Femke Maij

Temporal Information Can Influence Spatial Localization

To localize objects relative to ourselves, we need to combine various sensory and motor signals. When these signals change abruptly, as information about eye orientation does during saccades, small differences in latency between the signals could introduce localization errors. We examine whether independent temporal information can influence such errors. We asked participants to follow a randomly jumping dot with their eyes and to point at flashes that occurred near the time they made saccades. Such flashes are mislocalized. We presented a tone at different times relative to the flash. We found that the flash was mislocalized as if it had occurred closer in time to the tone. This demonstrates that temporal information is taken into consideration when combining sensory information streams for localization.

No Perisaccadic Mislocalization with Abruptly Cancelled Saccades

Every saccadic eye movement that we make changes the image of the world on our retina. Yet, despite these retinal shifts, we still perceive our visual world to be stable. Efference copy from the oculomotor system to the visual system has been suggested to contribute to this stable percept, enabling the brain to anticipate the retinal image shifts by remapping the neural image. A psychophysical phenomenon that has been linked to this predictive remapping is the mislocalization of a stimulus flashed around the time of a saccade. If this mislocalization is initiated by saccade preparation, one should also observe localization errors when a saccade is planned, but abruptly aborted just before its execution. We tested this hypothesis in human subjects using a novel paradigm that combines a flash localization task with a countermanding component that occasionally requires saccade cancellation. Surprisingly, we found no trace of mislocalization, even for saccades cancelled close to the point of no return. This strongly suggests that the actual execution of the saccade is a prerequisite for the typical localization errors, which rejects various models and constrains neural substrates. We conclude that perisaccadic mislocalization is not a direct consequence of saccade preparation, but arises after saccade execution when the flash location is constructed from memory.

Temporal uncertainty separates flashes from their background during saccades.

It is known that spatial localization of flashed objects fails around the time of rapid eye movements (saccades). This mislocalization is often interpreted in terms of a combination of shifts and deformations of the brains representation of space to account for the eye movement. Such temporary remapping of positions in space should affect all elements in a scene, leaving ordinal relationships between positions intact. We performed an experiment in which we presented flashes on a background with red and green regions to human subjects. We found that flashes that were presented on the green part of the background around the time of a saccade were readily reported to have been presented on the red part of the background and vice versa. This is inconsistent with the notion of a temporary shift and deformation of perceived space. To explain our results, we present a model that illustrates how temporal uncertainty could give rise to the observed spatial mislocalization. The model combines uncertainty about the time of the flash with a bias to localize targets where one is looking. It reproduced the pattern of mislocalization very accurately, showing that perisaccadic mislocalization can best be explained in terms of temporal uncertainty about the moment of the flash.

Peri-saccadic mislocalization is not influenced by the predictability of the saccade target location.

Flashes presented around the time of a saccade are often mislocalized. The precise pattern of mislocalization is influenced by many factors. Here we study one such factor: the predictability of the saccade targets location. The experiment examines two conditions. In the first the subject makes the same horizontal rightward saccade to the same target location over and over again. In the second the subject makes saccades to a target that is jumping in unpredictable radial directions. A dot is flashed in the vicinity of the saccade target near the time of saccade onset. Subjects are asked to localize the flash by touching its location on the screen. Although various saccade parameters differed, the errors that subjects made were very similar in both conditions. We conclude that the pattern of mislocalization does not depend on the predictability of the location of the saccade target.

Misjudging where you felt a light switch in a dark room.

Previous research has shown that subjects systematically misperceive the location of visual and haptic stimuli presented briefly around the time of a movement of the sensory organ (eye or hand movements) due to errors in the combination of visual or tactile information with proprioception. These briefly presented stimuli (a flash or a tap on the finger) are quite different from what one encounters in daily life. In this study, we tested whether subjects also mislocalize real (static) objects that are felt briefly while moving ones hand across them, like when searching for a light switch in the dark. We found that subjects systematically mislocalized a real bar in a similar manner as has been shown with artificial haptic stimuli. This demonstrates that movement-related mislocalization is a real world property of human perception.

Spatiotemporal integration for tactile localization during arm movements: a probabilistic approach.

It has been shown that people make systematic errors in the localization of a brief tactile stimulus that is delivered to the index finger while they are making an arm movement. Here we modeled these spatial errors with a probabilistic approach, assuming that they follow from temporal uncertainty about the occurrence of the stimulus. In the model, this temporal uncertainty converts into a spatial likelihood about the external stimulus location, depending on arm velocity. We tested the prediction of the model that the localization errors depend on arm velocity. Participants (n = 8) were instructed to localize a tactile stimulus that was presented to their index finger while they were making either slow- or fast-targeted arm movements. Our results confirm the models prediction that participants make larger localization errors when making faster arm movements. The model, which was used to fit the errors for both slow and fast arm movements simultaneously, accounted very well for all the characteristics of these data with temporal uncertainty in stimulus processing as the only free parameter. We conclude that spatial errors in dynamic tactile perception stem from the temporal precision with which tactile inputs are processed.

Luminance contrast in the background makes flashes harder to detect during saccades

To explore a visual scene we make many fast eye movements (saccades) every second. During those saccades the image of the world shifts rapidly across our retina. These shifts are normally not detected, because perception is suppressed during saccades. In this paper we study the origin of this saccadic suppression by examining the influence of luminance borders in the background on the perception of flashes presented near the time of saccades in a normally illuminated room. We used different types of backgrounds: either with isoluminant red and green areas or with black and white areas. We found that the ability to perceive flashes that were presented during saccades was suppressed when there were luminance borders in the background, but not when there were isoluminant color borders in the background. Thus, masking by moving luminance borders plays an important role in saccadic suppression. The perceived positions of detected flashes were only influenced by the borders between the areas in the background when the flashes were presented before or after the saccades. Moreover, the influence did not depend on the kind of contrast forming the border. Thus, the masking effect of moving luminance borders does not appear to play an important role in the mislocalization of flashes that are presented near the time of saccades.