Céline Paeye

REINFORCING SACCADIC AMPLITUDE VARIABILITY

Saccadic endpoint variability is often viewed as the outcome of neural noise occurring during sensorimotor processing. However, part of this variability might result from operant learning. We tested this hypothesis by reinforcing dispersions of saccadic amplitude distributions, while maintaining constant their medians. In a first experiment we reinforced the least frequent saccadic amplitudes to increase variability, and then reinforced the central part of the amplitude distributions to reduce variability. The target was placed at a constant distance from the fovea after the saccade to maintain the postsaccadic visual signal constant and an auditory reinforcement was delivered depending on saccadic amplitude. The second experiment tested the effects of the contingency. We reinforced high levels of variability in 4 participants, whereas 4 other participants were assigned to a yoked control group. On average, saccadic amplitude standard deviations were doubled while the medians remained mostly unchanged in the experimental participants in both experiments, and variability returned to baseline level when low variability was reinforced. In the control group no consistent changes in amplitude distributions were observed. These results, showing that variability can be reinforced, challenge the idea of a stochastic neural noise. We instead propose that selection processes constrain saccadic amplitude distributions.

Transsaccadic perceptual fusion.

Transsaccadic perceptual fusion is the integration of pre- and postsaccadic images into a single percept aligned in spatial coordinates. Several early studies reported an absence of transsaccadic fusion between dissimilar patterns, effectively stopping research on this question for three decades. We have now corrected two problematic aspects of these earlier studies and find robust evidence for transsaccadic perceptual fusion. First, we used simple pre- and postsaccadic targets, (|, ) for which spatial alignment is not critical. Second, we reduced the contrast of the postsaccadic stimulus, so that it would not suppress fusion. Participants reported seeing a superposition of the pre- and postsaccadic targets on 67% of trials. Importantly, we obtained similar results when the two stimuli were presented without an intervening eye movement, suggesting the existence of a general fusion mechanism. Directional biases in the saccade condition suggest that remapping might be the mechanism realigning the pre- and postsaccadic locations. Remapping may thus not only predict where targets will be located after a saccade but may also guide content, predicting what targets will look like. However, the constraints on the appearance of the fused percept suggest that it plays, at best, a limited role in visual stability across saccades.

Visual reinforcement shapes eye movements in visual search

We use eye movements to gain information about our visual environment; this information can indirectly be used to affect the environment. Whereas eye movements are affected by explicit rewards such as points or money, it is not clear whether the information gained by finding a hidden target has a similar reward value. Here we tested whether finding a visual target can reinforce eye movements in visual search performed in a noise background, which conforms to natural scene statistics and contains a large number of possible target locations. First we tested whether presenting the target more often in one specific quadrant would modify eye movement search behavior. Surprisingly, participants did not learn to search for the target more often in high probability areas. Presumably, participants could not learn the reward structure of the environment. In two subsequent experiments we used a gaze-contingent display to gain full control over the reinforcement schedule. The target was presented more often after saccades into a specific quadrant or a specific direction. The proportions of saccades meeting the reinforcement criteria increased considerably, and participants matched their search behavior to the relative reinforcement rates of targets. Reinforcement learning seems to serve as the mechanism to optimize search behavior with respect to the statistics of the task.

Reinforcing saccadic amplitude variability in a visual search task

Human observers often adopt rigid scanning strategies in visual search tasks, even though this may lead to suboptimal performance. Here we ask whether specific levels of saccadic amplitude variability may be induced in a visual search task using reinforcement learning. We designed a new gaze-contingent visual foraging task in which finding a target among distractors was made contingent upon specific saccadic amplitudes. When saccades of rare amplitudes led to displaying the target, the U values (measuring uncertainty) increased by 54.89% on average. They decreased by 41.21% when reinforcing frequent amplitudes. In a noncontingent control group no consistent change in variability occurred. A second experiment revealed that this learning transferred to conventional visual search trials. These results provide experimental support for the importance of reinforcement learning for saccadic amplitude variability in visual search.

Calibration of peripheral perception of shape with and without saccadic eye movements

The cortical representations of a visual object differ radically across saccades. Several studies claim that the visual system adapts the peripheral percept to better match the subsequent foveal view. Recently, Herwig, Weiß, and Schneider (2015, Annals of the New York Academy of Sciences, 1339(1), 97–105) found that the perception of shape demonstrates a saccade-dependent learning effect. Here, we ask whether this learning actually requires saccades. We replicated Herwig et al.’s (2015) study and introduced a fixation condition. In a learning phase, participants were exposed to objects whose shape systematically changed during a saccade, or during a displacement from peripheral to foveal vision (without a saccade). In a subsequent test, objects were perceived as less (more) curved if they previously changed from more circular (triangular) in the periphery to more triangular (circular) in the fovea. Importantly, this pattern was seen both with and without saccades. We then tested whether a variable delay between the presentations of the peripheral and foveal objects would affect their association—hypothetically weakening it at longer delays. Again, we found that shape judgments depended on the changes experienced during the learning phase and that they were similar in both the saccade and fixation conditions. Surprisingly, they were not affected by the delay between the peripheral and foveal presentations over the range we tested. These results suggest that a general associative process, independent of saccade execution, contributes to the perception of shape across viewpoints.

Predictive position computations mediated by parietal areas: TMS evidence

Abstract When objects move or the eyes move, the visual system can predict the consequence and generate a percept of the target at its new position. This predictive localization may depend on eye movement control in the frontal eye fields (FEF) and the intraparietal sulcus (IPS) and on motion analysis in the medial temporal area (MT). Across two experiments we examined whether repetitive transcranial magnetic stimulation (rTMS) over right FEF, right IPS, right MT, and a control site, peripheral V1/V2, diminished participants’ perception of two cases of predictive position perception: trans‐saccadic fusion, and the flash grab illusion, both presented in the contralateral visual field. In trans‐saccadic fusion trials, participants saccade toward a stimulus that is replaced with another stimulus during the saccade. Frequently, predictive position mechanisms lead to a fused percept of pre‐ and post‐saccade stimuli (Paeye et al., 2017). We found that rTMS to IPS significantly decreased the frequency of perceiving trans‐saccadic fusion within the first 10 min after stimulation. In the flash grab illusion, a target is flashed on a moving background leading to the percept that the target has shifted in the direction of the motion after the flash (Cavanagh and Anstis, 2013). In the first experiment, the reduction in the flash grab illusion after rTMS to IPS and FEF did not reach significance. In the second experiment, using a stronger version of the flash grab, the illusory shift did decrease significantly after rTMS to IPS although not after rTMS to FEF or to MT. These findings suggest that right IPS contributes to predictive position perception during saccades and motion processing in the contralateral visual field. HighlightsInvestigating visual predictive position mechanisms within FEF and IPS.Predictive object localization across saccades decrease after rTMS to IPS.Predictive object positioning during motion reduced after rTMS to IPS.IPS contributes to predictive localization during saccades and motion.Predictive position perception was not effected after rTMS to FEF.

Sensorimotor adaptation of size perception.

Sensorimotor adaptation is the process by which new associations between movements and their perceptual effects are learned. Previous work reported that the visual system learns associations between peripheral (coarse) and foveal (highly defined) images of objects to achieve feature constancy across eye movements (e.g., Cox et al., 2005). Here we investigated the ability to learn perceptual associations between peripheral and foveal object size across saccades. In a pre-adaptation phase, participants made saccades to a peripheral disk. During these saccades this disk was replaced with a bigger or smaller disk. Participants had to decide whether the post-saccadic (foveal) disk was bigger or smaller compared to the pre-saccadic (peripheral) disk. For each participant, we defined the critical size change that led to 75% correct performance. In the following 30-min adaptation phase, subjects made saccades to the peripheral disk. During the saccade, the disk was modified by the critical size change measured individually. The post-adaptation phase was identical to the pre-adaptation phase except that adaptation trials were interleaved to maintain the level of adaptation. Preliminary results on 5 participants showed a significant shift of the PSE after adaptation in the direction of the adapted size change. For example, after adapting to a small-to-large trans-saccadic size change, the post-saccadic target had to be slightly larger than the pre-saccadic target to appear as matched in size, while objects that did not change size during the saccade were seen as slightly shrinking. This suggests that a new trans-saccadic correspondence of object size was learned and that perhaps, like the trans-saccadic correspondence of object position (saccadic adaptation), the trans-saccadic correspondence of object size and other features might be adaptable. Meeting abstract presented at VSS 2015.