Infants’ responsiveness to rivalrous gratings

The study investigated the early development of responsiveness to rivalrous gratings. Infants were tested weekly between 6 and 16 weeks of age for their ability to discriminate between interocularly identical (fusible) lines and interocularly orthogonal (unfusible, rivalrous) lines. The stimuli were presented on an autostereoscopic monitor equipped with a face-tracking device. Two psychophysical techniques, the forced-choice preferential looking (FPL) method and measurement of looking times, were employed. Contrary to earlier findings, infants at all ages avoided looking at the rivalrous gratings instead of showing a developmental shift from a relative preference for unfusible, rivalrous gratings to a relative preference for fusible gratings. Avoidance of the rivalrous gratings became significant at 8–9 weeks of age, suggesting that infants clearly exhibit binocular rivalry from that age onwards. Control experiments secured that the infants’ preference for the fusible gratings was not governed by a natural preference for less over more complex line patterns.     

Do preferential looking techniques underestimate infant visual acuity?

Held et al. (1979) Vision Res.19, 1377–1379, have recently provided evidence for the existence of non-monotonic psychometric functions in infant acuity testing. We argue that these non-monotonicities are statistically significant in the original report, but are not robust across small variations of stimuli and testing techniques. The existence of such non-monotonicities in some laboratories does not imply that infant acuity has been universally underestimated by all PL techniques, nor does it argue against the general sensitivity of PL techniques as measures of other visual capacities in infants.     

Mechanisms of light adaptation in rat retina

Green, Dowling, Siegel and Ripps (1975)²¹J. gen. Physiol.65, 483–502 found that both receptors and post-receptoral elements regulate the process of light adaptation in skate retina. As a test of the generality of this conclusion, we repeated their experiments on the retina of the intact albino rat. Increment threshold and intensity-response functions of aspartate-isolated receptor potentials, ERG b-waves and single retinal ganglion cell axons were measured, and all were found to be similar to those described by Greenet al. (1975). In particular, the adaptive properties of the b-wave and ganglion cells were similar to each other, but different from those of the receptor potential. We conclude that the retinal mechanisms of light adaptation are similar in rat and skate.     

Cone lamellae and red and green rod outer segment disks contain a large intrinsic membrane protein on their margins: An ultrastructural immunocytochemical study of frog retinas

In addition to rhodopsin, the disk membranes of rod outer segments (ROS) contain a large integral membrane protein (mol. wt 290,000). This protein was previously localized by immunocytochemistry to the margins and incisures of disks in frog red ROS by specific antibody applied to thin sections of bovine serum albumin embedded retinas (Papermaster et al., 1978b, J. Cell Biol.78, 415–425).Upon further study of the reactions of this antibody with outer segments of other photoreceptor classes in frog retina, labeling of the short incisures and margins of green ROS and margins of cone outer segment lamellae is also observed. Thus the large protein participates in the structure of the edges of disks and lamellae of all photoreceptors in the frog. In addition, labeling of the inter-incisure surface of all photoreceptor classes was observed at high antibody concentration. In order to interpret this labeling, the effect of dilution on labeling density was determined and double reciprocal plots (Markham and Benton, 1931, J. Am. Chem. Soc.53, 497)were employed to evaluate the relative affinity and heterogeneity. There was considerable deviation from linearity in the plots of labeling of disk interiors compared to the relatively linear plots of disk incisure labeling which suggests that the interior sites contain a weakly cross-reacting antigen or that the serum contains a lower concentration of antibody weakly reactive with another antigen.     

The dependence of the induced effect on orientation and a hypothesis concerning disparity computations in general

The induced size effect is an apparent rotation about a vertical axis that results from binocularly viewing a target in which one half-image is vertically magnified. A previous paper (Arditiet al., 1981,Vision Res.21, 755–764) described a theory of this effect in terms ofhorizontal disparities that exist between vertically magnified images of oblique features and their unmagnified counterparts. The present studies test two aspects of that theory: the requirement of oblique features in stimuli eliciting the induced effect, and the assumption that binocular associations (inputs to disparity computations) are made across horizontal meridians. The former aspect was confirmed in a stereo discrimination experiment in which the direction of rotation (tilt) for crossed line patterns of varying orientation was judged, for a fixed vertical magnification of one half-image. The latter aspect was rejected on the basis of the results of that experiment, and of two experiments in which observers matched the apparent tilt of the lines with a horizontal adjustment line which could be stereoscopically rotated in depth. The data and some associated demonstrations suggest that stereoacuity and apparent depth of oblique lines vertically magnified in one half-image are determined by the horizontal separation between binocular points which arenearest in a fixed binocular coordinate map, rather than by purely horizontal point-matchings. This “nearest neighbor hypothesis” seems to be operative in classic measures of stereoacuity as well as in the induced effect.     

Discrimination and detection of changes in luminance

Prolonged viewing of fields varying in luminance as a sawtooth function of time selectively raises the detection threshold for incremental and decremental pulses and steps. The threshold changes are dependent on the sign of the sawtooth. This finding cannot be accounted for by the model proposed by Rashbass J. Physiol.210, 165–186. It is consistent with a model such as that of Roufs Vision Res.14, 831–852 which includes independent detectors for positive and negative internal responses. The latter model successfully predicts the changes in discrimination thresholds for transients of opposite sign as stimulus duration and adaptation level are varied if it is assumed that the independent detectors signal specific sensory qualities.     

Effect of focus on visual acuity of human infants

Recent theoretical arguments (Green et al. (1980) Viston Res.20, 827–835) predict that young human infants should have large depths of focus compared to adults. If so, optical blur should have relatively little effect on the resolving power of infant subjects. We tested this hypothesis by measuring the influence of optical blur on acuity thresholds in 6-week old infants and adults. Using the forcedchoice preferential looking (FPL) technique, we obtained acuity thresholds for each subject with five different lens powers (piano, ?14 D, ?3D, +6 D and +14 D). Acuity was differentially affected by lens power in all subjects, with the best acuity found with low power lenses and the worst with the highest power lenses. However, the amount of reduction in acuity produced by high power lenses, both plus and minus, was considerably less for infants than for adults. Although the effects of negative lenses are difficult to interpret without knowing the infants' exact accommodative state, the results with positive lenses support the theoretical predictions.     

Contrast thresholds for patterns of dynamic visual noise

Beurleet al. (1969) and Hodgsonet al. (1971) have suggested that when using a cascade image intensifier there is an optimum gain above which detection thresholds, i.e. threshold contrast, increase. This paper shows that this is only true for very low scintillation densities. At higher densities detection thresholds remain constant above the optimum gain which occurs when about 100 photons enter the pupil from each scintillation. The very low scintillation density regime appears to be a special case. We introduce the hypothesis that at low densities, where individual scintillations are visible, there is the possibility that detection is mediated by accurate scintillation counting (termed “subitizing” after Kaufmanet al. 1949).     

Dynamic properties of cat horizontal cell light responses

The presented sinewave squarewave light flicker responses of cat horizontal cells confirm the classification into Hn- and Hm-units described by Foersteret al. (1977a.b)Expl Brain Res.29, 347–385. However, we find somewhat higher order filter properties and develop a refinement of the model proposed by Foersteret al. to explain our new results. Using this model further analysis of Hm-unit responses suggests that cat retinal receptors have a smaller temporal bandwidth the smaller the spatial extent of the stimulus. Finally results from Gaussian white noise (GWN) stimulation are compared with those from classical flicker stimuli and Bode-plots. There are strong similarities in the results of these two types of analysis but some interesting differences are also found and discussed.     

Electroretinographic and psychophysical measures of cone spectral mechanisms using the two-color threshold technique

The two-color threshold pradigm of Stiles (1939) and a spatially alternating bar pattern stimulus were used to derive electrophysiological and psychophysical spectral sensitivity curves from the human retina. The test wavelength was kept constant at 600 nm and the background wavelength was varied from 450 to 650 nm. Three experiments were carried out with two color normal subjects. In the first experiment the original stimulus configuration of Stiles, a 1°, 500 msec test flash on a 10° background, was used to determine increment threshold curves. In the second experiment, ERGs were recorded with a bar pattern test field with a stripe width of 30′ alternating at a rate of 8Hz superimposed on a steady background. The diameter of both test and adaptation field was 15°. In the third experiment psychophysical thresholds were determined with the same stimulus conditions used to record the ERG.Stilesπ₅ mechanism provides a good fit to the spectral sensitivity curves derived from the increment threshold (ΔI/I) functions.     

Correlation of 55 samples of contact lens fluid and direct swabbings of the eye surface of symptomatic wearers of contact lenses

Background. The microbiological examination of contact lens storage solutions may reveal the etiological organisms resposible for keratitis and additionally harbour environmental contaminants. Patients and methods. From 1995 to 1998, conjunctival and corneal smears and scrapes, as well as the storage solutions of 55 soft contact lens wearers were examined microbiologically in a prospective non-randomised cohort study involving 53× unilateral and 2× bilateral cases of keratitis. Results. In 42 of the in-use contact lens storage solutions, the following organisms were found: 8×Bacillus sp., 7×P. aeruginosa and 8× other pseudomonads, 6×Alcaligenes xylosans, 4×Serratia liquefaciens, 4×Enterobacter sp., 4× coagulase negative staphylococci, 3×Serratia marcescens, 3×S. aureus, 2×Streptococcus faecalis, 1×Klebsiella oxytoca and 5×Acanthameba. A significant correlation of ocular cultures was found for Pseudomonas, Enterobacter, Serratia, S. aureus, S. faecalis and Acanthameba, but not for Bacillus, Alcaligenes, and coagulase negative staphylococci. Fungi were not found. The more pronounced the clinical picture, the more Gram-negative bacteria were isolated. Conclusions. The microbiological examination of soft contact lens storage solutions of 55 keratitis patients confirmed that the microbes responsible were to be found in the fluid, in addition to environmental contaminants.     

Ionic current model of the vertebrate rod photoreceptor

We describe voltage- and calcium-dependent ionic currents in the photoreceptor inner segments similar to the Hodgkin and Huxley (Journal of Physiology, 117, 500–544, 1952) equations. The model is used to describe both rods and cones by adjusting parameters. To simulate the light response, the inner segment model was connected with the phototransduction model proposed by Torreet al. (Cola Spring Harbor Symposia on Quantitative Biology, 55, 563–573, 1990). The role of individual ionic currents in the inner segment in shaping the light response was analyzed through computer simulations. The results suggest that: (1) the transient hyperpolarization to a bright flash is generated by I_h; (2) the oscillation after prolonged hyperpolarization in rods results from the interaction amongI_Ca,I_K(Ca), andI_Cl(Ca). Since the present model describes the biophysical processes from phototransduction to voltage response, the model can be used for analyzing the light response properties of the photoreceptors quantitatively.     

Blue-sensitive rod input to bipolar and ganglion cells of the Xenopus retina

Intracellular recordings were obtained from chromatic and non-chromatic bipolar cells, identified by Lucifer yellow injection in theXenopus retina. The chromatic cells, which lacked center-surround organization, were short wavelength hyperpolarizing (λ_max 445nm) and long wavelength depolarizing. Under photopic conditions the depolarizing component was driven by 612 nm cones, but under mesopic conditions it appeared that 524 nm rods also constituted an input to the response. The non-chromatic bipolars encountered were of the off-center (hyperpolarizing) variety, with an active antagonistic surround, and peak spectral sensitivity in the red portion of the spectrum. Extracellular recordings were obtained from color-coded ganglion cells classified as type 1 or 2 in frog retina by Maturanaet al. (1966) [J. gen. Physiol.43, 129–175] and Ba¨ckstro¨m and Reuter (1975) [J. Physiol.246, 79–107]. The spectral sensitivity of the long latency “on” component was matched by the density spectrum of the 445 nm rod. This response component lacked center-surround organization and showed a relatively broad area of spatial integration. In contrast, a short latency component had a spectral sensitivity matched by the 612 nm cone pigment under photopic conditions, was either “on” or “off” center, showed center-surround organization and had a relatively small area of spatial integration. We speculate that inXenopus retina, both chromatic and non-chromatic bipolar cells provide synaptic input to the class 1,2 ganglion cell.     

Visual acuity development in infants and young children,as assessed by operant preferential looking

The operant preferential looking (OPL) procedure was used to obtain psychophysical estimates of visual acuity for square-wave gratings in 50 children between 5 months and 5 yr of age. Acuity developed systematically with age, from 6 min arc (5 c/deg) at 5 months to 0.75 min arc (40 c/deg) at 5 yr, a value close to adult acuity tested with the OPL stimuli. In addition, psychometric functions became steeper with age, suggesting that criterion for responding varies with age. OPL offers the possibility of using a single procedure to follow acuity development across a wide age range.     

A binocular contribution to the production of optokinetic nystagmus in normal and stereoblind subjects

Viewing a large patterned field moving in one direction produces a regular pattern of eye movements known as Optokinetic Nystagmus (OKN). Fox et al. (1978) showed that the mechanism producing OKN can utilize purely binocular or “cyclopean” input. Experiments reported here confirm that finding and extend it to demonstrate that the binocularity of this mechanism is not disrupted in subjects who lack stereopsis. This result indicates that there exists more than one binocular process in the visual system.     

Cataract mutations and lens development

The lens plays an essential role for proper eye development. Mouse mutants affecting lens development are excellent models for corresponding human disorders. Moreover, using mutations in particular genes the process of eye and lens development can be dissected into distinct steps. Therefore, three mouse mutants will be described in detail and discussed affecting three essential stages: formation of the lens vesicle, initiation of secondary lens fiber cell formation, and terminal differentiation of the secondary fiber cells. The mutant aphakia (ak) has been characterized by bilaterally apakic eyes [Varnum and Stevens (1968)J. Hered. 59, 147–150], and the corresponding gene was mapped to chromosome 19 [Varnum and Stevens (1975)Mouse News Letters 53, 35]. Recent investigations in our laboratory refined the linkage 0.6±0.3 N cm proximal to the microsatellite marker D19Mit10. The linked gene Pax2, responsible for proper development of the posterior part of the eye and the optic nerve, was excluded as candidate gene by sequence analysis. Histological analysis of the homozygous ak mutants revealed a persisting lens stalk and subsequently the formation of lens rudiments. The lens defects led to irregular iris development and retinal folding. Congenital aphakia is known as a rare human anomaly. Besides a corneal dystrophy (CDTB), no corresponding disease is localized at the homologous region of human chromosome 10q23. The Cat3 mutations are characterized by vacuolated lenses caused by alterations in the beginning of secondary lens fiber cell differentiation at embryonic day 12.5. Secondary malformations develop at the cornea and the iris, but the retina remains unaffected. Two mutant alleles of the Cat3 locus have been mapped to mouse chromosome 10 very close to the microsatellite markers D10Mit41 and D10Mit95 (less than 0.3 cM). Since Cat3 is mapped to a position, which is homologous to human chromosome 12q21-24, the disorder cornea plana congenita can be considered as a candidate disease. The series of Cat2 mutations have been mapped close to the locus encoding the γ-crystallin gene cluster Cryg [Löster et al. (1994) Genomics 23, 240–242]. The Cat2^nop mutation is characterized by a deletion of 11 bp and an insertion of 4 bp in the 3rd exon of Crygb leading to a truncated γB-crystallin. The defect in the Crygb gene is causative for the stop of lens fiber cell differentiation from embryonic day 15.5 onward. Besides the lens, no further ocular tissue is affected. The Cat2 mouse mutants are interesting models for human cataracts caused by mutations in the γ-crystallin genes at human chromosome 2q32-35. The ak, Cat3 and Cat2 mutants are discussed in the context of other mutants affecting early eye and lens development. Additionally, human congenital cataracts are discussed, which have been characterized similar to the mouse models. The overview of the three types of mutants demonstrates that genes, which affect the early eye development, e.g. at the lens vesicle stage, have consequences for the development of the whole eye. In contrast, if the mutation influences later steps of lens differentiation, the consequences are restricted to the lens only. These data indicate a decreasing effect of the lens for the regulation of eye development during embryogenesis.     

Visual resolution and contour interaction in the fovea and periphery

The experiments provide quantitative data on the nature of contour interaction at the fovea and in the periphery to 10° eccentricity for observers with normal vision. Results with Landolt ring targets similar to those used by Flomet al. (J. opt. Soc. Am.53, 1026–1032) show that contour interaction effects are greater in peripheral vision. In central vision the increase in minimum angle of resolution (MAR) that occurred with the introduction of bars around the Landolt ring is small and much less than the experimental variation, whereas in peripheral vision bars at any of the ring-bar separations tested produced a consistent increase in MAR. Relationships between MAR and eccentricity are presented and compared with those in the literature. Reasons for the variation between previous studies are suggested.     

Infant visual acuity and its meridional variation

One hundred and four infants were tested using a preferential looking procedure. Results for subsets of these infants tested in three experiments were as follows: Median preference at the 75% level for vertical gratings over a homogeneous field increased monotonically from 3.0 c/deg at 17 weeks of age to 8.0 c/deg at 45 weeks of age; at the 58% level it increased from 3.7 c/deg to 11.8 c/deg. In the second experiment main axes gratings were directly paired with oblique gratings of the same spatial frequency. Results showed that the median spatial frequency at which main axes gratings were preferred over obliques (oblique effect) increased with age at a rate similar to the preference for vertical gratings. In the third experiment, vertical gratings were paired with the homogeneous field, and in separate sessions on the same infants, oblique gratings were paired with the homogeneous field. Preference thresholds for vertical gratings were similar to those for oblique gratings in very-young infants, but the preference threshold for vertical gratings increased more rapidly with age, becoming almost 1 octave greater by 11 months.     

Pre-stereoptic binocular vision in infants

In a preferential looking experiment, identical patterns (vertical stripes) were presented to both eyes on one of two screens while orthogonal patterns (vertical stripes in one eye and horizontal stripes in the other) were presented on the other screen. Most infants younger than 3.5 months of age originally showed a preference for the dichoptic (interocularly orthogonal) pattern. At an average age of 3.5 months, however, they showed a sudden shift of preference from this pattern to the interocularly identical pattern. The full shift from a preference for one stimulus to the other (both statistically significant) occurred within a few weeks in most cases. The onset age of the shift in preference agreed with the onset age of fusion-rivalry discrimination found in a previous study (Birch et al., 1985). The original preference for the bincularly orthogonal patterns may be interpreted as a preference for a grid (interocularly emergent intersections) over a grating, judging from results of two control experiments. These data suggest that the pre-stereoptic system non-selectively combines information from the two eyes without regard to edge orientation because it loses eye-of-origin information at a relatively early stage of binocular visual processing. Thus, the pre-stereoptic system does not have the capability of interocular suppression. The theoretical and clinical significance of the new findings are discussed along with a neuronal model of cortical development of ocular segregation and binocular pathways.     

Morphological correlates of Y, X and W type ganglion cells in the cat's retina

After physiological recordings with microelectrodes containing horseradish peroxidase (HRP). morphological properties of Y, X and W type ganglion cells were studied on flat-mounted cat's retinas. While all the Y cells (N = 9) and the X cells (N = 8) revealed α and β cell morphologies of Boycott and Wässle [J. Physiol. 240, 397–419 (1974)], respectively, W cells (N = 4) revealed various morphological types including their γ and δ cells. The Y cells were larger than X cells in soma diameter, but the W cells were of the same range as X cells. Electron microscopic observations of the cross-sectioned nerve fiber bundles provided evidence for the segregation of axon diameters into the three groups corresponding to Y, X and W axons. It was discussed that functional trichotomy of retinal ganglion cells is related to the differentiation more in axon diameter rather than in soma size.