Andrew B. Metha

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.

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.

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.

Calibration of a color monitor for visual psychophysics

It has become common for stimuli used in visual psychophysical experiments to be presented on high-resolution color cathode-ray tubes (CRTs) such as the Barco CDCT 6551. These enable a flexibility of color, spatial-frequency content, temporal-frequency content, duration, size, and position that is not provided by most other media. CRTs are, however, not perfect; they suffer from the effects of temporal instability, spatial variability, lack of phosphor constancy, gun interdependence, and gun nonlinearity. This paper describes methods of assessing these aspects of monitor performance with respect to how significant each may be in psychometric terms. Although every application of CRT use in visual psychophysics is different, some general rules can be formulated to help ensure that unwanted effects are kept to a minimum. For the CRT used in this study (Barco CDCT 6551), a warm-up time of 30-45 min is necessary before chromatic and luminous stability ensues. Restriction of individual gun outputs to within 10%-90% of the possible range ensures that the effects of gun interdependence and lack of phosphor constancy are negligible. Calibration methods dealing with the linearization of gun output are also discussed.

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.

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.

Two expressions of surround suppression in V1 that arise independent of cortical mechanisms of suppression

The preferred stimulus size of a V1 neuron decreases with increases in stimulus contrast. It has been supposed that stimulus contrast is the primary determinant of such spatial summation in V1 cells, though the extent to which it depends on other stimulus attributes such as orientation and spatial frequency remains untested. We investigated this by recording from single cells in V1 of anaesthetized cats and monkeys, measuring size-tuning curves for high-contrast drifting gratings of optimal spatial configuration, and comparing these curves with those obtained at lower contrast or at sub-optimal orientations or spatial frequencies. For drifting gratings of optimal spatial configuration, lower contrasts produced less surround suppression resulting in increases in preferred size. High contrast gratings of sub-optimal spatial configuration produced more surround suppression than optimal low-contrast gratings, and as much or more surround suppression than optimal high-contrast gratings. For sub-optimal spatial frequencies, preferred size was similar to that for the optimal high-contrast stimulus, whereas for sub-optimal orientations, preferred size was smaller than that for the optimal high-contrast stimulus. These results indicate that, while contrast is an important determinant of spatial summation in V1, it is not the only determinant. Simulation of these experiments on a cortical receptive field modeled as a Gabor revealed that the small preferred sizes observed for non-preferred stimuli could result simply from linear filtering by the classical receptive field. Further simulations show that surround suppression in retinal ganglion cells and LGN cells can be propagated to neurons in V1, though certain properties of the surround seen in cortex indicate that it is not solely inherited from earlier stages of processing.

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.

Variability in Bleach Kinetics and Amount of Photopigment between Individual Foveal Cones

PURPOSE To study the bleaching dynamics of individual foveal cone photoreceptors using an adaptive optics ophthalmoscope. METHODS After dark adaptation, cones were progressively bleached and imaged by a series of flashes of 545-nm to 570-nm light at 12 Hz. Intensity measurements were made within the foveal avascular zone (FAZ) to avoid confounding signals from the inner retinal blood supply. Over 1300 cones in this region were identified and tracked through the imaging sequences. A single subject was used who demonstrated the necessary steady fixation, wide FAZ, and resolvability of cones close to the foveal center. RESULTS The mean intensity of all cones was well-described by first-order kinetics. Individual cones showed marked differences from the mean, both in rate of bleach and amount of photopigment; there was an inverse correlation between these two parameters. A subset of the cones showed large oscillations in intensity consistent with interference from light scattered within the cone outer segment. These cones also bleached more quickly, implying that rapid bleaching induces greater amounts of scatter. CONCLUSIONS Neighboring cones in the fovea display high variability in their optical properties.

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.