The use of different orientation cues in vernier acuity

The role of orientation cues in vernier acuity was investigated in five experiments that varied the salience of different possible reference systems: (1) retinal, (2) subjective (vestibular), and (3) stimulus-defined ("intrinsic"). We compared the eight possible combinations of presence and absence of the three types of information about vertical by changing the orientation of the vernier stimulus, by changing the angle between the two segments of the stimulus, by reducing the stimulus to two dots rather than two lines, and/or by tilting the subject's head or by having the subject lie in a supine position. The results show that vestibular vertical plays no important role in vernier discriminations, whereas retinal vertical and, to a lesser degree, the intrinsic reference do. A control experiment excluded the possible role of a fourth possible reference system, the frame of the monitor. Our results are compatible with a model of vernier acuity detection based on oriented receptive fields with inhibitory surrounds. They show further that observers can produce, in the case of two-dot stimuli, an internal standard for comparison and detect deviations from this standard with high precision.