Another visual illusion involving orientation.

The orientation of a grating may be misperceived when that grating appears superimposed within one-dimensional visual noise of a different orientation. An adjustment procedure was employed to measure the size of this tilt illusion and its dependence on noise orientation. The illusion is maximum when the noise and test differ by 20 to 40 deg, depending on the spatial frequency of the test grating. Noise composed of spatial frequencies several octaves removed from the test spatial frequency can still cause illusory tilt, although the degree of tilt under these conditions depends on test spatial frequency. This orientation illusion, like the tilt aftereffect, simultaneous tilt illusion and angle expansion illusion, may reflect inhibitory interactions between orientation selective neurons in human vision.