Carbon disulfide effects on the visual system. II. Retinogeniculate degeneration

We examined the morphological effects of carbon disulfide exposure on neurons and vasculature of the visual system of macaque monkeys. Five monkeys were exposed to 256 ppm carbon disulfide (CS2) by inhalation for 6 hr a day, 5 days a week. One monkey, sacrificed immediately after exposure, had numerous axonal swellings in the distal optic tract. Four other monkeys survived the exposure period for at least 1 year and were found to have suffered marked degeneration of central retinal ganglion cells, with little or no effect on other neurons in the retina. No evidence was found for arteriosclerotic or aneurysmal changes, suggesting that visual system injury in primates induced by carbon disulfide exposure is not dependent on the occurrence of structural changes in retinal blood vessels.     

Acrylamide effects on the macaque visual system. I. Psychophysics and electrophysiology

Oral acrylamide produces axonal swelling and later degeneration and gliosis in the distal optic tract, especially within the lateral geniculate nucleus, of macaque monkeys. Measures of visual thresholds and cortical-evoked potentials were used to study the time course of visual changes during exposure to acrylamide in macaque monkeys. Contrast sensitivity, visual acuity, and flicker fusion frequency were reduced during exposure, and only flicker fusion recovered rapidly and completely after exposure. Pattern-reversal-evoked responses exhibited increased latency and reduced amplitude during dosing but substantially recovered after exposure. Visual acuity and contrast sensitivity for high spatial frequencies were decreased throughout the 140 days of testing after dosing. These results suggest an acute general depression of visual capacities as the initial effect of acrylamide exposure, whereas later effects were confined to high spatial frequencies.     

Residual eye-movements in macaque and their effects on visual responses of neurons

We recorded continuously, with high precision, the positions of the eyes in anesthetized macaque monkeys prepared for physiological recording. Most recordings were made after the infusion of muscle relaxant to immobilize the eyes; in some cases we also were able to record eye position for periods before the eyes were immobilized. In all monkeys, the eyes moved continuously by as much as 0.5 deg over a 10-min sampling period. The average distance moved was proportional to the square root of the sampling period, as would be expected from a random walk. The movements had three distinct components: slow drifts, and two rhythms driven by the pulse and respiration. The rhythmic movements occurred only under paralysis: they were not discernible in measurements made before the infusion of muscle relaxant. The movements of the eye in the paralyzed animal can have substantial effects on the measured physiological characteristics of neurons. For excursions in the midrange of those we observed, a neuron's sensitivity to a spatial frequency of 10 cycle/deg might be underestimated by as much as a factor of three, depending on the method by which responses were averaged. We show how the effects of eye-movements can be mitigated by appropriate data analysis.     

Carbon disulfide effects on the visual system. I. Visual thresholds and ophthalmoscopy

The visual effects of carbon disulfide exposure were studied in macaque monkeys with measurements of visual thresholds, fluorescein angiography and fundus photography. Five monkeys were exposed by inhalation for 6 hr a day, 5 days a week to 256 ppm carbon disulfide (CS2). The motor dysfunction observed in these monkeys appeared to be entirely reversible. All five suffered severe reductions in visual acuity and contrast sensitivity although flicker resolution was not affected. Visual loss was found to be irreversible, with degeneration of substantial numbers of retinal ganglion cells (companion paper) in those monkeys permitted to survive after the termination of exposure. None of the monkeys developed retinal microaneurysms or hemorrhages, major accepted signs of visual toxicity in CS2 exposed humans; thus, permanent visual loss may result from carbon disulfide exposure even in the absence of retinal vascular effects.     

Age-related maculopathy. I: A review of its morphology and effects on visual function

Age-related maculopathy (ARM) is a leading cause of permanent vision loss in elderly people. ARM therefore constitutes an important public health problem which will increase in magnitude as the number of aged people in the general population becomes greater. The consequences of this condition are exacerbated by the fact that treatment, especially of the atrophic form of the disease, is ineffective. While laser photocoagulation may be helpful in the exudative form of ARM, there is often an inexorable progression towards severe vision loss in these patients. Therefore considerable attention needs to be paid to the aetiology of ARM, the potential for its prevention or delayed onset and its recognition through functional disturbances. This is the first of three papers dealing with ARM and its effects on visual function. We review its morphology and the visual disturbances that may ensue. The second and third papers will discuss the nature and detection of the central visual field loss due to ARM.     

The effects of short-term experimental strabismus on the visual system in Macaca mulatta.

Experimental esotropia was produced surgically in infant rhesus monkeys (Macaca mulatta) for brief periods of time. Electrophysiological studies of the visual cortex showed that esotropia of only 2 weeks duration in an infant monkey is sufficient to cause a marked shift of dominance in favor of the fixating eye and to virtually extinguish cortical neuronal responses from the esotropic eye. In the lateral geniculate nucleus cell (LGN), shrinkage of 6% to 8% occurred in the parvocellular layers connected with the esotropic eye, the magnocellular layers showing no changes. After the fixating eye had been sutured for 3 weeks, a complete reversal of the cortical physiology in favor of the esotropic eye occurred, whereas no recovery in cell size was observed in the LGN. Surgical realignment of an esotropic eye caused recovery of cortical neuronal responses from the formerly esotropic eye, but the number of binocularly responsive cells remained reduced.     

The effects of Depo-Medrol preservative on the rabbit visual system

Periocular injections of corticosteroids play an important role in the management of various ophthalmologic diseases. The Depo-Medrol vehicle, injected into the vitreous, was shown to be toxic to the lens and to the retina when applied at double strength. The authors examined the effects of Depo-Medrol and one of the components of its vehicle, myristyl-gamma-picolinium chloride (MGP), on the functional integrity of the rabbit visual system. Visual function was assessed objectively from the electroretinogram (ERG) and the visual evoked potential (VEP). The experimental drugs were injected into the vitreous humor of one eye while saline was injected into the fellow eye for control. Depo-Medrol did not produce any measurable effects on the ERG or the VEP. When MGP solutions were injected in concentrations at least twice as large as that in the Depo-Medrol, significant reductions in the light- and dark-adapted ERG responses were seen. The effects of the drug on the ERG responses was seen as early as 3 days postinjection and developed to its maximal level within 1-2 weeks. No ERG recovery was seen over a period of more than 2 months. The VEP, elicited by applying light stimuli to the experimental eye, was characterized by low amplitude and delayed implicit time compared with the response obtained from the control eye.     

Reprint of: The effects of distractors and spatial precues on covert visual search in macaque

Covert visual search has been studied extensively in humans, and has been used as a tool for understanding visual attention and cueing effects. In contrast, much less is known about covert search performance in monkeys, despite the fact that much of our understanding of the neural mechanisms of attention is based on these animals. In this study, we characterize the covert visual search performance of monkeys by training them to discriminate the orientation of a briefly-presented, peripheral Landolt-C target embedded within an array of distractor stimuli while maintaining fixation. We found that target discrimination performance declined steeply as the number of distractors increased when the target and distractors were of the same color, but not when the target was an odd color (color pop-out). Performance was also strongly affected by peripheral spatial precues presented before target onset, with better performance seen when the precue coincided with the target location (valid precue) than when it did not (invalid precue). Moreover, the effectiveness of valid precues was greatest when the delay between precue and target was short (∼80–100 ms), and gradually declined with longer delays, consistent with a transient component to the cueing effect. Discrimination performance was also significantly affected by prior knowledge of the target location in the absence of explicit visual precues. These results demonstrate that covert visual search performance in macaques is very similar to that of humans, indicating that the macaque provides an appropriate model for understanding the neural mechanisms of covert search.     

Coding of visual stimulus velocity in area MT of the macaque

We have studied the interaction of the direction and speed selectivities of neurons in cortical visual area MT of the macaque monkey. For a given cell, preferred direction and the shape of the direction tuning curve for moving edges were similar at different stimulus speeds, and deviations from the optimal speed did not systematically alter direction tuning bandwidth. Similar speed tuning was obtained for responses to motion in the preferred and anti-preferred directions even when the response to anti-preferred motion was an inhibitory one. The results are discussed in terms of the unique contributions of area MT to visual motion analysis.     

Anatomical segregation of two cortical visual pathways in the macaque monkey

A number of lines of evidence suggest that, in the macaque monkey, inferior parietal and inferotemporal cortices process different types of visual information. It has been suggested that visual information reaching these two subdivisions follows separate pathways from the striate cortex through the prestriate cortex. We examined directly this possibility by placing injections of the retrograde fluorescent tracers, fast blue and diamidino yellow, in inferior parietal and inferotemporal cortex and examining the spatial pattern of cortical areas containing labeled cells in two-dimensional reconstructions of the cortex. The results of injections in inferotemporal cortex show that TEO receives afferents from areas V2, ventral V3, V3A, central V4, V4t, and DPL in prestriate cortex and from areas IPa, PGa, and FST in the superior temporal sulcus (STS). Area TEp receives afferents only from V4 in prestriate cortex and from IPa, PGa, and FST in the anterior STS. Area TEa receives no prestriate input and is innervated by IPa, PGa, FST, and TPO in the anterior STS. The results of injections in inferior parietal cortex demonstrate that POa receives afferents from dorsal V3, V3A, peripheral V4, DPL, and PO in prestriate cortex, from MST and *VIP and from IPa, PGa, TPO, and FST in anterior STS. Area PGc (corresponding to 7a) is innervated by PO, MST, and by TPO in the anterior STS. Examination of the two-dimensional reconstructions of the pattern of labeling after combined injections of fast blue and diamidino yellow in areas POa and TEO revealed that these areas are principally innervated by different prestriate areas. Only a small region, centered on area V3A and extending into V4 and DPL, contained cells labeled by either injection as well as a small number of double-labeled cells. In contrast, areas POa and TEO receive afferents from extensive common regions in the anterior STS corresponding to areas IPa, PGa, and FST. These results directly demonstrate that visual information from the striate cortex reaches inferior parietal and inferotemporal cortices through largely separate prestriate cortical pathways. On the other hand, both parietal and inferotemporal cortices receive common inputs from extensive regions in the anterior STS which map play a role in linking the processing occurring in these two cortical subdivisions of the visual system.     

Diazepam and its effects on visual fields.

A patient who experienced severe visual field loss whilst taking 100 mg of diazepam that reverted to normal on cessation of the drug is described. Diazepam affects GABA inhibitory neurones and the physiology of this in the retina and visual cortex is reviewed.     

Neurological syndromes involving the visual system.

Because optometrists are the primary deliverers of eye care, their role in pathology detection is of primary importance. The neurological syndromes of the visual system have been reviewed and put into a chart format as an aid to the optometric clinician.     

Background light adaptation of the retinal neuronal adaptive system. II. Dynamic effects

The dynamic effects of continuous exposure to light on the neuronal adaptive system of the retina, as indicated by the oscillatory response (OPs) of the electroretinogram (ERG) were studied in the albino rat. Digitally filtered OPs and the a- and b-waves of the corneal ERG were simultaneously recorded in dark adaptation, during continuous light adaptation to four levels of background light (BGL) changing in steps of two log units from 1.43×10^–6cd/m², referred to as low and high scotopic, low and high mesopic levels. Exposed to high scotopic BGL the total oscillatory response (SOP) significantly enhanced within the first minute, whereas the amplitudes of the a- and b-waves were unaffected. In low mesopic BGL the SOP increased within the first minute, whereas the a- and b-waves significantly decreased. High mesopic BGL instantaneously and profoundly reduced both the SOP and the slow potentials. The individual OPs changed in amplitudes mainly within the first minute of BGL. In general, the earlier OPs (O1 and O2) reacted more to the two scotopic BGL levels, whereas the later OPs (O3 and O4) were more affected by the relatively brighter two mesopic conditions. In conclusion, the rapid increase of the OPs within the first minute of high scotopic and low mesopic BGL exposure may represent a rudimentary light adaptational effect in the rod-dominated rat retina. These findings also suggest that the neuronal adaptive mechanism of the retina seems to be a robust system, probably attaining preservation of visual abilities in the rat on exposure to light.     

The UTAS-E1000 System in electrophysiological studies of the visual system. II. Programming and clinical use

The authors discuss a set of electrodiagnostic tests of the UTAS-E1000 system and their personal work on new tests. They present the scope of applications of the equipment in examinations of some pathological conditions of the visual system and--on the basis of their personal experience--the possibilities of introduction of new electrodiagnostic tests.