Experimental Brain Research

Ellis, R. R.; Flanagan, J. Randall; Lederman, Susan J.

doi: 10.1007/s002210050665pmid: 10204763

Visual size illusions have been shown to affect perceived object size but not the aperture of the hand when reaching to those same objects. Thus, vision for perception is said to be dissociated from vision for action. The present study examines the effect of visual-position and visual-shape illusions on both the visually perceived center of an object and the position of a grasp on that object when a balanced lift is required. The results for both experiments show that although the illusions influence both the perceived and the grasped estimates of the center position, the grasp position is more veridical. This partial dissociation is discussed in terms of its implications for streams of visual processing.

Pellizzer, Giuseppe; Richter, Hans; Georgopoulos, A. P.

doi: 10.1007/s002210050666pmid: 10204764

Six human subjects were asked to draw ellipses presented on a screen by moving a manipulandum that controlled the position of a cursor. Six visual templates were used, which comprised three different ellipses displayed either horizontally or vertically; the ratio between the major and minor axes was 2, 4, or 5. For each visual template, the gains were set such that the movement trajectories required to trace the template with the cursor corresponded to one of six ellipses. Thus these movement ellipses were horizontal or vertical with a ratio between major and minor axes of 2, 4, or 5. All 36 combinations of six visual ellipses and six required movement ellipses were used. Therefore, in some conditions the required movement ellipse had a different orientation (with respect to the major axis) than the visual template. These conditions were called orientation incongruent, whereas, when the orientation of the required movement ellipse matched the orientation of the visual template, the conditions were called orientation congruent. Similarly, eccentricity incongruent referred to conditions where the eccentricities of the visual ellipse and the required movement ellipse were different, as opposed to eccentricity congruent. The main results were as follows: (a) The perimeter of the traced ellipse always tended to be larger than that of the visual template. In addition, it was significantly larger in the orientation incongruent conditions than in the orientation congruent conditions. Nevertheless, the perimeter of the traced figure increased with the template in both orientation congruent and incongruent conditions. (b) The shape of the traced figure varied appropriately with the visual template, but differed significantly between the orientation congruent and incongruent conditions. It was closer to the one of the template in the orientation congruent than in the incongruent conditions. Finally, (c) the instantaneous speed was significantly correlated with curvature but more tightly so in the orientation congruent than in the orientation incongruent conditions. The parameters defining the relation between speed and curvature were affected by the required movement ellipse, but not by the particular visuomotor condition. These results showed that although spatial motor performance was affected by changes in the correspondence between visual and movement coordinates, the relation between the speed and curvature of the movement trajectory was stable despite drastic changes in this correspondence.

McGraw, P. V.; Whitaker, David

doi: 10.1007/s002210050667pmid: 10204765

The visual mechanism by which human observers determine the separation between objects has long been of interest. This study examines the extent to which separation in visual space can be misperceived in foveal and extrafoveal vision. Foveally, vertical separations were consistently overestimated relative to horizontal separations, a result which is consistent with the well-documented horizontal-vertical illusion (HVI). Extrafoveally, much larger misrepresentations of visual space were perceived. In addition, separations tangential to fixation were consistently perceived as being greater than separations in a radial direction. These marked misperceptions of visual space which occur in extrafoveal vision take the form of a radial/tangential anisotropy combined with an overestimation of vertical distance. The results have important implications for meridional anisotropies which have previously been documented in a number of visual performance tasks.

Wada, N.; Shikaki, N.

doi: 10.1007/s002210050668pmid: 10204766

We studied neuronal pathways for spinal reflexes activated by group-I and group-II muscle afferents in the spinal segments innervating the tail in unanesthetized and spinalized (L1) cats. Experiments were performed on 25 adult cats of both sexes. The effects of stimulating nerves innervating six tail muscles on both sides were recorded from tail motoneurons in the first coccygeal spinal segment (Co1) using glass microelectrodes. Stable recordings were obtained from 150 tail motoneurons. Stimulation of group-I muscle afferents (stimulus intensity <1.8 T) often produced EPSPs (82/150) after stimulating nerves innervating neighboring tail muscles. Motoneurons innervating the long-tendoned muscles, M. extensor caudae lateralis and M. flexor caudae longus (ECL and FCL), received heteronymous monosynaptic connections from group-I muscle afferents innervating the ipsilateral tail muscles. The motoneurons innervating segmental muscles, M. extensor caudae medialis and M. flexor caudae brevis (ECM and FCB), received heteronymous monosynaptic connections from group-I muscle afferents innervating tail muscles on both sides. The motoneurons innervating tail muscles originated from the Ossa coxae, M. abductor caudae externus and M. abductor caudae internus (ACE and ACI), received monosynaptic connection from group-I muscle afferents innervating most of the tail muscles on both sides. Crossed disynaptic inhibitory pathways activated by primary muscle afferent inputs were observed in ECM, ACE, FCL, and FCB motoneurons. The effects of group-II afferent inputs were not dependent on the kind of motoneuron, and alternative excitatory and inhibitory pathways were not clearly observed in the tail motoneuron pool. It is suggested that variability of the neuronal pathways from group-I and -II muscle afferents to tail motoneurons corresponds to functional relationships among tail muscles, depending on the tail movements.

Messier, Julie; Kalaska, J. F.

doi: 10.1007/s002210050669pmid: 10204767

The accuracy of reaching movements to memorized visual target locations is presumed to be determined largely by central planning processes before movement onset. If so, then the initial kinematics of a pointing movement should predict its endpoint. Our study examined this hypothesis by testing the correlation between peak acceleration, peak velocity, and movement amplitude and the correspondence between the respective spatial positions of these kinematic landmarks. Subjects made planar horizontal reaching movements to targets located at five different distances and along five radially arrayed directions without visual feedback during the movements.The spatial dispersion of the positions of peak acceleration, peak velocity, and endpoint all tended to form ellipses oriented along the movement trajectory. However, whereas the peaks of acceleration and velocity scaled strongly with movement amplitude for all of the movements made at the five target distances in any one direction, the correlations with movement amplitude were more modest for trajectories aimed at each target separately. Furthermore, the spatial variability in direction and extent of the distribution of positions of peak acceleration and peak velocity did not scale differently with target distance, whereas they did for endpoint distributions. Therefore, certain features of the final kinematics are evident in the early kinematics of the movements as predicted by the hypothesis that they reflect planning processes. However, endpoint distributions were not completely predetermined by the Initial kinematics. In contrast, multivariate analysis suggests that adjustments to movement duration help compensate for the variability of the initial kinematics to achieve desired movement amplitude. These compensatory adjustments do not contradict the general conclusion that the systematic patterns in the spatial variability observed in this study reflect planning processes. On the contrary, and consistent with that conclusion, our results provide further evidence that direction and extent of reaching movements are planned and determined in parallel over time.

Hay, L.; Redon, C.

doi: 10.1007/s002210050670pmid: 10204768

Any action performed by standing subjects is generally accompanied by compensatory postural activities, which reduce or abolish the postural disturbance generated by the movements and keep the subjects’ center of gravity within the supporting base. These postural activities are triggered by either anticipatory and/or feedback-based control processes, depending on the information available and on the behavioral context. To investigate the respective involvement of these two components in postural control during development, we studied the extent to which the postural equilibrium of children (3- to 10-year-olds) and adults was disturbed by the same physical event, an unloading, depending on whether it was initiated by the subject or externally imposed. The subjects were standing on a force platform with their eyes closed, holding a load (5% of their own body weight) in their hands, with arms vertical and forearms horizontal. Two conditions were applied: (1) the subjects voluntarily released the load and (2) the load was unpredictably removed. The unloading resulted in a backward movement of the center of pressure, which was smaller with self-initiated than imposed disturbances in all age groups. This difference varied depending mainly on the age-related changes in the relative amplitude of the self-initiated disturbance, which decreased between 3- to 5-, and 6- to 8-year-olds (who showed no marked postural instability after self-initiated unloading), and increased again in the two older groups (9- to 10-year-olds and adults), in which it also became more consistent . It was concluded that feedforward control becomes more efficient as children grow up, but that its relative contribution to postural control does not show a monotonic pattern of development.

Gu, WeiGang; Jiang, W.; Wester, P.

doi: 10.1007/s002210050671pmid: 10204769

In clinical thromboembolic stroke, spontaneous late recanalization is a common feature, but one which has been very sparsely studied experimentally. This study aimed at enabling the study of spontaneous reperfusion and exploring its consequences by modifying a recently developed photothrombotic-stroke model that focuses on the region-at-risk located within an ischemic ring-locus. The exposed crania of male Wistar rats (280–340 g) were subjected to a ring-shaped (5.0 mm outer diameter and 0.35 mm thick) laser-irradiation beam (514.5 nm; 0.89 W/cm 2 ) for 2 min simultaneously with intravenous erythrosin B (17 mg/kg) infusion for 30 s. Transcardial carbon-black perfusion experiments revealed a ring-shaped cortical perfusion deficit at 4 h post-irradiation, which progressively increased at 10, 24, and 48 h, at which time the whole region-at-risk was pale with single distal branches of the middle cerebral artery being extensively narrowed, but not occluded. At 72 h, spontaneous reperfusion was observed in the region-at risk, which was even more pronounced at 7 and 28 days. Cortical cerebral blood flow (cCBF), measured by laser-Doppler flowmetry, was distinctly reduced at 2 min post-irradiation and further decreased slightly during 4 h of recording to ca. 24% of baseline values at the ring locus and 40% in the region-at-risk. In the region-at-risk, cCBF flow values were 23–30% of the baseline at 24–48 h post-irradiation, followed by a relative cCBF increase to 71 and 77% at 72 and 96 h post-irradiation. Brain water content in the ischemic part of the cortex increased steadily from 4 to 48 h post-irradiation; at 72 h, it leveled off and returned to control values at 7 days. In conclusion, by employing a laser beam in the shape of a thin ring, critically sustained cCBF reduction was followed by late, consistent spontaneous reperfusion in the region-at-risk in this novel photochemically induced stroke-in-evolution model.

Gu, WeiGang; Brännström, Thomas; Jiang, W.; Wester, P.

doi: 10.1007/s002210050672pmid: 10204770

The photothrombotic ring stroke model with sustained underperfusion followed by late spontaneous reperfusion (Gu et al. 1999) was employed to study its morphological consequences. The exposed crania of adult male Wistar rats were subjected to a ring-shaped laser irradiation beam simultaneously with intravenous erythrosin B infusion. The ischemic volume was calculated from serial sections throughout the ischemic lesions at 4, 10, 24, 48, and 72 h and 7 days and 28 days after irradiation. The ischemic volume, expressed as a percentage of the ipsilateral hemispheric volume, increased steadily from 4 to 10 to 24 h to reach its maximum value at 48 h after irradiation; at 3 days, 7 days, and 28 days, the ischemic volume was reduced to 75%, 24%, and 22% of the value at 48 h. Evaluation of ischemic volumes at different anteroposterior levels revealed that the reduced ischemic volume at 72 h and later was mainly due to morphological restoration in the centrally located, nonirradiated region at risk. An initial enlargement and development of cystic coagulation necrosis was observed in the cortical areas corresponding to the ring lesion itself. In the region at risk, a gradually deteriorating neuropil and nerve cell morphology were observed over time, with maximum severity at 48 h postirradiation. At this time, most laminae II and III neurons in the region at risk exhibited eosinophilia and pyknosis but no incrustations, with small islands of less damaged neurons randomly scattered. At 72 h and up to 28 days after irradiation, these cell characteristics were no longer observed and the region at risk was well populated with neurons that had a chiefly unremarkable cytological appearance. Neuronal counts in the central part of the region at risk were performed; no significant difference in neuronal density was observed between sham-operated controls and at 28 days after irradiation. In conclusion, the consistent, late spontaneous reperfusion coincided with remarkable tissue recovery as assessed morphologically in the region at risk. The data suggest that nerve cell repair may occur even after the detection, by conventional morphological methods, of prolonged critical ischemic neuronal damage in the setting of acute ischemic stroke.

Martin, J. H.; Kably, Bouchra; Hacking, Antony

doi: 10.1007/s002210050673pmid: 10204771

Corticospinal (CS) axon terminations in several species are widespread early in development but are subsequently refined into a spatially more restricted distribution. We studied the role of neural activity in sensorimotor cortex in shaping postnatal development of CS terminations in cats. We continuously infused muscimol unilaterally into sensorimotor cortex to silence neurons during the postnatal CS refinement period (weeks 3–7). Using anterograde transport of WGA-HRP, we examined the laterality of terminations from the muscimol-infused (i.e., silenced) and active sides in the spinal cord, as well as in the cuneate nucleus and red nucleus. We found that CS terminations from the muscimol-infused cortex were very sparse and limited to the contralateral side, while those from the active cortex maintained an immature bilateral topography. Controls (saline infusion, noninfusion) had dense, predominantly contralateral, CS terminations. There was a substantial decrease in the spinal gray matter area occupied by terminations from the side receiving the blockade and a concomitant increase in the area occupied by ipsilateral terminations from the active cortex. Optical density measurements of HRP reaction product from the active cortex in muscimol-infused animals showed substantial increases over controls in the ratio of ipsilateral to contralateral CS terminations for all laminae examined (IV–V, VI, VII). Our findings suggest that ipsilateral dorsal horn terminations reflect new axon growth during the refinement period because they are not present there earlier in development. Those in the ventral horn are present earlier in development and thus could reflect maintenance of transient terminations. Increased ipsilateral terminations from active cortex were due to recrossing of CS axons in lamina X and not to an increase in labeled CS axons in the ipsilateral white matter. Examination of brain stem terminations suggested that, between postnatal weeks 3 and 7, development of corticocuneate terminations also is activity-dependent but that development of corticorubral terminations is not. Activity-dependent CS development is a plausible mechanism by which early motor experiences could shape the anatomical and functional organization of the motor systems during a critical postnatal period.

Adamovich, Sergei V.; Berkinblit, Michail B.; Fookson, Olga; Poizner, Howard

doi: 10.1007/s002210050674pmid: 10204772

The accuracy of visually guided pointing movements decreases with speed. We have shown that for movements to a visually defined remembered target, the variability of the final arm endpoint position does not depend on movement speed. We put forward a hypothesis that this observation can be explained by suggesting that movements directed at remembered targets are produced without ongoing corrections. In the present study, this hypothesis was tested for pointing movements in 3D space to kinesthetically defined remembered targets. Passive versus active acquisition of kinesthetic information was contrasted. Pointing errors, movement kinematics, and joint-angle coordination were analyzed. The movements were performed at a slow speed (average peak tangential velocity of about 1.2 m/s) and at a fast speed (2.7 m/s). No visual feedback was allowed during the target presentation or the movement. Variability in the final position of the arm endpoint did not increase with speed in either the active or the passive condition. Variability in the final values of the arm-orientation angles determining the position of the forearm and of the upper arm in space was also speed invariant. This invariance occurred despite the fact that angular velocities increased by a factor of two for all the angles involved. The speed-invariant variability supports the hypothesis that there is an absence of ongoing corrections for movements to remembered targets: in the case of a slower movement, where there is more time for movement correction, the final arm endpoint variability did not decrease. In contrast to variability in the final endpoint position, the variability in the peak tangential acceleration increased significantly with movement speed. This may imply that the nervous system adopts one of two strategies: either the final endpoint position is not encoded in terms of muscle torques or there is a special on-line mechanism that adjusts movement deceleration according to the muscle-torque variability at the initial stage of the movement. The final endpoint position was on average farther from the shoulder than the target. Constant radial-distance errors were speed dependent in both the active and the passive conditions. In the fast speed conditions, the radial distance overshoots of the targets increased. This increase in radial-distance overshoot with movement speed can be explained by the hypothesis that the final arm position is not predetermined in these experimental conditions, but is defined during the movement by a feedforward or feedback mechanism with an internal delay.