SENSITIVITY TO ORIENTATION MODULATION IN MICROPATTERN-BASED TEXTURES

We have measured the sensitivity of the human visual system to sinusoi dal modulations of orientation in micropattern-based textured stimuli. The result is the orientation modulation function, or OMF, which desc ribes this sensitivity as a function of the spatial frequency of orien tation modulation. We found that the OMF was bandpass with peak sensit ivity at spatial frequencies ranging between 0.06 acid 0.2 c/deg, depe nding on the size of the micropatterns. The OMF was found to be scale invariant, that is its position on the spatial frequency axis did not change with viewing distance when spatial frequency was measured in ob ject rather than retinal units. This scale invariance was shown to res ult from the visual system taking into account the scale rather than t he density of the micropatterns as viewing distance was changed. It ha s been argued by Bergen [(1991) Vision and visual dysfunction (Vol. 10 B) New York: Macmillan] that scale invariance in textures is a consequ ence of the coupling of mechanisms which detect textural features with those which detect local luminance contrasts. We reasoned that Gabor micropattern textures might therefore show narrower OMFs compared to l ine micropattern textures. However we found no difference in OMF bandw idth between the Gabor and line micropattern textures, suggesting that the line micropatterns were acting as selectively as the Gabor microp atterns for the spatial scale of the mechanisms which detected the ori entation modulation. Evidence is presented which suggests that the mec hanisms which detected the orientation modulation in our stimuli are n on-linear. Finally we showed similar OMFs for sine-wave and square-wav e modulations of micropattern orientation, and similar OMFs for modula tions of micropattern with orientation about the horizontal and about the vertical, the direction of modulation in both cases being horizont al. The implications of these findings for the mechanisms involved in orientation-defined texture processing is discussed.