The organization of eye and limb movements during unrestricted reaching to targets in contralateral and ipsilateral visual space

Summary The spatial and temporal organixation of unrestricted limb movements directed to small visual targets was examined in two separate experiments. Videotape records of the subjects' performance allowed us to analyze the trajectory of the limb movement through 3-dimensional space. Horizontal eye movements during reaching were measured by infrared corneal reflection. In both experiments, the trajectories of the different reaches approximated straight line paths and the velocity profile revealed an initial rapid acceleration followed by a prolonged period of deceleration. In Experiment 1, in which the target light was presented to the right or left of a central fixation point at either 10° or 20° eccentricity, the most consistent differences were observed between reaches directed across the body axis to targets presented in the contralateral visual field and reaches directed at ipsilateral targets. Ipsilateral reaches were initiated more quickly, were completed more rapidly, and were more accurate than contralateral reaches. While these findings suggest that hemispherically organized neural systems are involved in the programming of visually guided limb movements, it was not clear whether the inefficiency of the contralateral movements was due to reaching across the body axis or reaching into the visual hemifield contralateral to the hand being used. Therefore, in Experiment 2, the position of the fixation point was varied such that the effects of visual field and body axis could be disembedded. In this experiment, the kinematics of the reaching movement were shown to be independent of the point of visual fixation and varied only as a function of the laterality of the target position relative to the body axis. This finding suggests that the kinematics of a reaching movement are determined by differences in the processing of neural systems associated with motor output, after the target has been localized in space. The effect of target laterality on response latency and accuracy, however, could not be attributed to a single frame of reference, or to a simple additive effect of both. These findings illustrate the complex integration of visual spatial information which must take place in order to reach accurately to goal objects in extrapersonal space. Comparison of ocular and manual performance revealed a close relationship between movement latency for both motor systems. Thus, rightward-going eye movements to a given target were initiated more quickly when accompanied by reaches with the right hand than when they were accompanied by reaches with the left hand. The finding that the latency of eye movements in one direction was influenced by which hand was being used to reach suggests that reaching toward a target under visual control involves a common integration of both eye and hand movements.This study was supported by grant no. MA-7269 from the Medical Research Council of Canada to M. A. Goodale     

Spatial attention and eye movements

We previously showed that when attention is allocated to the right or left of the fixation point, saccades directed to targets located above or below the fixation point deviate contralateral to the attention locus. In the present study, we examined how general this phenomenon is and whether the amount of saccade deviation depends on the location of attention with respect to that of the saccade target. Three experiments were carried out. In experiment 1 the location of the imperative stimulus was uncued. Its presentation exogenously directed attention to its location. In experiment 2 the location of the imperative stimulus was cued by a central cognitive cue. In this experiment attention was endogenously directed to the imperative stimulus location before its presentation (expectancy paradigm). In experiment 3 all stimulus boxes contained a possible imperative stimulus at the display presentation. A central cue, presented subsequently, indicated which of them had to be used for the saccade. In this experiment attention was endogenously directed to the imperative stimulus, but after its presentation (no-expectancy paradigm). The results showed that, regardless of how attention was directed to the imperative stimulus, the vertical saccades deviated contralateral to the attention location. The deviation was larger when attention was in the upper field and the saccade was directed upward (same hemifield condition) than when attention was in the upper field and the saccade was directed downward (opposite hemifield condition). The same relationship between the same hemifield condition and opposite hemifield condition was found when attention was in the lower field. Saccadic reaction times (SRTs) were shortest in experiment 2 and longest in experiment 3. In experiment 2, SRTs of the same hemifield condition were significantly longer than those of the opposite hemifield condition. Taken altogether, these results strongly support the notion that attention allocation in space leads to an activation of oculomotor circuits, in spite of eye immobility. The possible mechanisms responsible for saccade deviations and for greater saccade deviations when attention is in the same hemifield as the programmed ocular saccade are discussed.     

Gaze effects in the cerebral cortex: reference frames for space coding and action

Visual information is mapped with respect to the retina within the early stages of the visual cortex. On the other hand, the brain has to achieve a representation of object location in a coordinate system that matches the reference frame used by the motor cortex to code reaching movement in space. The mechanism of the necessary coordinate transformation between the different frames of reference from the visual to the motor system as well as its localization within the cerebral cortex is still unclear. Coordinate transformation is traditionally described as a series of elementary computations along the visuomotor cortical pathways, and the motor system is thought to receive target information in a body-centered reference frame. However, neurons along these pathways have a number of similar properties and receive common input signals, suggesting that a non-retinocentric representation of object location in space might be available for sensory and motor purposes throughout the visuomotor pathway. This paper reviews recent findings showing that elementary input signals, such as retinal and eye position signals, reach the dorsal premotor cortex. We will also compare eye position effects in the premotor cortex with those described in the posterior parietal cortex. Our main thesis is that appropriate sensory input signals are distributed across the visuomotor continuum, and could potentially allow, in parallel, the emergence of multiple and task-dependent reference frames. Received: 21 September 1998 / Accepted: 19 March 1999     

Testing visually guided forepaw movements in the cat: training apparatus and procedure

An apparatus is described, for testing guided forepaw movements toward a moving target in the cat. It mainly consists of a lever passing at a controllable speed in front of the animal (at max speed, 27cm/sec, total time of accessibility is 750 msec). Animals are trained to pull the lever for food reward. Response times for efficient movements and percentages of various types of inadequate movements are computed for each session. The system also includes a logic circuit to be used to train the animal to a period of immobility prior to the movement.     

Anticipatory smooth-pursuit eye movements in man and monkey

A fundamental problem in the generation of goal-directed behaviour is caused by the inevitable latency of biological sensory systems. Behaviour which is fully synchronised with the triggering sensory event can only be executed if the occurrence of this event can be predicted based on prior information. Smooth-pursuit eye movements are a classical and well-established example of goal-directed behaviour. The execution of these eye movements is thought to be very closely linked to the processing of visual motion signals. Here, we show that healthy human subjects as well as trained rhesus monkeys are able to initiate smooth-pursuit eye movements in anticipation of a moving target. These anticipatory pursuit eye movements are scaled to the velocity of the expected target. Furthermore, we can exclude the possibility that anticipatory pursuit is simply an after-pursuit of the previous trial. Visually-guided pursuit is only marginally affected by the presence of a structured background. However, the presence of a structured background severely impedes the ability to perform anticipatory pursuit. More generally, our data provide additional evidence that the cognitive oculomotor repertoires of human and monkeys are similar, at least with respect of smooth-pursuit in the prediction of an appearing target.     

Influence of gap and overlap paradigms on saccade latencies and vergence eye movements in seven-year-old children

The latency of eye movements is influenced by the fixation task; when the fixation stimulus is switched off before the target presentation (gap paradigm) the latency becomes short and express movements occur. In contrast, when the fixation stimulus remains on when the target appears (overlap paradigm), eye movement latency is longer. Several previous studies have shown increased rates of express saccades in children; however the presence of an express type of latency for vergence and combined movements in children has never been explored. The present study examines the effects of the gap and the overlap paradigms on horizontal saccades at far (150 cm) and at close (20 cm) viewing distances, on vergence along the median plane, and on saccades combined with convergence or divergence in 15 normal seven-year-old children. The results show that the gap paradigm produced shorter latency for all eye movements than the overlap paradigm, but the difference was only significant for saccades at close viewing distances, for divergence (pure and combined), and for saccades combined with vergence. The gap paradigm produced significantly higher rates of express latencies for saccades at close viewing distances, for divergence, and for saccades combined with divergence; in contrast, the frequencies of express latencies for saccades at far viewing distances and for convergence (pure or combined) were similar in the gap and the overlap paradigms. Interestingly, the rate of anticipatory latencies (<80 ms) was particularly high for divergence in the gap paradigm. Our collective findings suggest that the initiation of saccades at close viewing distances and of divergence is more reflexive, particularly in the gap paradigm. The finding of frequent anticipatory divergence that occurs at similar rates for seven-year-old children (this study) and for adults (Coubard et al., 2004, Exp Brain Res 154:368–381) indicates that predictive initiation of divergence is dominant.     

Integration of visual and somatosensory target information in goal-directed eye and arm movements

 In this study, we compared separate and coordinated eye and hand movements towards visual or somatosensory target stimuli in a dark room, where no visual position information about the hand could be obtained. Experiment 1 showed that saccadic reaction times (RTs) were longer when directed to somatosensory targets than when directed to visual targets in both single- and dual-task conditions. However, for hand movements, this pattern was only found in the dual-task condition and not in the single-task condition. Experiment 1 also showed that correlations between saccadic and hand RTs were significantly higher when directed towards somatosensory targets than when directed towards visual targets. Importantly, experiment 2 indicated that this was not caused by differences in processing times at a perceptual level. Furthermore, hand-pointing accuracy was found to be higher when subjects had to move their eyes as well (dual task) compared to a single-task hand movement. However, this effect was more pronounced for movements to visual targets than to somatosensory targets. A schematic model of sensorimotor transformations for saccadic eye and goal-directed hand movements is proposed and possible shared mechanisms of the two motor systems are discussed. Received: 15 June 1998 / Accepted: 21 September 1998     

The activation pattern in normal humans during suppression,imagination and performance of saccadic eye movements

The distribution of activated cerebral regions was examined in nine normal subjects during four different eye movement-related conditions: (1) fixation – fixation on a central light emitting diode; (2) saccadic suppression – fixation on a diode in the presence of flashing lateral targets; (3) reflexive/volitional saccades – performance of overt eye movements to two laterally lit targets and back to the centre; and (4) imagined saccades – imagining, but not performing, the same eye movements. The regional neural activity was measured indirectly using repetitive bolus injections of oxygen-15-labelled water and positron emission tomography (PET) to yield time-integrated images of the normalized count distribution. These were aligned and anatomically normalized to a standard stereotactic space and the averages of each condition were analysed categorically using statistical parametric mapping. Compared to central fixation, reflexive/volitional saccades significantly activated regions in the classically known cortical oculomotor regions. The most notable activation during the saccade suppression task, compared to central fixation alone, was a bilateral activation of the parietal cortex with a right-sided preponderance, activation of the supplementary eye field/caudal cingulate regions, and activation of frontal regions close to the frontal eye fields. Imagined performance of eye movements without overt eye movements activated the supplementary eye field and frontal eye fields identically to regions involved in overt eye movements, thus demonstrating that overt eye movements are not a prerequisite of the activation of these regions in normal humans.     

Grip and load force coupling during discrete vertical arm movements with a grasped object in cerebellar atrophy

Control of isometric grip forces during manipulation of objects is an essential feature of all skilled manual performances. Recent studies suggested that the anticipation of movement-induced loads may be a cerebellar function. We analysed grip force adjustments to fluctuations of inertial loads during discrete vertical movements with a grasped object in five patients with cerebellar atrophy and five healthy control subjects. Normally grip force is precisely adapted to the load fluctuations, in particular to the maximum load force, which occurs early in upward and late in downward movements. Both groups produced similar accelerations of the grasped object and consequently similar maximum loads. However, cerebellar patients established increased static grip forces during stationary holding of the object and increased force ratios between grip and load force at the time of maximum acceleration. These findings are congruent with earlier studies analysing grip and load force coupling in patients with cerebellar lesions. In contrast to earlier studies, we found no significant differences in the timing of grip force onset and grip force maximum relative to the onset of movement and maximum acceleration, respectively, between normal controls and four of five cerebellar patients. However, a regression analysis between grip and load forces during the load increase and decrease phases of the movement suggested deficits in the close temporospatial coupling between the two forces in all cerebellar patients. Our findings give further support to the notion that the cerebellum plays a crucial role in the forward control of grip force magnitude and timing during voluntary object manipulation. Compared to earlier studies, the increase in grip forces may be interpreted as a general control strategy to compensate for motor deficits, whereas impairments of temporal grip force regulation may occur at different degrees of dysfunction during the progression of cerebellar atrophy.     

Improvement of the morphometric method and its use in electron-microscopy

Institute for Physicotechnical Problems, Moscow. (Presented by Academician of the Academy of Medical Sciences of the USSR A. P. Avtsyn.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 110, No. 7, pp. 100–103, July, 1990.     

Bilateral lesions of the medial longitudinal fasciculus in monkeys: Effects on the horizontal and vertical components of voluntary and vestibular induced eye movements

Summary Bilateral transections across the brainstem interrupted the medial longitudinal fasciculus (MLF) in three monkeys trained to make eye movements while subjected to horizontal or vertical angular accelerations. Eye movements measured before and after the lesion revealed deficits in both voluntary and vestibular compensatory eye movements; the deficits differed in the horizontal and vertical directions. Vertical saccades in both directions were normal but eccentric positions of fixation could not be maintained; a drift toward the midline followed by a corrective saccade produced vertical fixation nystagmus. Furthermore, the vertical vestibuloocular reflex (VOR) was abolished and vertical smooth pursuit was impaired. Along the horizontal meridian, adduction across the midline could not be achieved during either saccades, smooth pursuit, or the VOR. Temporal saccades were normal whereas nasal saccades were considerably slowed. If the eye was not required to cross the midline, the phase shift during the VOR was within 15 deg of normal in each eye. The gain of the VOR was reduced to about 0.4 immediately after the lesion, but recovered within one month. These findings suggest that the MLF transmits quite different kinds of information to horizontal and vertical oculomotoneurons and that deficits in vertical eye movements may be a sensitive indicator of anterior internuclear ophthalmoplegia.     

Nkx2.5‐negative myocardium of the posterior heart field and its correlation with podoplanin expression in cells from the developing cardiac pacemaking and conduction system

Recent advances in the study of cardiac development have shown the relevance of addition of myocardium to the primary myocardial heart tube. In wild‐type mouse embryos (E9.5–15.5), we have studied the myocardium at the venous pole of the heart using immunohistochemistry and 3D reconstructions of expression patterns of MLC‐2a, Nkx2.5, and podoplanin, a novel coelomic and myocardial marker. Podoplanin‐positive coelomic epithelium was continuous with adjacent podoplanin‐ and MLC‐2a‐positive myocardium that formed a conspicuous band along the left cardinal vein extending through the base of the atrial septum to the posterior myocardium of the atrioventricular canal, the atrioventricular nodal region, and the His‐Purkinje system. Later on, podoplanin expression was also found in the myocardium surrounding the pulmonary vein. On the right side, podoplanin‐positive cells were seen along the right cardinal vein, which during development persisted in the sinoatrial node and part of the venous valves. In the MLC‐2a‐ and podoplanin‐positive myocardium, Nkx2.5 expression was absent in the sinoatrial node and the wall of the cardinal veins. There was a mosaic positivity in the wall of the common pulmonary vein and the atrioventricular conduction system as opposed to the overall Nkx2.5 expression seen in the chamber myocardium. We conclude that we have found podoplanin as a marker that links a novel Nkx2.5‐negative sinus venosus myocardial area, which we refer to as the posterior heart field, with the cardiac conduction system. Anat Rec, 290:115–122, 2007. © 2006 Wiley‐Liss, Inc.     

Expression of Peste des petits ruminants virus nucleocapsid protein in prokaryotic system and its potential use as a diagnostic antigen or immunogen

In this study, both partial and full-length nucleocapsid (N) gene of Peste des petits ruminants virus (PPRV) were cloned into pET33b vector and expressed in Escherichia coli (BL21) with the objective of replacing live PPRV antigen with recombinant protein in ELISA. The expressed proteins were characterized by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and Western blot by using a PPRV N protein specific monoclonal antibody. The expressed histidine-tagged fusion proteins were purified using affinity Ni-NTA column and were assessed for their conformation in terms of reactivity by ELISA. The immunogenicity of recombinant proteins was also assessed in rabbits and anti-N antibody response against PPRV was observed in all the immunized rabbits, when tested by competitive and indirect ELISAs. In sandwich ELISA, a mean OD_492 nm of 1.4 and 0.90 was obtained for crude lysate having expressed the N protein and the PPRV antigen, respectively. Further, the N protein was tested as a coating antigen in competitive ELISA instead of PPRV antigen for serological diagnosis of PPR infection. This indicates the diagnostic potential of the PPRV recombinant N proteins, which are safe and better alternatives to live PPRV antigen in ELISA for clinical or sero-surveillance of PPR in enzootic or non-enzootic countries.