A recent study explored the challenges posed by nonrigid motion patterns on visual stability and ocular motor control. The researchers employed precise eye-tracking technology to analyze participants' responses to a vortex pattern moving across a field of static dots under various experimental conditions, including smooth pursuit, saccadic tasks, and assisted pursuit scenarios. They validated their findings using psychometric analyses and detailed statistical models.
Observers could perceive the motion of the nonrigid vortex stimulus accurately but were unable to track it with smooth pursuit. Instead, their eyes relied on frequent "catch-up" saccades. Attempting to pursue the nonrigid motion led to perceived instability. Observers reported that the vortex appeared to "jump" during saccades, which disrupted trans-saccadic visual continuity. This instability did not occur during stationary fixation, and this result was a stark contrast to typical motion tracking behaviors.
Introducing a secondary motion signal, such as a moving dot accompanying the vortex, restored smooth pursuit and visual stability. This finding demonstrated that coordinated engagement of pursuit and saccadic systems can overcome the inherent instability of nonrigid motion.
Despite accurate perception and saccadic targeting, the inability to pursue nonrigid motion revealed a dissociation between our visual motion processing systems and oculomotor control systems. The study suggested that the neural pathways responsible for motion perception, particularly nonrigid motion, may not overlap entirely with those that enable smooth pursuit. Specifically, the medial temporal and medial superior temporal areas, which are typically associated with motion processing, may lack sensitivity to the nonrigid motion cues that define the vortex stimulus. Other areas, such as the lateral intraparietal area and superior colliculus, appear critical for saccadic prediction and targeting.
“While the stimulus is inspired by the movement of substances such as smoke or water," the researchers wrote in their Science Advances article. "it purposefully does not contain all the possible visual cues that this stimuli would but only those cues that are specific to this type of motion and set it apart from rigid-object motion. When looking at the environmental stimuli that inspired our stimulus, it is likely that there are several other motion cues…supporting both pursuit and perception,” they noted when addressing the likelihood of these perceptual tasks with real-world stimuli.
They concluded, “We, thus, propose that the areas that integrate the eigenmotion of each object across saccades for perceptual stability differ from the area that integrates the motion of the saccade target and that these areas have no access to the vortex motion signal. A distinct perceptual mechanism and brain area can then also explain why the trans-saccadic visual stability is impaired, while perception during fixation is possible.”
A full list of author disclosures can be found in the published research.
