Derek H. Fender

Location of Sources of Evoked Scalp Potentials: Corrections for Skull and Scalp Thicknesses

The problem of locating the position of the source of evoked potentials from measurements on the surface of the scalp has been examined. It is shown that the position of the source in a head modeled by a sphere surrounded by two concentric shells of differing conductivities representing the skull and the scalp can be inferred from source localization calculations made on a homogeneous model.

Evaluation of Methods for Three-Dimensional Localization of Electrical Sources in the Human Brain

A quantitative evaluation has been made of methods designed to identify the intracranial sources of scalp recorded potentials. The scheme presented models the sources as equivalent dipoles and allows for the detection of multiple sources. The head is modeled by both homogeneous and inhomogeneous spheres. The results establish the technical feasibility of finding equivalent dipoles in the visual cortex for human visually evoked scalp potentials (VESPs).

The interplay of drifts and flicks in binocular fixation

Abstract A study of spontaneous human eye movements during monocular and binocular fixation was made in order to determine the corrective roles of flicks and drifts. It is found that both types of motions correct fixational errors although flicks are somewhat more active in this respect. Vergence error is a stimulus for correction by drifts but not by flicks, while binocular vertical discrepancy of the visual axes does not trigger corrective movements.

Nonlinearities of the human oculomotor system: Gain

Abstract Eye movements of two subjects have been measured while performing various two-dimensional tracking tasks. The stimuli consisted of sums of 3, 4, 7 or 13 sinusoids in each of the horizontal and vertical directions. On the average, the amplitude of the eye-movements decreased with frequency, but within a narrow band the motion increased with frequency. The system also responded preferentially to the highest frequency of a finite number of sinusoids. The behavior of the oculomotor system is non-linear in the sense that it is not possible to predict the response to one class of target motion by linear combination of the responses to other classes of stimuli; nevertheless the tracking eye-movements did not contain harmonics or sum and difference frequencies of the target motion.

Processing of direction and magnitude by the saccadic eye-movement system.

Abstract The function of the saccadic programming system has been studied using a doublestep target movement in two dimensions. The results of these experiments suggest the following hypotheses. The information processing in the programming of a saccade consists of direction computation and magnitude computation. If the new target-step arrives before the direction computation of the previous saccade is complete, this saccade will be cancelled. The partial program concerning the direction of the saccade is kept in a buffer memory: if the direction of the new saccade is not in a direction similar to the old one, this partial program has to be erased, which takes an extra 40–80 msec of processing time. There is a stage in which the direction of the saccade cannot be reprogrammed but the magnitude can still be reduced. In other words, the magnitude computation seems to finish after the direction computation. If the new target-step arrives after the computation of both direction and magnitude are complete, two full saccades will be observed.

Methods for the Localization of Electrical Sources in the Human Brain

Publisher Summary This chapter discusses the methods for the localization of electrical sources in the human brain. Although there has not been general agreement about the mechanism of evoked scalp potential generation, most investigators have proceeded from the belief that these potentials in some way directly or indirectly reflect the neuroelectric activity of the brain in the processing of sensory stimuli. As such, numerous authors have attempted to interpret evoked scalp potentials in terms of their neuroelectric sources in the brain. In strictly mathematical terms, this problem has long been known to be ill-posed and thus to possess no unique solution. Even with complete knowledge of the electrical and geometrical characteristics of the head itself, it is impossible to convert surface potential information alone into information concerning the actual distribution of sources in the brain. In spite of the limitations imposed by theoretical non-uniqueness, it is possible that useful information about the origin of evoked scalp potentials may be obtained by the solution of a less general problem that restricts the form of neural activity which might lead to scalp potentials.

Nonlinearities of the human oculomotor system: Time delays

Abstract Measurements of the gain of the human eye movement control system in two-dimensional tracking tasks show that the system is non-linear in the sense that it is not possible to predict the response to one class of target motion by linear combination of the responses to other classes of stimuli. It is therefore not valid to represent the system by a linear transfer function and hence a minimum phase lag cannot be calculated. There is thus no theoretical datum with which to compare the actual phase lags of the visual axes in tracking tasks and hence no evidence for a predictor mechanism. The lags of the visual axes can be satisfactorily explained in terms of simple delays which depend on the class of target motion.

Spatio-temporal visually evoked scalp potentials in response to partial-field patterned stimulation ☆

Visually evoked scalp potentials (VESP) have been recorded at 40 electrode sites from each of 3 subjects. Pattern appearance/disappearance was used for whole field and partial field stimulation. The data are displayed as equipotential maps. The topographical features of the equipotential maps show periods of stable organization followed by periods of relatively rapid change. The structure of the maps changes in a consistent pattern with the region of the retina stimulated; the first peak fits well within the framework of the cruciform model of striate cortex. The first and second peaks of the VESP appear to be caused by independent neural generators. The work reported in this paper shows major agreement with other authors and reconciles some points of disagreement between them.

Field studies of averaged visually evoked EEG potentials in a patient with a split chiasm

Abstract Averaged potentials evoked by monocular 3.7 f/sec light flash stimulation were studied in a subject with bitemporal hemianopsia caused by a traumatic split of the chiasm. In symmetrical recordings from bilateral parieto-occipital linkages, the evoked responses showed comparable wave forms but inverted polarity. A single source of electrical activity in the cortex ipsilateral to the stimulated eye, 3 cm from the midline, and 6 cm above the inion, accounted for almost all of the evoked response. A second minor source was active from 160 msec after the flash.

Component analysis of human averaged evoked potentials: dichoptic stimuli using different target structure.

In dichoptic viewing conditions, human subjects were shown subjectively superposed targets for the left and right eye. The left eye saw a steadily illuminated field containing different amounts of structure, and the right eye saw repetitive light flashes of constant parameters. Averaged EEG evoked responses were recorded from occipito-parietal electrodes; means responses and their standard deviations were computed for each subject, weighted average responses were computed for the whole subject population. It was found that the RMS value of the evoked responses decreased monotonically with increase in target structure. An analysis of the average responses into Gaussian components showed that peak latencies and relative peak amplitudes did not change; i.e., that no change in shape of the responses of individual subjects, analysis applied responses of individual subjects, however, leads to the conclusion that shape changes do occur; but these changes are different from one subject to another and are not characteristic of the entire subject population. The reduction of the evoked responses was interpreted as a consequence of increased informational load upon the neural populations whose activity is synchronized by the flash stimulus to produce the evoked response.