Evidence for tilt normalization can be explained by anisotropic orientation sensitivity.

Some data have been taken as evidence that after prolonged viewing, near-vertical orientations "normalize" to appear more vertical than they did previously. After almost a century of research, the existence of tilt normalization remains controversial. The most recent evidence for tilt normalization comes from data suggesting a measurable "perceptual drift" of near-vertical adaptors toward vertical, which can be nulled by a slight physical rotation away from vertical (Müller, Schillinger, Do, & Leopold, 2009). We argue that biases in estimates of perceptual stasis could, however, result from the anisotropic organization of orientation-selective neurons in V1, with vertically-selective cells being more narrowly tuned than obliquely-selective cells. We describe a neurophysiologically plausible model that predicts greater sensitivity to orientation displacements toward than away from vertical. We demonstrate the predicted asymmetric pattern of sensitivity in human observers by determining threshold speeds for detecting rotation direction (Experiment 1), and by determining orientation discrimination thresholds for brief static stimuli (Experiment 2). Results imply that data suggesting a perceptual drift toward vertical instead result from greater discrimination sensitivity around cardinal than oblique orientations (the oblique effect), and thus do not constitute evidence for tilt normalization.