Human visual processing during walking: Dissociable pre- and post-stimulus influences.

Walking influences visual processing but the underlying mechanism remains poorly understood. In this study, we investigated the influence of walking on pre-stimulus and stimulus-induced visual neural activity and behavioural performance in a discrimination task while participants were standing or freely walking. The results showed dissociable pre- and post-stimulus influences by the movement state. Walking was associated with a reduced pre-stimulus alpha power, which predicted enhanced N1 and decreased P3 components during walking. This pre-stimulus alpha activity was additionally modulated by time on the task, which was paralleled by a similar behavioural modulation. In contrast, the post-stimulus alpha power was reduced in its modulation due to stimulus onset during walking but showed no evidence of modulation by time on the task. Additionally, stimulus parameters (eccentricity, laterality, distractor presence significantly influenced post-stimulus alpha power, whereas the visually evoked components showed no evidence of such an influence. There was further no evidence of a correlation between pre-stimulus and post stimulus alpha power. We conclude that walking has two dissociable influences on visual processing: while the walking induced reduction in alpha power suggests an attentional state change that relates to visual awareness, the post-stimulus influence on alpha power modulation indicates changed spatial visual processing during walking.

Rapid and Accumulated Modulation of Action-Effects on Action.

Auditory feedback to a keypress is used in many devices to facilitate the motor output. The timing of auditory feedback is known to have an impact on the motor output, yet it is not known if a keypress action can be modulated on-line by an auditory feedback or how quick an auditory feedback can influence an ongoing keypress. Furthermore, it is not clear if the prediction of auditory feedback already changes the early phase of a keypress action independent of sensory feedback, which would suggest that such prediction changes the motor plan. In the current study, participants pressed a touch-sensitive device with auditory feedback in a self-paced manner. The auditory feedback was given either after a short (60 msec) or long (160 msec) delay, and the delay was either predictable or not. Our results showed that the keypress peak force was modulated by the amount of auditory feedback delay even when the delay was unpredictable, thus demonstrating an on-line modulation effect. The latency of the on-line modulation was suggested to be as low as 70 msec, indicating a very fast sensory to motor mapping circuit in the brain. When the auditory feedback delay was predictable, a change in the very early phase of keypress motor output was found, suggesting that the prediction of sensory feedback is crucial to motor control. Therefore, even a simple keypress action contains rich motor dynamics, which depend on expected as well as on-line perceived sensory feedback.

Action force modulates action binding: evidence for a multisensory information integration explanation.

Action binding refers to the observation that the perceived time of an action (e.g., a keypress) is shifted towards the distal sensory feedback (usually a sound) triggered by that action. Surprisingly, the role of somatosensory feedback for this phenomenon has been largely ignored. We fill this gap by showing that the somatosensory feedback, indexed by keypress peak force, is functional in judging keypress time. Specifically, the strength of somatosensory feedback is positively correlated with reported keypress time when the keypress is not associated with an auditory feedback and negatively correlated when the keypress triggers an auditory feedback. The result is consistent with the view that the reported keypress time is shaped by sensory information from different modalities. Moreover, individual differences in action binding can be explained by a sensory information weighting between somatosensory and auditory feedback. At the group level, increasing the strength of somatosensory feedback can decrease action binding to a level not being detected statistically. Therefore, a multisensory information integration account (between somatosensory and auditory inputs) explains action binding at both a group level and an individual level.

Differential effects of walking across visual cortical processing stages.

Perceptual processes are almost exclusively investigated and understood under marked movement restriction, while natural behaviour includes pronounced movements. Recent human studies have indicated a profound influence of body movement on early visual responses (e.g., evoked components around 100 msec in EEG, electroencephalogram). However, very little is known about the influence of free walking on later visual responses (e.g., responses related to visual selective attention in a later time window than the stimulus evoked N1 component). In the current study, we measured neural signals (EEG) and behavioural performance in a visual selective attention task while participants were standing or freely walking. The results showed that walking was associated with an amplification of early sensory-evoked potential as indicated by the N1 component. Interestingly, neural indexes of the succeeding processing stages of stimulus discrimination and identification, namely the N2pc component and alpha oscillations, and the eventual behavioural measures were comparable between standing and walking. Additionally, in both standing and walking conditions, an overall advantage in target processing for the right visual field was observed. Our work provides evidence that the early sensory processing is enhanced during locomotion while the succeeding processing steps in a later time window are not modulated by locomotion. We conclude that walking has differential effects across visual cortical processing stages.

Delusional thinking and action binding in healthy individuals.

Action binding is the effect that the perceived time of an action is shifted towards the action related feedback. A much larger action binding effect in schizophrenia compared to normal controls has been shown, which might be due to positive symptoms like delusions. Here we investigated the relationship between delusional thinking and action binding in healthy individuals, predicting a positive correlation between them. The action binding effect was evaluated by comparing the perceived time of a keypress between an operant (keypress triggering a sound) and a baseline condition (keypress alone), with a novel testing method that massively improved the precision of the subjective timing measurement. A positive correlation was found between the tendency of delusional thinking (measured by the 21-item Peters et al. delusions inventory) and action binding across participants after controlling for the effect of testing order between operant and baseline conditions. The results indicate that delusional thinking in particular influences action time perception and support the notion of a continuous distribution of schizotypal traits with normal controls at one end and clinical patients at the other end.

Overground Walking Decreases Alpha Activity and Entrains Eye Movements in Humans.

Experiments in animal models have shown that running increases neuronal activity in early visual areas in light as well as in darkness. This suggests that visual processing is influenced by locomotion independent of visual input. Combining mobile electroencephalography, motion- and eye-tracking, we investigated the influence of overground free walking on cortical alpha activity (~10 Hz) and eye movements in healthy humans. Alpha activity has been considered a valuable marker of inhibition of sensory processing and shown to negatively correlate with neuronal firing rates. We found that walking led to a decrease in alpha activity over occipital cortex compared to standing. This decrease was present during walking in darkness as well as during light. Importantly, eye movements could not explain the change in alpha activity. Nevertheless, we found that walking and eye related movements were linked. While the blink rate increased with increasing walking speed independent of light or darkness, saccade rate was only significantly linked to walking speed in the light. Pupil size, on the other hand, was larger during darkness than during light, but only showed a modulation by walking in darkness. Analyzing the effect of walking with respect to the stride cycle, we further found that blinks and saccades preferentially occurred during the double support phase of walking. Alpha power, as shown previously, was lower during the swing phase than during the double support phase. We however could exclude the possibility that the alpha modulation was introduced by a walking movement induced change in electrode impedance. Overall, our work indicates that the human visual system is influenced by the current locomotion state of the body. This influence affects eye movement pattern as well as neuronal activity in sensory areas and might form part of an implicit strategy to optimally extract sensory information during locomotion.