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1. Using experimental curves relating the threshold contrast of sinusoidal grating patterns to their spatial frequency, the expected threshold contrast curves for three aperiodic patterns, viz. a single half-cycle sinusoid bar, a single full-cycle sinusoid bar, and the boundary between an extended sinusoidal grating and a 50% grey surround, are calculated. In this calculation the assumption is made that the system is linear near the threshold.

2. Experiments are described in which the actual threshold contrast curves are determined for these aperiodic patterns by three observers. The patterns were generated on the face of an oscilloscope and could be varied in size and contrast.

3. These experimental curves agree well with the predictions in the high frequency region (i.e. above about 10 c/deg), but below this various complicating factors restrict the validity of the calculations.

4. Thus there is no reason to suppose that a linear theory cannot be used to predict visibility of aperiodic patterns near threshold.