frames. The contrast of the target sine wave grating will vary throughout the experiment.
· Your task is to determine which frame contained the sine-wave grating: the 1st or in the 2nd.
· You can enter your decision in one of two ways.
- Use the mouse and click on the either the 1st Interval or 2nd Interval button
- Press either the "1" or "2" in the row of numbers at the top of the keyboard.
DO NOT USE THE NUMBER PAD!!!
· You can move onto the next trial in one of the following ways.
- Use the mouse and click on the Present Stimuli button.
- Press the space bar
· You will notice a + symbol in the center of the screen during the experiment. It is advisable that you maintain fixation on this point during the experiment. Therefore, it may be easier (as well as faster) if you use the keyboard method to complete this experiment.
Important things to remember:
You will be presented with six different stimulus spatial frequencies (in separate blocks of trials). After you have completed a set of trials at one of the spatial frequencies, the screen will inform you when a new target spatial frequency is being selected.
Be careful when entering your decisions into the computer, once you input your decision into the computer, it is locked in and can not be changed.
If you need to examine the current trial stimulus again before you input your decision, just choose "Present Stimuli" again. The trial will be repeated (Note, however, that the target stimulus may be presented in either frame).
· The results will contain the stimulus spatial frequency (cycles/image) as well as the your contrast sensitivities for those stimuli.
· Graph these data points.
1. What is the independent variable?
2. What is the dependent variable?
3. What psychophysical method was used to assess your contrast sensitivity?
4. What is the relationship between the independent variable and the dependent variable?
(Hint: Plot the Contrast Sensitivity Function.)
Do this for your individual results as well as the class average results.
5. If there is a difference between these two graphs, provide an explanation.
Braddick, O., Campbell, F.W. & Atkinson, J. (1978). Channels in vision: Basic aspects. In R.Held, H.W. Leibowitz & H. Teuber (Eds.) Perception. Berlin: Springer-Verlag. pp. 3-38.
Ginsburg, A.P., Evans, D. Sekuler, R. & Harp, S. (1982). Contrast sensitivity predicts pilot's performance in aircraft simulators. American Journal of Optometry and Physiological Optics, 59, 105-109.
Maffei, L. (1978). Spatial frequency channels: Neural mechanisms. In R.Held, H.W. Leibowitz & H. Teuber (Eds.) Perception. Berlin: Springer-Verlag. pp. 39-66.
Olzak, L.A. & Thomas, J.P. (1985). Seeing spatial patterns. In K. Boff, et al. (Eds.), Handbook of perception and human performance. New York: Wiley. pp. 7:1-7:56.
Schieber, F. (1992). Aging and the senses. In J.E. Birren, R.Sloan & G. Cohen (Eds.), Handbook of mental health and aging. New York: Academic Press. pp. 251-306.
Watson, A.B., BArlow, H.B. & Robson, J.G. (1983). What does the eye see best? Nature, 302, 419-422.
End of The Contrast Sensitivity Function.