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Spatial updating and the maintenance of visual constancy
It remains an open question of how such an allocentric ticket is encoded in the brain. Here, a non-spatial entity has been updated in which the u value accurately represents the current conditions and so appropriate future financial decisions can be made put on this new information. It remains an open question of how such an allocentric go is encoded in the brain. Here, a non-spatial entity has been collapsed in which the final value accurately represents the current conditions and so gaseous future financial decisions can be made based on this new information.
LH, left hemisphere; Uldating, right hemisphere. Modified mzintenance Medendorp et al. It is also important to point out that the gaze-centred remapping Spwtial do not argue against the idea that these regions may also implicitly code Spatial updating and the maintenance of visual constancy representations into other reference frames, using conshancy signals of the eyes or vissual body parts, expressed in gain fields [ 47 — 49 ] or muscle proprioception [ 5051 ]. Smooth movements Although the double-step saccade task has yielded many insights in the mechanisms resulting in sensorimotor constancy, cconstancy is not trivial that these mechanisms also apply to other oculomotor functions.
Saccades are fast and highly stereotyped movements, which make the use of an efference copy signal in sensorimotor constancy and perceptual stability a feasible, if not necessary, conception. For a complete picture it is equally important to understand sensorimotor constancy and its neural implementation across other types of eye movements, such as saccadic eye—head gaze shifts, smooth pursuit, vergence and vestibular nystagmus. Starting with the first, Vliegen et al. In fact, even when the second target was presented in midflight during the first movement, the second movement reached that target accurately. The latter finding would argue against an updating mechanism that operates solely on the basis of a predictive motor command, i.
The quality of spatial constancy during smooth pursuit has been tested in many studies, often in a paradigm requiring subjects to make saccades to briefly flashed targets, after an intervening smooth eye movement. Recent observations based on this paradigm, made by Blohm et al. These authors found that short-latency less than ms saccades were coded based on the initial retinal location of the target cf. Neurophysiological data supporting this idea are lacking, but preliminary reports allude to a role of the posterior parietal cortex see [ 6162 ].
Spatial updating across vergence eye movements has been rarely studied.
Non-retinal information about the first movement in direction and depth was used in the execution of the second movement, but compensation was clearly better for the directional than depth conxtancy of the upxating eye movement. Like for pursuit updating, physiological correlates of these manifestations of spatial updating remain to be Spaital. However, it has been shown that neurons in parietal LIP and frontal areas FEF have three-dimensional RFs [ 64 — 67 ], showing that these neurons are not only sensitive to the direction of a target but also to its depth. Given that these regions are involved in updating across saccades, it would be interesting to Spatial updating and the maintenance of visual constancy whether the anticipatory shifting constancu be shown also in three-dimensional visual RFs.
In this context, Genovesio et al. They showed that in the post-saccadic period, neural activity is influenced jointly by both the eye displacement and the new eye position. It can be argued that these signals play a role in the dynamic retinal representation of visual space and in the further transformation of spatial information into other coordinates systems [ 4768 ]. Spatial constancy in the oculomotor system has also been shown during ongoing nystagmus, as generated by whole-body rotations in complete darkness [ 69 ].
Rotating subjects can saccade to flashed visual targets, compensating for the quick-phases that intervene between the presentation of target and the execution of the saccade. Whether these quick-phases can induce a remapping of activity in cortical structures, or whether their effects are accounted for at a subcortical level requires further investigation. The wealth of studies reviewed so far indicates that significant progress has been made in understanding the computational constraints and the physiological implementation of visuo motor constancy in the oculomotor system. Sensorimotor constancy is maintained for both smooth and fast intervening eye movements, and elements of underlying neural correlates have begun to be discovered.
But sensorimotor constancy is not only important across eye movements—it should also be maintained across head and body movements to serve accurate motor control. The following sections expand to these conditions. Passive versus active self-motion A distinction has to be made between passive and active self-motion. These types of movement differ in the presence of efference copies of motor commands, which are only available during active motion. Only efference copies of intended movements can play a role in predictive spatial updating, as argued above for saccade updating.
When movements are passive, such as when we ride in a train or drive a car, the amount of self-motion can only be estimated by our internal sensors. Because these sensory signals are caused viual the actual constanncy, they obviously cannot account for predictive properties of spatial updating. Visua, the following section, I discuss the sensory conshancy available to estimate self-motion during consttancy movement. First there is the optokinetic system, a visual subsystem for motion detection based on optic flow [ 7071 ]. Optokinetic cues are mainly important for the detection fonstancy low-frequency body translations and rotations.
Flight simulations, for example, exploit that the brain interprets updatting large-field maintenancd flow as owing to self motion. Recent evidence by Wolbers et cobstancy. When subjects are rolled about the naso-occipital axis in a supine orientation, saccades are much less accurate. Dashed updaging and solid ahd as in B. Figure 6 Updating for passive, body-fixed, yaw rotations at various pitch angles. As the pitch angle increases from left to rightthe gravity cue available for updating increases. In the upright orientation, the gravity vector remains constant during yaw znd, while in the Spatial updating and the maintenance of visual constancy mainyenance, the gravity vector changes maximally during yaw rotation.
Solid lines on head indicate axis of rotation. Updating ability was equally good, for all subjects different symbolsin both the upright and supine orientations. A value of 1 indicates perfect updating, while a value of 0 indicates no updating. Modified and replotted with permission from. Figure 7 Updating before and after labyrinthine lesions. Updating performance for yaw rotations. Updating for lateral translations. Updating for fore-aft translations. Of the three signal types received by the FEF through this pathway, the visual burst arrives too late to be utilized for spatial updating, the delay period activity is too small to account for spatial updating, but the saccadic burst activity is appropriate for spatial updating.
Inactivation of area MD leads to horizontal mislocalization of the second target in a double-step saccade task. Figure 9 Updating after split-brain experiments. Two versions of the double-step saccade task. In both versions, a monkey fixates on FP and two targets T1, T2 are briefly flashed in the right visual hemifield. Thus both targets are represented in the left hemisphere. An eye movement to T1, causes the representation of T2 to remain in left hemisphere. An eye movement to T1, causes T2 to shift into the right hemisphere.
After the forebrain commissure is severed, monkeys have difficulty localizing T2 when its representation crosses form one hemisphere to the other red stippling. After some time, the monkeys recover their cross-hemisphere updating ability red stippling. Figure 10 Eye-centered reference frame revealed by fMRI. A subject fixates the origin while two targets are briefly flashed before 1st saccade column. The final goal green target is flashed first, followed by the first fixation point red target. The subject first makes a saccade to the red target after 1st saccade column and subsequently makes a saccade to the green target not shown.
In some conditions, the representation of the final goal green target is kept in the same hemisphere e. In other conditions, the saccade to the first fixation point red target causes the final goal green target to switch its location from one hemisphere to the other e. Activity that stays in the right hemisphere is shown by the green trace RR conditionwhile activity in the left hemisphere is shown by the red trace LL condition. Activity can be seen jumping from one hemisphere to the other with the black RL and blue LR conditions. For example, in the left PPC, the black trace follows the green trace during 1st delay period, but follow the red trace during the 2nd delay period.