Walking on Visual Illusions

Perception

Participants reported to have perceived depth not only whilst looking at but also whilst walking over the Primrose, Lisbon Straight, and Lisbon Angled patterned floors. For both Lisbon patterns, floors were perceived as three-dimensional, rising up and down in “ridges” or “bumps” as one would expect for geometrical illusions. The Primrose pattern was described as appearing to “ripple” and “shimmer,” in line with earlier descriptions of combining geometrical and motion illusion effects. Walking over the floors evoked additional sensations of movement; the Primrose pattern was referred to as dynamically “wavy,” providing a gentle sense of movement like walking over a meadow or on water. The “ridges” and “furrows” of the Lisbon patterns were described as dynamically moving up and down in the direction of their respective orientations, sometimes seemingly reversing height for the Lisbon Angled pattern during the walk. Even the Café Wall pattern was perceived as “weaving” with its parallel lines dynamically converging and diverging as participants walked.

Walking Experience

Lisbon Angled, Café Wall, and Primrose patterns further affected participants’ perceived ability to walk in a straight line, evoking a sense of being “pulled”/veering to the left in the main direction of the patterns. This perceived veering mirrors the objectively measured veering when people walk over a floor with oblique lines (Leonards et al., 2015). Perceived veer is thus most probably related to the high-contrast oblique patterns rather than the presence of illusions. More interesting was the description of the two Lisbon patterns as “disorientating”: particularly older participants—irrespective of whether participating at the community event or at activities in the context of the Science Conferences—said they felt uncertain of the height of the floor surface and where to place their feet in relation to the patterns. For the Lisbon Straight pattern, a quarter of participants felt uncomfortable and uncertain whether to place their feet “within a furrow” or “on top of a ridge,” with 16 participants stating that they intentionally walked on the “ridges” of the two Lisbon patterns to account for the ambiguity of the perceived surface level.

In addition to walking discomfort, participants reported feelings of increased instability, expressed in words such as “unsteady,” “unstable,” “uncertainty,” “need to walk more slowly,” “walk more carefully,” “walking instability,” and “feeling dizzy”.

Emotional Response

Most participants described negative feelings when walking over the high-contrast Lisbon and Café Wall patterns, even though participants generally agreed that the patterns per se were intriguing. Some participants even commented for the Lisbon patterns that, in the real world, they “would avoid looking at”/”walking on such patterns” or that they found the walking experience “horrible” or “uncomfortable”. Participants were far more likely to describe walking on the Primrose pattern as a walking experience they enjoyed, with statements such as “pleasant movement, relaxing and comfortable”, “like walking on water”, “like gliding over the floor”, or wanting “to dance and play on it”. Several participants even wondered whether the Primrose patterns were printed on a softer, more padded material than the other floors.

Overall, this exploration and the differences in experiences it provoked for different patterns suggest that walking over floors containing high-contrast patterns such as the visual illusions used here might affect people’s walking experience -- often, but by no means always, in a negative way. The lack of adequate control floors without illusions does not allow us to disentangle how much of these effects described here were due to the presence of illusions per se, the specific type of illusion or how much was simply the effect of high-contrast patterns. Nor can we draw conclusions about how reported effects were impacted by the exact environment, the speed with which people walked, where participants looked relative to the patterns, how quickly they adapted to the floors, or whether they would have felt similar effects without a perceptual scientist asking questions.

Despite the study’s obvious limitations, we feel encouraged to call for a new line of research into the parameters that underpin the link between floor pattern characteristics and human gait, and perception of instability of the walking surface. In particular, the impact of the degree of perception of depth and movement of patterns in designed walkways should be investigated further, in relation to feelings of disorientation and instability, positive experiences, and how such experiences are related to quantifiable adaptations of gait kinematics themselves. Recent evidence supports the notion of a direct impact of floor pattern on gait kinematics: Certain aspects of floor patterns (such as the orientation of tiling or the spatial frequency of stripes) have been shown to influence locomotion characteristics, such as lateral veer (Leonards et al., 2015) and walking speed (Ludwig et al., 2018). In addition, perceived scene motion has been shown to modulate horizontal trunk displacement (Logan et al., 2010), suggesting a decline in stability during bipedal locomotion. This might affect the responses in leg kinematics, which, in turn, would disrupt the gait cycle and lead to walking instability (Frost et al., 2015). Other research might want to take an approach less common to the perception sciences by collecting data of people walking over existing patterned floors in the real world—both with and without visual illusions—using CCTV footage to measure changes in gait. Moreover, one could investigate whether problems have been reported to authorities about particularly striking floor patterns in public spaces.

Why should vision scientists care? As a result of the evolution of architectural design and increases in modular design, high-contrast and repetitive patterns are much more pervasive in urban environments (Wilkins et al., 2018), where increasingly more of the global population live (Park & Burgess, 1925). Architectural design choices, as well as solutions to practical problems, (such as, e.g., for barrier matting; Harle et al., 2006), have also increased the amount of visual illusions that are present in urban environments (Penacchio & Wilkins, 2015). To date, vision research has paid little attention to how such patterns impact the way we move and, consequently, feel in everyday life contexts. Given the increasingly aging population which is much more reliant on visual information for postural control (Li et al., 2018), it seems crucial to understand how visual aspects of the built environment impact on our walking behaviour.

Last, but not least, do we think that the original Lisbon pattern in Lisbon’s Rossio Square (see Figure 1C) negatively affects people’s gait? Most likely not: Lisbon’s Rossio Square pattern consists of far lower spatial frequencies and luminance contrasts than the ones used in our design. Yet, this remains to be tested.