Israel Abramov

Analysis of Response Patterns of LGN Cells

A detailed analysis was made of the response characteristics of single cells in the lateral geniculate nucleus of the macaque monkey. The goal was to understand how these cells contribute to the processing of visual information. Data were analyzed from a representative sample of 147 cells, whose responses to equal-energy spectra (presented as diffuse flashes of monochromatic light) were recorded at three radiance levels. On the basis of their responses, the cells were divided into two general classes: (a) spectrally nonopponent cells which respond to all wavelengths with either an increase or decrease in firing rate, (b) spectrally opponent cells (about two-thirds of the sample) which respond with an increase in firing rate to some parts of the spectrum and a decrease to other parts. Four types of opponent cells were found: (i) red excitatory and green inhibitory (+R−G), (ii) green excitatory and red inhibitory (+G−R), (iii) yellow excitatory and blue inhibitory (+Y−B), (iv) blue excitatory and yellow inhibitory (+B−Y). Comparisons with psychophysical data indicated that nonopponent cells transmit brightness information; opponent cells, however, carry information about color, the hue of a light being determined by the relative responses of the four types. The saturation of spectral lights appears to be related to the differences in responses of opponent and non-opponent cells.

Color vision in the peripheral retina. II. Hue and saturation.

Hue and saturation of spectral lights were measured (direct scaling) in the fovea and at 45 degrees in the periphery; all lights were of equal photopic retinal illuminance (1200 trolands). At each retinal location both large and small targets were used. As shown by previous studies, small peripheral targets appear desaturated and of uncertain hue, except long wavelengths which appear red. However, if target size is increased, saturation increases and a full range of hues is seen; the hue functions for large peripheral targets are comparable to foveal ones for very small targets. From a modified form of color matching, it was concluded that the color deficiency in the periphery is more tritanlike than deutanlike; this is strengthened by the observation, that, for small peripheral targets, hues are generally apportioned between two hue categories and the change from one to the other is at about 580 nm.

Color appearance in the peripheral retina: effects of stimulus size

Hue and saturation scaling were used to measure the appearance of spectral lights as a function of stimulus size for nine loci across the horizontal retinal meridian. At a given locus, each hue (R, Y, G, and B) grew as a function of stimulus size up to some asymptotic value. The parameter values of Michaelis-Menten growth functions fitted to the hue data were used to derive the sizes of the so-called perceptive fields of the hue mechanisms. The fields for all mechanisms increased with eccentricity, and this increase was greater on the temporal than on the nasal retina. By increasing stimulus size it was possible to achieve fovealike color vision to eccentricities of 20 deg. However, even the largest stimuli failed to produce fully saturated hues at 40 deg. The retinal size scales of the four hue mechanisms were not the same; those for R and B were similar, and these mechanisms had the smallest perceptive fields everywhere. The perceptive fields of the hue mechanisms at all loci were larger than anatomical estimates of the sizes of retinal receptive fields.

Characteristics of saccades in human infants

Infants (14-151 days) and adults were shown two-dimensional geometric forms or stimuli from a set of highly textured patterns. Their eye movements were recorded by an infrared corneal reflection eye movement recorder as they freely scanned the stimuli. For both infants and adults, linear relationships were found between the peak velocities of fast eye movements and their amplitudes (main sequences). Infants viewing texture stimuli had main sequences with slopes comparable to those of adults. Infants viewing simple geometric forms made slower saccades. They also showed more eye movement oscillations which analyses showed were probably back-to-back saccades. Both the slower saccades and saccadic oscillations were attributed to factors related to the attentional value of the stimuli.

Color vision in the peripheral retina. I. Spectral sensitivity

Spectral sensitivity was measured by heterochromatic flicker photometry both in the fovea and at 45 degrees in the periphery, using a 1200 troland standard. At each location, sensitivity functions were obtained using both large and small targets. While the foveal functions were normal, the peripheral ones showed a large enhancement in sensitivity to short wavelengths relative to long wavelengths. Similar results in the past had been criticized on the ground that they might have been due to differential chromatic adaptation of cone mechanisms. In this study such an interpretation was ruled out by additional control experiments including varying luminance and wavelength of the standard light. The possibility of a rod contribution to the peripheral functions could not be eliminated although several different techniques, including the Stiles-Crawford effect, were used to try to isolate cone mechanisms.

The distribution of fixation durations in infants and naive adults

The distributions of durations of fixations from infants and free-viewing adults are shown to be basically exponential for different stimulus conditions. It is found that fixation duration can be divided into two periods. One, the alpha-period, is a refractory period during which a saccade does not occur and fluctuates across fixations. The other, the beta-period, is a random variable intrinsic to each fixation and constitutes a waiting-time for a saccade that occurs with constant probability per unit time. It is shown that mean duration decreases when stimulus size increases. These results suggest that fixations are terminated by saccades triggered by non-foveal stimulation.

Development of accommodation and convergence in infancy

Paraxial photorefraction was used to assess the development of accommodation and convergence in a large sample of infants under 1 year of age. The infants viewed small dolls placed at various distances (200-25 cm). The majority of infants at all ages demonstrated appropriate convergence for target distance, regardless of manifest refractive error. However, accommodation lagged behind convergence in development. Infants under 2 months tended to demonstrate either flat accommodation responses with a fixed plane of focus at around 30 cm, or accommodated appropriately for near targets, but failed to relax their accommodation sufficiently for the more distant targets. Thus, the focussing error increased with increasing target distance. Since the manifest refractive error was estimated by extrapolating the accommodation function to 0 diopters demand, these infants demonstrated spuriously myopic behavior. After 2 months, the majority of infants showing emmetropic behavior had accommodation responses that changed appropriately with target distance. However, infants with myopic or hyperopic manifest refractive errors displayed a variety of accommodative styles.

Describing color appearance: Hue and saturation scaling

Most of the fully elaborated systems for describing color appearance rely on matching to samples from some standard set. Since this is not satisfactory in all situations, various forms of direct linguistic description have been used, ranging from color naming to continuous numerical scaling of sensations. We have developed and extensively applied a particular variant in which subjects use percentage scales to describe their sensations of the four unique hue sensations (red, yellow, green, blue) and of the apparent saturation of colored lights. In this paper we explore the properties of this procedure, including its statistical properties and reliability both between and within subjects, in different contexts. We conclude that the technique is robust, easy to use, and provides direct access to sensory experience.

Photopigments and pseudo-pigments

Abstract The spectral properties of photopigments are often inferred from physiological measures of spectral sensitivity. From these measures pigments have recently been inferred with properties not corresponding to those of any pigment measured by spectrophotometry. It is proposed that in all these cases the properties of single pigments are not being measured; rather, they are pseudo pigments resulting from a specific form of neural interaction involving several real, known, photopigments. These pseudo-pigments were accepted as being related to single, but unusual, pigments largely because they obeyed the commonly accepted but incomplete criteria for identifying single pigments. Starting from the essential features of a pseudo-pigment, an analysis is developed; the conclusion is that pseudo-pigments can be resolved into standard photopigments, as identified by spectrophotometry, and that these interact in a specified form. A method is described for identifying the number andλmaxs of the photopigments involved in any physiological spectral sensitivity function, as well as certain details of the physiological interactions involved.

Sex & vision I: Spatio-temporal resolution

BackgroundCerebral cortex has a very large number of testosterone receptors, which could be a basis for sex differences in sensory functions. For example, audition has clear sex differences, which are related to serum testosterone levels. Of all major sensory systems only vision has not been examined for sex differences, which is surprising because occipital lobe (primary visual projection area) may have the highest density of testosterone receptors in the cortex. We have examined a basic visual function: spatial and temporal pattern resolution and acuity.MethodsWe tested large groups of young adults with normal vision. They were screened with a battery of standard tests that examined acuity, color vision, and stereopsis. We sampled the visual system’s contrast-sensitivity function (CSF) across the entire spatio-temporal space: 6 spatial frequencies at each of 5 temporal rates. Stimuli were gratings with sinusoidal luminance profiles generated on a special-purpose computer screen; their contrast was also sinusoidally modulated in time. We measured threshold contrasts using a criterion-free (forced-choice), adaptive psychophysical method (QUEST algorithm). Also, each individual’s acuity limit was estimated by fitting his or her data with a model and extrapolating to find the spatial frequency corresponding to 100% contrast.ResultsAt a very low temporal rate, the spatial CSF was the canonical inverted-U; but for higher temporal rates, the maxima of the spatial CSFs shifted: Observers lost sensitivity at high spatial frequencies and gained sensitivity at low frequencies; also, all the maxima of the CSFs shifted by about the same amount in spatial frequency. Main effect: there was a significant (ANOVA) sex difference. Across the entire spatio-temporal domain, males were more sensitive, especially at higher spatial frequencies; similarly males had significantly better acuity at all temporal rates.ConclusionAs with other sensory systems, there are marked sex differences in vision. The CSFs we measure are largely determined by inputs from specific sets of thalamic neurons to individual neurons in primary visual cortex. This convergence from thalamus to cortex is guided by cortex during embryogenesis. We suggest that testosterone plays a major role, leading to different connectivities in males and in females. But, for whatever reasons, we find that males have significantly greater sensitivity for fine detail and for rapidly moving stimuli. One interpretation is that this is consistent with sex roles in hunter-gatherer societies.