Saccade control without a fovea.

Prior research has suggested that two types of fast eye movements (FEMs) can be distinguished behaviorally. Foveating saccades respond to salient peripheral targets by directing the target image to the fovea. Non-foveating saccades include other FEMs such as nystagmus quick phase, saccades without visual stimuli and visually-directed saccades that direct target images to eccentric retina. Foveating saccades have a shorter initiation latency and are faster than non-foveating saccades. Following adaptation to central scotoma, patients tend to use preferred retinal loci for fixation (PRL). If PRL acquire the foveal characteristic of a retino-motor center then visually guided saccades would acquire the properties of foveating saccades. Using an objectively-calibrated 2-dimensional search coil, we measured saccades in response to salient, unpredictable targets. The saccades of normal observers were compared to the saccades of patients with long-standing macular scotomas. Although the saccades of patients consistently directed images to PRL, the saccades still had the latency and dynamic characteristics of non-foveating saccades. Moreover, the non-foveating saccades of patients were found to be less accurate than foveating saccades, showing a range effect (larger saccades undershoot with greater error than do smaller saccades). Apparently, patients with macular scotoma suppress rather than adapt a foveating saccade mechanism.

Foveating saccades.

A review of the literature revealed that foveating saccades were found to be faster than other fast eye movements (FEMs) except optokinetic nystagmus (OKN) quick phase. In the present experiment, foveating saccades were compared to OKN quick phase in humans and were found to have higher maximum speeds and shorter durations. Unlike previous experiments, foveating saccades were made to targets at unpredictable distances, and active pursuit during OKN was discouraged. Previously reported differences between the speeds of foveating saccades and saccades to remembered target positions were replicated. Foveating saccades, therefore, can be distinguished from other FEMs on the basis of speed. This behavioral difference suggests that a distinctive mechanism exists for foveating targets.

Eccentric fixation with macular scotoma.

People with macular scotoma tend to read and visually scan more slowly than others with equivalently reduced visual acuity but intact central fields. We measured fixation eye movements and considered the contribution of fixation variability and centripetal eye drift to poor visual performance. These factors might confound efforts to consistently use an optimum retinal locus outside of the macula. We measured monocular horizontal and vertical eye movements using a search coil eyetracker while subjects with naturally occurring central scotomata or control subjects with simulated scotomata eccentrically fixated a single character that was sized to their visual acuity. Motivated subjects with long-standing stable maculopathies were chosen to estimate attainable performance limits. During attempts to eccentrically fixate, an ubiquitous foveal pursuit or centripetal drift tendency was not found; rather a pattern of drift was idiosyncratic from subject to subject. This finding was confirmed by an analysis of eye drift of 32 eyes with long-standing bilateral macular scotomata. Moreover, the eye drift speeds (15-200 minarc/sec) were too low to be of functional significance. Drift speeds during eccentric fixation with a visible target were not significantly different than those after the target was extinguished; however, drift speeds were greater than during foveal fixation. This suggests that the fovea has a specialized control of slow eye movements. Fixation variability increased with scotoma size for both simulated and real scotomata, with an abrupt rise when scotomata diameters exceeded 20 degrees C. A significant minority of subjects (39%) adopted two or more distinct preferred retinal loci (PRL) during fixation. Multiple PRL were also more likely if scotoma size exceeded 20 degrees C. Reasonably steady fixation is thus attainable when central scotoma sizes are smaller than approximately 20 degrees C.

Scanning characters and reading with a central scotoma.

Among the patients we tested, most of those who had lost normal macular function developed extrafoveal retinal loci which resulted in strongly preferred viewing angles for steady fixation. These patients used these extrafoveal retinal loci to scan lines of characters in patterns similar to patients with normal macular function. Although the scanning speed of these patients was adversely affected by their large size scotomas, their reading accuracy was not necessarily reduced. The assessment of scanning and reading performance in this population requires careful measurement of the visual skills involved in reading.

Pseudoretinitis pigmentosa.

A 52-year-old black male with Duane's retraction syndrome and fundus pigmentation was examined. The patient had high hyperopia and reported ocular problems in other family members. A battery of tests indicated that the pigmentary anomalies were not due to retinitis pigmentosa.

An accessory fill-in flash for the Nikon photo-slit lamp.

High-quality slit lamp photographs require a balance between slit beam exposure and exposure of surrounding ocular structures. Proper exposure of surrounding ocular structure is difficult to obtain with the Nikon photo-slit lamp. The purpose of this research was to design an accessory fill-in flash for this slit lamp, which would provide proper exposure of over all ocular structures. In addition, this paper describes the procedures used to obtain proper exposure values and provides an exposure table for this accessory flash unit.

An improved electrode for electroretinography: design and standardization.

An electrode for electroretinography was developed that fits over the patient's lower lid. The electrode incorporates a platinum strip embedded in a piece of shaped plexiglass. The electrode was standardized for clinical use by comparing ERG recordings from this platinum lid electrode with those recorded from a Burian-Allen electrode. The a- and b-wave implicit times and amplitudes were measured in four subjects, using both electrodes under varying luminance conditions, to provide a measure of the validity of the platinum lid electrode. The same experimental procedure was repeated five times on two individuals to provide a measure of the reliability of both electrodes. Results indicate amplitudes approximately 65% of those recorded with a Burian-Allen electrode but which are otherwise in good agreement.

Recording monocular VEPs under binocular conditions.

This report describes a technique for recording the visual evoked potential (VEP) arising from 1 eye under conditions of binocular pattern stimulation. The subject views a sinusoidally modulated vectographic checkerboard with orthogonally oriented polarizing filters before the eyes. This technique results in waveforms that contain monocular VEPs distributed in time. The technique is sensitive to interocular suppression and controls for the influence of various extraneous sources of variability. It is useful in clinical VEP testing.