The use of synchronous demodulation for the measurement of eye movements by means of an ocular magnetic search coil

Eye-movement recording in freely moving animals

A new method is described for precise recording of eye movements in freely moving animals using Hall-effect devices. This inexpensive system, of small size and low weight, allows the analysis of horizontal and vertical components of saccadic eye movements, optokinetic nystagmus, slow tracking movements, eye vergence, etc., in unrestrained animals. A set of Hall-effect devices mounted in the skull is used to sense variations in the position of high-power miniature magnets fixed to the eye sclera. The output of the Hall-effect devices is amplified by operational amplifiers and collected through an analog-to-digital converter to be displayed on-line in a personal computer and stored for later analysis by specific software. Some examples of simultaneous body- and eye-movement recordings obtained in freely moving goldfish in different experimental situations are presented. This method would be useful in the recording of eye and gaze movements under natural conditions and for behavioural studies in freely moving animals.

Analysis of head movement and position using hall effect devices

We describe a system for the analysis of head displacement and angle. This system utilizes an inexpensive array of Hall effect transducers and associated electronics. Computer analysis of the output of the system permits real time display of head angle, position and associated velocity measures.

Interactions between natural and electrically evoked saccades. I. Differences between sites carrying retinal error and motor error signals in monkey superior colliculus

Fixed-vector saccades evoked by electrical stimulation may result from the elicitation of a retinal error signal directing the eyes toward a goal, or from the elicitation of a motor error signal determining the vector itself. Theoretically, the two mechanisms can be differentiated by delivering the stimulation while the eyes are already in motion (colliding saccade paradigm), thereby changing the eye position from which the evoked saccade starts. Only in the first case is the trajectory of the evoked saccade expected to be modified to compensate for part of the ongoing eye movement. An attempt was made to distinguish retinal vs. motor error mechanisms by applying the colliding saccade paradigm of stimulation to 29 sites throughout the superior colliculus (SC) of two trained monkeys. Compensatory evoked saccades, as predicted by the retinal error hypothesis, were obtained consistently in the superficial layers and at deeper sites where visual unit responses could be recorded. Conversely, in deep layers where only presaccadic activity was found, evoked saccades either were not affected by collision or summed their vectors with that of the ongoing movement. These last observations are both consistent with the hypothesis that the signal produced from deep sites was an initial motor error. A second observation was incidentally made: when stimulation was applied to the most superficial SC region, it definitively erased the goal of the ongoing saccade, and the latter did not resume its interrupted course. The colliding saccade paradigm may be useful in clarifying the role of structures involved in oculomotor function.

Saccades to visual targets are uncompensated after cerebellar stimulation

Electrical stimulation of the posterior vermis produces a saccadic perturbation of the eyes. If this stimulation occurs after presentation of a visual target but before a trained saccade is initiated, the ensuing movement misses the target by an amount approximating the perturbation (uncompensated). These observations differ from those obtained after stimulation of the superior colliculus or frontal eye fields which result in compensated saccades that land on target. It is suggested that vermal stimulation may be acting outside the pontine burst feedback system.

Measuring eye movements with a search coil: non-linear filter allows simultaneous recording of horizontal and vertical eye position by means of the phase modulation method

Measuring eye position from the phase angle of the signal induced by a rotating magnetic field in a search coil presents a number of advantages. So far, however, existing schemes using this principle either record eye position in one dimension only or require two search coils. We describe here a simple analog circuit which singles out a signal of given frequency from a frequency composite without changing the signal's phase. This makes it feasible to measure horizontal and vertical eye position with a single search coil by using two orthogonal rotating fields of different frequency.

Horizontal optokinetic nystagmus in the cat: recovery from cortical lesions

We have examined recovery from the acute effects of unilateral and bilateral visual cortex lesions on horizontal optokinetic nystagmus (OKN) in the cat. A single bilateral cortical lesion virtually abolishes monocular OKN in response to temporalward stimulation, and severely reduces OKN in response to nasalward stimulation at higher drum velocities. A unilateral cortical lesion causes deficits in OKN toward the side of the lesion, but these changes are less substantial than those observed after a bilateral lesion. The nature of the acute changes in OKN caused by cortical lesions supports the notion that the visual cortex has two important roles in the OKN system of the cat: first, it mediates OKN in response to temporalward motion of the visual environment during monocular viewing; second, it expands the response range of the OKN system to include higher stimulus velocities. Finally, the results from unilaterally lesioned cats suggest that a single hemisphere can mediate OKN in both directions, with an emphasis on the direction ipsilateral to that hemisphere. The patterns of recovery of OKN in lesioned cats follow two major trends: a small, gradual increase in the overall gain of OKN from its severely depressed level immediately after a large bilateral lesion, and a 'balancing' process that reduces the marked asymmetry between rightward and leftward OKN caused by a unilateral lesion. This balancing of the reflex is accomplished by an improvement in performance in one direction while performance in the opposite direction actually declines. Recovery is faster and more pronounced in lesioned cats with one cortical hemisphere intact, suggesting that the surviving visual cortex compensates for the effects of the lesion by modifying activity in subcortical pathways. These experiments indicate that the visual system of the cat has the capacity to recover at least partially from the deficits in OKN behavior caused by cortical lesions, and that this recovery is the outcome of an interaction between cortical and subcortical pathways mediating this important visuomotor reflex.

Simple spike activity of Purkinje cells in the posterior vermis of awake cats during spontaneous saccadic eye movements

Extracellular recordings were made from 151 cerebellar cortical cells in the posterior vermis of 12 awake cats. Thirty-two percent (n = 48) of these cells modulated their activity with respect to the onset of spontaneous saccadic eye movements. Thirty-five cells in this group were positively identified as Purkinje cells and manifested changes in simple spike activity that were related to saccade onset. These included short excitatory, inhibitory, or biphasic changes that were superimposed on background tonic firing rates (avg. = 54 spikes/sec). Such changes were recorded before as well as after the onset of a saccade. Sixty-five percent (n = 22) of these cells were related to horizontal and vertical saccades in more than one direction of motion. These cells were randomly distributed throughout the posterior vermis and manifested no anatomical topographic organization with respect to the direction of saccadic eye movement. The results of this study suggest that lobules VI and VII of the cerebellar vermis participate in both the initiation and execution of spontaneous saccades in preferred directions.