Implicit and explicit learning of Bayesian priors differently impacts bias during perceptual decision-making.

Knowledge without awareness, or implicit knowledge, influences a variety of behaviors. It is unknown however, whether implicit knowledge of statistical structure informs visual perceptual decisions or whether explicit knowledge of statistical probabilities is required. Here, we measured visual decision-making performance using a novel task in which humans reported the orientation of two differently colored translational Glass patterns; each color associated with different orientation probabilities. The task design allowed us to assess participants' ability to learn and use a general orientation prior as well as a color specific feature prior. Classifying decision-makers based on a questionnaire revealed that both implicit and explicit learners implemented a general orientation bias by adjusting the starting point of evidence accumulation in the drift diffusion model framework. Explicit learners additionally adjusted the drift rate offset. When subjects implemented a stimulus specific bias, they did so by adjusting primarily the drift rate offset. We conclude that humans can learn priors implicitly for perceptual decision-making and depending on awareness implement the priors using different mechanisms.

Differential effect of a distractor on primary saccades and perceptual localization.

A distractor presented nearby the target of a goal-directed short latency saccade leads to spatial averaging, that is, the saccade lands between the target and the distractor. This so-called global effect is a characteristic feature of the spatial processing underlying the programming of saccadic eye movements. To determine whether this effect of near distractors on saccade metrics is also reflected in perceptual localization, subjects performed a saccade task and a perceptual localization task using identical, briefly flashed visual stimuli. To make the available visual processing time for saccades and perception more similar, we followed the target with a mask. Without the mask, primary saccades with short latency landed between target and distractor. The distractor had less effect on primary saccades with longer latencies (>200 ms) and did not affect the final eye position after late secondary saccades in the dark. This indicates that the oculomotor system can correctly use information about the target location 200 ms after the target flash even if no visual stimulus is present during this period. Likewise the presence of a distractor did not affect perceptual localization. Under the masking condition a similar global effect occurred for primary saccades with short latencies, but the latency dependence of the global effect was weakened. Secondary saccades and perceptual localization still did not show a global effect. The results suggest that the primary saccade is based on a specific target acquisition process that differs from that used for spatial perception and for the programming of memory-guided corrective saccades.

Using high frame rate CMOS sensors for three-dimensional eye tracking.

A novel three-dimensional eye tracker is described and its performance evaluated. In contrast to previous devices based on conventional video standards, the present eye tracker is based on programmable CMOS image sensors, interfaced directly to digital processing circuitry to permit real-time image acquisition and processing. This architecture provides a number of important advantages, including image sampling rates of up to 400/sec measurement, direct pixel addressing for preprocessing and acquisition,and hard-disk storage of relevant image data. The reconfigurable digital processing circuitry also facilitates inline optmization of the front-end, time-critical processes. The primary acquisition algorithm for tracking the pupil and other eye features is designed around the generalized Hough transform. The tracker permits comprehensive measurement of eye movement (three degrees of freedom) and head movement (six degrees of freedom), and thus provides the basis for many types of vestibulo-oculomotor and visual research. The device has been qualified by the German Space Agency (DLR) and NASA for deployment on the International Space Station. It is foreseen that the device will be used together with appropriate stimulus generators as a general purpose facility for visual and vestibular experiments. Initial verification studies with an artificial eye demonstrate a measurement resolution of better than 0.1 degrees in all three components (i.e.,system noise for each of the components measured as 0.006 degrees H, 0.005 degrees V, and 0.016 degrees T. Over a range of +/-20 degrees eye rotation, linearity was found to be <0.5% (H), <0.5% (V), and <2.0% (T). A comparison with the scleral search coil technique yielded near equivalent values for the system noise and the thickness of Listing's plane.

Cerebellar lesions impair rapid saccade amplitude adaptation.

This study investigated whether the cerebellum is essential for rapid saccade adaptation. Saccade adaptation was elicited by 30% backward target steps during the primary saccade. Patients with cerebellar lesions adapted less than normal subjects, but saccade adaptation was most impaired in the group of patients with cerebellar degeneration. As the variability of the saccade gain in patients with cerebellar degeneration did not significantly differ from that in the other patients, the increased variability in motor performance due to a cerebellar lesion cannot alone explain this impaired adaptation.

Improving the homogeneity of the magnetic field in the magnetic search coil technique.

The magnetic search coil technique is used to obtain accurate eye-movement measurements. For the data analysis it must be assumed that the magnetic field does not change over time (apart from the required modulation). To comply with this assumption either no translational movements of the eye coil(s) are allowed or the magnetic field has to be perfectly homogeneous. Both are normally not the case, i.e., measurement errors occur. These errors can be minimized by keeping the magnetic field as homogeneous as possible. Larger field coils are typically chosen to achieve this aim. However, sometimes the experimental setup imposes limitations on the size of the configuration. We present a method for improving the homogeneity of the magnetic field by adding supplementary field coils without changing the size of the configuration.

Mislocalization of peripheral targets during fixation.

To investigate the effect of the visual stimulus configuration on localization when oculomotor performance is excluded, we evaluated the errors made when subjects compare the horizontal location of two sequentially presented peripheral targets while looking at a visual or memorized fixation spot. Eye position was monitored by means of an infrared eye tracker. Significant localization errors were observed. As long as the fixation spot stayed on or off during the entire presentation time of both peripheral targets, the localization error did not depend on the presence or absence of the fixation spot. A significant change in the localization error was observed only if the fixation spot was presented together with the first peripheral target but disappeared before the presentation of the second one. The localization error did not depend on: (1) the visual asymmetry (unilateral versus bilateral target presentation); (2) the distribution of visual attention (cued versus non-cued test location); or (3) the time interval between the two targets. These results suggest that the mislocalization observed during fixation is partially due to a mismatch between egocentric and exocentric localization mechanisms.

Relation between the metrics of the presaccadic attention shift and of the saccade before and after saccadic adaptation.

A shift of the visual attention focus is known to precede saccades. However, how the metrics of both this presaccadic attention shift and the saccade are coupled is still unclear. We altered the saccade size by short-term saccadic adaptation to determine whether the attention focus would still be shifted to the location of the saccade target or to the modified postsaccadic eye position. The results showed that saccadic adaptation had no influence on the presaccadic attention shift. Thus either different processes determine the metrics of the attention shift and of the saccade or saccadic adaptation causes only modifications on a lower hierarchical level of saccade programming, thereby not influencing the metrics of the attention shift.

Spatial and temporal aspects of eye-hand coordination across different tasks.

The way in which saccadic eye movements are elicited influences their latency and accuracy. Accordingly, different tasks elicit different types of saccades. Using the tasks steps, gap, memory, scanning and antisaccade, we analyzed combined eye and hand movements to determine whether both motor systems share control strategies. Errors and latencies were measured to examine whether changes in eye motor behavior are reflected in hand motor behavior. Directional and variable errors of eye and hand changed differently according to the tasks. Moreover, errors of the two systems did not correlate for any of the tasks investigated. Contrary to errors, mean latencies of eye movements were organized in the same pattern as hand movements. A correlation of latencies indicates that both motor systems rely on common information to initiate movement. Temporal coupling was stronger for intentional tasks than for reflexive tasks.

Can a motion-blind patient reach for moving objects?

It has been claimed that the visual brain is organized in two separate processing streams for spatial vision: one for perception and one for action. To determine whether motion vision is also divided into vision for action and for perception we examined the interceptive behaviour of the motion-blind patient LM. The task for LM and three age-matched control subjects was to reach-and-grasp for an object that moved away. Three experiments were conducted to examine the effects on perfomance of target speed (Expt 1), observation time (Expt 2) and visual feedback (Expt 3). As LM is only able to reach for objects which move at 0.5 m/s or less, her performance is inferior to that of controls who can reach for objects moving at 1.0 m/s, but it is better than would be expected from her performance in psychophysical experiments on her motion vision. Kinematic analysis of LM's reaching movements showed that she adapted the speed of her moving hand to the speed of the target but only when full vision was available. In contrast to normal subjects, LM required long observation times and vision of her moving hand to produce successful reaching responses. Thus, the impairment of both perception and action in LM suggests that the motion area MT/V5 is located at an early stage of the extrastriate hierarchy and provides input to both the perception and the action processing streams.

Size distortion in spatial neglect.

One of the suggested theories to explain some aspects of neglect in patients with right parietal lesions is the spatial distortion hypothesis. To determine whether a distorted representation of space can account for the performance of neglect patients in visuospatial tasks we asked 27 neglect patients to compare the width of two horizontally aligned bars. The bars were presented symmetric to the body midline or either on the left or right side of egocentric space. The size comparison data are in accordance with the hypothesis of a distorted egocentric representation based on a dynamic remapping of space. The results support the idea that the abnormalities observed in the size comparison tasks are due to a distorted internal representation of the outside world. There are some hints that this distortion could be based on a dynamic mapping of space determined by the distribution of visuospatial attention.

The role of the attention focus in the visual information processing underlying saccadic adaptation.

Three experiments were performed to determine how an error signal for driving saccadic adaptation is derived from visual information processing. The first experiment demonstrated that an intrasaccadic displacement of a visual background does not influence saccadic adaptation when a small foveal target is used. The second experiment showed that when a different type of target, a 4.8 deg annulus, is used an intrasaccadic background shift influences the adaptive process. The third experiment showed that the size of the saccade target determines the size of the attention focus around the time of a saccade. These findings suggest that the attention focus selects the visual information used for a trans-saccadic comparison in order to generate the error signal.

Intrasaccadic target steps during the deceleration of primary saccades affect the latency of corrective saccades.

This study investigates how visually guided saccades and subsequent corrective saccades are affected by a secondary target step occurring at different times during the primary saccade. Eye movements of human subjects were measured by means of a differential infrared light reflection technique while the subjects performed visually guided saccades to a laser spot in darkness. The target was stepped backward or onward during the targeting saccade. While the intrasaccadic target step did not influence gain, peak velocity or skewness of the primary saccade, it had a significant effect on the subsequent corrective saccade when the secondary target step occurred during the deceleration phase of the primary saccade: the latency of the corrective saccade was significantly increased compared with the one performed under the single-step control condition. This increase also occurred when single target steps were presented randomly intermixed with backward and onward double target steps and even between selected sub-samples of saccades with identical postsaccadic visual error. If the target step occurred early during the primary saccade, the latency of the corrective saccade was not changed. This indicates that visual information sampled during the deceleration phase of a saccade can lead to a cancellation of the normal trigger mode of corrective saccades.

Fixation errors and timing in sequences of memory-guided saccades.

We analyzed the relation between position and amplitude errors during the performance of sequences of saccades to previously memorized target positions in complete darkness. Although a complete compensation (on the average) for fixation errors was observed, groups of successive saccades could be identified which showed propagation of position errors. These groups are characterized by a long fixation time prior to the first saccade and short fixations prior to the remaining saccades. These findings indicate that sequences of eye movements can be performed in two different modes: (1) extraretinal information about the actual eye position is used to correct fixation errors; (2) pre-programmed groups of saccades with limited length can be performed with fixed amplitudes neglecting the actual eye position. These groups tended to occur predominantly at the end of a sequence.

Electrical stimulation of the posteroventral pallidum influences internally guided saccades in Parkinson's disease.

Chronic electrical stimulation of the posteroventral internal pallidum, a recently developed treatment option in advanced Parkinson's disease (PD), improves bradykinesia, rigidity and medication-induced dyskinesia. Apart from disturbances of the somatic motor system, PD is also characterized by disturbances of saccadic eye movements with hypometric and delayed internally guided saccades. We examined these internally guided eye movements (memory-guided and anti-task saccades) in a patient with bilaterally implanted stimulation electrodes, when stimulation was turned on and off. The electrical stimulation not only improved the bradykinesia and rigidity, but also the internally guided saccades by shortening the latency of the anti-saccades and increasing the gain of the memory-guided saccades. This finding supports the idea that the oculomotor pathways through the basal ganglia are organized like the somatic motor pathways and that stimulation of the posteroventral pallidum influences both.