Although the visual system is known to provide relevant information to guide stair locomotion, there is less understanding of the specific contributions of foveal and peripheral visual field information. The present study investigated the specific role of foveal vision during stair locomotion and ground-stairs transitions by using a dual-task paradigm to influence the ability to rely on foveal vision. Fifteen healthy adults (26.9±3.3 years; 8 females) ascended a 7-step staircase under four conditions: no secondary tasks (CONTROL); gaze fixation on a fixed target located at the end of the pathway (TARGET); visual reaction time task (VRT); and auditory reaction time task (ART). Gaze fixations towards stair features were significantly reduced in TARGET and VRT compared to CONTROL and ART. Despite the reduced fixations, participants were able to successfully ascend stairs and rarely used the handrail. Step time was increased during VRT compared to CONTROL in most stair steps. Navigating on the transition steps did not require more gaze fixations than the middle steps. However, reaction time tended to increase during locomotion on transitions suggesting additional executive demands during this phase. These findings suggest that foveal vision may not be an essential source of visual information regarding stair features to guide stair walking, despite the unique control challenges at transition phases as highlighted by phase-specific challenges in dual-tasking. Instead, the tendency to look at the steps in usual conditions likely provides a stable reference frame for extraction of visual information regarding step features from the entire visual field.

In contrast, foveal vision and executive demands could play a crucial role while making the transition from level ground to stairs and vice-versa (transitions). Given that transitions are commonly associated with accidents [6] – [8] , the present study also focused on the specific role of gaze and executive demands during locomotion on stair transitions. It was expected that, during dual-tasking, gaze fixations on the steps would be preserved, particularly in the phases preceding the transitions. Additionally, with increasing challenge in the dual-task context, it was expected that individuals would adopt more conservative movement strategies characterized by increased handrail use and slower walking speed. It was also anticipated a reduction in reaction time task performance (i.e., longer reaction time; lower accuracy) specifically during transition phases where the visual and executive demands are expected to be the greatest.

The current study explored the role of foveal vision during stair locomotion by investigating the impact of diverting gaze in order to perform a concurrent visual task. Although this study was designed to explore the particular role of foveal vision on stair walking, dual-tasking could also have a confounding effect on the executive function [5] . In order to control for the influences of gaze direction and executive challenge, three levels of dual-task conditions were compared in this study: (1) visual reaction time (RT) task (gaze fixation and executive challenge), (2) stationary target fixation (gaze fixation but no executive challenge), and (3) auditory RT task (no gaze fixation and executive challenge). Overall, it was hypothesized that gaze directed towards the stairs would be less frequent when dual-task requires gaze fixations whether or not there was an executive challenges (i.e., target fixation comparable to visual RT task, and both different from auditory RT task).

The prevalence of gaze behaviour documented in previous stair-related studies [2] – [4] may simply be the product of natural gaze tendencies in a familiar task and predictable environment rather than an index of reliance on foveal inputs. For example, the increased amount of time that people spend looking at the steps during ascent compared to descent [2] could be the result of the steps being naturally available in the visual field for longer time during stair ascent. Consequently, the overall gaze behaviour may overestimate the importance of foveal information during the control of stair locomotion.

Many accidents during stair walking are attributed to perceptual errors and distractions [1] illustrating the importance of visual information during stair walking. Gaze behaviour studies indicate that people evenly look across the steps in a staircase, and that the gaze fixation point is maintained a few steps ahead in the path supporting the importance of foveal vision in continuously guiding immediate stepping [2] , [3] . Additionally, fixating on the tread of the steps seems a gaze strategy that plays a dual role in providing visual input for appropriate foot placement and balance control [4] . Despite this potential role for foveal vision, there are times in everyday life when successful stair walking can be performed with the view of the steps unavailable (e.g., walking with boxes, laundry basket).

Overall, only 3/15 participants contacted the handrail during the experiment in a few small number of trials, and these only occurred in TARGET (3 participants in 1, 2, and 5 trials, respectively) or VRT (1 participant in 9 trials) conditions. Interestingly, all the 3 participants used the handrail in their first trial of these task conditions. Inspection of the video recordings revealed that participants contacted the handrail when stepping on the first step, and slid their hands (left hand) across the length of the handrail until reaching the last step. Participants did not fixate on the handrails in any trial during this study even if they used the handrail.

Task condition significantly influenced the time to walk on the stairs (F(3,42) = 4.94, p = 0.005; Figure 3 ). Total walk time on stairs was increased in VRT (6.64±1.16s) compared to CONTROL (6.01±0.69s; p = 0.002). ART (6.31±0.67s) and TARGET (6.26±0.93s) showed walk time values between CONTROL and VRT. Step time was different between tasks (F(3,42) = 4.82, p = 0.0056), the location of the step (F(9,126) = 32.56, p<0.0001) and there was a significant interaction between task condition and step location (F(27,378) = 2.95, 0<0.0001). Specifically, step time was longer in VRT compared to CONTROL, and on steps 1, 2, 4, 5, and 6.

Positive step numbers are the steps on the stairs. Frequency represents all fixations observed across all participants. Numbers at the top of the bars represent the number of participants contributing with fixations. Step “zero” represents the step ending with the last foot contact on the ground prior to the stairs. CONTROL and ART (A), and TARGET and VRT (B) were plotted in two difference graphs due to the large difference in scale.

Figure 2 shows the frequency of gaze fixations directed to specific steps on the stairs referenced to participants’ stepping location. For CONTROL and ART ( Figure 2a ), the greatest number of fixations occurred during the approach to the stairs and progressively reduced as participants continued ascent. When participants reached the last steps, they most often fixated at the end of the pathway following the stairs. For TARGET and VRT, fixation frequency was greater during the approach steps and initial transition ( Figure 2b ). However, the frequency of fixations and the number of subjects (numbers on the top of the bars) performing gaze fixations on the stairs were lower during TARGET and VRT compared to CONROL and ART.

Total gaze time on the stairs (as percentage of trial time) was significantly influenced by task conditions (F(3,42) = 56.38, p<0.0001). Total gaze time was lower in TARGET and VRT compared to CONTROL and ART conditions ( Figure 1a ). Similarly, there were task related differences in total fixation time (F(3,42) = 42.92, p<0.0001), number of fixations (F(3,42) = 58.03, p<0.0001), and fixation duration (F(3, 35) = 5.33, p<0.005). Overall, TARGET and VRT showed reduced fixation time ( Figure 1b ) and number of fixations ( Figure 1c ) compared to ART and CONTROL. Conversely, fixations were significantly longer during ART compared to all other conditions ( Figure 1d ).

Discussion

This study investigated the role of gaze fixations during stair walking. This study showed that gaze fixations on stair features drastically decreased when individuals performed a visual task while stair walking. Despite the shift in gaze, participants were still able to successfully ascent the stairs without transiently shifting gaze back to the steps even at transition phases. Handrail use was rare and there was modest change in walk speed associated with the diverted gaze. The most profound differences in behaviour were evident at transitions phases but only when the dual-tasking involved both executive and visual challenges. The increase in reaction time (auditory and visual dual-tasks) on the transition steps highlights the phase specific demands for executive function when an individual transitions on and off the stairs. In contrast, walking on the middle steps appeared immune to dual task and visual task demands.

Previous studies demonstrated that individuals spend a significant amount of time looking at the steps during stair walking suggesting that foveal fixations are required to guide locomotion [2], [3]. However, using a dual-task paradigm, the current study revealed that foveal vision may not be an essential requirement to extract visual information to control locomotion. Why do individuals direct their foveal gaze at the stairs if it is not essential? There are likely advantages in keeping foveal vision directed to the steps that are independent of foveal information. One possibility is that centering foveal vision to the stairs ensures that the entire visual field can be used to extract relevant information. It has been demonstrated, for instance, that peripheral visual information is sufficient to implement alternate foot placement, even when an obstacle suddenly appears in the travel path [9], as well as to provide information for support surfaces in the event of loss of balance [10]. It is possible, therefore, that peripheral vision was the primary source of visual information to guide stair walking specifically when vision was diverted. In this context, peripheral visual information likely provided online exproprioceptive information to fine tune limb trajectory on the steps, similarly to reports from obstacle avoidance studies [9], [11], [12].

While the findings from this study appear to diminish the potential importance of foveal vision, we did observe an increase in gaze fixations during the auditory dual-task condition. While this may have been linked to increased demands for foveal information during dual-tasking, most of the fixations were directed to the last steps in the staircase which coincides with a comfortable height for line of gaze. It remains unclear if these fixations served to provide a stable frame of reference to use optic flow and full visual field information to guide locomotion or simply reflected a reduction in gaze shifts when executive function was directed to non-visual tasks.

Overall, foveal fixation to an external target had a relatively small effect on walk time suggesting that locomotion can be sustained even when foveal vision is not used to monitor the stairs. Thus, foveal vision (including fixations) may not be the major source of visual information to guide stair locomotion and detect step edges as previously suggested [4]. Alternatively, the peripheral visual field may provide sufficient visual information to guide locomotion, as participants walked upstairs looking at the computer monitor, the view of the stairs was at least partially available in the lower peripheral field. The auditory task had a similar small effect on step time, which is in agreement with a previous study on obstacle avoidance that found that young adults kept gait parameters (gait velocity and stride time) constant while performing an auditory Stroop task [13]. This modest dual-task cost on walking speed and lack of influence on RT and accuracy, demonstrates that the current auditory task and stair walking did not pose a high collective demand for attention resources. However, when the secondary task included vision as well as executive requirements (VRT), participants walked slower. This could not be simply attributed to the fixation to a target since similar changes in walking speed were not observed in the gaze fixation task. So one possible explanation for this finding is that the load in executive function caused a narrowing in the attentional visual field. Previous studies demonstrated that the useful visual field reduces when individuals have their central visual field engaged in attentional tasks [14], [15], which could have been the case in the VRT condition leading to a reduction in gait speed. Therefore, the control of locomotion is most greatly affected when the concurrent dual task places demands on gaze control, reliance on visual inputs and executive function resources. While the present study revealed that foveal vision may not be as critical to stair walking, it did highlight phase-specific demands on executive function. The increase in reaction time in the transition steps suggest that transitions may impose additional executive demands compared to the middle (steady state) steps. The fact that reaction time was not increased in the middle steps could be associated with an overall reduction of executive/visual challenges in the steady state region reflecting a reliance on an internal prediction and working memory of stair dimension. In contrast, the executive demands of transitions phases may be linked to the adaptive control of stepping to accommodate for changes in foot placement (vertical and horizontal). Similar accommodation is also observed in gait parameters, such as foot clearance, which is reduced in the mid steps in comparison with the first step [16].

The weak support for the reliance on foveal vision for stair walking and handrail use does lead to the view that peripheral field information may play an important role during stair walking. This may be related in the lab and everyday life to the familiarity of the environmental characteristics. The stairs and handrails in this study were specifically selected to match standard stair design guidelines. As a result, reliance on an internal representation to predictively guide actions on stairs may account for the lack of any meaningful dual-task cost during the middle steady-state phases of stair walking and reduced reliance on foveal vision. One may certainly expect that in situations of environmental uncertainty, though not typical in everyday stair walking, the reliance on foveal or peripheral vision may vary.