O I Ukkonen | PubMed Health

PubMed2000

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.

Contrast Sensitivity↗

PubMed1999

Second-order motion discrimination by feature-tracking.

When a plaid pattern (the sum of two high spatial frequency gratings oriented +/- 84 degrees from vertical) jumps horizontally by 3/8 of its spatial period its contrast envelope, a second-order pattern, moves in the opposite direction to its luminance waveform. Observers report that the pattern moves in the direction of the contrast envelope when the jumps are repeated at intervals of more than 125 ms and in the direction of the luminance profile when they are repeated at longer intervals. When a pedestal [Lu, Z.-L. & Sperling, G. (1995). Vision Research, 35, 2697-2722] is added to the moving plaid a higher contrast is required to see motion of the contrast envelope but not to see the motion of the luminance profile, suggesting that the motion of the contrast envelope is sensed by a mechanism that tracks features. Static plaids with different spatial parameters from the moving pattern are less effective at raising the contrast required to see the motion of the contrast envelope and simple gratings of low or high spatial frequency are almost completely ineffective, suggesting that the feature-tracking mechanism is selective for the type of pattern being tracked and rejects distortion products and zero-crossings.

Contrast Sensitivity↗