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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.
Sensory input as well as cognitive factors can drive the modulation of blinking. Our aim was to dissociate sensory driven bottom-up from cognitive top-down influences on blinking behavior and compare these influences between the auditory and the visual domain.
Using an oddball paradigm, we found a significant pre-stimulus decrease in blink probability for visual input compared to auditory input. Sensory input further led to an early post-stimulus blink increase in both modalities if a task demanded attention to the input. Only visual input caused a pronounced early increase without a task. In case of a target or the omission of a stimulus (as compared to standard input), an additional late increase in blink rate was found in the auditory and visual domain. This suggests that blink modulation must be based on the interpretation of the input, but does not need any sensory input at all to occur.
Our results show a complex modulation of blinking based on top-down factors such as prediction and attention in addition to sensory-based influences. The magnitude of the modulation is mainly influenced by general attentional demands, while the latency of this modulation allows to dissociate general from specific top-down influences that are independent of the sensory domain.