Searchers adjust their eye-movement dynamics to target characteristics in natural scenes
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Abstract When searching a target in a natural scene, it has been shown that both the target’s visual properties and similarity to the background influence whether and how fast humans are able to find it. So far, it was unclear whether searchers adjust the dynamics of their eye movements (e.g., fixation durations, saccade amplitudes) to the target they search for. In our experiment, participants searched natural scenes for six artificial targets with different spatial frequency content throughout eight consecutive sessions. High-spatial frequency targets led to smaller saccade amplitudes and shorter fixation durations than low-spatial frequency targets if target identity was known. If a saccade was programmed in the same direction as the previous saccade, fixation durations and successive saccade amplitudes were not influenced by target type. Visual saliency and empirical fixation density at the endpoints of saccades which maintain direction were comparatively low, indicating that these saccades were less selective. Our results suggest that searchers adjust their eye movement dynamics to the search target efficiently, since previous research has shown that low-spatial frequencies are visible farther into the periphery than high-spatial frequencies. We interpret the saccade direction specificity of our effects as an underlying separation into a default scanning mechanism and a selective, target-dependent mechanism.Keywords:
Visual Search
Spatial frequency
Microsaccade
Under natural viewing conditions, small movements of the eyes prevent the maintenance of a steady direction of gaze. It is unclear how the spatiotemporal content of the fixated scene has an impact on the properties of miniatures, fixational eye movements. We have investigated the characteristics of fixational eye movements recorded while human subjects are instructed to fixate natural statistics random textures (Motion Clouds) in which we manipulated the spatial frequency content. We used long presentations (5 sec) of Motion Clouds stimuli (Schrater et al. 2000) of varying spatial frequency bandwidths (Bsf) around different central spatial frequency (Sf0). We found that central spatial frequency has an effect upon microsaccadic eye movements. In particular, smaller saccadic amplitudes were associated with high spatial frequencies, and larger saccades with low spatial frequencies. Broadening the spatial frequency bandwidth also changed the distribution of microsaccade amplitudes. A lower spatial frequencies, larger Bsf resulted in a large reduction of microsaccades amplitude while fixation behavior for high spatial frequencies texture was not affected. Relationship between microsaccade rate and intersaccadic timing was also dependent upon Bsf. These results suggest that the spatial frequency content of the fixated images have a strong impact upon fixation instability. Paul R. Schrater, David C. Knill and Eero P. Simoncelli (2000) " Mechanisms of visual motion detection" Nature Neuroscience 3, 64 - 68. Meeting abstract presented at VSS 2012
Microsaccade
Spatial frequency
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Operator (biology)
Movement control
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Microsaccade
Motor Control
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Visual search is thought to be guided by an active visual working memory (VWM) representation of the task-relevant features, referred to as the search template. In three experiments using a probe technique, we investigated which eye movement metrics reveal which search template is activated prior to the search, and distinguish it from future relevant or no longer relevant VWM content. Participants memorized a target color for a subsequent search task, while being instructed to keep central fixation. Before the search display appeared, we briefly presented two task-irrelevant colored probe stimuli to the left and right from fixation, one of which could match the current target template. In all three experiments, participants made both more and larger eye movements towards the probe matching the target color. The bias was predominantly expressed in microsaccades, 100-250 ms after probe onset. Experiment 2 used a retro-cue technique to show that these metrics distinguish between relevant and dropped representations. Finally, Experiment 3 used a sequential task paradigm, and showed that the same metrics also distinguish between current and prospective search templates. Taken together, we show how subtle eye movements track task-relevant representations for selective attention prior to visual search.
Visual Search
Microsaccade
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Recent studies have demonstrated a close relationship between the frequency of microsaccades and covert attention shift. However, there are no arguments of attentional effect on drift movements during attentive or inattentive fixation. We examined whether visual attention has any influence on drift eye movements using statistical analysis of a time series of fixation eye movements. The results indicated that the power in the 3-4 Hz frequency range was enhanced when visual attention was dispersed over the parafoveal visual field. Furthermore, the amplitudes of the drift eye movements were reinforced immediately after the microsaccades, especially after square-wave jerks. These results suggest that microsaccades and drift eye movements are both controlled by higher order brain functions to acquire detailed visual information from the peripheral vision.
Microsaccade
Covert
Gaze-contingency paradigm
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Although we are rarely aware of it, our ability to visually perceive and successfully interact with the world depends on a rapid and carefully orchestrated sequence of eye movements. Roughly three times a second, large high-velocity movements known as saccades drastically alter the spatial and temporal stream of visual input. To investigate the temporal constraints on saccadic eye movements and identify biases in oculomotor behavior, I developed a simple but novel task: rapid alternating saccades (RAS). Human participants are asked to make a series of eye movements back and forth between stationary targets as quickly as possible.In Chapter 2, I investigate the characteristics of one of the most prominent and well-known biases in eye guidance, inhibition of return. Participants made RAS between two targets or following an “hourglass” pattern in which the same location is only sampled every fourth eye movement. The experiments revealed that both saccade dwell times and secondary saccade characteristics were dramatically altered when the eye returned directly to a previously viewed location. The effects depended on the direction of movement and the angular difference between subsequent saccades. The results further explicate the inhibition of return phenomenon and provide novel insight into motor and attentional constraints governing the rate of sequential eye movements.Chapter 3 explores interactions between the eye and the hand. Findings indicate that the maximum rate of RAS increases when concurrent and directionally compatible hand movements accompany the eye movements. The increase is the result of shorter dwell times, higher saccade peak velocity, and a decrease in secondary saccade occurrence. The findings occur independently of changes in saccade amplitude or direction. Hand movements in the opposite direction of the eye result in longer dwell times and more frequent secondary saccades. The experiments illustrate the tight temporal coordination between saccades and arm motor systems during sequential movements.
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Saccadic eye movement
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Smooth pursuit
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Abstract When searching a target in a natural scene, it has been shown that both the target’s visual properties and similarity to the background influence whether and how fast humans are able to find it. So far, it was unclear whether searchers adjust the dynamics of their eye movements (e.g., fixation durations, saccade amplitudes) to the target they search for. In our experiment, participants searched natural scenes for six artificial targets with different spatial frequency content throughout eight consecutive sessions. High-spatial frequency targets led to smaller saccade amplitudes and shorter fixation durations than low-spatial frequency targets if target identity was known. If a saccade was programmed in the same direction as the previous saccade, fixation durations and successive saccade amplitudes were not influenced by target type. Visual saliency and empirical fixation density at the endpoints of saccades which maintain direction were comparatively low, indicating that these saccades were less selective. Our results suggest that searchers adjust their eye movement dynamics to the search target efficiently, since previous research has shown that low-spatial frequencies are visible farther into the periphery than high-spatial frequencies. We interpret the saccade direction specificity of our effects as an underlying separation into a default scanning mechanism and a selective, target-dependent mechanism.
Visual Search
Spatial frequency
Microsaccade
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Microsaccades are small-amplitude (typically <1°), ballistic eye movements that occur when attempting to fixate gaze. Initially thought to be generated randomly, it has recently been established that microsaccades are influenced by sensory stimuli, attentional processes, and certain cognitive states. Whether decision processes influence microsaccades, however, is unknown. Here, we adapted two classic economic tasks to examine whether microsaccades reflect evolving saccade decisions. Volitional saccade choices of monkey and human subjects provided a measure of the subjective value of targets. Importantly, analyses occurred during a period of complete darkness to minimize the known influence of sensory and attentional processes on microsaccades. As the time of saccadic choice approached, microsaccade direction became the following: 1) biased toward targets as a function of their subjective value and 2) predictive of upcoming, voluntary choice. Our results indicate that microsaccade direction is influenced by and is a reliable tell of evolving saccade decisions. Our results are consistent with dynamic decision processes within the midbrain superior colliculus; that is, microsaccade direction is influenced by the transition of activity toward caudal saccade regions associated with high saccade value and/or future saccade choice.
Microsaccade
Superior colliculus
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Visual physiology suggests that the contrast sensitivity for a flickering or drifting grating should fall off sharply at temporal frequencies above 20 Hz (as in a typical DeLange curve). But when the spatial frequency of the stimulus is higher than 2 or 3 cycles/deg, the flicker response curve flattens unexpectedly beyond 20 Hz. Even with stabilized (drifting) gratings and careful fixation, our subjects detect such stimuli at frequencies as high as 200 Hz. We attribute this high-frequency sensitivity, not to a neural mechanism, but to the destabilizing effect of microsaccades. Similar microsaccade effects also appear to influence the thresholds for steady stabilized gratings.
Microsaccade
Spatial frequency
Stimulus (psychology)
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