Why do movements take a characteristic amount of time, and whydo diseases that affect the reward system alter control of movements?Suppose that the purpose of any movement is to position ourbody in a more rewarding state. People and other animals discountfuture reward as a hyperbolic function of time. Here, we showthat across populations of people and monkeys there is a correlationbetween discounting of reward and control of movements. We considersaccadic eye movements and hypothesize that duration of a movementis equivalent to a delay of reward. The hyperbolic cost of thisdelay not only accounts for kinematics of saccades in adults,it also accounts for the faster saccades of children, who temporallydiscount reward more steeply. Our theory explains why saccadevelocities increase when reward is elevated, and why disordersin the encoding of reward, for example in Parkinson's diseaseand schizophrenia, produce changes in saccade. We show thatdelay of reward elevates the cost of saccades, reducing velocities.Finally, we consider coordinated movements that include motionof eyes and head and find that their kinematics is also consistentwith a hyperbolic, reward-dependent cost of time. Therefore,each voluntary movement carries a cost because its durationdelays acquisition of reward. The cost depends on the valuethat the brain assigns to stimuli, and the rate at which itdiscounts this value in time. The motor commands that move oureyes reflect this cost of time.
Oculomotor prediction of accelerative target motion during occlusion: long-term and short-term effects.
The present study examined the influence of long-term (i.e., between-trial) and short-term (i.e., within-trial) predictive mechanisms on ocular pursuit during transient occlusion. To this end, we compared ocular pursuit of accelerative and decelerative target motion in trials that were presented in random or blocked-order. Catch trials in which target acceleration was unexpectedly modified were randomly interleaved in blocked-order trials. Irrespective of trial order, eye velocity decayed following target occlusion and then recovered towards the different levels of target velocity at reappearance. However, the recovery was better scaled in blocked-order trials than random-order trials. In blocked-order trials only, the reduced gain of smooth pursuit during occlusion was compensated by a change in saccade amplitude and resulted in total eye displacement (TED) that was well matched to target displacement. Subsidiary analysis indicated that three repeats of blocked-order trials was sufficient for participants to modify eye displacement compared to that exhibited in random-order trials, although more trials were required before end-occlusion eye velocity was better scaled. Finally, we found that participants exhibited evidence of a scaled response to an unexpected change in target acceleration (i.e., catch trials), although there were also transfer effects from the preceding blocked-order trials. These findings are consistent with the suggestion that on-the-fly prediction (short-term effect) is combined with memorised information from previous trials (long-term effect) to generate a persistent and veridical prediction of occluded target motion.
Orban de Xivry J.-J., Coppe S, Lefevre P and Missal M, Biological motion drives perception and action, Journal of Vision, 10(2):6, 1-11, 2010.
Presenting a few dots moving coherently on a screen can yield to the perception of human motion. This perception is based on a specific network that is segregated from the traditional motion perception network and that includes the superior temporal sulcus (STS). In this study, we investigate whether this biological motion perception network could influence the smooth pursuit response evoked by a point-light walker. We found that smooth eye velocity during pursuit initiation was larger in response to the point-light walker than in response to one of its scrambled versions, to an inverted walker or to a single dot stimulus. In addition, we assessed the proximity to the point-light walker (i.e. the amount of information about the direction contained in the scrambled stimulus and extracted from local motion cue of biological motion) of each of our scrambled stimuli in a motion direction discrimination task with manual responses and found that the smooth pursuit response evoked by those stimuli moving across the screen was modulated by their proximity to the walker. Therefore, we conclude that biological motion facilitates smooth pursuit eye movements, hence influences both perception and action.
Smooth pursuit performance during target blanking does not influence the triggering of predictive saccades
Orban de Xivry J.-J., Missal M and Lefevre P, Smooth pursuit performance during target blanking does not influence the trigger of predictive saccades. Journal of Vision, 9(11):7, 1-16, 2009.
Visually guided catch-up saccades during the pursuit of a moving target are highly influenced by smooth pursuit performance. For example, the decision to execute a saccade and its amplitude is driven by the difference in velocity between the eye and the target. In previous studies, we have demonstrated that the predictive saccades that occur during the blanking of the moving target compensate for the variability of the smooth pursuit response. Therefore, we wondered whether the predictive smooth pursuit response during target blanking influenced the occurrence of predictive saccades, which is the case for visually guided catch-up saccades. To answer this question, we asked subjects to track visually a target moving along a circular path. From time to time, the target was unexpectedly blanked for some randomized durations and disappeared from the screen. Surprisingly, we did not find any differences in smooth pursuit performance between the blanks that did and those that did not contain predictive saccades. In addition, during the blanks, the differences in smooth pursuit performance across the sessions or across the subjects did not correlate with the differences in the number of predictive saccades. Therefore, this study demonstrates that smooth pursuit performance does not influence the occurrence of predictive saccades. We interpret these results in light of the possible minimization of position error at target reappearance, which heavily depends on the saccadic amplitudes but not on their timing.
Orban de Xivry J.-J., Lefevre P, Interactions between saccades and pursuit. Encyclopedia of Neuroscience (L.R. Squire, Editor), Oxford: Academic Press, 2009
Classically, saccadic and pursuit systems were considered largely independent.
However, on the basis of cat and primate studies, scientists accumulated evidence showing that both oculomotor systems are intimately linked. It was demonstrated that both saccadic and pursuit systems use position and velocity inputs and share common initiation, cancellation and target selection mechanisms. Moreover, neuronal populations in several cortical and sub-cortical brain areas were shown to convey both saccade and pursuit signals. All these results are discussed in the present chapter showing that saccade and pursuit systems are much more integrated than previously
Orban de Xivry J.-J., Missal M and Lefevre P. A dynamic internal representation of target motion drives predictive smooth pursuit during target blanking Journal of Vision, 8(15):6, 1-13, 2008
Moving objects are often occluded by neighboring objects. In order for the eye to smoothly pursue a moving object that is transiently occluded, a prediction of its trajectory is necessary. For targets moving on a linear path, predictive eye velocity can be regulated on the basis of target motion before and after the occlusions. However, objects in a more dynamic environment move along more complex trajectories. In this condition, a dynamic internal representation of target motion is required. Yet, the nature of such an internal representation has never been investigated. Similarly, the impact of predictive saccades on the predictive smooth pursuit response has never been considered. Therefore, we investigated the predictive smooth pursuit and saccadic responses during the occlusion of a target moving along a circular path. We found that the predictive smooth pursuit was driven by an internal representation of target motion that evolved with time. In addition, we demonstrated that in two dimensions, the predictive smooth pursuit system does influence the amplitude of predictive saccades but not vice versa. In conclusion, in the absence of retinal inputs, the smooth pursuit system is driven by the output of a short-term velocity memory that contains the dynamic representation of target motion.
Yuksel D, Orban de Xivry J.-J. and Lefevre P Binocular coordination of saccades in Duane Retraction Syndrome Vision Research 48(19): 1972-1979, 2008
Disconjugate oculomotor adaptation is driven by the need to maintain binocular vision. Since binocular vision in Duane Retraction Syndrome (DRS) patients is normal in half of their horizontal field of gaze (i.e., sound-side of gaze), we wondered whether oculomotor adaptive capabilities are efficient despite such a severe impairment of eye motility towards the other half of the horizontal field of gaze (i.e., affected-side gaze). We compared properties of horizontal saccades of patients with congenital unilateral Duane Retraction Syndrome type I in binocular viewing and monocular viewing conditions by simultaneously recording both eyes with the search coil technique. Our results show a mismatch between the pulse and the step signal of the innervation for saccades. When tested in the affected eye viewing condition (sound eye covered), the eyes showed not only similarly-directed increases of the saccadic gain (pulse signal) in the two eyes but also disjunctive post-saccadic drifts (step signal). This behavior suggests that visuomotor errors presented only to the affected eye were transferred to the sound eye, producing conjugate changes of the saccadic command. The post-saccadic command remained unchanged, however, and controlled the final position of each eye separately. This suggests that monocular adaptation is possible only for the step of innervation (i.e., controlling the final eye position) but not for the pulse of innervation (i.e., controlling the saccadic gain), even though the peculiarity of unilateral DRS type I offers a clear advantage for separate pathways of control for the two eyes.
Orban de Xivry J.-J. and Ethier V Neural correlates of internal models Journal of Neuroscience 28 (32),7931-7932, 2008
Orban de Xivry J.-J., Ramon M, Lefevre P, Rossion B Reduced fixation on the upper area of personally familiar faces following acquired prosopagnosia Journal of Neuropsychology 2 (1): 245-268, 2008. Here, you will find some explanation about prosopagnosia.
Selective impairment of face recognition following brain damage, as in acquired prosopagnosia, may cause a dramatic loss of diagnosticity of the eye area of the face and an increased reliance on the mouth for identification (Caldara et al., 2005). To clarify the nature of this phenomenon, we measured eye fixation patterns in a case of pure prosopagnosia (PS, Rossion et al., 2003) during her identification of photographs of personally familiar faces (27 children of her kindergarten). Her age-matched colleague served as a control. Consistent with previous evidence, the normal control identified the faces within two fixations located just below the eyes (central upper nose). This pattern (location and duration) of fixations remained unchanged even by increasing difficulty by presenting anti-caricatures of the faces. In contrast, the great majority of the patient's fixations, irrespective of her accuracy, were located on the mouth. Overall, these observations confirm the abnormally reduced processing of the upper area of the face in acquired prosopagnosia. Most importantly, the prosopagnosic patient also fixated the area of the eyes spontaneously in between the first and last fixation, ruling out alternative accounts of her behaviour such as, for example, avoidance or failure to orient attention to the eyes, as observed in autistic or bilateral amygdala patients. Rather, they reinforce our proposal of a high-level perceptual account (Caldara et al., 2005), according to which acquired prosopagnosic patients have lost the ability to represent multiple elements of an individual face as a perceptual unit (holistic face perception). To identify a given face, they focus very precisely on local features rather than seeing the whole of a face from its diagnostic centre (i.e. just below the eyes). The upper area of the face is particularly less attended to and less relevant for the prosopagnosic patient because it contains multiple features that require normal holistic perception in order to be the most diagnostic region. Consequently, prosopagnosic patients develop a more robust representation of the mouth, a relatively isolated feature in the face that may contain more information than any single element of the upper face area, and is thus sampled repeatedly for resolving ambiguity in the process of identification
Orban de Xivry J.-J. and Lefevre P. Saccades and pursuit: two outcomes of a single sensorimotor process J Physiol. 584(1): 11-23, 2007.
Saccades and smooth pursuit eye movements are two different modes of oculomotor control. Saccades are primarily directed toward stationary targets whereas smooth pursuit is elicited to track moving targets. In recent years, behavioural and neurophysiological data demonstrated that both types of eye movements work in synergy for visual tracking. This suggests that saccades and pursuit are two outcomes of a single sensorimotor process that aims at orienting the visual axis.
Enclosed is my list of publications. Each of them is presented in a separate "blog post" so that each of them can be commented. Only comments with name and valid email address will be posted. Messages such as "you'll never be a good scientists will be discarded :-) "