Motion Fading and the Motion Aftereffect Share a Common Process of Neural Adaptation

Article excerpt

After prolonged viewing of a slowly drifting or rotating pattern under strict fixation, the pattern appears to slow down and then momentarily stop. Here, we show that this motion fading occurs not only for slowly moving stimuli, but also for stimuli moving at high speed; after prolonged viewing of high-speed stimuli, the stimuli appear to slow down but not to stop. We report psychophysical evidence that the same neural adaptation process likely gives rise to motion fading and to the motion aftereffect.

After prolonged viewing of a slowly drifting or rotating pattern under strict fixation, the pattern appears to slow down and then momentarily stop, even though the stationary form of the pattern remains visible. This motion fading has been reported to occur over slowly rotating gratings and spinning sector disks (Campbell & Maffei, 1979, 1981; Cohen, 1965; Hunzelmann & Spillmann, 1984; Lichtenstein, 1963; MacKay, 1982). Several factors, including retinal eccentricity, number of sectors, speed of rotation (Hunzelmann & Spillmann, 1984), and salience of trackable features (such as corners; Hsieh & Tse, 2007), have been shown to affect the time required for motion fading.

According to the motion aftereffect (MAE) hypothesis tested here, motion fading arises because of adaptation among cortical motion-tuned neurons that are the same as those that underlie the well-known MAE, where illusory motion is perceived to occur over a stationary object or image following prolonged exposure to visual motion (Wohlgemuth, 1911). For example, it has been shown that the MAE is based on neural adaptation (i.e., modulation of gain control) near or at the input of MT (Kohn & Movshon, 2003). Van de Grind, van der Smagt, and Verstraten (2004; see also van de Grind, Lankheet, & Tao, 2003) showed how such adaptation might occur, on the basis of the gain-control model of Grunewald and Lankheet (1996). Moreover, Kohn and Movshon showed that the MAE mechanism is realized in the motion pathway, not the form pathway, which is consistent with the phenomenology of motion fading, where only the motion component appears to vanish from consciousness, while the form component remains visible. Together, these findings are consistent with the hypothesis that adaptation among motion-tuned neurons underlies motion fading.

In this article, we first will examine whether the motion- fading effect can occur over stimuli at high speed. To our knowledge, motion fading over stimuli moving at high speeds has not been carefully examined. However, it has been shown that a flickering spot/grating presented in the peripheral visual field appears to lose contrast and stop flickering (Anstis, 1996; Frome, MacLeod, Buck, & Williams, 1981; Hammett & Smith, 1990; Harris, Calvert, & Snelgar, 1990; Schieting & Spillmann, 1987). Given that moving and flickering stimuli share similar spatiotemporal properties, it is reasonable to hypothesize that motion fading can also occur over stimuli moving at high speed. This possibility was examined in Experiment 1. In Experiment 2, the degree to which motion fading occurs was mapped across the visual field. In Experiment 3, we measured motion fading as a function of stimulus spatial frequency. In Experiment 4, we tested whether the subjective decrement of perceived luminance in motion fading is correlated with the subjective decrement of perceived speed. In Experiments 5 and 6, we directly tested the MAE hypotheses stated above.

Experiment 1

Factors Affecting Motion Fading

In Experiment 1, rather than using a slowly moving target, as has been tested previously (Campbell & Maffei, 1979, 1981; Cohen, 1965; Hsieh & Tse, 2007; Hunzelmann & Spillmann, 1984; Lichtenstein, 1963; MacKay, 1982), we examined whether motion fading can occur over stimuli moving at high speed. We measured the perceived speed of the adapted motion pattern by asking subjects to adjust the speed on a nonadapted motion pattern presented in the mirror-opposite location (relative to the vertical axis) of the adapted motion pattern (Figure 1A). …