Vivian Holten

Occlusion-related lateral connections stabilize kinetic depth stimuli through perceptual coupling

Local sensory information is often ambiguous forcing the brain to integrate spatiotemporally separated information for stable conscious perception. Lateral connections between clusters of similarly tuned neurons in the visual cortex are a potential neural substrate for the coupling of spatially separated visual information. Ecological optics suggests that perceptual coupling of visual information is particularly beneficial in occlusion situations. Here we present a novel neural network model and a series of human psychophysical experiments that can together explain the perceptual coupling of kinetic depth stimuli with activity-driven lateral information sharing in the far depth plane. Our most striking finding is the perceptual coupling of an ambiguous kinetic depth cylinder with a coaxially presented and disparity defined cylinder backside, while a similar frontside fails to evoke coupling. Altogether, our findings are consistent with the idea that clusters of similarly tuned far depth neurons share spatially separated motion information in order to resolve local perceptual ambiguities. The classification of far depth in the facilitation mechanism results from a combination of absolute and relative depth that suggests a functional role of these lateral connections in the perception of partially occluded objects.

Decreasing perceived optic flow rigidity increases postural sway

Optic flow simulating self-motion through the environment can induce postural adjustments in observers. Some studies investigating this phenomenon have used optic flow patterns increasing in speed from center to periphery, whereas others used optic flow patterns with a constant speed. However, altering the speed gradient of an optic flow stimulus changes the perceived rigidity of such a stimulus. Optic flow stimuli that are perceived as rigid can be expected to provide a stronger sensation of self-motion than non-rigid optic flow, and this may well be reflected in the amount of postural sway. The current study, therefore, examined, by manipulating the speed gradient, to what extent the rigidity of an optic flow stimulus influences posture along the anterior–posterior axis. We used radial random dot expanding or contracting optic flow patterns with three different speed profiles (single-speed, linear speed gradient or quadratic speed gradient) that differentially induce the sensation of self-motion. Interestingly, most postural sway was observed for the non-rigid single-speed optic flow pattern, which contained the least self-motion information of the three profiles. Moreover, we found an anisotropy in that contracting optic flow produced more postural sway than expanding optic flow. In addition, the amount of postural sway increased with increasing stimulus speed, but for contracting optic flow only. Taken together, the results of the current study support the view that visual and sensorimotor systems appear to be tailored toward compensating for rigid optic flow stimulation.

Illusory Motion of the Motion Aftereffect Induces Postural Sway

It remains an open question, however, whether this perception-action cycle is the result of direct visual stimulation only, or whether postural adjustments also occur when the motion of the visual stimulus is illusory. Here, we show that the latter is the case. Prolonged viewing of visual motion results in neural adaptation, and subsequent viewing of a stationary stimulus normally results in illusory motion in the opposite direction, a famous phenomenon known as the motion aftereffect (MAE; Anstis, Verstraten, & Mather, 1998). Surprisingly, this sequence of stimulation also causes postural sway in the direction consistent with the perceived illusory motion. Control test patterns that do not generate an MAE after identical adaptation do not induce sway. This suggests that the visuo-vestibular interactions that govern postural control are not influenced by visual stimulation per se, but can be modulated by an illusory motion signal (e.g., the internal neural signal responsible for the MAE).

Visual Directional Anisotropy Does Not Mirror the Directional Anisotropy Apparent in Postural Sway

Presenting a large optic flow pattern to observers is likely to cause postural sway. However, directional anisotropies have been reported, in that contracting optic flow induces more postural sway than expanding optic flow. Recently, we showed that the biomechanics of the lower leg cannot account for this anisotropy (Holten, Donker, Verstraten, & van der Smagt, 2013, Experimental Brain Research, 228, 117–129). The question we address in the current study is whether differences in visual processing of optic flow directions, in particular the perceptual strength of these directions, mirrors the anisotropy apparent in postural sway. That is, can contracting optic flow be considered to be a perceptually stronger visual stimulus than expanding optic flow? In the current study we use a breaking continuous flash suppression paradigm where we assume that perceptually stronger visual stimuli will break the flash suppression earlier, making the suppressed optic flow stimulus visible sooner. Surprisingly, our results show the opposite, in that expanding optic flow is detected earlier than contracting optic flow.

Interaction effects of visual stimulus speed and contrast on postural sway

Manipulating the characteristics of visual stimuli that simulate self-motion through the environment can affect the resulting postural sway magnitude. In the present study, we address the question whether varying the contrast and speed of a linear translating dot pattern influences medial–lateral postural sway. In a first experiment, we investigated whether the postural sway magnitude increases with increasing dot speed, as was previously demonstrated for expanding and contracting stimuli. In a second experiment, we also manipulated the contrast of the stimuli. For reasons that high-contrast stimuli can be considered ‘perceptually’ stronger, we expect that higher-contrast stimuli induce more sway than lower-contrast stimuli. The results of the first experiment show that dot speed indeed influences postural sway, although in an unexpected way. For higher speeds, the sway is in the direction of the stimulus motion, yet for lower speeds the sway is in a direction opposite to the stimulus motion. The results of the second experiment show that dot contrast does affect postural sway, but that this depends on the speed of the moving dots. Interestingly, the direction of postural sway induced by a relatively low dot speed (4°/s) depends on dot contrast. Taken together, our results suggest that interactions between the visual, vestibular and proprioceptive system appear to be influenced by an internal representation of the visual stimulus, rather than being influenced by the external visual stimulus characteristics only.

Grouping of optic flow stimuli during binocular rivalry is driven by monocular information.

During binocular rivalry, perception alternates between two dissimilar images, presented dichoptically. Although binocular rivalry is thought to result from competition at a local level, neighboring image parts with similar features tend to be perceived together for longer durations than image parts with dissimilar features. This simultaneous dominance of two image parts is called grouping during rivalry. Previous studies have shown that this grouping depends on a shared eye-of-origin to a much larger extent than on image content, irrespective of the complexity of a static image. In the current study, we examine whether grouping of dynamic optic flow patterns is also primarily driven by monocular (eye-of-origin) information. In addition, we examine whether image parameters, such as optic flow direction, and partial versus full visibility of the optic flow pattern, affect grouping durations during rivalry. The results show that grouping of optic flow is, as is known for static images, primarily affected by its eye-of-origin. Furthermore, global motion can affect grouping durations, but only under specific conditions. Namely, only when the two full optic flow patterns were presented locally. These results suggest that grouping during rivalry is primarily driven by monocular information even for motion stimuli thought to rely on higher-level motion areas.