Saccadic suppression of image displacement

Adaptive changes to saccade amplitude and target localization do not require pre-saccadic target visibility

The accuracy of saccadic eye movements is maintained by saccadic adaptation, a learning mechanism that is proposed to rely on visual prediction error, i.e., a mismatch between the pre-saccadically predicted and post-saccadically experienced position of the saccade target. However, recent research indicates that saccadic adaptation might be driven by postdictive motor error, i.e., a retrospective estimation of the pre-saccadic target position based on the post-saccadic image. We investigated whether oculomotor behavior can be adapted based on post-saccadic target information alone. We measured eye movements and localization judgements as participants aimed saccades at an initially invisible target, which was always shown only after the saccade. Each such trial was followed by either a pre- or a post-saccadic localization trial. The target position was fixed for the first 100 trials of the experiment and, during the following 200 trials, successively shifted inward or outward. Saccade amplitude and the pre- and post-saccadic localization judgements adjusted to the changing target position. Our results suggest that post-saccadic information is sufficient to induce error-reducing adaptive changes in saccade amplitude and target localization, possibly reflecting continuous updating of the estimated pre-saccadic target location driven by postdictive motor error.

Mislocalization after inhibition of saccadic adaptation

Saccadic eye movements are often imprecise and result in an error between expected and actual retinal target location after the saccade. Repeated experience of this error produces changes in saccade amplitude to reduce the error and concomitant changes in apparent visual location. We investigated the relationship between these two plastic processes in a series of experiments. Following a recent paradigm of inhibition of saccadic adaptation, in which participants are instructed to look at the initial target position and to continue to look at that position even if the target were to move again, our participants nevertheless perceived a visual probe presented near the saccade target to be shifted in direction of the target error. The location percept of the target gradually shifted and diverged over time from the executed saccade. Our findings indicate that changes in perceived location can be the same even when changes in saccade amplitude differ according to instruction and can develop even when the amplitude of the saccades executed during the adaptation procedure does not change. There are two possible explanations for this divergence between the adaptation states of saccade amplitude and perceived location. Either the intrasaccadic target step might trigger updating of the association between pre- and post-saccadic target positions, causing the localization shift, or the saccade motor command adjusts together with the perceived location at a common adaptation site, downstream from which voluntary control is exerted upon the executed eye movement only.

Contextual saccade adaptation induced by sequential saccades

Saccade adaptation is the gradual adjustment of saccade end point to maintain spatial accuracy. Contextual adaptation refers to a situation in which the adaptation-related change in saccade end point is contingent on the behavioral context in which the saccade is made. For example, in some situations, the same saccade to the same retinotopic location can be simultaneously adapted in opposite directions depending on the context in which it is made. Saccade adaptation has traditionally been studied in isolated movements, but in everyday life, saccades are often planned and executed in sequences. The oculomotor system may therefore have adaptive mechanisms specific to sequential saccades. Here, in five experiments, we investigated contextual saccade adaptation in sequences of saccades. In the first experiment, we demonstrate that saccades to a given retinotopic location can be simultaneously adapted in opposite directions depending on whether they occur in isolation or in a sequence. In the other experiments, we measured the extent to which properties of the previous and following saccades in a sequence can induce contextual saccade adaptation. Overall, we find that the existence, direction, and amplitude of previous and subsequent saccades, as well as the order of the current saccade within a movement sequence, can all induce contextual adaptation. These novel findings demonstrate the surprising flexibility of the system in maintaining end point accuracy, and support the idea that saccades made in a movement sequence are planned concurrently rather than independently.NEW & NOTEWORTHY This study reveals a new type of contextual saccade adaptation: sequential saccades are able to induce contextual saccade adaptation when direction, amplitude, or the existence of preceding and following saccades are used as contexts. These novel findings are also consistent with the idea that saccades made in a sequence are planned concurrently rather than independently.

Saccade Adaptation and Visual Uncertainty

Visual uncertainty may affect saccade adaptation in two complementary ways. First, an ideal adaptor should take into account the reliability of visual information for determining the amount of correction, predicting that increasing visual uncertainty should decrease adaptation rates. We tested this by comparing observers' direction discrimination and adaptation rates in an intra-saccadic-step paradigm. Second, clearly visible target steps may generate a slower adaptation rate since the error can be attributed to an external cause, instead of an internal change in the visuo-motor mapping that needs to be compensated. We tested this prediction by measuring saccade adaptation to different step sizes. Most remarkably, we found little correlation between estimates of visual uncertainty and adaptation rates and no slower adaptation rates with more visible step sizes. Additionally, we show that for low contrast targets backward steps are perceived as stationary after the saccade, but that adaptation rates are independent of contrast. We suggest that the saccadic system uses different position signals for adapting dysmetric saccades and for generating a trans-saccadic stable visual percept, explaining that saccade adaptation is found to be independent of visual uncertainty.

What muscle variable(s) does the nervous system control in limb movements?

To control force accurately under a wide range of behavioral conditions, the central nervous system would either require a detailed, continuously updated representation of the state of each muscle (and the load against which each is acting) or else force feedback with sufficient gain to cope with variations in the properties of the muscles and loads. The evidence for force feedback with adequate gain or for an appropriate central representation is not sufficient to conclude that force is the major controlled variable in normal limb movements.

Morton's hypothesis, that length is controlled by a follow-up servo, has a number of difficulties related to the delays, gains, variability, and specificity in feedback pathways comprising potential servo loops. However, experimental evidence is consistent with these pathways providing servo assistance for some movements produced by coactivation of α- and static γ-motoneurons. Dynamic γ-motoneurons may provide an additional input for adaptive control of different types of movements.

The idea that feedback is used to compensate for changes in muscle stiffness has received experimental support under static postural conditions. However, reflexes tend to increase rather than decrease the range of variation in muscle stiffness during some cyclic movements. Theoretical problems associated with the regulation of stiffness are also discussed. The possibilities of separate control systems for velocity or viscosity are considered, but the evidence is either negative or lacking. I conclude that different physical variables can be controlled depending on the type of limb movement required. The concept of stiffness regulation is also useful under some conditions, but should probably be extended to the regulation of the visco-elastic properties (i.e., the mechanical impedance) of a muscle or joint.

Apparent motion during saccadic suppression periods

Sensitivity to many visual stimuli, and, in particular, image displacement, is reduced during a change in fixation (saccade) compared to when the eye is still. In these experiments, we studied the sensitivity of observers to ecologically relevant image translations of large, complex, real world scenes either during horizontal saccades or during fixation. In the first experiment, we found that such displacements were much less detectable during saccades than during fixation. Qualitatively, even when trans-saccadic scene changes were detectable, they were less salient and appeared slower than equivalent changes in the absence of a saccade. Two further experiments followed up on this observation and estimated the perceived magnitude of trans-saccadic apparent motion using a two-interval forced-choice procedure (Experiment 2) and a magnitude estimation procedure (Experiment 3). Both experiments suggest that trans-saccadic displacements were perceived as smaller than equivalent inter-saccadic displacements. We conclude that during saccades, the magnitude of the apparent motion signal is attenuated as well as its detectability.

Parietal encoding of action in depth

The posterior parietal cortex is a crucial node in the process of coordinates transformation for the visual control of eye and hand movements. This conviction stems from both neurophysiological studies in the behaving monkey and from the analysis of the consequences of parietal lobe lesions in humans. Despite an extensive literature concerning varying aspects of the composition and control of eye and hand movements, there is little information about the physiological processes responsible for encoding target distance and hand movement in depth or about their control and impairment in parietal patients. This review is an attempt to provide a comprehensive picture from the fragmentary material existing on this issue in the literature. This should serve as a basis for discussion of what we consider to be a prototypical function of the dorsal visuomotor stream in the primate brain, that of encoding eye and hand movement in depth.

Postsaccadic activities in the posterior parietal cortex of primates are influenced by both eye movement vectors and eye position

Primates explore their visual environment by redirecting the gaze to objects of interest by alternating eye movements and periods of steady fixation. During this task, the fixation point changes frequently in depth. Therefore, the representation of object location based on retinal disparity requires frequent updating. Neural activity was recorded in the lateral intraparietal (LIP) area while monkeys performed saccades between targets in different depths. We report that in the early postsaccadic period, posterior parietal neurons continue to encode the difference in depth between fixation point and targets. About one-third of these neurons are, during the same period, modulated by eye position in depth as well. In the late postsaccadic period, the influence of the previous movement vector dissipates, and parietal neurons are modulated only by the new fixation distance. This result suggests that the postsaccadic activity of area LIP contributes to the dynamic representation of the visual space, and it is compatible with the presence of both a vector subtraction computation and eye-position-dependent gain fields.

Maintenance of Visual Stability in the Human Posterior Parietal Cortex

Visual stability refers to our stable visuospatial perceptions despite the unstable visual input caused by saccades. Functional neuroimaging results, studies on patients with posterior parietal cortex (PPC) lesions, and single-unit recordings in the lateral intraparietal sulcus of primates indirectly suggest that the PPC might be a potential locus of visual stability through its involvement with spatial remapping. Here we directly explored the role of the PPC in visual stability by applying transcranial magnetic stimulation (TMS) while participants performed a perisaccadic displacement detection task. We show that TMS over the PPC but not a frontal control site alters sensitivity to displacement detection when administered just before contralateral saccades and that a general impairment in attention or in the perception of apparent motion cannot account for the decreased sensitivity. The specific relationship between the timing of TMS and saccade direction demonstrates that saccadic suppression of displacement (SSD) is likely a consequence of noisy contralateral spatial representations in the PPC around the time of a saccade. The same mechanism may keep the unstable visual world in the temporal proximity of saccades from reaching our consciousness.

Saccadic suppression of displacement: separate influences of saccade size and of target retinal eccentricity

The threshold for detection of displacements of visual objects is higher during voluntary saccades than it is during steady gaze ("saccadic suppression of displacement"; SSD). Relative contributions to SSD of extraretinal and retinal factors were investigated by measuring displacement thresholds in four experiments in which three observers judged whether a test flash, presented after a saccade or a period of fixation, was located to the left or right of a reference point viewed earlier. The experiments, involving saccades ranging from 4 to 12 deg in length, separated the effects of saccade size from the effects of retinal eccentricity of the reference point, and also separated the effects of retinal eccentricity of the test flash from both. The influences of the three are nearly linearly independent. Approximately 20% of the total influence on SSD derives from retinal influences of test flash and reference point; 80% is due to extraretinal influence associated with saccade size. A signal/noise model that accounted well for our previous on SSD (Li & Matin, 1990a,b) was extended to account for the present results. The model also provides a unified treatment of SSD and of the saccadic suppression of visibility (SSV).

Saccadic suppression relies on luminance information

To determine whether saccadic suppression of image displacement uses information from luminance channels, we measured spatial displacement detection thresholds with equiluminant and non-equiluminant targets during saccades. We compared these saccadic thresholds with displacement thresholds measured during fixation by making ratios of saccadic thresholds to fixation thresholds. Ratios were lower in the equiluminant condition than in the non-equiluminant. This surprising result indicates that detection of equiluminant target displacements during saccades was better than detection of nonequiluminant targets, compared with the detection abilities during fixation. Thus, saccadic suppression of image displacement, which should increase displacement thresholds during saccades over fixation thresholds, was more effective with nonequiluminant targets. Because of target flicker, displacement thresholds were anisotropic in the nonequiluminant condition; thresholds were greater when target and eye moved in the same direction than when they moved in opposite directions, consistent with earlier results. These two effects (flicker-induced anisotropy and greater suppression in nonequiluminance) canceled when the eye moved opposite the displacement, yielding equal thresholds and summed when eye and target moved in the same direction, yielding large threshold differences. We conclude that saccadic suppression of image displacement uses mechanisms sensitive to luminance contrast.

Visual stability with goal-directed eye and arm movements toward a target displaced during saccadic suppression

This experiment tested whether the perceived stability of the environment is altered when there is a combination of eye and visually open-loop hand movements toward a target displaced during the eye movements, i.e., during saccadic suppression. Visual-target eccentricity randomly decreased or increased during eye movements and subjects reported whether they perceived a target displacement or not, and if so, the direction of the displacement. Three experimental conditions, involving different combinations of eye and arm movements, were tested: (a) eye movements only; (b) simultaneous eye and rapid arm movements toward the target; and (c) simultaneous eye and arm movements with a restraint blocking the arm as soon as the hand left the starting position. The perceptual threshold of target displacements resulting in an increased target eccentricity was greater when subjects combined eye and arm movements toward the target object, specially for the no-restraint condition. Subjects corrected most of their arm trajectory toward the displaced target despite the short movement times (average MT = 189 ms). After the movements, the null error feedback of the hand's final position presumably overlapped the retino-oculomotor signal error and could be responsible for the deficient perception of target displacements. Thus, subjects interpreted the terminal hand positions as being within the range of the endpoint variability associated with the production of rapid arm movements rather than as a change of the environment. These results suggest that a natural strategy adopted for processing spatial information, especially in a competing situation, could favour a constancy tendency avoiding systematic perception of a change of environment for any noise or variability at the central or peripheral levels.

Motion perception during saccades

Although the retinal image is displaced by each saccade performed we do not perceive the visual environment moving concordant with the saccades. In this study experiments were designed in which additional movement of most of the visual scene was applied during saccades. The subjects perceived the intrasaccadic movement after the saccade. The perceived speed of this movement was decreased and the threshold amplitude was increased compared to perception during fixation. The intrasaccadic movement perception was based on a novel aftereffect of motion perception. The velocity of retinal slip did not affect the threshold. If the retinal slip speed during saccades was temporally reduced by an intrasaccadic movement parallel to the saccade, the threshold amplitude was identical to the threshold amplitude obtained by intrasaccadic movement opposite to the saccade increasing retinal slip speed. Horizontal intrasaccadic movements were detected at lower thresholds than vertical movements independent of saccade direction. In addition, the thresholds were not effected by the saccade amplitude suggesting that neither speed, duration, nor direction of eye movement related retinal slip affects the amount of suppression. Our results suggest that saccadic suppression is related to delayed central processing of retinal information during saccades. This processing does not involve saccade parameters such as direction and amplitude.

Flicker distorts visual space constancy

Effects of flicker on space perception were measured by displacing a flickering target during saccadic eye movements. A small target was flickered at 33, 66, 130 or 260 Hz. Using a 2-interval forced-choice design, sensitivity to the displacement was about twice as great when the target was moved in the direction opposite the eye movement as when it was moved in the same direction. This would be expected from a partial breakdown of space constancy--the world should seem to jump in the direction opposite an eye movement. Even if a suppression of displacement detection during saccades prevents this jump from being perceived; it should be easier to detect a target displacement in the direction opposite the eye movement than in the same direction: when movement is opposite, the imposed displacement adds to the illusory displacement, making detection easier. Displacements were more easily detected at lower flicker rates. Results imply that both masking and extraretinal signals are important in suppressing the detectability of target displacements during saccades, and that flicker on video display terminals may distort space perception.