Optimality of position commands to horizontal eye muscles: A test of the minimum-norm rule. (1/1100)

Six muscles control the position of the eye, which has three degrees of freedom. Daunicht proposed an optimization rule for solving this redundancy problem, whereby small changes in eye position are maintained by the minimum possible change in motor commands to the eye (the minimum-norm rule). The present study sought to test this proposal for the simplified one-dimensional case of small changes in conjugate eye position in the horizontal plane. Assuming such changes involve only the horizontal recti, Daunicht's hypothesis predicts reciprocal innervation with the size of the change in command matched to the strength of the recipient muscle at every starting position of the eye. If the motor command to a muscle is interpreted as the summed firing rate of its oculomotor neuron (OMN) pool, the minimum-norm prediction can be tested by comparing OMN firing rates with forces in the horizontal recti. The comparison showed 1) for the OMN firing rates given by Van Gisbergen and Van Opstal and the muscle forces given by Robinson, there was good agreement between the minimum-norm prediction and experimental observation over about a +/-30 degrees range of eye positions. This fit was robust with respect to variations in muscle stiffness and in methods of calculating muscle innervation. 2) Other data sets gave different estimates for the range of eye-positions within which the minimum-norm prediction held. The main sources of variation appeared to be disagreement about the proportion of OMNs with very low firing-rate thresholds (i.e., less than approximately 35 degrees in the OFF direction) and uncertainty about eye-muscle behavior for extreme (>30 degrees ) positions of the eye. 3) For all data sets, the range of eye positions over which the minimum-norm rule applied was determined by the pattern of motor-unit recruitment inferred for those data. It corresponded to the range of eye positions over which the size principle of recruitment was obeyed by both agonist and antagonist muscles. It is argued that the current best estimate of the oculomotor range over which minimum-norm control could be used for conjugate horizontal eye position is approximately +/-30 degrees. The uncertainty associated with this estimate would be reduced by obtaining unbiased samples of OMN firing rates. Minimum-norm control may result from reduction of the image movement produced by noise in OMN firing rates.  (+info)

Electrical stimulation as a therapeutic option to improve eyelid function in chronic facial nerve disorders. (2/1100)

PURPOSE: To establish whether it is possible to improve orbicularis oculi muscle function in the eyelids of patients with a chronic seventh cranial nerve palsy by using transcutaneous electrical stimulation to the point at which electrical stimulation induces a functional blink. METHODS: Ten subjects (one woman, nine men) aged 36 to 76 with chronic, moderate to severe facial nerve palsy were recruited into the study. Voluntary and spontaneous eyelid movements were assessed, using an optical measuring system, before, during, and after a 3-month treatment period. Voluntary and spontaneous lid velocities were also measured and compared with eyelid kinematic data in normal subjects (12 women, 18 men; age range, 22-56 years). RESULTS: Therapeutic electrical stimulation applied over 3 months produced improvement in eyelid movement (>2 mm) in 8 of 10 patients during voluntary eyelid closure. However, there was no significant improvement recorded in spontaneous blink amplitudes or peak downward-phase velocity of the upper eyelid. This regimen of stimulation failed to recover function well enough that a functional blink could be induced in the paretic eyelid by electrical stimulation. CONCLUSIONS: Electrical stimulation using transcutaneous electrical nerve stimulators units can improve voluntary eye closure, apparently because of a reduction in stiffness of eyelid mechanics, rather than an improvement of muscle function. Investigation of alternative stimulation regimens is warranted.  (+info)

Microstimulation of the lateral wall of the intraparietal sulcus compared with the frontal eye field during oculomotor tasks. (3/1100)

We compared the effects of intracortical microstimulation (ICMS) of the lateral wall of the intraparietal sulcus (LIP) with those of ICMS of the frontal eye field (FEF) on monkeys performing oculomotor tasks. When ICMS was applied during a task that involved fixation, contraversive saccades evoked in the LIP and FEF appeared similar. When ICMS was applied to the FEF at the onset of voluntary saccades, the evoked saccades collided with the ongoing voluntary saccade so that the trajectory of voluntary saccade was compensated by the stimulus. Thus the resultant saccade was redirected and came close to the endpoint of saccades evoked from the fixation point before the start of voluntary saccade. In contrast, when ICMS was applied to the LIP at the onset of voluntary saccades, the resultant saccade followed a trajectory that was different from that evoked from the FEF. In that case, the colliding saccades were redirected toward an endpoint that was close to the endpoint of saccades evoked when animals were already fixating at the target of the voluntary saccade. This finding suggests that the colliding saccade was directed toward an endpoint calculated with reference to the target of the voluntary saccade. We hypothesize that, shortly before initiation of voluntary saccades, a dynamic process occurs in the LIP so that the reference point for calculating the saccade target shifts from the fixation point to the target of a voluntary saccade. Such predictive updating of reference points seems useful for immediate reprogramming of upcoming saccades that can occur in rapid succession.  (+info)

Oculomotor evidence for neocortical systems but not cerebellar dysfunction in autism. (4/1100)

OBJECTIVE: To investigate the functional integrity of cerebellar and frontal systems in autism using oculomotor paradigms. BACKGROUND: Cerebellar and neocortical systems models of autism have been proposed. Courchesne and colleagues have argued that cognitive deficits such as shifting attention disturbances result from dysfunction of vermal lobules VI and VII. Such a vermal deficit should be associated with dysmetric saccadic eye movements because of the major role these areas play in guiding the motor precision of saccades. In contrast, neocortical models of autism predict intact saccade metrics, but impairments on tasks requiring the higher cognitive control of saccades. METHODS: A total of 26 rigorously diagnosed nonmentally retarded autistic subjects and 26 matched healthy control subjects were assessed with a visually guided saccade task and two volitional saccade tasks, the oculomotor delayed-response task and the antisaccade task. RESULTS: Metrics and dynamics of the visually guided saccades were normal in autistic subjects, documenting the absence of disturbances in cerebellar vermal lobules VI and VII and in automatic shifts of visual attention. Deficits were demonstrated on both volitional saccade tasks, indicating dysfunction in the circuitry of prefrontal cortex and its connections with the parietal cortex, and associated cognitive impairments in spatial working memory and in the ability to voluntarily suppress context-inappropriate responses. CONCLUSIONS: These findings demonstrate intrinsic neocortical, not cerebellar, dysfunction in autism, and parallel deficits in higher order cognitive mechanisms and not in elementary attentional and sensorimotor systems in autism.  (+info)

Ocular development and involution in the European cave salamander, Proteus anguinus laurenti. (5/1100)

The anatomy and development of the eye of Proteus anguinus are described. The relationships between organogenesis of the eye in embryos and larva and its involution in the young and the adult are discussed. The availability (in breeding cultures) of a significant number of Proteus embryos (which are normally rare) allowed experimental analysis of the effects of light, xenoplastic differentiation and thyroid hormones on the development of the eye. The results of this study suggest that development and involution of the eye of Proteus are controlled by genetic factors which are not greatly influenced by environment, and one can, therefore, consider the microphthalmy of Proteus as a relict characteristic which is the result of a specific development with disturbance of the normal ontogenic process.  (+info)

Discharge profiles of abducens, accessory abducens, and orbicularis oculi motoneurons during reflex and conditioned blinks in alert cats. (6/1100)

The discharge profiles of identified abducens, accessory abducens, and orbicularis oculi motoneurons have been recorded extra- and intracellularly in alert behaving cats during spontaneous, reflexively evoked, and classically conditioned eyelid responses. The movement of the upper lid and the electromyographic activity of the orbicularis oculi muscle also were recorded. Animals were conditioned by short, weak air puffs or 350-ms tones as conditioned stimuli (CS) and long, strong air puffs as unconditioned stimulus (US) using both trace and delayed conditioning paradigms. Motoneurons were identified by antidromic activation from their respective cranial nerves. Orbicularis oculi and accessory abducens motoneurons fired an early, double burst of action potentials (at 4-6 and 10-16 ms) in response to air puffs or to the electrical stimulation of the supraorbital nerve. Orbicularis oculi, but not accessory abducens, motoneurons fired in response to flash and tone presentations. Only 10-15% of recorded abducens motoneurons fired a late, weak burst after air puff, supraorbital nerve, and flash stimulations. Spontaneous fasciculations of the orbicularis oculi muscle and the activity of single orbicularis oculi motoneurons that generated them also were recorded. The activation of orbicularis oculi motoneurons during the acquisition of classically conditioned eyelid responses happened in a gradual, sequential manner. Initially, some putative excitatory synaptic potentials were observed in the time window corresponding to the CS-US interval; by the second to the fourth conditioning session, some isolated action potentials appeared that increased in number until some small movements were noticed in eyelid position traces. No accessory abducens motoneuron fired and no abducens motoneuron modified their discharge rate for conditioned eyelid responses. The firing of orbicularis oculi motoneurons was related linearly to lid velocity during reflex blinks but to lid position during conditioned responses, a fact indicating the different neural origin and coding of both types of motor commands. The power spectra of both reflex and conditioned lid responses showed a dominant peak at approximately 20 Hz. The wavy appearance of both reflex and conditioned eyelid responses was clearly the result of the high phasic activity of orbicularis oculi motor units. Orbicularis oculi motoneuron membrane potentials oscillated at approximately 20 Hz after supraorbital nerve stimulation and during other reflex and conditioned eyelid movements. The oscillation seemed to be the result of both intrinsic (spike afterhyperpolarization lasting approximately 50 ms, and late depolarizations) and extrinsic properties of the motoneuronal pool and of the circuits involved in eye blinks.  (+info)

Ocular microtremor in patients with idiopathic Parkinson's disease. (7/1100)

Abnormalities in the oculomotor control mechanism of patients with idiopathic Parkinson's disease are well recognised. In this study the effect of Parkinson's disease on tonic output from oculomotor nuclei was studied by using oculomicrotremor as an index of such output. Oculomicrotremor readings were taken from 22 parkinsonian patients and 22 normal healthy volunteers using the piezoelectric strain gauge technique. There was a slower overall tremor frequency, baseline, and burst frequency in the parkinsonian group. There was also a significant increase in the duration of baseline, with a decrease in the number of bursts a second and a decrease in average duration of bursts in the patient group compared with the normal group. One patient, whose medication was withdrawn, showed a marked decrease in mean frequency and baseline frequency with a decrease in number of bursts and increase in baseline duration compared with readings taken when treatment recommenced. These results suggest that variables measured in oculomicrotremor are altered compared with normal subjects, reflecting altered tonic output from oculomotor nuclei in patients with idiopathic Parkinson's disease.  (+info)

Diplopia in a swimmer due to badly fitting goggles. (8/1100)

An unusual effect of badly fitting swimming goggles is described. The goggles pressed on the trochlea of the left eye, interfering with the action of the superior oblique muscle. Diplopia resulted, which took several weeks to resolve.  (+info)