Citation

Ledgeway T, Hutchinson CV. Vision Res. 2005; 45(16): 2081-2094.

Affiliation

School of Psychology, University of Nottingham, University Park, Nottingham, NG7 2RD, United Kingdom.

Copyright

(Copyright © 2005, Elsevier Publishing)

DOI

10.1016/j.visres.2005.02.005

PMID

15845240

Abstract

Thresholds for identifying the direction of second-order motion (contrast-modulated dynamic noise) are consistently higher than those for identifying spatial orientation, unlike first-order gratings for which the two thresholds are typically the same. Two explanations of this phenomenon have been proposed: either first-order and second-order patterns are encoded by separate mechanisms with different properties, or dynamic noise selectively impairs ("masks") sensitivity to second-order motion direction but not orientation. The former predicts the two thresholds should remain distinct for second-order patterns, irrespective of the temporal structure (static vs. dynamic) of the noise carrier. The latter predicts direction thresholds should be higher than orientation thresholds, for both second-order and first-order motion patterns, when dynamic (but not static) noise is present. To resolve this issue we measured direction and orientation thresholds for first-order (luminance) and second-order (contrast or polarity) modulations of static or dynamic noise. Results were decisive: The two thresholds were invariably the same for first-order stimuli but markedly different (direction thresholds approximately 50% higher) for second-order stimuli, regardless of the temporal properties (static or dynamic) and the overall contrast of the noise, or the drift temporal frequency of the envelope. This suggests that first-order and second-order motion are encoded separately and that the mechanisms encoding second-order stimuli cannot determine direction at the absolute threshold for spatial form.