The influence of spatial and temporal noise on the detection of first-order and second-order orientation and motion direction.
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.
Ledgeway
Timothy
T
School of Psychology, University of Nottingham, University Park, UK. txl@psychology.nottingham.ac.uk
Hutchinson
Claire V
CV
D006801:Humans; D009039:Motion Perception / Q000502:physiology*; D009949:Orientation; D010775:Photic Stimulation; D011601:Psychophysics; D012684:Sensory Thresholds; D017603:Signal Detection, Psychological