Elwin Marg

Good Visual Function after Neonatal Surgery for Congenital Monocular Cataracts

We treated eight neonates who had total, monocular, congenital cataracts with surgery, occlusion, and contact lenses. Visual results in all eight patients were good. In five patients visual acuities improved to 6/9 (20/30) or better in the aphakic eye. In three patients visual acuities improved to 6/24 (20/80) or better. Problems with contact lenses probably accounted for the poorer results in two of these three patients. Binocularity was not demonstrated in any of our patients.

Finding the depth of magnetic brain stimulation: a re-evaluation

The depth of threshold magnetic nerve stimulation can be estimated by using thresholds from two different-sized stimulus coils and plotting their induced electric field vs. depth profiles. Stimulation is presumed to take place where the two field profiles are equal. If the two coils have unequal inductances, however, there is a relative shift in threshold between coils that alters the intersection point and the apparent stimulus depth. This systematic error arises from two sources: (1) there is a difference in the fraction of stimulator energy reaching each coil, and (2) pulse durations are different, causing threshold shifts governed by the nerve strength-duration curve. Both sources of error are additive. If the larger coil has the lesser inductance, stimulus depth is underestimated; if it has the greater inductance, it is overestimated. This can lead to large disparities in the measured depth, depending on the sets of coils used. In this paper, we show how to correct for errors introduced by unequal inductance and how this resolves discrepancies in depth measurement. Our own depth measurements in the motor area for threshold finger movements, and recalculated depths from Epstein et al., indicate that stimulation is slightly deeper (18-21 mm, average 19+ mm) than previously thought. This suggests that threshold magnetic stimulation in the motor area may arise from large, tangentially oriented fibers in the superficial white matter, or in the gray matter at the upper sulcus or lip of the gyrus.

Phosphenes induced by magnetic stimulation over the occipital brain: description and probable site of stimulation.

Phosphenes were elicited by brief, intense magnetic pulses directed to the occipital area of the brain with two different magnetic stimulators and various coils. The observed phosphenes were described or sketched by the subject. Phosphenes were usually wedgeshaped flashes of light in the midperiphery, although occasionally structured phosphenes were reported (stripes or grids). The depth of effective stimulation was measured by determining the phosphene threshold for two different size coils. Additional depth measurements were made at the cortical motor strip for threshold finger twitches. The visual stimulation site was clearly deeper (~4 cm) than the site for motor stimulation (~2 cm), and lay near the midline. Both foveal and peripheral phosphenes had identical stimulation depths, implying a subcortical stimulation site, possibly in the optic radiation fibers adjacent to the posterior tip of the lateral ventricles. Fibers closest to the ventricle, representing the horizontal meridian of the visual field, would be preferentially stimulated, in agreement with experimental results.

FAST, AUTOMATIC, ELECTRONIC TONOMETERS BASED ON AN EXACT THEORY

We have been concerned about the need for a fast, automatic, direct-reading tonometer, which is accurate, repeatable and gentle. An instrument with any of these characteristics is desirable but one with them all would be extremely useful. Such an instrument would be of great value not only to the ophthalmologist in his diagnosis and treatment of glaucoma but also to the optometrist who would be able to determine intraocular pressure in his patients without using the topical anesthetics present corneal tonometers demand. Previous tonometers, mechanical or electronic, measure either the indentation of the cornea produced by a certain size and shape plunger of a given weight, or they measure the force required to flatten a given area of the cornea. W e will not discuss these classical instruments, details of which are available in the textbooks (1). Measurement with all classical tonometers requires experience, they are somewhat tedious in their use, and are based upon questionable assumptions. Some of the tonometers to be discussed are subject to some of these same restrictions while others are quite free of them and involve basically different principles.

Automatic Tonometer with Exact Theory: Various Biological Applications

Methods for externally measuring pressure within body cavities exist. In tonometer form they are fast and gentle not requiring anesthetics for the cornea of the human eye. Readings are accurate and independent of tissue tension, corneal stiffness, astigmatic curvatures, and surface tension. There are also separate indications of corneal rigidity and relaxation, and tonographic fluid expression. Other applications include monitoring blood pressure, uterine contractions, and infant intracranial pressure.

MAGNETOSTIMULATION OF VISION : DIRECT NONINVASIVE STIMULATION OF THE RETINA AND THE VISUAL BRAIN

The history of magnetophosphenes and their closely related predecessor, electrophosphenes, is described from the mid-18th century to the present time. The current era of magnetic stimulation started in 1985 with the development of a practical capacitor-discharge electromagnetic stimulator by Barker and his colleagues at the University of Sheffield, and their application of it to the brain with Merton and Morton at the National Hospital, London. The safety of magnetostimulation of the brain is discussed as well as the advantages of magnetostimulation over electrostimulation. Principles of magnetostimulation of nerves and magnetic measurement are considered. Effects on motor and sensory systems of the brain are described including magnetic perceptual suppression in the visual cortex and other pioneering work of Amassian, Cracco and Maccabee at SUNY Health, Brooklyn. Magnetophosphenes from retinal and cortical magnetostimulation are distinguished. Now that visual cortical stimulation is possible with the strong magnetic pulses generated by capacitor-discharge instruments, the functional viability of the visual cortex may be tested directly and noninvasively.

THE ACCESSORY OPTIC SYSTEM.

Classically, three pairs of retinal projections to the brain are described : to the lateral geniculate body, to the superior colliculus, and to the pretectal nucleus. In our laboratory we have been studying the less-known accessory optic system which projects from the retina to the midbrain. Three pairs of such tracts have been described: the anterior accessory optic tracts (Bochenek) , the posterior accessory optic tracts (tractus peduncularis transversus of Marburg), and the transpeduncular tracts (tractus peduncularis of Gudden) . In present day nonmammalian vertebrates these paths are represented by only one discreet pair of tracts, the basal optic roots. Thus the accessory optic system is known in all classes of vertebrates.l It might be asked, is the accessory optic system a possible (or probable) photo-neural input for the triggering of circadian or other rhythms? In attempting to answer this question we shall draw largely on research done in our laboratory by Roland A. Giolli, Duco I. Hamasaki, Richard M. Hill, Roc E. Walley, and myself.

Positron-emission tomographic study of human amblyopia with use of defined visual stimuli.

PURPOSE The purpose of this study was to use positron emission tomography (PET) to evaluate effects of amblyopia on cerebral blood flow and glucose metabolism in humans viewing defined visual stimuli and to correlate these effects with specific behavioral and electrophysiologic measures of visual function. METHODS One subject with normal vision and five patients with amblyopia were prospectively studied. During monocular viewing of a checkerboard reversal stimulus by each subject, we performed PET imaging of relative cerebral glucose metabolism with use of [18F]fluorodeoxyglucose, PET imaging of relative cerebral blood flow with use of H2(15)O, and visual evoked potentials. Control studies were also performed with use of binocular occlusion and during presentation of stationary and horizontally drifting checkerboards. These data were correlated with letter acuities and contrast sensitivity functions for each eye. RESULTS Although spatial resolution was superior for glucose metabolic imaging, PET readily demonstrated activation of calcarine cortex with use of both metabolic and blood flow tracers. Even in patients with mild amblyopia, functional activation of calcarine cortex was reduced in amblyopic eyes compared with sound eyes to a degree more closely correlated with visual acuity than were visual evoked potential amplitudes to the same stimulus. When responses to drifting versus stationary stimuli were compared, a putative motion processing center was identified in the right temporoparietal region. Activity in this motion center was relatively preserved during viewing of drifting stimuli by the affected eye of an anisometropic amblyopic subject, but was attenuated during viewing of the same stimulus by the affected eye of a strabismic amblyopic subject. CONCLUSIONS PET imaging of blood flow and metabolism can quantitatively evaluate functional deficits resulting from amblyopia in striate as well as extrastriate visual areas. Calcarine cortical function correlates closely with severity of amblyopia, but function in a putative motion processing area may vary according to the type of amblyopia present.

Physiological Nystagmus in the Cat

Cats were prepared by the encephale isole technique. A small plane-mirror mount was sutured to each cornea without obstructing vision, and eye movements were recorded using the optical-lever method. The records showed physiological nystagmus similar to that in man, although the cats had fewer and smaller saccades. Some saccades were binocular, but usually they were uniocular. The fine tremor varied in frequency from 35–65 cps, averaging 50 cps, and in amplitude from 4–52 sec of arc, averaging 22 sec. Curare decreased and ultimately abolished eye movements and physiological nystagmus, whereas neostigmine increased them. Physiological nystagmus is therefore mediated by efferent neural stimulation of the eye muscles. Tremor was also recorded from the detached inferior oblique muscle, as well as from the eyeball with most of the extraocular muscles detached. Fine tremor was also found in finger pointing in man. The significance of physiological nystagmus to vision is briefly discussed.

Indwelling multiple micro-electrodes in the brain

Abstract The free-cluster indwelling micro-electrode bundle effectively records single neural cells in an array in the brain. Application to neurosurgery with an implantable micro-drive is described.