Contrast Sensitivity Handout
PSYC 230-Burnham
Spatial Acuity and Contrast
Recall, spatial frequency refers to the number of times that a pattern (dark/light cycles in a
grating) repeats in a given unit of space. For example, to the right, if the width of each box
represents 1° of visual angle, then the upper pattern has a spatial frequency of 4 cycles/° and the
lower patter has a spatial frequency of 8 cycles/°.
The size of a retinal ganglion cell’s receptive field is ‘tuned’ for certain spatial frequencies. That
is, a ganglion cell will fire a maximum number of action potentials when light is reflected into its
ON-area and nothing is reflected into its OFF-area (the OFF-area is in darkness). For example, in
the figure to the right, the hypothetical firing rate of a retinal ganglion cell is displayed for three
spatial frequencies. (The firing rate is to the right, where each vertical line marks an action potential over a
given unit of time.) The spatial frequencies in (a) and (c) do not elicit a strong response from the ganglion cell.
This is, because, in (a) the spatial frequency is to low and the receptive field fits within the lighted area, and in
(c) several cycles fit in the ON-area and also
Firing Rate
the OFF-area. But, the frequency of (b) elicits
a near-maximum response rate from the
ganglion cell, because the ON-area is
contained within the lighted area, and the
OFF-area is contained within the darkened
areas. Thus, this ganglion cell’s receptive
filed best ‘fits’ the spatial frequency in (b).
Interestingly, spatial acuity is influenced by
both spatial frequency and contrast. The
contrast threshold is the minimum difference
in intensity, between the darkened bars and
the lighted bars of a grating pattern, which is
needed to detect the pattern. Thus, it is the
absolute threshold for detecting a grating
pattern against a background. Typically, to
measure the contrast thresholds, a person is
presented with a grating pattern against a uniform gray background (see examples below), and is asked to adjust
the contrast of the pattern until it is just noticeable. For example, if the dark stripes of a grating patch must be
2% darker than the light stripes, then if the light stripes reflect 1000 photons, the dark stripes need to reflect 980
photons (or fewer) for the pattern to be detected. This 2% difference is the contrast threshold.
Contrast Sensitivity Handout
PSYC 230-Burnham
The contrast sensitivity function is a plot of the reciprocal of the contrast threshold of an individual against the
spatial frequency of various grating patches. The contrast sensitivity function resembles an upside-down U, and
is presented in the figure to the below. The x-axis of the figure is the spatial frequency of grating patches (in
cycles per degree). The y-axis if the contrast sensitivity to a particular frequency, which is the reciprocal of the
contrast threshold. Using the example from above, say that the dark stripes of a grating patch must be 2% darker
than the light stripes for the pattern to be detected. This 2% (0.2) difference is the contrast threshold, the
reciprocal of which is 1/.02 = 200, which is the contrast sensitivity. Higher values on the y-axis are associated
with contrasts that we are more sensitive to.
From the graph above, you should see that we are most sensitive to spatial frequencies between 6 and 10
cycles/°. Any pattern whose spatial frequency and contrast falls within the area under the upside-down U is
visible, and those patterns outside of this area would be invisible (i.e., not visible).
The figure below represents the contrast sensitivity function with sine-wave gratings. In the figure below, you
should see a sine-wave grating whose contrast increases from the top of the figure to the bottom, and whose
spatial frequency increases from left to right along the x-axis. If you hold this paper at about 2-meters, you
should, be able to see the upside-down U in the figure. This demonstrates the relationship between spatial
frequency and contrast sensitivity.