Evidence for high-velocity smooth pursuit in the trained cat

It is generally accepted that in cats smooth pursuit velocity of the eye never exceeds a few degrees per second. This is in contrast with observations in primates, where smooth pursuit velocity can reach values as high as 100°/s. Cats were trained to fixate and pursue spots of light appearing on a translucent screen. Spots were moved in the horizontal and vertical planes at different constant velocities up to 80°/s. Eye position was recorded with the sclerai search coil technique. Naive cats did not pursue moving targets with high efficiency. Smooth eye movement velocity saturated at 5°/s. After a few days of training, smooth-pursuit eye velocity increased with target velocity and saturated at 25°/s on average. However, velocities twice as high have been observed frequently. When the target was unexpectedly extinguished, smooth eye movement velocity dropped to values close to 0°/s in approximately 350 ms. After a short training period (usually 5 times the same target presentation), the eye continued to move smoothly until the target reappeared. These data suggest that smooth pursuit eye movements of the cat are qualitatively similar to those of primates, but reach lower velocities and are more variable in their characteristics.     

Suppression of charge movement by calcium antagonists is not related to calcium channel block

The calcium channel-inhibiting drugs nitrendipine and diltiazem represent two important classes of organic calcium antagonists. In the present study, the effect of these drugs on calcium currents and charge displacement currents in bullfrog semitendinosus muscle fibers was examined using a vaseline gap voltage clamp. Nitrendipine (10 M) reduced the quantity of charge that moved both during the ON phase (Q_ON) and the OFF phase (Q_OFF) of charge movement. This action appeared to be most selective for Q_ON. However, at this same concentration, nitrendipine had no blocking action on inward calcium currents. In contrast to these findings, diltiazem blocked calcium currents in a concentration-dependent manner, while slightly increasing the quantity of charge moved during Q_ON and Q_OFF. The enhancement of charge movement by diltiazem resulted from two actions. First, diltiazem shifted the voltage-dependence of charge movement to more negative potentials. Second, diltiazem increased the maximum amount of charge moved. (Supported by NIH NS 03178 and HL 07382.)     

Eye movement disorders after frontal eye field lesions in humans

Eye movements were recorded electro-oculographically in three patients with a small ischemic lesion affecting the left frontal eye field (FEF) and in 12 control subjects. Reflexive visually guided saccades (gap and overlap tasks), antisaccades, predictive saccades, memory-guided saccades, smooth pursuit and optokinetic nystagmus (OKN) were studied in the three patients. Staircase saccades and double step saccades were also studied in one of the three patients. For both leftward and rightward saccades, latency in the overlap task (but not in the gap task) and that of correct antisaccades and of memory-guided saccades was significantly increased, compared with the results of controls. There was a significant decrease in the amplitude gain of all rightward saccades programmed using retinotopic coordinates (gap and overlap tasks, predictive and memory-guided saccades), whereas the amplitude gain of corresponding leftward saccades was preserved. Such an asymmetry between leftward and rightward saccades was significant. In the staircase paradigm as well as for the first saccade in the double step paradigm (with the use of retinotopic coordinates in both cases), the amplitude gain of rightward saccades was also significantly lower than that of leftward saccades. Moreover, in the double step paradigm, the amplitude gain of the first rightward saccade was significantly lower than that of the second rightward saccade (programmed using extraretinal signals), which was preserved. The percentage of errors in the antisaccade task did not differ significantly from that of normal subjects. In the predictive saccade paradigm, the percentage of predictive rightward saccades was significantly decreased. The left smooth pursuit gain for all tested velocities, the right smooth pursuit gain for higher velocities, and the left OKN gain were significantly decreased. The results show, for the first time in humans, that the FEF plays an important role in (1) the disengagement from central fixation, (2) the control of contralateral saccades programmed using retinotopic coordinates, (3) saccade prediction and (4) the control of smooth pursuit and OKN, mainly ipsilaterally. In contrast, the left FEF did not appear to be crucial for the control of the only type of saccades programmed using extraretinal signals studied here.     

The involvement of glutamatergic transmission in the mechanism of movement disorders induced by reversive rotation of white mice

The ability of the selective non-competitive NMDA receptor blocker MK-801 and a series of new glutamate antagonists—the adamantane derivatives IEM-1754 and IEM-1857 and phencyclidine (IEM-1925)—to prevent movement disorders induced by reversive rotation in mice was studied. I.p. MK-801 at a dose of 0.15 ml and IEM-1754 at a dose of 5.0 mg/kg prevented the development of akinesia in response to reversive rotation, as effectively as scopolamine, a known agent which provides effective prophylaxis for movement diseases. IEM-1857, the quaternary analog of IEM-1754, was not effective. IEM-1925 significantly increased the responses of mice to reversive rotation, possibly because of its high activity in relation to other subtypes of glutamate receptors. These data provide evidence for the involvement of glutamatergic transmission in the mechanism of movement disorders of vestibular origin. Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 85, No. 4, pp. 497–501, May, 1999.     

Hitting moving objects: is target speed used in guiding the hand?

We investigated what information subjects use when trying to hit moving targets. In particular, whether only visual information about the target's position is used to guide the hand to the place of interception or also information about its speed. Subjects hit targets that moved at different constant speeds and disappeared from view after varying amounts of time. This prevented the subjects from updating position information during the time that the target was invisible. Subjects hit further ahead of the disappearing point when the target moved faster, but not as much as they should have on the basis of the target's speed. This could be because more time is needed to perceive and use the correct speed than was available before the target disappeared. It could also be due to a speed-related misperception of the target's final position. The results of a second experiment were more consistent with the latter hypothesis. In a third experiment we moved the background to manipulate the perceived speed. This did not affect the hitting positions. We conclude that subjects respond only to the changing target position. Target speed influences the direction in which the hand moves indirectly, possibly via a speed-related misperception of position.     

Eye Movements and Visually Active Dreams

There are a number of reports suggesting an association between profusion of eye movements and active dreaming. It has however been suggested that this relationship might only be evident in comparisons across the night and would not be evident in comparisons within one REM period. Data from 20 subjects taking placebo, amylobarbitone, and nitrazepam were used to test this. Dream reports were collected from REM awakenings and rated blind as visually active or passive. Eye movement profusion (number of 2 sec epochs) was assessed for each REM period. Correlation between dream content and eye movement was low but significant in comparisons including the whole night, and including data from drug, withdrawal, and placebo conditions. A significant correlation was not consistently obtained, however, when data from each REMP were considered separately. Correlations based on data from non-drug nights only were also small and could have been due to chance effects alone. The low correlations were not explicable solely by poor reliability of content ratings. It is concluded that the relationship between visually active dreaming and eye movement is slight, and may not hold when time of night is adequately controlled.     

Joint angle variability in 3D bimanual pointing: uncontrolled manifold analysis

The structure of joint angle variability and its changes with practice were investigated using the uncontrolled manifold (UCM) computational approach. Subjects performed fast and accurate bimanual pointing movements in 3D space, trying to match the tip of a pointer, held in the right hand, with the tip of one of three different targets, held in the left hand during a pre-test, several practice sessions and a post-test. The prediction of the UCM approach about the structuring of joint angle variance for selective stabilization of important task variables was tested with respect to selective stabilization of time series of the vectorial distance between the pointer and aimed target tips (bimanual control hypothesis) and with respect to selective stabilization of the endpoint trajectory of each arm (unimanual control hypothesis). The components of the total joint angle variance not affecting (V_COMP) and affecting (V_UN) the value of a selected task variable were computed for each 10% of the normalized movement time. The ratio of these two components R_V=V_COMP/V_UN served as a quantitative index of selective stabilization. Both the bimanual and unimanual control hypotheses were supported, however the R_V values for the bimanual hypothesis were significantly higher than those for the unimanual hypothesis applied to the left and right arm both prior to and after practice. This suggests that the CNS stabilizes the relative trajectory of one endpoint with respect to the other more than it stabilizes the trajectories of each of the endpoints in the external space. Practice-associated improvement in both movement speed and accuracy was accompanied by counter-intuitive lack of changes in R_V. Both V_COMP and V_UN variance components decreased such that their ratio remained constant prior to and after practice. We conclude that the UCM approach offers a unique and under-explored opportunity to track changes in the organization of multi-effector systems with practice and allows quantitative assessment of the degree of stabilization of selected performance variables.     

Fast, anticipatory smooth-pursuit eye movements appear to depend on a short-term store

Anticipatory smooth pursuit before the expected appearance of a moving target can reduce the initial retinal blur caused by the 100-ms delay of visual feedback. Humans, though, can only voluntarily generate smooth velocities up to about 5°/s without a moving target. However, previous experiments have shown that repetitive brief presentations of a moving target every few seconds appear to charge an internal store, the contents of which can later be released to generate higher velocity anticipatory movements. This store’s longevity was assessed here by repetitively presenting a moving target for 500 ms at different known intervals up to 7.2 s. Target motion at 25°/s or 50°/s was tested, with presentations in alternate directions or the same direction. Anticipatory velocity, measured 100 ms after target onset, decreased with increasing interval for all target motion conditions. A decrease was still seen when accurate timing cues were given before each presentation, suggesting that the drive for anticipatory pursuit is held in a short-term store lasting a few seconds which can enhance the low velocities produced by volition alone. The results also demonstrate that high-velocity anticipatory pursuit helps to overcome the temporal delays in the system and allows target velocity to be matched at an earlier time. Received: 27 August 1997 / Accepted: 22 December 1997     

Sensory components facilitating jaw-closing muscle activities in the rabbit

Summary The role of oral and facial sensory receptors in the control of masticatory muscle activities was assessed from the effect of acute deafferentiation on cortically induced rhythmic jaw movements (CRJMs) in anesthetized rabbits. When a thin polyurethane-foam strip (1.5, 2.5 or 3.5 mm thick) was placed between opposing molars during CRJMs, masseteric activities were facilitated in association with an increase in the medial excursion of the mandible during the power phase. The effects varied with the pattern of CRJMs, and the rate of facilitation was greater for small circular movements than for the crescent-shaped movements. Furthermore, the response of the masseter muscle was greater in the anterior half of the muscle, where muscle spindles are most dense, than in its posterior half. It was also demonstrated that the response increased with an increase in the thickness of the test strip. In contrast, the activities of the jaw-opening muscle were not affected significantly. The duration of masseteric bursts increased during application of the test strip and the chewing rhythm tended to slow down. However, the latter effect was not significant. After locally anesthetizing the maxillary and inferior alveolar nerves, the facultative responses of the masseter muscle to the test strip was greatly reduced but not completely abolished. Lesioning of the mesencephalic trigeminal nucleus (Mes V) where the primary ganglion cells of muscle spindle afferents from jaw-closing muscles and some periodontal afferents are located, also reduced the facilitative effects. Similar results were obtained in the animals with the kainic acid injections into the Mes V 1 week before electrical lesioning of this nucleus. In these animals the effects of electrical lesioning of the Mes V could be attributed to the loss of muscle receptor afferents since the neurons in the vicinity of the Mes V were destroyed and replaced by glial cells, whereas the Mes V neurons are resistant to kainic acid. When electrical lesioning of the Mes V and sectioning of the maxillary and inferior alveolar nerves were combined in animals with a kainic acid injection into the Mes V, the response of the masseter muscle to application of the strip was almost completely abolished. From these findings, we conclude that both periodontal receptors and muscle spindles are primarily responsible for the facilitation of jaw-closing muscle activities. Furthermore, it is suggested that the transcortical loop may not be the only path producing this facilitation since similar effects were induced in animals with ablation of the cortical masticatory area (CMA), when the test strip was placed between the molars during rhythmic jaw movements induced by pyramidal tract stimulation.