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Color differences required for fast parallel searches were measured for small and large display fields. The main purpose of the measurement was to test the hypothesis that serial searches obtained with small color differences in large display fields are due to poor discrimination in the peripheral visual field and to the need for foveal fixation. Results do not support this hypothesis but show that the color differences required for parallel search are just as large in a display confined to an area roughly the size of the fovea as in a large display. However, results also show that the color difference required for a fast, parallel search is dependent on the size of the stimuli in a large display field. This result is consistent with the possibility that poor discrimination in the periphery may contribute to the size of the required differences if the stimuli are small.

A quantitative multiple-channels model can account for changes in perceived spatial frequency between foveally and peripherally viewed patterns; these frequency shifts are as large as 0.47 octave (38.5%). Factors that change as a function of eccentricity and that can alter the appearance of visual patterns include (1) scaling of receptive-field sizes, (2) overall sensitivity, and (3) steepness and nonlinearity of contrast-transfer functions (CTF's). Computations based on receptive-field size scaling, combined with fixed perceptual labels, predict higher perceived spatial frequency for peripheral patterns. Computations based on reduced sensitivity, however, predict the opposite result and thus partially offset the effect of receptive-field size scaling. The predicted perceived-frequency shifts based on changes in CTF's are quite small and can be ignored. Both individual differences and visual-field-location differences in magnitudes of perceived-frequency shifts are predicted by the model. An alternative perceptual explanation based on size-distance invariance and size constancy is also evaluated.

For the purpose of understanding the influence of the use of magnifying loupes on the reading process, the reading field is a relevant concept. Three possible reading fields are distinguished: the monocular reading field, the binocular reading field, and the composite reading field. Theoretical expressions for the widths of reading fields are derived as a function of the physical parameters of the loupe and the geometry of the reading situation. Variation of reading-field width as a function of magnification is illustrated. For verification, experimentally determined reading-field widths were compared with the theoretical ones. There were small systematic deviations, probably caused by achromatic aberrations. In order to learn about the strategies that subjects use when reading with the aid of a loupe, loupe displacement was measured while subjects read text under conditions that provided a variety of reading-field widths. It was found that individual subjects use different strategies (i.e., they use different reading-field widths).

Performance of three-dot bisection was determined as a function of the feature separation at eccentricities of 0-10 deg along the inferior vertical field meridian by using dot stimuli scaled in size to compensate for eccentricity. For these briefly flashed dot stimuli, the entire function of three-dot bisection acuity against dot separation worsens away from the fovea with a single scaling factor that is compatible with the change of the cortical magnification factor in area V1. When the presentation duration of the stimulus was lengthened from 150 msec to 1 sec, the improvement was much greater in the fovea than at 10-deg eccentricity for closely separated stimuli. We attribute this difference to a luminance cue (detection of a brightness change) that is present for a long stimulus duration at small separations in the fovea, but not in the periphery.

Performance of three-dot bisection was determined as a function of orientation for a variety of feature separations and field meridians at eccentricities of 0-10 deg for two observers. The dot stimuli and separations were scaled in size to compensate for eccentricity. The precision of three-dot bisection was found to depend on the direction of test-feature offset. In the fovea, horizontal and vertical bisections were better than oblique bisections, while at eccentricities of 5-20 deg, isoeccentric (on a tangent to a circle of a given eccentricity) bisection was better than radial bisection. The direction of offset was more important than the orientation of the stimulus. Large separations showed a stronger effect than small separations. The anisotropy of bisection appears different from the meridional effect for resolution and is unlikely to be simply related to a local anisotropy of the cortical magnification factor.

In an additivity experiment, increment thresholds were determined for 510 and 660 nm and for 470 and 570 nm. These thresholds were determined for each wavelength separately and for the wavelengths of each pair mixed. These measurements were made against a 1000-Td white background using fields subtending 0.16 degrees and 1.04 degrees visual angles and at 10- and 200-msec durations. For most conditions with the 510- and 660-nm pair, additivity of the reduction type was obtained. Generally, the larger field and longer duration produced larger additivity failures than the smaller field and shorter duration. For the 470- and 570-nm pair, two of the three observers showed additivity for the 0.16 degrees field in both the long- and the short-duration conditions. The larger field with longer durations yielded additivity failure for the three observers, but these failures were smaller than for the 510- and 660-nm pair.

In this paper we describe four experiments concerned with spatial-interval discrimination. The first experiment demonstrates that perturbations in the internal light distribution of two bright bars alter the perceived separation of the bars without influencing the precision of spatial-interval discrimination. Specifically, alterations in the centroid of the luminance distribution of the bars were found to influence their perceived separation systematically. In the second experiment we investigated separation discrimination for a pair of bright lines, a pair of dark lines, and a pair of lines with opposite polarity (a bright-dark pair). For large interline separations, the thresholds for separation discrimination of the three stimuli were equivalent. However, for small separations (less than 4 arcmin) the thresholds for the opposite-polarity pair were significantly higher than those for either bright or dark lines. The third experiment showed that separation discrimination is also impaired when the direction-of-brightness change of adjacent edges is the same. In the fourth experiment, random luminance perturbations were introduced in the interval between two bright bars separated by 1.5 to 8 arcmin by the addition of a dim line within the interval. The effect of these perturbations was also to alter the perceived separation of the bars with no effect on the precision of spatial-interval discrimination. Increasing the intensity of the perturbating line (to equal that of the outer lines) elevated thresholds for small separations (less than 4 arcmin) to approximately 6 arcsec but had no effect on the threshold for larger separations. Taken together, these results suggest that large and small separations are processed differently and place strong constraints on models for spatial-interval discrimination.

Intensive testing of spatial-frequency discrimination thresholds for 21% contrast gratings between 2.9 and 7.8 cycles/degree (c/deg) reveals, on average, smooth functions conforming to Weber's law. Discrimination overall is very sensitive, indicating highly skilled performance, but there are no consistent peaks or troughs. Individual, single-run functions do vary, resulting in candidate peaks and troughs that might be proposed to reflect true variations in an observer's sensory discrimination capacities. However, these features do not replicate from session to session nor from observer to observer. We do find consistent differences in overall sensitivity level between testing methods, with randomized presentation of reference frequencies giving significantly higher thresholds than blocked reference frequencies.

Temporal processing is different in the periphery than in the fovea. Two measures of temporal sensitivity, increment thresholds for onset asynchrony and temporal modulation thresholds (flicker), were studied in the fovea and at 20 deg in the lower visual field. The ability to detect incremental changes in the time between the onsets of two vertical lines was generally more precise in the fovea than at the peripheral locus, at least for asynchronies ranging from 10 to 80 msec. Flicker thresholds were measured from 1 to 40 Hz with a vertical-line target used for the asynchrony thresholds. The peripheral flicker function peaks at a higher temporal frequency (15-20 Hz) than the foveal function (8-10 Hz); this suggests that the peripheral temporal impulse response follows a faster time course than the foveal impulse response. Asynchrony discrimination would potentially provide a simple estimate of the temporal sensitivity underlying velocity discrimination. We find that asynchrony discrimination is not a good predictor of velocity thresholds; it fails at the high velocities at which velocity discrimination is most precise.

In a recent publication, Westheimer concluded that the foveal line-separation discrimination function was smooth [J. Opt. Soc. Am. A 1, 683-684 (1984)]. However, when the Westheimer data are replotted in the Weber-fraction (delta s/s) form, segments in the discrimination function similar to those previously described by Hirsch and Hylton [J. Opt. Soc. AM. 72, 1367-1374 (1984)] can be observed.

The detection of coherent motion was studied in stroboscopically displayed moving random-dot patterns disturbed by incoherent noise. We determined the threshold signal-to-noise ratio S as a function of velocity V at eccentricities of 0 degrees, 3 degrees, 6 degrees, 12 degrees, 24 degrees, 48 degrees in the temporal visual field of the right eye. At each eccentricity the measurements of S = f(V) were repeated for a range of rms contrast values from 60% (0 dB) in steps of 3 dB down to 1.9% (-30 dB). All stimuli were scaled with eccentricity to keep the ratio of pixel size to acuity constant (about 2). It is shown that the S values in our paradigm are never determined by contrast-threshold effects. They are true correlational thresholds. Bilocal movement detectors are assumed to underlie the detection of coherent motion. The bilocal correlation proves to be rather insensitive to rms contrast down to contrast levels of about 10%. Despite the eccentricity scaling, which is quite effective at high contrast levels, differences between the eccentricities become noticeable at lower contrast levels (below about 30-20%). The fovea is the least, and the far periphery the most, resistent to contrast degradation.

Temporal-frequency characteristics were measured as a function of retinal location, with test-field size scaled to provide equivalent sensitivity at each eccentricity. The results showed that the temporal-frequency limits increased uniformly by about a factor of 2 between the fovea and 45 degrees eccentricity, corresponding to a decrease in the response-time constant from 70 to 35 msec. These data were compared with the change in inner- and outer-segment diameters of the cones across the same retinal range. The data conform to the hypothesis that visual time constant varies inversely with cone outer-segment diameter. A second hypothesis that visual sensitivity increases in proportion to come outer-segment length in the central fovea was also supported.

Vernier acuity (delta v) as a function of two-dot separation (s) was measured at five retinal locations between the foveal center and 2.0 deg of eccentricity. We compare these results with average cone spacing at each of the corresponding retinal eccentricities and find that the angular dot separations at which the delta v versus s function intersects the angular cone spacing at each eccentricity remain nearly constant. Further, we define a vernier Weber fraction delta v/s and find that the average Weber fraction increases by nearly a factor of 2 from the fovea to 2.0 deg of retinal eccentricity. We suggest that both receptor factors and postreceptor factors contribute to limits of vernier acuity within 2 deg of the foveal center.

Because the discriminability of spatial intervals may give a pointer to possible underlying detection mechanisms for hyperacuity, a careful measurement was undertaken of how this function behaves with the width of the spatial intervals in the 6- to 12-min-of-arc range. Data on two subjects failed to produce convincing evidence for significant segmentation in the line-separation discrimination curve.

Spectral sensitivities of mechanisms dominated by short-, middle-, or long-wavelength-sensitive cones were measured for 76 observers ranging in age from 10 to 84 years. The short-wavelength mechanism was isolated with a yellow adapting background and five test wavelengths between 420 and 550 nm modulated at 2 Hz. Sensitivity declined with age, but the slopes of the functions varied as a function of wavelength. When the data were corrected for light losses in the ocular media, the slopes were similar for lambda less than or equal to 500 nm and still significantly correlated with age. At 440 nm, the sensitivity of the short-wavelength mechanism, specified at the retinal level, declined at a rate of 0.08 log unit per decade. Sensitivity at 550 nm under these conditions was dependent on middle- and/or long-wavelength-sensitive cones and was not correlated significantly with age. To study isolated middle- and long-wavelength mechanisms, sensitivities were measured at six wavelengths between 500 and 650 nm, using a 20-Hz test stimulus and appropriate chromatic adapting backgrounds. The sensitivities at all wavelengths were correlated negatively with age. When specified at the retinal level, the sensitivity at 560 nm declined at a rate of 0.11 log unit per decade for both middle- and long-wavelength mechanisms. These data support the view that the sensitivities of all three cone types and/or at least one of their postreceptoral pathways declines from 10 to 84 years of age.

Photopic temporal contrast sensitivity for healthy eyes of observers 65 years old and older is compared with retinal-illuminance-matched sensitivity of younger eyes. The older observers are significantly less sensitive for frequencies between 10 and 45 Hz. Although there is a slight shift to slower flicker rates in the mean contrast sensitivity function for older observers, this trend is not statistically significant, suggesting that there is relatively little loss of temporal resolving power of the visual system with healthy aging. These are preliminary results from an ongoing study of temporal contrast sensitivity in healthy aging eyes.

In principle, the ability to detect a luminance increment is lowered when there is uncertainty for its spatial location. Frequency-of-seeing curves were generated for small foveal targets. When fixed in space the target's detectability was more than 10 times higher than when it could occur at one of 140 locations.

Sensitivity to motion was studied by using sinusoidal gratings of contrast 0.4 that were displaced sinusoidally over time. At low motion temporal frequencies, thresholds were proportional to velocity and were independent of spatial frequency. Sensitivity, defined as the reciprocal of the threshold displacement amplitude, declined exponentially with temporal frequencies above the peak. Peak motion sensitivities decreased with visual field eccentricity but could be equalized by applying a spatial scaling function. However, this left a small relative superiority in the periphery for high temporal frequencies of motion, and possible explanations for this residual temporal inhomogeneity are considered.

Subjects estimated the perceived contrast of 2 degrees-diameter sine-wave grating patches for spatial frequencies of 2, 4, 8, and 16 cycles/deg, at eccentricities from 0 degrees to 40 degrees and contrasts up to 0.8. The data were well fitted in all cases by power functions of contrast minus threshold, with exponents of the order of 0.5 implying similar mechanisms in both fovea and periphery. The data also demonstrate that, at high physical contrast, the visual system is generally driven toward an operating state in which two stimuli of equal physical contrast have equal perceived contrast even if the thresholds are quite different. As a consequence, peripheral perceived contrasts produced by high physical contrasts show almost no change with eccentricity, whereas thresholds increase by at least an order of magnitude. This implies that mechanisms mediating threshold detection and suprathreshold perception may be different.

The concept of local scale asserts that for a given class of psychophysical measurements, performance at any two visual field locations is equated by magnifying the targets by the local scale associated with each location. Local scale has been hypothesized to be equal to cortical magnification or alternatively to the linear density of receptors or ganglion cells. Here, we show that it is possible to estimate local scale without prior knowledge about the scale or its physiological basis.

A grating viewed in the periphery usually appears to be of higher frequency than the same grating viewed at the fovea, for frequencies below the Nyquist frequency of the periphery. Systematic shifts in perceived frequency between fovea and periphery were found under several experimental conditions: spatially localized or extended sine-wave patterns, test stimuli presented in the superior or the inferior visual field, and stimuli of high or low mean luminance at several different contrasts. A multiple-spatial-frequency channels model can qualitatively account for our results, if it is assumed that each channel has a receptive-field size that increases as a function of eccentricity but has a perceived frequency label that does not change as a function of eccentricity.

Spectral sensitivity and wavelength discrimination are determined along the nasal horizontal meridian of the human peripheral retina. The target size as a function of eccentricity is varied according to a particular cortical magnification factor. Spectral sensitivity is measured by flicker photometry parameterized for the flicker frequency (10-20 Hz) and is found to be independent of the eccentricity (0-80 degrees) for 20-Hz flicker photometry after correction of the foveal spectral sensitivity for macular pigment absorption. This 20-Hz function is chosen as being representative for the peripheral luminous-efficiency function and is used in the wavelength-discrimination experiments. The peripheral retina can perform wavelength discrimination up to an eccentricity of 80 degrees. If field-size scaling according to the eccentricity-dependent cone density, the cortical magnification factor, or the reciprocal of the interganglion cell distance is applied, then wavelength-discrimination performance from 8 degrees to 80 degrees eccentricity is roughly the same. Foveal wavelength discrimination is considerably better than peripheral wavelength discrimination.

We compared discrimination and detection in the parafovea for oblique, cathode-ray-tube-generated, square-wave gratings. There is a band of frequencies between roughly 10 and 20 cycles/deg wherein such gratings are detected only in an apparently aliased form. The existence of aliasing is consistent with the cone density and optical quality of the parafoveal visual system, but the spatial frequency at which the aliasing is observed is only about half of what is suggested by cone density. The reason for this result is unclear, but one provocative possibility is that the site of the aliasing is neural rather than receptoral.

A new apparatus for in vivo retinal-scattering experiments incorporating Mueller-matrix ellipsometry is described. The basic principle is that the state of polarization of the entrance beam is modulated, after which the Stokes vector of the exit beam is assessed. Results show that nearly 90% of the degree of polarization of the entrance beam is preserved after double passage of the ocular media and retinal scattering. Changes in the state of polarization are studied in terms of a rotation around an eigenvector on the Poincaré sphere. These studies show that the type of change in the state of polarization of the totally polarized component is probably caused by a linearly birefringent process.

The information used in the output of the visual filters was examined for a blur discrimination task for which it had previously been claimed that the best human performance was with pedestal blurs of intermediate magnitude. This conclusion was verified for parameters leading to asymptotic performance for foveal and parafoveal locations. The nature of the computations underlying these visual discriminations was then assessed experimentally and by simulations. The results suggest that a frequency-based model in its simplest form cannot explain these findings. A model based on the spatial properties of filter outputs is discussed.

The perception of suprathreshold luminance contrast was investigated by forced-choice psychophysical procedures that were designed to define contrast equivalence relations. Observers compared the perceived contrast of rectangular bars that were presented for 500 msec at 3.9 deg on opposite sides of the fovea. The results show a nearly symmetrical relation between the perception of negative and positive contrast that is largely invariant over four decades of background luminance. Thus, for any fixed background luminance, equal absolute contrasts evoke approximately equal perceived contrasts. Symmetry also held with variations in the width, the eccentricity, and the focus of the bars. Symmetry was investigated further by determining equivalent contrast relations for negative contrasts as a function of background luminance and by contrast scaling. These results show evidence for nearly perfect contrast constancy for targets of low to moderate contrast and departures form constancy for high-contrast targets. These new findings on negative contrast, symmetry, and contrast constancy are discussed in relation to underlying mechanisms for contrast perception and classic experiments on brightness and lightness constancy.