Robert P. Scobey

Intracranial pressure during epileptic seizures.

A comatose 31-year-old male with presumed viral encephalitis and frequent partial motor seizures was paralyzed with pancuronium in an attempt to reduce recurrent elevation of intracranial pressure (ICP) associated with each seizure. ICP was continuously monitored with a Richmond Bolt and 5 electrographic seizures originating in the left frontal area were recorded. Each ictal episode was associated with stable blood pressure and an increase of ICP. The average seizure duration was 78 +/- 17 sec (mean +/- S.D.) and the average maximum increase of ICP above baseline during the seizures was 6.5 +/- 0.6 mm Hg with average peak ICP of 16.0 +/- 0.86 mm Hg. A simple mathematical model predicts the rate of increase of ICP, the peak ICP, the phase difference between maximum spike frequency and maximum ICP, and the rate at which ICP returns to pre-ictal values after termination of the seizure. The predicted values of ICP closely approximate the experimentally derived data. Therefore, the time course of the ICP appears to be determined by the frequency of the fundamental units of abnormal synchronized activity (the epileptogenic spike) and the CSF pressure-volume dynamics existing at the time of the seizure. An average increment of ICP per spike can be calculated for each seizure. The model also predicts that patients may develop high ICPs due to prolonged seizures. Prolonged unrecognized seizures may occur in patients who are therapeutically paralyzed as demonstrated by the case described here.

Foveal and peripheral displacement thresholds as a function of stimulus luminance, line length and duration of movement

Abstract Displacement thresholds for moving line targets were determined in the fovea and periphery (18° eccentricity) as a function of duration of movement, line length and stimulus luminance. At both visual field locations, displacement thresholds were essentially constant for durations of movement between 5 and 500 msec. Longer durations of movement produced progressively higher displacement thresholds and greater response variability. Stimulus luminance and line length displayed little or no influence on foveal displacement thresholds. In contrast, peripheral displacement thresholds became significantly reduced as a function of both increasing line length and higher luminance values. Preliminary comparisons are drawn between the current psychophysical findings and retinal receptive field properties.

Effects of reference lines on displacement thresholds at various durations of movement

Displacement thresholds were determined for durations of movement between 10 msec and 2.5 sec, with and without the presence of a reference line. For all durations of movement, displacement thresholds were lower when a reference line was present. The magnitude of this effect was essentially constant across all durations of movement. These data suggest that previous reports of differential effects of reference lines on long and short durations of movement depend upon the stimulus paradigm employed.

Properties of K+ Conductances in Cat Retinal Ganglion Cells During the Period of Activity‐mediated Refinements in Retinofugal Pathways

During ontogeny retinal ganglion cells manifest pronounced changes in excitable membrane properties. To further our understanding of the ionic conductances underlying such functional changes, the whole‐cell voltage‐clamp variation of the patch‐clamp technique was used to record potassium currents in 220 ganglion cells dissociated from cat retinas ranging in age from embryonic day 31 to postnatal day 10. Potassium currents were isolated by blocking voltage‐gated Na+ and Ca2+ currents with tetrodoxin (TTX) and CoCl2 respectively and were characterized by their pharmacology, kinetics and voltage‐dependence of activation and inactivation. In all cases, a combination of three currents accounted for the total outward calcium‐independent K+ current: (i) a steady linear conductance; (ii) a voltage‐gated transient current, lA, and (iii) a voltage‐gated sustained current, lk. Both voltage‐gated currents were affected by the application of 4‐aminopyridine and tetraethylammonia (TEA): lA showed a greater sensitivity to 4‐aminopyridine, while lk was more sensitive to TEA. Both voltagegated currents were present throughout the developmental period examined; however, the percentage of retinal ganglion cells (RGCs) expressing lA showed a marked decline from 82% at E31 to 45% at postnatal ages. During this developmental period there was an increase in the density of the two voltage‐gated and the linear conductance. Additionally, with maturation, significantly slower inactivation kinetics were observed for lK. These findings, and our previous results dealing with maturational changes in the TTX‐sensitive voltage‐gated Na current, are related to the generation of excitability in developing retinal ganglion cells. Furthermore, the presence of cells with and without transient K+ conductance throughout development suggests that the different spiking patterns observed in RGC classes may be partially due to differences in their membrane properties.

Displacement thresholds for unidirectional and oscillatory movement

Abstract Displacement thresholds were determined for unidirectional linear movement (A-B motion) and oscillatory movement (A-B-A motion) for various periods of motion. Both functions exhibited similar increases in motion thresholds for durations of movement greater than 800 msec, and nearly constant motion thresholds for durations of movement between 300 and 800 msec. At shorter durations, A-B motion thresholds remained essentially constant, whereas A-B-A (oscillatory) motion thresholds displayed a dramatic increase. The introduction of a pause at point B of the oscillatory movement decreased motion thresholds at short durations, supporting the hypothesis that temporal integration of the stimulus luminance at point B was a critical factor underlying the degraded performance. The temporal integration hypothesis was also supported by a subsequent experiment in which luminancetime trade-offs were demonstrated for motion thresholds at short durations of movement. These findings indicate that stimulus displacement is a primary determinant of movement detection sensitivity.

Detection of image displacement by phasic cells in peripheral visual fields of the monkey

Abstract Impulse histograms for monkey phasic ganglion cells were measured following either the rapid displacement of a small luminous spot or an incremental change in intensity of the same spot. Phasic cells responded with a short burst of action potentials during a 100-msec interval. The smallest distance that the luminous spot must move to evoke a brief audible increase in the ongoing neural activity, the displacement threshold, was found to be less than human thresholds at equivalent retinal sites. Analytic expressions are given for receptive field sensitivity profiles to stationary flashing light, and for the displacement thresholds through the receptive fields.

A horizontal stripe of displacement sensitivity in the human visual field

Displacement thresholds of peripheral sites in monocular human vision were obtained. The average of 12 directional thresholds at different visual field sites was used to define isometric lines of average displacement threshold about central vision. Isometric lines extended further into the temporal visual fields along the horizontal meridian than along other meridians. At any single site in the peripheral visual field the thresholds were not the same in all directions; they were larger toward and away from central vision. These two psychophysical findings vary in a qualitatively similar manner across the retinal field, as does the average size and the collected orientation bias of dendritic fields of retinal ganglion cells.

Movement sensitivity of retinal ganglion cells in monkey

Abstract The responses of on-center monkey retinal ganglion cells to small displacements of a spot light within the receptive field center were studied as a function of spot luminance. One group of cells reached peak rates of firing near 1000 spikes per sec with small displacements of spots with moderate and high contrast. These units had non-opponent color characteristics, relatively large receptive fields and phasic responses to a stationary flashing light. Easily distinguished from these cells were a group which had a maximum firing rate to displacement at moderate contrast. Above and below this optimum luminance, the firing rate decreased to zero. These cells had lower peak firing rates (100–200 spikes per sec), opponent color characteristics, tonic responses to stationary flashing light and relatively small receptive fields. All retinal ganglion cells isolated were potential candidates for detecting and transmitting movement information. For any individual units, histograms of responses to small displacements were very similar to histograms of responses to the stationary flashing spot. An ensemble code (cross-fiber analysis) is proposed to account for transmission of visual form and motion information to the central nervous system via tonic (x-type) and phasic (y-type) units.

The detection of small image displacements by cat retinal ganglion cells

Abstract The sensitivity of the cats retinal ganglion cells to rapid displacement of a small luminous spot was measured by determining the minimum distance that the luminous spot must move to evoke an audible increase in the ongoing neural activity (the displacement threshold). The displacement threshold was not directly related to the gradient of receptive field sensitivity nor was the minimum displacement threshold a simple function of receptive field size. Rather, the smallest displacement threshold was found within, but not throughout, the border of the central zone which had an exponentially decreasing sensitivity to stationary flashing light. At the optimal location, “on center” and “off center” retinal ganglion cells could encode a small displacement of a constant intensity image through a difference in receptive field sensitivity of 0.07 log units.

Effects of retinal eccentricity on displacement thresholds for unidirectional and oscillatory stimuli

Motion sensitivity was determined for line stimuli undergoing unidirectional and oscillatory displacements at various retinal eccentricities. Oscillatory motions of short duration were found to produce consistently greater motion thresholds than unidirectional stimuli at each eccentricity tested. Increasing retinal eccentricity elevated motion thresholds similarly for both forms of stimulus motion. The observed differences between unidirectional and oscillatory stimuli are attributed to differences in the temporal summation of the two forms of stimuli at short duration. These findings also suggest that differences in motion sensitivity between central and peripheral vision are quantitative rather than qualitative.