Multiconjugate adaptive optics applied to an anatomically accurate human eye model.

Aberrations of both astronomical telescopes and the human eye can be successfully corrected with conventional adaptive optics. This produces diffraction-limited imagery over a limited field of view called the isoplanatic patch. A new technique, known as multiconjugate adaptive optics, has been developed recently in astronomy to increase the size of this patch. The key is to model atmospheric turbulence as several flat, discrete layers. A human eye, however, has several curved, aspheric surfaces and a gradient index lens, complicating the task of correcting aberrations over a wide field of view. Here we utilize a computer model to determine the degree to which this technology may be applied to generate high resolution, wide-field retinal images, and discuss the considerations necessary for optimal use with the eye. The Liou and Brennan schematic eye simulates the aspheric surfaces and gradient index lens of real human eyes. We show that the size of the isoplanatic patch of the human eye is significantly increased through multiconjugate adaptive optics.

Information conveyed by onset transients in responses of striate cortical neurons.

Normal eye movements ensure that the visual world is seen episodically, as a series of often stationary images. In this paper we characterize the responses of neurons in striate cortex to stationary grating patterns presented with abrupt onset. These responses are distinctive. In most neurons the onset of a grating gives rise to a transient discharge that decays with a time constant of 100 msec or less. The early stages of response have higher contrast gain and higher response gain than later stages. Moreover, the variability of discharge during the onset transient is disproportionately low. These factors together make the onset transient an information-rich component of response, such that the detectability and discriminability of stationary gratings grows rapidly to an early peak, within 150 msec of the onset of the response in most neurons. The orientation selectivity of neurons estimated from the first 150 msec of discharge to a stationary grating is indistinguishable from the orientation selectivity estimated from longer segments of discharge to moving gratings. Moving gratings are ultimately more detectable than stationary ones, because responses to the former are continuously renewed. The principal characteristics of the response of a neuron to a stationary grating-the initial high discharge rate, which decays rapidly, and the change of contrast gain with time-are well captured by a model in which each excitatory synaptic event leads to an immediate reduction in synaptic gain, from which recovery is slow.

Maintaining the cornea and the general physiological environment in visual neurophysiology experiments.

Neurophysiologists have been investigating the responses of neurons in the visual system for the past half-century using monkeys and cats that are anesthetized and paralyzed, with the non-blinking eyelids open for prolonged periods of time. Impermeable plastic contact lenses have been used to prevent dehydration of the corneal epithelium, which would otherwise occur in minutes. Unfortunately, such lenses rapidly introduce a variety of abnormal states that lead to clouding of the cornea, degradation of the retinal image, and premature termination of the experiment. To extend the viability of such preparations, a new protocol for maintenance of corneal health has been developed. The protocol uses rigid gas permeable contact lenses designed to maximize gas transmission, rigorous sterile methods, and a variety of methods for sustaining and monitoring the overall physiology of the animal. The effectiveness of the protocol was evaluated clinically by ophthalmoscopy before, during, and after the experiments, which lasted 8-10 days. Histopathology and quantitative histology were performed on the corneas following the experiment. Our observations showed that this protocol permits continuous contact lens wear without adversely affecting the corneas. Thus, it is possible to collect data 24 h each day, for the entire duration of the experiment.

Packing arrangement of the three cone classes in primate retina.

We describe a detailed analysis of the spatial arrangement of L, M and S cones in the living eyes of two humans and one monkey. We analyze the cone mosaics near 1 degrees eccentricity using statistical methods that characterize the arrangement of each type of cone in the mosaic of photoreceptors. In all eyes, the M and L cones are arranged randomly. This gives rise to patches containing cones of a single type. In human, but not in monkey, the arrangement of S-cones cannot be distinguished from random.

Transmission of spatial information in S-cone pathways.

The mosaics of S-cones and the neurons to which they are connected are relatively well characterized, so the S-cone system is a good vehicle for exploring how the sampling of the retinal image controls visual performance. We used an interferometer to measure the grating acuity of the S-cone system in the fovea and at a range of eccentricities out to 20 deg. We also developed a simple model observer that, by assuming only that cone pathways are noisy and that signals are subject to eccentricity-dependent postreceptoral pooling, predicts the measured acuities from the sampling properties of the S-cone mosaic. The amount of pooling required to explain performance is consistent with that suggested by anatomical and physiological measurements.

Evidence for mild blue-yellow colour vision deficits immediately following fluorescein angiography.

AIMS: We have investigated the short term effects of fluorescein angiography on the blue-yellow, red-green, and luminance contrast sensitivity of patients with early age-related macular degeneration (ARMD). METHODS: Nine ARMD patients with no exudative complications and a visual acuity of 20/60 or better in the tested eye were selected. Cardinal colour directions for the isolation of the red-green, blue-yellow and achromatic (luminance) visual mechanisms were determined for each patient. Contrast sensitivity was measured in each cardinal colour direction immediately before and 20 min after standard 20-flash fluorescein angiography. RESULTS: A significant, albeit mild, reduction for blue-yellow contrast sensitivity following angiography was observed (ANOVA, alpha = 0.05). Fluorescein angiographic exposure had no significant effect on red-green or luminance contrast sensitivity. CONCLUSION: Our results show that fluorescein angiography causes at least a short term deficit selective to blue-yellow contrast sensitivity in our patient group.

Rapid adaptation in visual cortex to the structure of images.

Complex cells in striate cortex of macaque showed a rapid pattern-specific adaptation. Adaptation made cells more sensitive to orientation change near the adapting orientation. It reduced correlations among the responses of populations of cells, thereby increasing the information transmitted by each action potential. These changes were brought about by brief exposures to stationary patterns, on the time scale of a single fixation. Thus, if successive fixations expose neurons' receptive fields to images with similar but not identical structure, adaptation will remove correlations and improve discriminability.

Enhanced motion aftereffect for complex motions.

We measured the magnitude of the motion after effect (MAE) elicited by gratings viewed through four spatial apertures symmetrically positioned around fixation. The gratings were identical except for their orientations, which were varied to form patterns of global motion corresponding to radiation, rotation or translation. MAE magnitude was estimated by three methods: the duration of the MAE; the contrast required to null the MAE and the threshold elevation for detecting an abrupt jump. All three techniques showed that MAEs for radiation and rotation were greater than those for translation. The greater adaptability of radiation and rotation over translation also was observed in areas of the display where no adapting stimulus had been presented. We also found that adaptation to motion in one direction had equal effects on sensitivity to motion in the same and opposite directions.

Psychophysical evidence for a functional hierarchy of motion processing mechanisms.

Current models of motion perception typically describe mechanisms that operate locally to extract direction and speed information. To deal with the movement of self or objects with respect to the environment, higher-level receptive fields are presumably assembled from the outputs of such local analyzers. We find that the apparent speed of gratings viewed through four spatial apertures depends on the interaction of motion directions among the apertures, even when the motion within each aperture is identical except for direction. Specifically, local motion consistent with a global pattern of radial motion appears 32% faster than that consistent with translational or rotational motion. The enhancement of speed is not reflected in detection thresholds and persists in spite of instructions to fixate a single local aperture and ignore the global configuration. We also find that a two-dimensional pattern of motion is necessary to elicit the effect and that motion contrast alone does not produce the enhancement. These results implicate at least two serial stages of motion-information processing: a mechanism to code the local direction and speed of motion, followed by a global mechanism that integrates such signals to represent meaningful patterns of movement, depending on the configuration of the local motions.

Red-green and achromatic temporal filters: a ratio model predicts contrast-dependent speed perception.

We simultaneously measured detection and identification performance by using isoluminant red-green (RG) and achromatic flickering stimuli and fitted these data with a modified line-element model that does not make high-threshold assumptions. The modeling shows that detection and identification data are adequately described by postulating only two underlying temporal filters each for RG and achromatic vision, even when more than two threshold classifications are evident. We use a spatial frequency of 1.5 cycles per degree (c/deg) and compare the derived temporal impulse response functions with those obtained previously with the use of 0.25 c/deg stimuli under otherwise identical conditions [J. Opt. Soc. Am. A 13, 1969 (1996)]. We find that at 1.5 c/deg the luminance impulse response functions peak later and integrate out to longer times compared with those measured at 0.25 c/deg. For RG stimuli, although their relative overall sensitivities change, the impulse response functions are similar across spatial frequency, indicating a constancy of chromatic temporal properties across spatial scales. In a second experiment, we measured RG and achromatic flicker discrimination over a wide range of suprathreshold contrasts. These data suggest a common nonlinear contrast response function operating after initial temporal filtering. Using a ratio model of speed perception in which both RG and achromatic filters are combined at a common motion site, we can predict (1) the perceived slowing of RG stimuli compared with the perceived drift of achromatic drifting stimuli, (2) the contrast dependency of speed perception for RG and achromatic drifting stimuli, and (3) how this dependency changes with base speed. Thus we conclude that there is no need to postulate separate mechanisms for fast and slow motion [Nature (London) 367, 268 (1994)], since a unified ratio model can explain both RG and achromatic contrast-speed dependency.

Temporal mechanisms underlying flicker detection and identification for red-green and achromatic stimuli.

We have simultaneously measured detection and temporal frequency identification for both red-green isoluminant and achromatic stimuli over a range of temporal frequencies for two observers. Results show that temporal frequency identification can be made along the temporal frequency dimension for both red-green and achromatic stimuli at contrasts close to detection threshold. In general, temporal frequency identification was better for the achromatic than for the red-green stimuli; however, the level of chromatic identification performance was still sufficient to permit us to reject the notion that the red-green mechanism embodies a single temporal filter. We have developed a model based on signal detection theory that assumes that detection and identification both depend on the properties of the temporal filters underlying each mechanism. From this we have derived putative underlying shapes and sensitivities for the temporal filters of the red-green and achromatic mechanisms that comprise a low-pass and a bandpass filter for red-green color vision and two bandpass filters for luminance vision. Finally, we suggest that the relative perceived slowing of isoluminant stimuli may be accounted for by a common motion analysis subserved by different front-end temporal filters for red-green and achromatic motion signals.

Detection and discrimination of moving stimuli: the effects of color, luminance, and eccentricity.

Psychophysical detection and direction discrimination thresholds for 1c/o, 1-Hz Gabors are plotted in a Weberian long-middle-wavelength-sensitive cone contrast plane. The shape of these threshold contours suggests linear cone contributions to additive (delta L/Lb + delta M/Mb) and opponent (delta L/Lb - delta M/Mb) postreceptoral mechanisms. The opponent mechanism dominates thresholds at the fovea, but sensitivity decreases rapidly with eccentricity in comparison with the additive mechanism. Cone contributions to the mechanisms vary in a small and nonsystematic manner across the retina. The experiments show that the additive mechanism is directionally sensitive at detection threshold. At all eccentricities studied (0-24 degrees), 0.3-log-unit suprathreshold contrasts are necessary for the opponent mechanism to signal direction of motion.