No abstract

Most of our knowledge about vision comes from experiments in which stimuli are presented to immobile human subjects or animals. In the case of human subjects, movement during psychophysical, electrophysiological, or neuroimaging experiments is considered to be a source of noise to be eliminated. Animals used in visual neuroscience experiments are typically restrained and, in many cases, anesthetized. In reality, however, vision is often used to guide the motion of awake, ambulating organisms. Recent work in mice has shown that locomotion elevates visual neuronal response amplitudes (Niell and Stryker, 2010; Erisken et al., 2014; Fu et al., 2014; Lee et al., 2014; Mineault et al., 2016) and reduces long-range gain control (Ayaz et al., 2013). Here, we used both psychophysics and steady-state electrophysiology to investigate whether similar effects of locomotion on early visual processing can be measured in humans. Our psychophysical results show that brisk walking has little effect on subjects' ability to detect briefly presented contrast changes and that co-oriented flankers are, if anything, more effective masks when subjects are walking. Our electrophysiological data were consistent with the psychophysics indicating no increase in stimulus-driven neuronal responses while walking and no reduction in surround suppression. In summary, we have found evidence that early contrast processing is altered by locomotion in humans but in a manner that differs from that reported in mice. The effects of locomotion on very low-level visual processing may differ on a species-by-species basis and may reflect important differences in the levels of arousal associated with locomotion.SIGNIFICANCE STATEMENT Mice are the current model of choice for studying low-level visual processing. Recent studies have shown that mouse visual cortex is modulated by behavioral state: primary visual cortex neurons in locomoting mice tend to be more sensitive and less influenced by long-range gain control. Here, we tested these effects in humans by measuring psychophysical detection thresholds and electroencephalography (EEG) responses while subjects walked on a treadmill. We found no evidence of increased contrast sensitivity or reduced surround suppression in walking humans. Our data show that fundamental measurements of early visual processing differ between humans and mice and this has important implications for recent work on the links among arousal, behavior, and vision in these two species.

People make saccades--rapid eye movements to a new fixation--approximately three times per second. This would seemingly disrupt perceptual continuity, yet our brains construct a coherent, stable view of the world from these successive fixations. There is conflicting evidence regarding the effects of saccades on perceptual continuity: some studies report that they are disruptive, with little information carryover between saccades; others report that carryover is substantial. Here we show that saccades actively contribute to perceptual continuity in humans in two different ways. When bistable stimuli are presented intermittently, saccades executed during the blank interval shorten the duration of states of ambiguous figures, indicating that saccades can erase immediately past perceptual states. On the other hand, they prolong the McCollough effect, indicating that saccades strengthen learned contingencies. Our results indicate that saccades help, rather than hinder, perceptual continuity.

People make rapid, goal-directed movements to interact with their environment. Because these movements have consequences, it is important to be able to control them with a high level of precision and accuracy. Our hypothesis is that vision guides rapid hand movements, thereby enhancing their accuracy and precision. To test this idea, we asked observers to point to a briefly presented target (110 ms). We measured the impact of visual information on endpoint precision by using a shutter to close off view of the hand 50, 110 and 250 ms into the reach. We found that precision was degraded if the view of the hand was restricted at any time during the reach, despite the fact that the target disappeared long before the reach was completed. We therefore conclude that vision keeps the hand on the planned trajectory. We then investigated the effects of a perturbation of target position during the reach. For these experiments, the target remained visible until the reach was completed. The target position was shifted at 110, 180 or 250 ms into the reach. Early shifts in target position were easily compensated for, but late shifts led to a shift in the mean position of the endpoints; observers pointed to the center of the two locations, as a kind of best bet on the position of the target. Visual information is used to guide the hand throughout a reach and has a significant impact on endpoint precision.

We studied the effects of visual references and the level of illumination on the localization of stimuli flashed briefly near the start of saccades. A translucent shutter made it possible to remove visual references, but admit light, at different times after saccadic onset. The results show that post-saccadic visual references are not necessary for compression: a consistent compression of verbally reported relative stimulus distances is found at all shutter latencies and at all post-shutter levels of illumination. They also show that positions indicated by blind pointing show no compression except when visual references remain in view for a substantial time after saccades. These results confirm that the visual system uses multiple representations of space and suggest that it weights them differently for different tasks and different viewing conditions. No single map is used exclusively for conscious perception or for motor action, and conscious perception is always subject to compression at the time of saccades.