Can we use peripheral vision to create a visuospatial map for compensatory reach-to-grasp reactions?

Perturbation-induced reach-to-grasp reactions are dependent on vision to capture environmental features of potential support surfaces. Previous research proposed the use of an intrinsic visuospatial map of the environment to reduce delays in motor responses (e.g., stepping, grasping a handrail). Forming such a map from foveal vision would be challenging during movement as it would require constant foveal scanning. The objective of this study was to determine if compensatory reach-to-grasp reactions could be successfully executed while relying on a visuospatial map acquired using peripheral vision. Subjects were instructed to respond to a perturbation by grasping a handle randomly located at 0°, 20° or 40° in their field of view under three visual conditions: full vision throughout the entire trial (FV), vision available prior to perturbation only (MAP), and vision available post-perturbation only (ONLINE). Electromyography was used to determine reaction time and kinematic data were collected to determine initial reach angle. Overall, participants were successful in arresting whole-body motion across all visual conditions and handle locations. Initial reach angles were target specific when vision was available prior to perturbation onset (FV and MAP). However, the 40° handle location produced a greater initial reach angle in MAP, suggesting some limitations for mapping in the further visual periphery. These findings suggest that peripheral vision contributes to the ability to spatially locate targets by building an a priori visuospatial map, which benefits the control of rapid compensatory reach-to-grasp reactions evoked in the response to unpredictable events of instability.

A role for the lower visual field information in stair climbing.

BACKGROUND: Locomotion on stairs is challenging for balance control and relates to a significant number of injurious falls. The visual system provides relevant information to guide stair locomotion and there is evidence that peripheral vision is potentially important. RESEARCH QUESTION: This study investigated the role of the lower visual field information for the control of stair walking. It was hypothesized that restriction in the lower visual field (LVF) would significantly impact gaze and locomotor behaviour specifically during descent and during transition phases emphasizing the importance of the LVF information during online control. METHODS: Healthy young adults (n = 12) ascended and descended a 7-step staircase while wearing customized goggles that restricted the LVF. Three visual conditions were tested: full field of view (FULL); 30° (MILD), and 15° (SEVERE) of lower field of view available. Stride time, head pitch angle and handrail use were assessed during approach, transition steps (two steps at the top and bottom of the stairs) and middle step phases. RESULTS: Transient downward head pitch angle increased with LVF restriction, while walk speed decreased and handrail use increased. Occlusion impaired stair descent more strongly than ascent reflected by a larger downward head pitch angles and slower walk times. LVF restriction had a greater influence on stride time and head angle during the approach and first transition compared to other stair regions. SIGNIFICANCE: Information from the lower visual field is important to guide stair walking and particularly when negotiating the first few steps of a staircase. Restriction in the lower visual field during stair walking results in more cautious locomotor behaviour such as walking slower and using the handrails. In daily activities, tasks or conditions that restrict or alter the lower visual field information may elevate the risk for missteps and falls.

Gaze shifts during dual-tasking stair descent.

To investigate the role of vision in stair locomotion, young adults descended a seven-step staircase during unrestricted walking (CONTROL), and while performing a concurrent visual reaction time (RT) task displayed on a monitor. The monitor was located at either 3.5 m (HIGH) or 0.5 m (LOW) above ground level at the end of the stairway, which either restricted (HIGH) or facilitated (LOW) the view of the stairs in the lower field of view as participants walked downstairs. Downward gaze shifts (recorded with an eye tracker) and gait speed were significantly reduced in HIGH and LOW compared with CONTROL. Gaze and locomotor behaviour were not different between HIGH and LOW. However, inter-individual variability increased in HIGH, in which participants combined different response characteristics including slower walking, handrail use, downward gaze, and/or increasing RTs. The fastest RTs occurred in the midsteps (non-transition steps). While gait and visual task performance were not statistically different prior to the top and bottom transition steps, gaze behaviour and RT were more variable prior to transition steps in HIGH. This study demonstrated that, in the presence of a visual task, people do not look down as often when walking downstairs and require minimum adjustments provided that the view of the stairs is available in the lower field of view. The middle of the stairs seems to require less from executive function, whereas visual attention appears a requirement to detect the last transition via gaze shifts or peripheral vision.

Standing still: is there a role for the cortex?

In humans, standing still appears so automatic that high-level cortical processes seem unnecessary. However, by measuring cortical activity time-locked to reactive control events arising from naturally occurring instability while standing still, we detected cortical involvement in the form of an evoked N1 potential prior to the onset of balance reactions. Peak amplitude and spectral power of this event-related activity increased as postural challenges and demand for reactive control increased.

Speed of processing in the primary motor cortex: a continuous theta burst stimulation study.

'Temporally urgent' reactions are extremely rapid, spatially precise movements that are evoked following discrete stimuli. The involvement of primary motor cortex (M1) and its relationship to stimulus intensity in such reactions is not well understood. Continuous theta burst stimulation (cTBS) suppresses focal regions of the cortex and can assess the involvement of motor cortex in speed of processing. The primary objective of this study was to explore the involvement of M1 in speed of processing with respect to stimulus intensity. Thirteen healthy young adults participated in this experiment. Behavioral testing consisted of a simple button press using the index finger following median nerve stimulation of the opposite limb, at either high or low stimulus intensity. Reaction time was measured by the onset of electromyographic activity from the first dorsal interosseous (FDI) muscle of each limb. Participants completed a 30 min bout of behavioral testing prior to, and 15 min following, the delivery of cTBS to the motor cortical representation of the right FDI. The effect of cTBS on motor cortex was measured by recording the average of 30 motor evoked potentials (MEPs) just prior to, and 5 min following, cTBS. Paired t-tests revealed that, of thirteen participants, five demonstrated a significant attenuation, three demonstrated a significant facilitation and five demonstrated no significant change in MEP amplitude following cTBS. Of the group that demonstrated attenuated MEPs, there was a biologically significant interaction between stimulus intensity and effect of cTBS on reaction time and amplitude of muscle activation. This study demonstrates the variability of potential outcomes associated with the use of cTBS and further study on the mechanisms that underscore the methodology is required. Importantly, changes in motor cortical excitability may be an important determinant of speed of processing following high intensity stimulation.

Role of peripheral vision in rapid perturbation-evoked reach-to-grasp reactions.

Onset and execution of compensatory reaches are faster than the most rapid voluntary reaches. With onset latencies near 100 ms, it is proposed that initial control of compensatory reaches cannot rely on visual information obtained after perturbation onset; rather, they rely on a visuospatial map acquired prior to instability. In natural conditions, it is not practical to direct gaze toward every potential support surface in preparation for a perturbation, suggesting that peripheral vision may be uniquely important. This study aimed to determine whether visuospatial mapping achieved using only peripheral visual information could be used to control reach-to-grasp reactions. Participants sat in an unstable chair. Whole body perturbations were used to evoke rapid reach-to-grasp reactions. A handle was positioned at midline or to the right of the participant. Gaze was directed toward the center or right to view the handle in peripheral or central visual fields. Electromyographic and kinematic data were recorded. Peripheral information acquired prior to perturbation was sufficient for successful execution of reach-to-grasp without delay. Differences in reach kinematics, however, did exist between vision conditions (e.g., maximum lateral wrist displacement and magnitude of hand overshoot relative to the handle were greater for peripheral vs. central vision). Handle location led to target-specific differences in initial muscle recruitment revealing information acquired prior to perturbation were used to guide initial limb trajectory. Results reveal the capacity to rely on a visuospatial map constructed from peripheral visual information for compensatory reaching but also highlight limitations leading to more conservative reach trajectories.

'Priming' the brain to generate rapid upper-limb reactions.

Evoked autonomic nervous system (ANS) activity may be an important modulator of rapid reactions, generated in the face of urgency and may serve to augment the parallel somatosensory processing to adjust speed of processing. The primary objective of the current study was to temporally pair auditory stimuli with whole body perturbations to determine if conditioning could 'prime' the central nervous system (CNS) to respond faster and with greater ANS reactivity to the auditory stimulus alone. Healthy young participants (n = 19) were seated in a custom chair, which tilted backwards upon the release of an electromagnet and were instructed to reach to grasp a handle located in front of their arm as fast as possible following an auditory cue. Three conditions were completed in the following order: (1) baseline-auditory cue alone (5 trials); (2) paired-auditory cue, followed by a chair tilt 110 ms later (20 trials); and (3) post-pairing-auditory cue alone (5 trials). Participants were not informed of the switch from paired to auditory-only stimuli in the first trial of the post-pairing task condition. Reaction time was measured using electromyography, and autonomic nervous system activity was monitored via the electrodermal response (EDR). The first trial post-pairing had significantly faster reaction time (Δ = 21 ms) and significantly greater EDR amplitude compared to the last trial prior to pairing (baseline). The amplitude of contraction and overall time to handle contact were not significantly different between the first trial post-pairing and the last trial prior to pairing. This study demonstrates that the CNS can be 'primed' to generate rapid reactions and an elevated autonomic response in the absence of whole body instability. This indicates that afferent volume generated following whole body instability is not the only determinant of rapid reactions and emphasizes the importance of physiologic measures of autonomic activity with respect to stimulus-evoked reaction time.

Use of mobility aids reduces attentional demand in challenging walking conditions.

While mobility aids (e.g., four-wheeled walkers) are designed to facilitate walking and prevent falls in individuals with gait and balance impairments, there is evidence indicating that walkers may increase attentional demands during walking. We propose that walkers may reduce attentional demands under conditions that challenge balance control. This study investigated the effect of walker use on walking performance and attentional demand under a challenged walking condition. Young healthy subjects walked along a straight pathway, or a narrow beam. Attentional demand was assessed with a concurrent voice reaction time (RT) task. Slower RTs, reduced gait speed, and increased number of missteps (>92% of all missteps) were observed during beam-walking. However, walker use reduced attentional demand (faster RTs) and was linked to improved walking performance (increased gait speed, reduced missteps). Data from two healthy older adult cases reveal similar trends. In conclusion, mobility aids can be beneficial by reducing attentional demands and increasing gait stability when balance is challenged. This finding has implications on the potential benefit of mobility aids for persons who rely on walkers to address balance impairments.

Does it really matter where you look when walking on stairs? Insights from a dual-task study.

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.

Electrophysiological correlates of changes in reaction time based on stimulus intensity.

BACKGROUND: Although reaction time is commonly used as an indicator of central nervous system integrity, little is currently understood about the mechanisms that determine processing time. In the current study, we are interested in determining the differences in electrophysiological events associated with significant changes in reaction time that could be elicited by changes in stimulus intensity. The primary objective is to assess the effect of increasing stimulus intensity on the latency and amplitude of afferent inputs to the somatosensory cortex, and their relation to reaction time. METHODS: Median nerve stimulation was applied to the non-dominant hand of 12 healthy young adults at two different stimulus intensities (HIGH & LOW). Participants were asked to either press a button as fast as possible with their dominant hand or remain quiet following the stimulus. Electroencephalography was used to measure somatosensory evoked potentials (SEPs) and event related potentials (ERPs). Electromyography from the flexor digitorum superficialis of the button-pressing hand was used to assess reaction time. Response time was the time of button press. RESULTS: Reaction time and response time were significantly shorter following the HIGH intensity stimulus compared to the LOW intensity stimulus. There were no differences in SEP (N20 & P24) peak latencies and peak-to-peak amplitude for the two stimulus intensities. ERPs, locked to response time, demonstrated a significantly larger pre-movement negativity to positivity following the HIGH intensity stimulus over the Cz electrode. DISCUSSION: This work demonstrates that rapid reaction times are not attributable to the latency of afferent processing from the stimulated site to the somatosensory cortex, and those latency reductions occur further along the sensorimotor transformation pathway. Evidence from ERPs indicates that frontal planning areas such as the supplementary motor area may play a role in transforming the elevated sensory volley from the somatosensory cortex into a more rapid motor response.

Does the movement matter?: determinants of the latency of temporally urgent motor reactions.

BACKGROUND: Extremely rapid movements are frequently executed in response to novel, potentially threatening stimuli. The mechanism by which these sophisticated responses are generated is a topic of debate. The current study investigates: 1) the importance of stimulus-response congruence in rapid responses and 2) the relationship between the autonomic nervous system (ANS) and response time. METHODS: Sixteen participants were seated in a chair that could tilt backwards 13°. Participants were instructed to react as fast as possible in response to either an auditory cue (AUD) or balance perturbation (chair tilt) (PERT) and completed one of three different tasks: reach-to-grasp a fixed handle (FIXED), reach-to-grasp a free moving handle (FREE) or plantar flex the left foot (FOOT). Electromyography and electrodermal activity were recorded. RESULTS: For all tasks, muscle onset latency was shorter and muscle response amplitude was greater following the PERT cue compared to the AUD cue. In contrast, there were no differences in onset latency between motor response conditions. Electrodermal response amplitude was greater in the FIXED and FREE conditions than in the FOOT condition. DISCUSSION: Even in situations where the stimulus was incongruent with the response, muscle onset latencies were evoked faster following the perturbation. The response latencies were determined by stimulus characteristics and the most rapid responses were not reliant on stimulus-response congruence. It remains unclear how it is possible to achieve such rapid response latencies to whole body perturbations but we speculate there may exist similar pathways that are uniquely facilitated by a stimulus dependent ANS response.

Where do we look when we walk on stairs? Gaze behaviour on stairs, transitions, and handrails.

Stair walking is a challenging locomotor task, and visual information about the steps is considered critical to safely walk up and down. Despite the importance of such visual inputs, there remains relatively little information on where gaze is directed during stair walking. The present study investigated the role of vision during stair walking with a specific focus on gaze behaviour relative to (1) detection of transition steps between ground level and stairs, (2) detection of handrails, and (3) the first attempt to climb an unfamiliar set of stairs. Healthy young adults (n = 11) walked up and down a set of stairs with 7 steps (transitions were defined as the two top and bottom steps). Gaze behaviour was recorded using an eye tracker. Although participants spent most part of the time looking at the steps, gaze fixations on stair features covered less than 20% of the stair walking time. There was no difference in the overall number of fixations and fixation time directed towards transitions compared to the middle steps of the stairs. However, as participants approached and walked on the stairs, gaze was within 4 steps ahead of their location. The handrail was rarely the target of gaze fixation. It is noteworthy that these observations were similar even in the very first attempt to walk on the stairs. These results revealed the specific role of gaze behaviour in guiding immediate action and that stair transitions did not demand increased gaze behaviour in comparison with middle steps. These findings may also indicate that individuals may rely on a spatial representation built from previous experience and/or visual information other than gaze fixations (e.g. dynamic gaze sampling, peripheral visual field) to extract information from the surrounding environment.