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1. Sinusoidal curvature was produced in a vertical stimulus line generated on an oscilloscope screen. Sensitivity to sinusoidal curvature (periodic vernier acuity) was measured by the method of adjustment as a function of spatial frequency of the curvature.

2. Sensitivity was maximal at about 3 c/deg and reduced steeply for both higher and lower spatial frequencies of curvature.

3. Sensitivity at 10° in the periphery was greatly reduced but showed very little low frequency reduction.

4. The effect of number of cycles present was checked by reduction of the field size with spatial frequency such that a fixed number of cycles of the stimulus were visible. This procedure did not affect the decline in sensitivity at spatial frequencies below 0·3 c/deg but reduced sensitivity at higher spatial frequencies.

5. The results were interpreted in terms of cortical orientation detectors. It is suggested that the low frequency sensitivity is limited by a fixed value for the maximum difference in orientation in the stimulus (i.e. 20 and 30' for the present subjects). The high frequency sensitivity was limited by grating acuity and required spatial integration of the stimulus information over about 2·5°.