Nathan H. Heller

Perception of an ambiguous apparent motion is influenced by the immediately preceding motion. In positive priming, when an observer is primed with a slow-pace (1–3 Hz) sequence of motion frames depicting unidirectional drift (e.g., Right–Right–Right–Right), subsequent sequences of ambiguous frames are often perceived to continue moving in the primed direction (illusory Right–Right …). Furthermore, priming an observer with a slow-pace sequence of rebounding apparent motion frames that alternate between opponently coded motion directions (e.g., Right–Left–Right–Left) leads to an illusory continuation of the two-step rebounding sequence in subsequent random frames. Here, we show that even more arbitrary two-step motion sequences can be primed; in particular, two-step motion sequences that alternate between non-opponently coded directions (e.g., Up–Right–Up–Right; staircase motion) can be primed to be illusorily perceived in subsequent random frames. We found that staircase sequences, but not drifting or rebounding sequences, were primed more effectively with four priming frames compared with two priming frames, suggesting the importance of repeating the sequence element for priming arbitrary two-step motion sequences. Moreover, we compared the effectiveness of motion primes to that of symbolic primes (arrows) and found that motion primes were significantly more effective at producing prime-consistent responses. Although it has been proposed that excitatory and rivalry-like mechanisms account for drifting and rebounding motion priming, current motion processing models cannot account for our observed priming of staircase motion. We argue that higher order processes involving the recruitment and interaction of both attention and visual working memory are required to account for the type of two-step motion priming reported here.

The double-drift illusion produces a large deviation in perceived direction that strongly dissociates physical position from perceived position. Surprisingly, saccades do not seem to be affected by the illusion (Lisi & Cavanagh, 2015). When targeting a double-drift stimulus, the saccade system is driven by retinal rather than perceived position. Here, using paired double-drift targets, we test whether the smooth pursuit system is driven by perceived or physical position. Participants (n = 7) smoothly pursued the inferred midpoint (Steinbach, 1976) between two horizontally aligned Gabor patches that were separated by 20° and moving on parallel, oblique paths. On the first half of each trial, the Gabors’ internal textures were static while both drifted obliquely downward. On the second half of each trial, while the envelope moved obliquely upward, the internal texture drifted orthogonally to the envelope's motion, producing a large perceived deviation from the downward path even though the upward and downward trajectories always followed the same physical path but in opposite directions. We find that smooth pursuit eye movements accurately followed the nonillusory downward path of the midpoint between the two Gabors, but then followed the illusory rather than the physical trajectory on the upward return. Thus, virtual targets for smooth pursuit are derived from perceived rather than retinal coordinates.

In the double-drift illusion, the combination of the internal and external motion vectors produces large misperceptions of both position and direction of motion. Here, we investigate the role that speed plays in determining how these two sources of motion are combined to produce the double-drift illusion. To address this question, we measure the size of the illusion at seven internal speeds combined with six external speeds. We find that the illusion increases with increasing internal speed and decreases with increasing external speed. We model this by combining the external and internal vectors to produce the resulting, illusory direction (Tse & Hsieh, 2006). The relative effect of the two vectors is specified by a constant K in this model and the data reveal that K decreases linearly as external speed increases. This critical role of external speed in modulating the vector combination uncovers new details about how the visual system combines different sources of motion information to produce a global motion percept.

It has been shown that adaptation to superimposed, balanced opposite motion directions (i.e. transparent motion) induces unidirectional motion aftereffects along the axis orthogonal the adaptation directions, not parallel to them (Grunwald & Lankheet, 1996). However, we recently showed in a striking new effect that adaptation to alternating (rather than superimposed) balanced motion directions leads to percepts of rebounding or back-and- forth motion that is parallel to the adaptation directions. In order for adaptation to produce these parallel rebounding aftereffects, the transition between alternate opposing directions must be sufficiently slow, around 1–3Hz (Davidenko, Heller, Cheong, & Smith, 2017; Davidenko & Heller, 2018; Heller & Davidenko, 2018). Here we asked whether adaptation to alternating directions at higher speeds would produce orthogonal rebounding after-effects. In this study participants adapted for 2, 2.667, or 3.333 seconds to rapidly alternating horizontal or vertical motion (9 Hz alternating speed; 18, 24, or 30 alternations) or slowly alternating motion (1.5 Hz; 4, 5, or 6 alternations). Participants then reported perceived direction on two ambiguous test frame transitions composed of uncorrelated random dots presented at 1.5 Hz. We coded responses as parallel rebounding aftereffects if reports followed the same axis as the adaptation (e.g. LEFT-RIGHT-LEFT-RIGHT followed by LEFT-RIGHT response). When responses switched axis (e.g. LEFT-RIGHT-LEFT-RIGHT followed by UP-DOWN response) we coded those as orthogonal rebounding aftereffects. The proportion of orthogonal and parallel rebounding aftereffects were significantly modulated by alternation speed [paired t test: t(33) = 2.9, p = .007], with the orthogonal effect reported on 23.1 % of fast trials and 15.1% of slow trials, and parallel effect reported on 16.4% of fast trials and 20.3% of slow trials. Thus, adapting to opposing directions not only result in unidirectional orthogonal aftereffects, but also leads to rebounding orthogonal or parallel aftereffects, depending on the speed of the adapting, rebounding motion.