Dean Yager

Accommodation responds to changing contrast of long, middle and short spectral-waveband components of the retinal image.

We simulated the effects of longitudinal (axial) chromatic aberration and defocus on contrast of the long-, middle- and short-wavelength components of the retinal image to determine whether the effects of chromatic aberration are sufficient to drive accommodation. Accommodation was monitored continuously while subjects (12) viewed a 3 c/deg white sine-wave grating (0.92 contrast) in a Badal stimulus system. The contrasts (amplitudes) of the red, green and blue components of the white grating changed independently to simulate a grating oscillating from 1 D behind the retina to 1 D in front of the retina at 0.2 Hz. Subjects responded strongly to the chromatic simulation but poorly to a luminance control. The results support the hypothesis that focus is specified by the contrast of spectral-wavebands of the retinal image, and that conventional color mechanisms, monitoring chromatic contrast at luminance borders (1-8 c/deg), mediate the signals that specify dioptric vergence.

Signal-detection models for multidimensional stimuli: probability distributions and combination rules

Abstract Probabilistic independence among multiple random variables (e.g., among the outputs of multiple spatial-frequency channels) has been invoked to explain two effects found with many kinds of stimuli: increments in detection performance due to “probability summation” and decrements in detection and identification performance due to “extrinsic uncertainty.” Quantitative predictions of such effects, however, depend on the precise assumptions. Here we calculate predictions from multidimensional signal-detection theory assuming any of several different probability distributions characterizing the random variables (including two-state, Gaussian, exponential, and double-exponential distributions) and either of two rules for combining the multiple random variables into a single decision variable (taking the maximum or summing them). In general, the probability distributions predicting shallower ROC curves predict greater increments due to summation but smaller decrements due to extrinsic uncertainty. Some probability distributions yield steep-enough ROC curves to actually predict decrements due to summation in blocked-summation experiments. Probability distribution matters much less for intermixed-summation than for blocked-summation predictions. Of the two combination rules, the sum-of-outputs rule usually predicts both greater increments due to summation and greater decrements due to extrinsic uncertainty. Put another way, of the two combination rules, the sum-of-outputs rule usually predicts better performance on the compound stimulus under any condition but worse performance on simple stimuli under intermixed conditions.

High and low luminance letters, acuity reserve, and font effects on reading speed

We compared reading speed with two fonts, Dutch (serif) and Swiss (sans serif). Text was displayed on a computer monitor, white letters on black, with the RSVP method. Luminance of the letters was either 146.0 or 0.146 cd m-2. Lower-case x-height of the fonts was approximately 5.5 times as large as letter acuity. At the high luminance, there was no difference between reading rates. There was a significant advantage for the Swiss font at the low luminance. The acuity reserve for Swiss was higher than for Dutch at the low luminance, which may account for the difference in reading speeds.

Psychophysical studies of pigeon color vision--I. Photopic spectral sensitivity.

Abstract A two-choice operant discrimination procedure was used to determine the spectral sensitivity function for the red field of the pigeon visual system under light adaptation. The data are consistent with a proposed model of the visual system which assumes that spectral sensitivity is determined by an additive combination of responses from cones containing the 562 pigment, and cones containing a postulated pigment with a peak absorption at 415 nm.

Visual function in goldfish with unilateral and bilateral tectal ablation

Following complete bilateral tectal ablation, the ability to detect light recovers in about three weeks. The non-tectal retinal connections which may mediate detection are about one log unit less sensitive than the retinotectal connections. At moderate illumination levels, tectumless fish do not react visually to objects. Tectumless fish integrate luminous flux over a very wide area (at least 25 degrees diameter) while the critical diameter for the normal in this experiment is smaller. Subcortical structures may mediate the interocular transfer of a brightness discrimination. The ipsilateral retinotectal projection following unilateral tectal removal is functional, with normal sensitivity to detection of light.

Detection and identification of spatial frequency: models and data.

Detection and identification of up to four simple sinusoidal gratings were studied. The experimental results were quantitatively compared to predictions from several models. The models all assumed probabilistically independent channels sensitive to different ranges of spatial frequency. The models differed in the shapes of their underlying distributions and, for detection, their decision rule. Detection and identification of far-apart spatial frequencies were consistent with these models. Thus, uncertainty effects for both detection and identification (the decrease in performance with an increase in the number of possible spatial frequencies) can be explained without assuming that attention capacity is limited.

Psychophysical Methods for Investigations of Vision in Fishes

A great deal of information about fish visual systems has been obtained through physiological, anatomical, and biochemical studies; however, very few quantitative data have been obtained about the visual discriminating capacities of the same species on which these studies have been conducted. Historically, psychophysical investigations have pointed the way for a variety of physiological investigations. The input — output relations revealed by psychophysics have led to theoretical models which attempt quantitatively to account for these relations; these models in turn suggest the expected form of physiological functions. A notable example of this logic is the prediction by Hering of opponent-colour-coding many decades before it was demonstrated at the electrophysiological level. The ultimate task for investigators of any sensory system is to develop linking hypotheses among psychophysics, electrophy-siology, photochemistry, and anatomy.

Spatial-frequency difference thresholds for central and peripheral viewing.

We measured spatial-frequency difference thresholds with a two-alternative forced-choice technique. On each trial a given standard frequency was presented in one temporal interval and a higher or lower frequency in the other. The subject indicated which interval contained the higher frequency of the two presented in that trial. The variable frequencies were chosen using a decision rule combined with a double-random-alternating staircase that converged on stimulus values that produced 70.7% correct choices. The proportional difference in frequency for criterion responding was computed from the mean of all stimulus values presented in a block of trials. This fraction was plotted as a function of standard spatial frequency. The resulting function shows local maxima and minima that are greater than the standard errors of the means across blocks of trials at a given standard. Functions obtained at 10 deg off the fovea showed a greater range of values than those obtained with central viewing. The results are consistent with the notion that there is a limited number of spatial-frequency channels whose bandwidths are narrow relative to their separation in the spatial-frequency spectrum.

Shifts in perceived spatial frequency of low-contrast stimuli: data and theory

We studied shifts in perceived spatial frequency of foveally viewed stimuli as a function of contrast, using lower contrasts and/or a lower range of spatial frequencies than used previously by others. Reliable shifts in perceived spatial frequency were found, but the direction of the shift at low frequencies was not the same as at high frequencies for some observers. Models based on either light scatter or an early compressive nonlinearity (transducer function) probably cannot account for these results. A multiple spatial-frequency-channels model in which a nonlinear contrast-transfer function (CTF) follows the output of each channel, however, is consistent with most of the results for suprathreshold contrasts. We considered several versions of this latter model, differing in their assumptions about the peak frequencies and bandwidths of the underlying channels, the shape of the CTF, and the combination rule by which the outputs of the channels are labeled and combined according to a weighted average.

Effects of chromatic adaptation on saturation discrimination in goldfish

Abstract Saturation discrimination for the goldfish is better at short wavelengths with short wavelength adaptation and at long wavelengths with long wavelength adaptation. These results are accounted for by a model of color vision that predicts the appearance of the diluting light for different states of adaptation. The analysis also accounts for the results from a human subject in the same procedure.