Motion of contrast-modulated gratings is analysed by different mechanisms at low and at high contrasts.
We used a pedestal test [Lu & Sperling (1995a). Vision Research, 35, 2697-2722] to determine whether motion discrimination of contrast-modulated gratings has different properties at low contrast (4.5%) and at high contrast (45%). The amplitude-modulated gratings consisted of a 5 c/deg static carrier modulated by a moving 1 c/deg contrast envelope. We found that when contrast is low direction discrimination for contrast-modulated gratings is vulnerable to pedestals and becomes impossible at about 4 Hz. At high contrast contrast-modulated gratings are unaffected by pedestals and modulation sensitivity in a motion direction-discrimination task remains high up to 12 Hz. These results are consistent with the hypothesis that separate mechanisms analyse motion of contrast-modulated gratings at low and at high contrast; at low contrast motion analysis is based on feature tracking, whereas at high contrast, contrast-modulated gratings are analysed by spatio-temporal filters.
Ukkonen
O I
OI
School of Psychology, University of Nottingham, University Park, NG7 2RD, Nottingham, UK. oiu@psychology.nottingham.ac.uk
Derrington
A M
AM
D015350:Contrast Sensitivity / Q000502:physiology*; D004192:Discrimination, Psychological; D006801:Humans; D009039:Motion Perception / Q000502:physiology*; D011601:Psychophysics; D014795:Visual Pathways / Q000502:physiology*