Transformation of sensory signals into commands for saccadic eye movements: a neural network study

This report describes the distribution and localization of thrombomodulin (TM) in the rat eye by light and electron microscopic immunocytochemistry. In addition to the endothelium of the entire vasculature, TM was found on the non-vascular structures lining the cavities of the posterior and anterior chambers and the limbus. TM was localized on the basal, lateral, and apical plasma membranes of the inner and outer ciliary epithelium, and the posterior iris epithelium in which there was no polarized expression of TM. In the anterior chamber, TM was localized on the luminal surface of the corneal endothelium, but was negative on the anterior border layer of the iris, which is composed of a discontinuous layer of fibroblasts and collagen fibers. Thus, TM was present at sites of cell-to-cell contact. TM was also present on the endothelia of the trabecular meshwork and the Schlemm's canal in the limbus. TM was localized not only on the luminal plasma membrane, but also on the cytoplasmic giant vacuoles in the endothelial cells of the Schlemm's canal. These findings extend the importance of anticoagulant mechanisms to the systems of secretion, circulation, and drainage of the aqueous humor.     

Premotor neurons for vertical eye movements in the rostral mesencephalon of monkey and human: histologic identification by parvalbumin immunostaining

In the monkey, premotor neurons for vertical gaze are located in the mesencephalic reticular formation: the rostral interstitial nucleus of the medial longitudinal fascicle (riMLF) contains medium-lead burst neurons, and the interstitial nucleus of Cajal (iC) acts as integrator for the eye-velocity signals to eye-position signals. Both nuclei lie adjacent to each other and are similar in appearance at the transition zone in Nissl-stained sections, which makes a delineation of the functionally different nuclei difficult in human. For a neuropathologic analysis of degenerative changes in saccadic disorders of patients, the histologic identification of the riMLF and the iC is important. The aim of this study is to identify both nuclei in human by using parvalbumin as a histologic marker. First, in monkeys the premotor neurons in riMLF and iC were identified by trans-synaptic labelling after injections of tetanus toxin fragment C into vertical-pulling eye muscles. Premotor neurons were found in the riMLF mainly ipsilateral to the corresponding eye muscle motoneurons and on both sides within the iC, but here the labelled cell populations differed: the contralateral side contained more medium-sized cells compared with the mainly small-sized cell population on the ipsilateral side. Double labelling showed that almost all premotor neurons in the iC and all premotor neurons in the riMLF were parvalbumin-immunoreactive. The immunocytochemical staining of human brainstem sections revealed the riMLF as a cluster of medium-sized, elongated parvalbumin-positive cells, with a similar appearance and at a similar location as that in monkey: a wing-shaped nucleus dorsomedial to the red nucleus, rostral to the traversing tractus retroflexus, dorsally bordered by the thalamo-subthalamic paramedian artery. The adjacent iC could be distinguished easily by its more densely packed, round parvalbumin-immunoreactive neurons. The exact identification of premotor neurons of the vertical system in the normal human brain provides a reference basis for the neuropathologic analysis of vertical gaze disorders at a cellular level.     

Voluntary eye movements and alcohol

Human voluntary horizontal eye-movements are studied prior and subsequent to the ingestion of alcohol. Such movements are important, especially during reading and driving. Infrared monitoring techniques were employed to record eye movements and eye-movement latency histograms were tabulated. Blood alcohol levels were continually monitored. Alcohol increases eye-movement latency but the maximum latency occurred in most cases at different elapsed times from the time of maximum blood alcohol level. In general, a roughly 20% increase in latency occurs subsequent to alcohol. Whether this added delay is a result of a general depression of the oculomotor system or a specific increase in some oculomotor computing element is unresolved. Within the limits of our experimentation, there was no difference between latency change observed in moderate and heavy drinkers.     

Estimating heading during eye movements

In eight experiments, we examined the ability to judge heading during tracking eye movements. To assess the use of retinal-image and extra-retinal information in this task, we compared heading judgments with executed as opposed to simulated eye movements. In general, judgments were much more accurate during executed eye movements. Observers in the simulated eye movement condition misperceived their self-motion as curvilinear translation rather than the linear translation plus eye rotation that was simulated. There were some experimental conditions in which observers could judge heading reasonably accurately during simulated eye movements; these included conditions in which eye movement velocities were 1 deg/sec or less and conditions which made available a horizon cue that exists for locomotion parallel to a ground plane with a visible horizon. Overall, our results imply that extra-retinal, eye-velocity signals are used in determining heading under many, perhaps most, viewing conditions.     

Programming saccadic eye movements.

This article addresses questions about the preparatory processes that immediately precede saccadic eye movements. Saccade latencies were measured in a task in which subjects were provided partial advance information about the spatial location of a target fixation. In one experiment, subjects were faster in initiating saccades when they knew either the direction or amplitude of the required movement in advance compared to a condition with equal uncertainty about the number of potential saccade targets but without knowledge of the parameters required to execute the movement. These results suggest that the direction and amplitude for an upcoming saccade were calculated separately, and not in a fixed serial order. In another experiment, subjects appear to have programmed the saccades more holistically—with computations of direction and amplitude parameters occurring simultaneously. The implications of these results for models of eye movement preparation are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Premotor potentials

The topic of this article is probably unfamiliar to most electromyographers. These potentials can be seen at high amplification immediately preceding the median or ulnar compound muscle action potential (CMAP) during standard motor nerve conduction studies. A chronological review of premotor potentials is provided. After longstanding debate, eventual determination of their electrophysiologic origin employed analysis of these potentials using volume conduction principles. The significance of premotor potentials is addressed.     

Cerebellar complex spikes encode both destinations and errors in arm movements

Purkinje cells of the cerebellum discharge complex spikes, named after the complexity of their waveforms, with a frequency of approximately 1 Hz during arm movements. Despite the low frequency of firing, complex spikes have been proposed to contribute to the initiation of arm movements or to the gradual improvement of motor skills. Here we recorded the activity of Purkinje cells from the hemisphere of cerebellar lobules IV-VI while trained monkeys made short-lasting reaching movements (of approximately 200 milliseconds in duration) to touch a visual target that appeared at a random location on a tangent screen. We examined the relationship between complex-spike discharges and the absolute touch position, and between complex-spike discharges and relative errors in touching the screen. We used information theory to show that the complex spikes occurring at the beginning of the reach movement encode the absolute destination of the reach, and the complex spikes occurring at the end of the short-lasting movements encode the relative errors. Thus, complex spikes convey multiple types of information, consistent with the idea that they contribute both to the generation of movements and to the gradual, long-term improvement of these movements.     

Saccadic eye movements in Graves' disease

PURPOSE: To describe the saccades made by patients with Graves' disease (GD) and to attempt to distinguish these objectively and quantitatively from the saccades in control subjects. METHODS: In 12 euthyroid patients with GD, the saccades of both eyes were recorded simultaneously with electromagnetic search coils. Subjects were asked to alternate their gazes between two fixed targets that were 10 degrees, 20 degrees, 30 degrees, or 40 degrees apart along each horizontal or vertical meridian. The data from the patients with GD and those from the similarly recorded group of 12 control subjects were examined in two ways. First, the difference in saccadic sizes between the two eyes was assessed. Second, the saccadic dynamics--that is, the maximum velocity and the saturation constant of the main sequence--were determined for each eye. Repeated measurement analysis of variance was used to test observed differences between the two groups. Finally, through exact logistic regression analysis, classification of the saccades as those of a patient with GD or of a control subject was carried out. RESULTS: The saccades of patients with GD were generally less conjugate than those of control subjects (P < 0.05). On average, the maximum main sequence velocities in patients with GD were lower than in control subjects (P < 0.05). The saccades of patients with GD were well differentiated from those of control subjects. CONCLUSIONS: The saccades in GD may differ markedly from normal saccades, and the two can be reliably distinguished.     

The aftereffect of tracking eye movements

When observers tracked moving stripes across a background either of stationary stripes, or of stripes moving in the opposite direction, they saw a clear motion aftereffect when the stripes stopped moving. The direction of this aftereffect was opposite to that of the previously tracked stripes, and was thus the same as the direction of the retinal movement of the non-tracked stripes. This aftereffect of tracking was shown not to depend upon slippage of the tracked contours on the retina during tracking, or upon the saccadic phase of optokinetic nystagmus. The effect showed storage over a period of time with the eyes shut. It appears that the effect is due to induced movement, and arises originally from stimulation of the retina by background contours in the tracking phase. This was shown by confining the view of the moving target to one eye, while permitting both eyes to be exposed to background stimulation during tracking. After such stimulation the magnitude of the aftereffect was equal in the two eyes.     

Information integration across eye movements.

The visual world contains more information than can be perceived in a single glance. Consequently, humans make eye and head movements and somehow construct a stable and continuous representation of the visual environment from these successive views. How this is accomplished has puzzled psychologists for over a century. The present research investigated the properties of transsaccadic integration, the integration of information across saccadic eye movements. The results of several experiments suggest that the mental representation of the environment that is constructed across eye movements is surprisingly schematic and undetailed, and based more on the contents of the current fixation than on one's memory for the contents of previous fixations. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Photoelectric methods of recording eye movements.

Presents 2 photoelectric methods of recording eye movements to investigate the oculomotor system of hospitalized patients with local brain lesions. The general appearance, operation, and advantages of (a) recording horizontal eye movements with direct ink-writing registration, and (b) simultaneous recording of horizontal and vertical eye movements are described. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dynamic asymmetries in disparity convergence eye movements

Vergence eye movements have traditionally been considered the product of a single neural control center and are usually studied by combining the movements of each eye into a single 'vergence' response. In the present experiment, disparity-driven eye movements were produced by symmetrical step stimuli, and the dynamic properties of each eye movement were analyzed separately. Although the final positions of the two eyes were symmetrical, large dynamic asymmetries often occurred. The timing between the two eyes showed fair synchrony as they attained maximum velocity at approximately the same time. Since the final static positions were symmetrical, asymmetries occurring during the initial dynamic component must necessarily be compensated by offsetting asymmetries in the latter portion of the response.     

Shared motor error for multiple eye movements

Most natural actions are accomplished with a seamless combination of individual movements. Such coordination poses a problem: How does the motor system orchestrate multiple movements to produce a single goal-directed action? The results from current experiments suggest one possible solution. Oculomotor neurons in the superior colliculus of a primate responded to mismatches between eye and target positions, even when the animal made two different types of eye movements. This neuronal activity therefore does not appear to convey a command for a specific type of eye movement but instead encodes an error signal that could be used by multiple movements. The use of shared inputs is one possible strategy for ensuring that different movements share a common goal.     

Linguistic focus affects eye movements during reading

OBJECTIVE: To clarify the diagnostic significance of selective mutism (elective mutism in DSM-III-R). METHOD: Fifty children with selective mutism were evaluated systematically by means of semistructured clinical interviews and rating scales to obtain detailed diagnostic information. RESULTS: All 50 children met DSM-III-R criteria for social phobia or avoidant disorder and 24 (48%) had additional anxiety disorders. Clinical measures of anxiety and behavioral symptoms supported the presence of anxiety disorders as a characteristic of selectivity mute children. Only one case each of oppositional defiant disorder and attention-deficit hyperactivity disorder was found. CONCLUSIONS: Persistent selective mutism typically presents in the context of anxiety disorders.     

Adaptive modification of saccadic eye movements.

Experiments are reported in which the target for a saccadic eye movement was displaced during the saccade. Ss adapted to the displacement by altering the amplitudes of subsequent saccades to compensate for it. Analysis of kinematic details of the saccade trajectories revealed that the adaptation did not arise from a simple remapping of perceived target locations. Instead, the adaptation appeared to be accomplished by a change in the gain of the saccadic system. The gain change arose primarily from a change in the magnitude of the force pulse for the saccade, not a change in the duration of the pulse. These results have implications for the mechanisms that underlie saccades in normal situations. In particular, people can separately adjust the magnitudes and durations of the force pulses used to produce saccades. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Integrating pictorial information across eye movements.

Six experiments are reported dealing with the types of information integrated across eye movements (EMs) in picture perception. A line drawing of an object was presented in peripheral vision, and the 12 Ss (members of the university community) made an EM to it. During the saccade, the initially presented picture was replaced by another that the S was instructed to name as quickly as possible. The relation between the stimulus on the 1st fixation and the stimulus on the 2nd fixation was varied. Across experiments, there was about 100–230 msec facilitation when the pictures were identical compared with a control condition in which only the target location was specified on the 1st fixation. This finding implies that information about the 1st picture facilitated naming the 2nd picture. When the pictures represented the same concept (e.g., 2 pictures of a horse), there was a 90-msec facilitation effect that could have been the result of either the visual or conceptual similarity of the pictures. However, when the pictures had different names, only visual similarity produced facilitation; there appeared to be inhibition from the competing names. Results of all experiments are consistent with a model in which the activation of both the visual features and the name of the picture seen on the 1st fixation survive the saccade and combine with the information extracted on the 2nd fixation to produce identification and naming of the 2nd picture. (32 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)