Stephen J. Fricker

Velocity and acceleration statistics of pseudorandomly timed saccades in humans

Abstract A system is described for the recording and analysis of the angular velocities and accelerations associated with 20° horizontal saccades. The acceleration data are included in order to bring out details of the velocity waveforms which otherwise might be ignored. The saccades are induced by a target system which operates on a pseudorandom time base, so that the timing of the saccades is not predictable. Statistical results for average peak velocities and accelerations of each eye are given for a group of 40 normal subjects. In addition an analysis is presented of the differences of pairs of individual velocities and accelerations, to provide additional statistical data which emphasize the interrelationships of these dynamic parameters of the two eyes. The significance and possible application of these sets of normative data are discussed.

Internuclear ophthalmoplegia and associated abnormalities in eye motion timing (differential delays)

Studies of the dynamic characteristics of horizontal saccadic eye motion give velocity, acceleration, and timing information. Average time differences between the motion of the eyes on left and right gaze, termed “differential delays,” are statistical measures of the average differences of right-going and left-going velocity waveforms, and can be determined to within 0.5 msec. These relative time measurements give information that is not necessarily apparent from the velocity or acceleration data, and provide a sensitive measure of early oculomotor dysfunction. Patients with internuclear ophthalmoplegia have been shown to give characteristic abnormal values, and two such patients are described in detail to illustrate how these measurements may be used both to help in diagnosis and to characterize the presumed lesions.

IS LINEAR ANALYSIS SUFFICIENT TO REVEAL ABNORMALITIES OF THE VISUAL SYSTEM UNDER DICHOPTIC STIMULATION CONDITIONS

Measurement of evoked potentials with low signal-to-noise ratios usually is carried out with conventional signal averaging. For visual evoked potentials repetitive stimuli are used, with timing intervals of the order of one-half to one second. Thus the first few hundred milliseconds of the averaged response can be viewed as the visual system’s impulse response to a single stimulus. Because of the anatomic intermixing of the nerve transmission pathways from the two eyes, this type of measurement has to be carried out testing one eye at a time (monocular test condition). Some use has been made of steady-state VER testing, using fixed rate repetitive stimuli (e.g., flash stimuli a t 30 Hz). In such cases it is possible to stimulate each eye independently by using two sets of stimuli at slightly different frequencies.’ Thus the system is being tested under steady-state conditions with two fixed but different frequencies. While this does allow some degree of resolution of the steady-state responses from the two eyes, a more general approach to obtaining dichoptic VER measurements would be to stimulate each eye separately using independent stimuli covering a range of frequencies. Provided that the timing of the two sets of stimuli is randomized, then cross-correlation analysis allows the recovery of VER responses representing the impulse-response of the visual system to each of the two sets of noiselike stimuli. It is of interest to ask whether linear cross-correlation analysis of such dichoptically induced VERs demonstrates the same type of behavior of the visual system as observed under conditions of monocular stimulation, and whether the dichoptic test results suggest the existence of a significant degree of cross-coupling between the two visual pathways. For some years now we have been applying cross-correlation techniques to the measurement of VERs (and ERGS) in human subjects.’X3 For VER measurements either flash stimuli or video presentations of alternating checkerboards have been used. Regardless of the type of stimulus used the timing of the stimulus occurrenceflash or pattern-alternation-is controlled by a pseudorandom binary sequence, so that the intervals between stimuli vary in a statistically definable manner. The use of two independent pseudorandom binary sequences to control two separate stimulus presentations permits “true” dichoptic stimulation of the two eyes! With this test arrangement the subject views two similar but independently driven displays-one for each eye. The amplified occipital signal, measured from a single pair of midline electrodes, then is cross-correlated with each of two independent reference waveforms, corresponding to the two independent pseudorandom binary sequences used to control the stimuli. The two cross-correlation VERs obtained in this manner from the occipital signal represent the first.-order responses of the visual system to the stimuli presented to each eye. Using a “true” dichoptic stimulation system, we can investigate differences in the

Advantages of a Cross-Correlation Technique for ERG Measurement in Children

The measurement of ERGs of infants and children suspected of having retinal dysfunction can present some significant practical problems. Often the degree of cooperation is less than ideal, the stimulus actually reaching the retina may vary, and the level of background electrical activity can mask small responses. Single flash ERGs often are not adequate in such circumstances, and usually it is advantageous to use some form of noise reduction. Conventional averaging techniques can be used, but one disadvantage is the time required to average a significant number of responses. The use of rapid, pseudorandomly timed stimuli and a cross-correlation signal recovery procedure offers some theoretical and practical advantages. Usually a better defined response (with a higher signal-to-noise ratio) can be obtained in a relatively short time, sometimes in a few seconds. In addition, the pseudorandomly timed flash stimuli inherently contain a wide range of frequencies, so that in effect the frequency response of the outer layers of the retina is being measured. Some clinical examples will be described to illustrate the practical application of this procedure for measuring the cone and rod ERGs of young patients.

The Corneal Light Reflex: A Guide to Binocular Disorders.

This reviewers initial impression on receiving the book was to wonder what the contents would be in a book that was devoted to the corneal light reflex. It rapidly became clear that we were to be concerned with estimates of the position of the light reflex in all possible combinations and permutations. In many places, the text is repetitious to the point of tedium. There are the expected photographs of patients with ocular deviations, and the position of the light reflex is duly noted. For someone who has never considered the light reflex as a means of estimating eye position, this may be of some use. The chapters are supposed to take one through a strabismic eye examination, with emphasis upon this particular technique for estimating deviations. Some case examples are given in the last part of the book. In places, it appears that some effort had to be made

Symposium on Horizontal Ocular Deviations.

This volume has as its basis symposia held at the Wills Eye Hospital in Philadelphia in 1968 and 1969. It is stated in the foreword that it is neither a factual reporting of a symposium nor a textbook, but a combination. One would like to think that such a combination should add something new to the literature, preferably presented in a readable and attractive form. Unfortunately, it is difficult for even the most eminent authors to produce something new and readable on the subject of strabismus every year. In this volume there are discussions of diagnostic methods, surgical procedures, collected surgical results, and suggestions concerning the treatment of undercorrection or overcorrection. From a technical viewpoint not everyone will agree with many of the statements made in this book. For example, it is stated that because at times the operative field is small when working on the medial rectus muscle, a