Spectacle lenses alter eye growth and the refractive status of young monkeys

The influence of visual experience on ocular development in higher primates is not well understood. To investigate the possible role of defocus in regulating ocular growth, spectacle lenses were used to optically simulate refractive anomalies in young monkeys (for example, myopia or nearsightedness). Both positive and negative lenses produced compensating ocular growth that reduced the lens-induced refractive errors and, at least for low lens powers, minimized any refractive-error differences between the two eyes. These results indicate that the developing primate visual system can detect the presence of refractive anomalies and alter each eye's growth to eliminate these refractive errors. Moreover, these results support the hypothesis that spectacle lenses can alter eye development in young children.

Behavioral studies of local stereopsis and disparity vergence in monkeys

Investigations on macaque monkeys have provided much of our knowledge of the neural mechanisms of binocular vision, but there is little psychophysical data on the accuracy of vergence responses or the precision of stereoscopic depth perception in these primates. We have conducted comparative behavioral studies of binocular disparity processing in rhesus monkeys and humans via measurements of prism-induced fixation disparities (disparity vergence) and relative depth discrimination for spatially localized stimuli (local stereopsis). The results of these studies demonstrated a remarkable similarity in both the oculomotor and the sensory aspects of binocular vision in the two species when the stimulus dimensions were specified in visual angles, which were independent of interocular separation. The disparity vergence functions for the two species revealed fusion responses over the same range of prism-induced vergence and comparable vergence errors for stimuli near their fusional limits. Disparity vergence responses were independent of the spatial frequency of the binocular fusion stimulus. Stereothresholds as a function of the spatial frequency of the difference-of-Gaussian stimuli were of the same form, with equivalent stereoacuities, in monkey and human observers. The presence of substantial vergence errors had only a small effect on the precision of stereoscopic depth perception. We conclude that, after compensation for the differences in the lateral separation of their eyes, the operating characteristics of disparity vergence and stereoscopic vision are virtually identical in rhesus monkeys and humans and, consequently, the performance limits for these visual functions must be determined by anatomical and/or neural constraints that are similar in both species.

Receptive-field properties of deafferentated visual cortical neurons after topographic map reorganization in adult cats

When neurons in primary visual cortex of adult cats and monkeys are deprived of their normal sources of activation by matching lesions in the two retinas, they are capable of acquiring new receptive fields based on inputs from regions of intact retina around the lesions. Although these "reactivated" neurons respond to visual stimuli, quantitative studies of their response characteristics have not been attempted. Thus, it is not known whether these neurons have normal or abnormal features that could contribute to or disrupt an analysis of a visual scene. In this study, we used extracellular single-unit recording methods to investigate their stimulus selectivity and responsiveness. Specifically, we measured the sensitivity of individual neurons to stimulus orientation, direction of drift, spatial frequency, and contrast. Over 98% of all units in the denervated zone of cortex acquired new receptive fields after 3 months of recovery. Newly activated units exhibited strikingly normal orientation tuning, direction selectivity, and spatial frequency tuning when high-contrast (< 40%) stimuli were used. However, contrast thresholds of most neurons were abnormally elevated, and the maximum response amplitude under optimal stimulus conditions was significantly reduced. The results suggest that the striate cortical neurons reactivated during topographic reorganization are capable of sending functionally meaningful signals to more central structures provided that the visual scene contains relatively high contrast images.

Binocular interactions in striate cortical neurons of cats reared with discordant visual inputs

The postnatal development of cortical binocularity is known to be adversely affected by early abnormal visual experience. However, little information exists on how the signals from the two eyes are combined in individual cortical neurons of animals reared with early discordant binocular visual experience. Since this is a fundamental issue in understanding visual cortical development, we used extracellular single-unit recording methods to study binocular integration in striate cortical neurons of strabismic cats. Specifically, we measured the sensitivity of individual cells to the relative interocular spatial phase of dichoptically presented drifting sinusoidal gratings (i.e., to binocular retinal image disparity). Clear alterations in ocular dominance were observed in all strabismic subjects. Nevertheless, the majority of cortical neurons exhibited some form of binocular interactions when both eyes were stimulated together. The most prominent aspect of cortical physiology in the strabismic animals was the relatively high prevalence of suppressive binocular interactions. Suppression was most frequently found in kittens reared with 2 weeks of early optical dissociation and among adult cats that received 2 weeks of early optical dissociation and a prolonged recovery period. However, substantial excitatory binocular interactions were also maintained in these animals. With an extended period of interocular misalignment (3 or 8 months), all forms of binocular interactions, excitatory and suppressive, were drastically reduced and a greater number of neurons were truly monocular. Although the reduction in the strength of binocular interactions occurred in all units irrespective of their monocular spatial properties, the effect was more pronounced among those units tuned to higher spatial frequencies and this spatial-frequency-dependent effect was larger in the subjects receiving longer periods of binocular dissociation. The results suggest that the "breakdown" of cortical binocular properties in strabismic subjects is not an all-or-none process, and that suppressive binocular interactions may be closely associated with the abnormal binocular interactions exhibited by strabismic humans. Furthermore, our findings are consistent with the notion that cortical disparity-detecting mechanisms are spatial-frequency dependent and, thus, can be selectively altered depending on an animal's early visual experience.

Interocular suppression produced by rivalry stimuli: a comparison of normal and abnormal binocular vision

This study compares interocular suppression in subjects with early strabismus and/or anisometropia with binocular rivalry suppression in subjects with normal binocular vision. A psychophysical test-probe paradigm was used to measure the changes in luminance-increment detection thresholds associated with periods of phenomenal suppression. In subjects with normal binocular vision, rivalry suppression induced by viewing orthogonally oriented grating pairs produced a distinctive wavelength-dependent change in visual sensitivity; specifically, there was a greater reduction in sensitivity for short (e.g., 450 nm) vs. middle or long wavelength stimuli (e.g., 560 nm). In contrast, subjects with abnormal binocular vision, regardless of the type of early abnormal visual experience, showed more reduction in sensitivity for a 560-nm stimulus than for the short-wavelength, 450-nm stimulus. Moreover, the pattern of sensitivity change in subjects with abnormal binocular vision was the same for suppression induced by either rivalry stimuli or stimuli that would normally promote fusion. The results clearly indicate that interocular suppression in subjects with abnormal binocular vision is qualitatively different from normal binocular rivalry. Evidently, the processes that mediate binocular rivalry are very susceptible to environmental influences during early vision development and can be disrupted easily.

Early discordant binocular vision disrupts signal transfer in the lateral geniculate nucleus

The mammalian lateral geniculate nucleus (LGN) is known to regulate signal transfer from the retina to the brain neocortex in a highly complex manner. Besides inputs from the brainstem, extraretinal inputs via corticogeniculate projections and local inhibitory neurons modulate signal transfer in the LGN. However, very little is known about whether the postnatal development of LGN signal-transfer mechanisms is influenced by early discordant binocular vision. By intraunit comparisons of responses between individual X-LGN cells and their direct retinal inputs, the efficiency of signal transfer was found permanently reduced due to an early interocular misalignment (strabismus). The contrast sensitivity and spatial resolution of cat LGN cells were significantly lower relative to their retinal inputs, and there was substantial decrease in signal-transfer speed. The observed physiological deficits were associated with immature X-retinogeniculate axon arbors. Thus, contrary to previous ideas, conflicting binocular inputs can produce neural deficits in subcortical visual structures.

Transfer characteristics of lateral geniculate nucleus X-neurons in the cat: effects of temporal frequency

The dependency of intrageniculate signal transfer on stimulus temporal frequency was investigated by comparing responses of individual X-relay cells with their direct retinal inputs in anesthetized and paralyzed cats. Temporal frequency response functions of lateral geniculate nucleus (LGN) X-cells were more narrowly tuned than those of their retinal inputs. The efficiency of signal transfer was consistently highest at or around the geniculate cells' optimal temporal frequency, and the degree of signal transfer, which was more closely related to the LGN cells' firing rate than to the firing rate of their retinal input, decreased for both lower and higher temporal frequencies. The high temporal frequency cut-offs were significantly lower in geniculate cell responses than those of their direct retinal inputs. This reduction in temporal resolution was exaggerated for relatively low stimulus spatial frequencies. The present results provide clear evidence for the notion that LGN cells function as nonlinear temporal filters and that this stimulus-dependent signal transmission appears to be regulated by complex local mechanisms.

Effects of optically induced blur on the refractive status of young monkeys

In each of eight rhesus monkeys, one eye was defocused with a -9 D contact lens beginning before 1 month of age for periods of 2-3 months. At the end of the rearing period, interocular comparisons showed that one subject had developed a relative axial myopia (3.0 D), however, five monkeys had developed a relative axial hyperopia (2.0-3.5 D). After discontinuing the contact-lens rearing procedure, the induced refractive errors diminished over time in all subjects. These results indicate that the defocus threshold for form-deprivation myopia is relatively high and that substantial levels of optical defocus which do not exceed this threshold typically produce axial hyperopia. The recovery data suggests that monkeys have an emmetropization mechanism which is sensitive to optical defocus, but the failure of this mechanism to compensate for the refractive errors simulated during the lens-rearing procedures suggests that this mechanism has a limited operating range.

Does weight-bearing exercise affect non-weight-bearing bone?

It was previously reported that weight-bearing exercise increases femur periosteal formation in 3-year-old, 230 kg cross-bred sows. To determine if this weight-bearing exercise also stimulated non-weight-bearing bone, bone formation in the seventh rib from these same sows was measured histomorphometrically on the periosteal, endocortical, and osteonal envelopes. The sows were randomly assigned by body weight to basal (B, n = 6), control (C, n = 7), or trained (T, n = 7) groups. After 3 weeks of exercise adaptation, T walked on a treadmill for 17 weeks at 5 km/h, 20 minutes/day, 5 days/week, at 5% grade. Groups were sacrificed initially (B) or after 20 weeks (C and T). Periosteal mineral apposition rate (MAR) was calculated over 136 days. Osteonal and endocortical MAR were calculated over the 14 days prior to sacrifice. There were no effects of exercise on the periosteal or endocortical percentage double-labeled surface (dLS/BS), osteonal remodeling frequency (N.dL.On/B.Ar), or MAR in any bone envelope of the rib. In conclusion, weight-bearing exercise sufficient to stimulate periosteal formation in the femur did not activate formation in the rib. Bone response to weight-bearing exercise appears to be specific to the loaded bones.

Mechanisms mediating visual detection in static perimetry

PURPOSE

The usual stimuli in static perimetry are white-light luminance increments. However, the specific visual detection mechanisms involved in perimetry are unknown because all classes of neural mechanisms are sensitive to spectrally broadband stimuli. The objective of this study was to determine the relative sensitivities of nonopponent and opponent detecting mechanisms under standard perimetry test conditions.

METHODS

Using trained rhesus monkey subjects, the relative sensitivities of the vision mechanisms for the detection of perimetry test stimuli were determined through psychophysical measurements of spectral sensitivity at each of the test field locations of the C24-2 threshold program on the Humphrey Field Analyzer (Allergan Humphrey, San Leandro, CA). The spectral sensitivity functions were analyzed by a three-channel model that incorporated independent short-wavelength-sensitive, nonopponent (luminance), and opponent (chromatic) spectral sensitivity mechanisms.

RESULTS

The visual detection mechanisms for perimetry thresholds varied as a function of the size and wavelength of the test field. With the perimeter's standard stimulus (Goldmann Size III) and bowl illumination (31.5 asb), the presence of a short-wavelength-sensitive mechanism was clearly evident at all field locations, but its relative sensitivity systematically declined with eccentricity. Under these conditions, the sensitivities of the opponent and nonopponent mechanisms were approximately equal at most field locations. With a larger stimulus (Goldmann Size V), however, the contribution of the opponent spectral sensitivity mechanism was more apparent over most of the central field and the alterations of sensitivity with eccentricity were less pronounced. In contrast, a small test field (Goldmann Size II) appeared to bias detection toward nonopponent mechanisms.

CONCLUSION

The results of these investigations indicate that detection thresholds during perimetry can be effectively biased toward different photopic, visual processing channels through the appropriate selection of size and wavelength of the test stimulus.

Rheumatology writing

The interaction of C 60 molecules with a graphite surface is modelled using molecular dynamics. At normal incidence it is found that the C 60 molecule is reflected intact at energies up to about 250 eV and a depression wave spreads radially from the point of impact across the graphite surface preceded by hypersonic fronts which transmit small amounts of energy. At energies of 1 keV and 6 keV the molecule implodes as it enters the crystal creating a particle of dense amorphous carbon beneath the surface. Very little sputtering occurs at normal incidence at energies of up to 6 keV. At 6 keV and at an incidence angle of 60° to the normal, the molecule breaks up on impact and some sputtering is observed. At 15 keV the sputtering yield increases and the surface ruptures in the central region where carbon clusters and chains are ejected. Outside this region the bonds remain intact but surface begins to separate from the second layer with a raised travelling wave propagating from the impact point.

Synergistic induction of cytochrome P450 by ethanol and isopentanol in cultures of chick embryo and rat hepatocytes

The purpose of this study was to investigate whether isopentanol, the most abundant higher chain alcohol in alcoholic beverages, contributes to induction of different forms of hepatic P450s associated with consumption of these beverages. We have previously reported that ethanol and isopentanol each induce P450 2H1/2 in cultured chick hepatocytes and that ethanol induced P450 2B1/2, as well as P450 2E, in cultured rat hepatocytes. Here we investigated the induction of P450 by isopentanol alone and in combination with ethanol in cultured chick and rat hepatocytes. The forms of induced P450 were identified both enzymatically and immunochemically. In cultured chick hepatocytes, both isopentanol and ethanol induced P450 2E, but combined treatment with these alcohols had no greater effect than treatment with ethanol alone. In cultured rat hepatocytes, isopentanol alone did not induce P450 2E or 2B1/2 and had no effect on the ethanol-mediated induction of P450 2E. However, isopentanol combined with ethanol caused a synergistic induction of P450 2B1/2 in cultured rat hepatocytes and an additive to synergistic induction of P450 2H1/2 in cultured chick hepatocytes. The levels of enzyme activities induced by the combined alcohol treatment approached those induced by potent barbiturates. The results suggest that the pentanols in alcoholic beverages may be responsible for a barbiturate-like induction of P450 in hepatocytes.

Colour vision anomalies following experimental glaucoma in monkeys

Spectral sensitivity defects, associated with chronic elevated intraocular pressure (IOP) produced by Argon laser trabeculoplasty, were studied in monkeys. Increment-threshold spectral sensitivity (ITSS) and threshold versus intensity (TVI) functions were measured using a behavioural model. Elevated IOP resulted in short wavelength (SW) sensitivity losses characteristic of many ocular diseases. The amount of SW sensitivity loss for ITSS functions depended upon the intensity level and chromatic composition of the background field. The optimum condition identifying the greatest SW sensitivity reduction was a yellow background of moderate intensity (100-1000 Td). In the early stages of experimental glaucoma, the cone mechanisms and the rod mechanism typically showed decreased test and field sensitivities. The SW cone pathway has slightly greater threshold elevation (approximately 0.3 log unit) compared to the rod and cone pathways. On the other hand, in the advanced stages of experimental glaucoma, the largest sensitivity losses were in the longer-wavelength, red-green opponent mechanisms, with the rod and SW cone pathways showing smaller losses. The similarities of the colour vision anomalies in this animal model with those of patients with glaucoma, provides support for its use as an experimental model for human glaucoma.

Behavioral perimetry in monkeys

PURPOSE

Normative data on the systematic changes in visual sensitivity as a function of retinal eccentricity have provided the basis for efficient threshold strategies and data analysis routines for static perimetry. The standard methods of assessing visual field changes in patients also could be used for monkeys with experimentally induced ocular disorders if the normal visual fields of monkeys and humans were similar.

METHODS

Normal visual field data from three rhesus monkeys were compared to data from eight human subjects using the standard threshold programs of the Humphrey Field Analyzer.

RESULTS

The experimental paradigm developed for these measurements provided excellent behavioral control for the monkeys, with reliability indices well within acceptable limits. The visual field data from monkeys were comparable to those from humans with respect to: (1) sensitivity as a function of stimulus field size; (2) the derived Statpac global indices; and (3) the variance of threshold measurements across the visual field.

CONCLUSION

The visual fields of monkeys and humans are similar, and the techniques of computerized perimetry may be applied to monkey subjects without significant modification.

Keeping an eye on the brain: the role of visual experience in monkeys and children

The quality of visual experience during infancy determines the functional sensitivity and precision of the mature primate visual system. Infant monkeys subjected to monocular form deprivation show a period of critical visual development that, though decreasing in sensitivity, lasts throughout the first 2 years of life. Photopic threshold spectral sensitivity appears to have a briefer critical period, which is essentially complete by 6 months old, whereas scotopic visual functions appear well developed by 3 months old. Binocular visual functions seem to have the longest period of sensitivity to abnormal visual experience because periods of monocular form deprivation initiated during the first 2 years affect visual functions. Viewing the world through prisms, which mimics the condition of strabismus, causes a permanent loss of cortical binocular cells and stereopsis in monkeys. This result explains stereoblindness in children having equivalent clinical histories.

2-Amino-3,4-dimethylimidazo[4,5-f]quinoline induces and inhibits cytochrome P450 from the IA subfamily in chick and rat hepatocytes

Several heterocyclic amines, found in cooked food, are powerful mutagens in the Ames Salmonella mutagenicity test system. One of these, 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) is one of the most mutagenic chemicals tested in this assay. In primary cultures of chick and rat hepatocytes, MeIQ, by itself, induced cytochrome P450 from the IA subfamily but was a weak inducer compared to 3-methylcholanthrene. However, in both chick and rat hepatocytes in culture, MeIQ decreased the amount of 3-methylcholanthrene-induced ethoxyresorufin deethylase activity, which is catalyzed by cytochrome P450 IA. The protein moiety of cytochrome P450 IA was decreased at MeIQ concentrations of 2.5 micrograms/ml or greater in chick hepatocytes and 25 micrograms/ml in rat hepatocytes. In hepatic microsomes from methylcholanthrene-treated chicks and rats, MeIQ was a competitive inhibitor of both ethoxyresorufin deethylase activity, a reaction catalyzed mainly by rodent cytochrome P450 IA1, and uroporphyrinogen oxidation, a reaction catalyzed by rodent P450 IA2. In cultured chick hepatocytes, MeIQ also decreased cytochrome P450-mediated oxidation of uroporphyrinogen by intact cells. The ability of MeIQ to inhibit as well as to induce cytochrome P450s of the IA subfamily may be important in assessing the mutagenic and carcinogenic effects of MeIQ in mammals.

Rapid reorganization of cortical maps in adult cats following restricted deafferentation in retina

The retinotopic map in the visual cortex of adult mammals can reorganize in response to a small injury in a restricted region of retina. Although the mechanisms underlying this neural plasticity in adults are not well understood, it is possible that rapid, adaptive alterations in the effectiveness of existing connections play a key role in the reorganization of cortical topography following peripheral deafferentation. In order to test this hypothesis, a small retinal lesion was made in one eye of adult cats and the visual cortex was mapped before and immediately after enucleating the non-lesioned eye. We found that substantial reorganization takes place within hours of enucleation.

Prior binocular dissociation reduces monocular form deprivation amblyopia in monkeys

The profound visual deficits associated with early monocular form deprivation (MD) are caused largely by competitive binocular interactions in the visual cortex. We tested the hypothesis that disrupting normal cortical binocularity prior to the onset of MD would reduce the degree of form deprivation amblyopia compared to that produced in animals that had normal binocular vision prior to MD. An optical strabismus was maintained in two rhesus monkeys between 30 and 90 d old, a rearing strategy that substantially reduces excitatory cortical binocular interactions. Subsequently, the lids of one eye were sutured closed for 9 mo. Comparison data were obtained from a series of form-deprived monkeys reared in a normal manner prior to the onset of MD. Psychophysical procedures were employed to measure the effects of the rearing procedures on the spatial contrast sensitivity functions for each eye. By itself, MD resulted in a severe amblyopia. The spatial resolution of the deprived eyes of monkeys lid sutured by 5 mo of age was about 6 octaves lower than normal values (Snellen acuities of about 20/2000). In contrast, equivalent periods of MD had a much smaller effect on the spatial vision of the prism-reared monkeys. In both cases, the deprived-eye cut-off spatial frequencies were within 1 octave of the nondeprived-eye values and were comparable to cut-offs for binocularly form-deprived monkeys. The results demonstrate that prior binocular image dissociation protects young monkeys from the effects of MD, probably by reducing the potential for antagonistic, competitive binocular interactions in the visual cortex.

Effects of interocular suppression on spectral sensitivity

A stable form of interocular suppression, referred to as "permanent suppression," can be produced by presenting a contoured field to one eye and a spatially homogenous field to the fellow eye, a viewing condition analogous to the classic "hole-in-the-hand" illusion. We examined the relation between permanent suppression and binocular rivalry suppression by comparing the changes in the increment-threshold spectral sensitivity function produced by these two forms of suppression. Permanent suppression produced a reduction in spectral sensitivity; however, in contrast to binocular rivalry suppression, the sensitivity alterations associated with permanent suppression were independent of the test-probe wavelength. The different patterns of sensitivity loss observed during binocular rivalry and permanent suppression indicate that different neural mechanisms mediate these two forms of interocular suppression.

Binocular beat VEPs: losses of cortical binocularity in monkeys reared with abnormal visual experience

Binocular beat VEPs were recorded from anesthetized macaque monkeys with diverse visual rearing histories, including surgically induced esotropia, optical prism dissociation, optical anisometropia, monocular form deprivation (MD), and normal rearing. Dichoptic visual stimulation was produced by temporally modulating the luminances of uniform fields presented to each eye. Five pairs of temporal frequencies were used, all of which had interocular differences of 2 Hz. While normally reared animals exhibited robust binocular beat responses strongly tuned to temporal frequency, the responses from monkeys with abnormal rearing experiences showed losses in beat signal-to-noise ratios that correlated with the age of onset or duration of the abnormal visual experience. Surgical esotropia induced early in life (2 months of age) produced a virtually complete loss of the binocular beat response; the cortical losses were less severe as the age of surgery rose to 10 months. Monkeys reared with either anisometropia or optical dissociation also manifested substantial reductions in evoked beat nonlinearity. MD monkeys sutured relatively late in development (8 and 25 months) showed mild reductions. The correspondence of these results to earlier psychophysical data obtained from these animals, and the similarity of these results to previous findings with binocularly normal and abnormal humans, supports the use of the binocular beat as an objective, noninvasive index of binocular neural integrity.