Karine Doré-Mazars

Motor space structures perceptual space: Evidence from human saccadic adaptation

Saccadic adaptation is the progressive correction of systematic saccade targeting errors. When a saccade to a particular target is adapted, saccades within a spatial window around the target, the adaptation field, are affected as a function of their distance from the adapted target. Furthermore, previous studies suggest that saccadic adaptation might modify the perceptual localization of objects in space. We investigated the localization of visual probes before and after saccadic adaptation, and examined whether the spatial layout of the observed mislocalizations was structurally similar to the saccadic adaptation field. We adapted a horizontal saccade directed towards a target 12 degrees to the right. Thirty-eight saccades towards the right visual hemifield were then used to measure the adaptation field. The adaptation field was asymmetric: transfer of adaptation to saccades larger than the adapted saccade was greater than transfer to smaller saccades. Subjects judged the localization of 39 visual probes both within and outside the adaptation field. The perceived localization of a probe at a given position was proportional to the amount of transfer from the adapted saccade to the saccade towards that position. This similar effect of saccadic adaptation on both the action and perception representations of space suggests that the system providing saccade metrics also contributes to the metric used for the perception of space.

Eye movement signals influence perception: evidence from the adaptation of reactive and volitional saccades.

Information about upcoming saccadic eye movements is used to orient visuo-spatial attention across the visual field. Different eye movement signals (intended or actual) could be used according to the intentionality of the saccade in preparation (Reactive or Volitional), and can be dissociated by saccadic adaptation. Gap 0 and overlap paradigms were contrasted to elicit the two saccade populations with different latencies and an asymmetric transfer of saccadic adaptation. Preparation of both saccade types caused a concomitant shift in the attentional focus (indexed by relative perceptual performance) to the actual, not intended, eye position. The attentional shift emerged progressively, earlier for V-saccades but reaching a maximal level around saccade onset for both saccade types. These results suggest that information about actual eye movements mediates the pre-saccadic shift of attention.

Saccade dynamics before, during, and after saccadic adaptation in humans.

PURPOSE When saccade amplitude is systematically inadequate relative to the desired target position, the saccadic system adaptively modifies the amplitude of subsequent saccades so as to recover precise targeting capabilities. The effect of saccadic adaptation on saccade metrics (amplitude, direction) is well documented, but the effect on dynamics (velocity, duration, acceleration, deceleration) remains to be fully elucidated. METHODS The dynamics of adapted saccades were compared with that of baseline saccades of similar amplitudes. RESULTS The peak deceleration and skewness (duration of the acceleration period/duration of the deceleration period) were modified by adaptation. CONCLUSIONS The results point toward involvement of the cerebellum rather than the brain stem saccade generator in human saccadic adaptation.

Effect of a Dual Task on Postural Control in Dyslexic Children

Several studies have examined postural control in dyslexic children; however, their results were inconclusive. This study investigated the effect of a dual task on postural stability in dyslexic children. Eighteen dyslexic children (mean age 10.3±1.2 years) were compared with eighteen non-dyslexic children of similar age. Postural stability was recorded with a platform (TechnoConcept®) while the child, in separate sessions, made reflex horizontal and vertical saccades of 10° of amplitude, and read a text silently. We measured the surface and the mean speed of the center of pressure (CoP). Reading performance was assessed by counting the number of words read during postural measures. Both groups of children were more stable while performing saccades than while reading a text. Furthermore, dyslexic children were significantly more unstable than non-dyslexic children, especially during the reading task. Finally, the number of words read by dyslexic children was significantly lower than that of non-dyslexic children and, in contrast to the non-dyslexic children. In line with the U-shaped non-linear interaction model, we suggest that the attention consumed by the reading task could be responsible for the loss of postural control in both groups of children. The postural instability observed in dyslexic children supports the hypothesis that such children have a lack of integration of multiple sensorimotor inputs.

Saccadic adaptation shifts the pre-saccadic attention focus

The well-documented phenomenon of the spatial coupling between saccadic programming and the orienting of attention refers to the fact that visual attention is directed toward the location that the eyes are aiming for. However, the question remains open as to whether saccades and attention are two independent processes that can be directed concurrently toward a common goal, or whether their relationship is tighter, with the motor components of the saccade program influencing the selection of the position towards which visual attention is directed. To investigate this issue, an experiment was carried out in which the initial saccade goal was dissociated from the final executed motor vector. This was done by using a saccadic adaptation paradigm and a discrimination task. Results showed that best perceptual performance, which is taken to be an indicator of the locus of visual attention, followed the motor modifications arising from saccadic adaptation. This suggests that visual attention is directed toward the actual saccade landing position and that the perceptual system must have access to information regarding the motor vector before saccade execution.

Are There Any Left-Right Asymmetries in Saccade Parameters? Examination of Latency, Gain, and Peak Velocity

PURPOSE Hemispheric specialization in saccadic control is still under debate. Here we examine the latency, gain, and peak velocity of reactive and voluntary leftward and rightward saccades to assess the respective roles of eye and hand dominance. METHODS Participants with contrasting hand and eye dominance were asked to make saccades toward a target displayed at 5°, 10°, or 15° left or right of the central fixation point. In separate sessions, reactive and voluntary saccades were elicited by Gap-200, Gap-0, Overlap-600, and Antisaccade procedures. RESULTS Left-right asymmetries were not found in saccade latencies but appeared in saccade gain and peak velocity. Regardless of the dominant hand, saccades directed to the ipsilateral side relative to the dominant eye had larger amplitudes and faster peak velocities. CONCLUSIONS Left-right asymmetries can be explained by naso-temporal differences for some subjects and by eye dominance for others. Further investigations are needed to examine saccadic parameters more systematically in relation to eye dominance. Indeed, any method that allows one to determine ocular dominance from objective measures based on saccade parameters should greatly benefit clinical applications, such as monovision surgery.

Decision and metrics of refixations in reading isolated words

Eye movements were recorded during the reading of long words presented in isolation. Overall, the decision to refixate was found to depend on both length and frequency of the word, while refixation amplitude depended only on word length. This finding corroborates the assumption that most refixation saccades are preplanned on the basis of the parafoveal word length. However, cancellation of such a plan is possible and could be linked to the lexical processing during the first fixation into the word. Finally, a small proportion of refixations are corrective saccades, related to an oculomotor error. Theoretical implications for models of eye movement control during reading are discussed.

Visual Versus Motor Vector Inversions in the Antisaccade Task: A Behavioral Investigation With Saccadic Adaptation

In the antisaccade task, subjects must execute an eye movement away from a visual target. Correctly executing an antisaccade requires inhibiting a prosaccade toward the visual target and programming a movement to the opposite side. This movement could be based on the inversion of the visual vector, corresponding to the distance between the fixation point and the visual target, or the motor vector of the unwanted prosaccade. We dissociated the two vectors by means of saccadic adaptation. Adaptation can be observed when systematic targeting errors are caused by the displacement of the visual target during the saccade. Adaptation progressively modifies saccade amplitude (defined by the motor vector) such that it becomes appropriate to the postsaccadic stimulus position and thus different from the visual vector of the target. If antisaccade preparation depended on visual vector inversion, rightward prosaccade adaptation should not transfer to leftward antisaccades (which are based on the same visual vector) but should transfer to rightward antisaccades (which are based on a visual vector inside the adaptation field). If antisaccade preparation depended on motor vector inversion, rightward prosaccade adaptation should transfer to leftward antisaccades (which are based on the same, adapted motor vector) but should not transfer to rightward antisaccades (which are based on a nonadapted motor vector). The results are in line with the first hypothesis, showing that vector inversion precedes saccadic adaptation and suggesting that antisaccade preparation depends on the inversion of the visual target vector.

The use of recurrent signals about adaptation for subsequent saccade programming depends on object structure

Executing sequences of accurate saccadic eye movements supposes the use of signals carrying information about the first saccade for updating the predetermined motor plan of the subsequent saccades. The present study examines the signals used in planning a second saccade when subjects made two successive saccades towards one long or two short peripheral objects displayed before the first saccade execution. Different first eye movement signals could be used: desired eye movement signals, representing the movement necessary for attaining the intended target, or actual eye movement signals, representing the movement actually executed. Experimental dissociation of desired and actual eye movement signals is made possible by adaptive modifications of the first saccade, obtained by transfer of single saccade adaptation, during which the motor vector was progressively modified in response to the systematic intra-saccadic step of a single target. Whether the second saccade used the actual eye movement signal to compensate or not for the adaptive changes in the first saccade depended on which object properties were relevant for saccade planning. Compensation was observed for saccades that aimed for a new object (between-object saccades) because adaptation modifies relative object location. No compensation was observed for saccades that explored an extended object (within-object saccades). Implications for the on-line control of subsequent eye movements are discussed.

Saccadic adaptation depends on object selection: Evidence from between- and within-object saccadic eye movements

The accuracy of saccadic eye movements is maintained by saccadic adaptation. Post-saccadic visual feedback about the error between the target position and the saccade endpoint is crucial to the adaptive process. The present experiments examine the adaptation of saccades that select a new target object (between-object saccades) and that of saccades that would not aim for a selected target but execute a fixed motor vector (within-object saccades). We show that the post-saccadic visual error, induced by the intra-saccadic back step, leads to the adaptation of between-object saccades but not of within-object saccades. Furthermore, between-object saccade adaptation does not transfer to within-object saccades. These results suggest that saccadic adaptation depends on the selection of a precise target object.