Gordon E. Legge

Contrast masking in human vision

Contrast masking was studied psychophysically. A two-alternative forced-choice procedure was used to measure contrast thresholds for 2.0 cpd sine-wave gratings in the presence of masking sine-wave gratings. Thresholds were measured for 11 masker contrasts spanning three log units, and seven masker frequencies ranging +/- one octave from the signal frequency. Corresponding measurements were made for gratings with horizontal widths of 0.75 degrees (narrow fields) and 6.0 degrees (wide fields). For high contrast maskers at all frequencies, signal thresholds were related to masking contrast by power functions with exponents near 0.6. For a range of low masking contrasts, signal thresholds were reduced by the masker. For the wide fields, high contrast masking tuning functions peaked at the signal frequency, were slightly asymmetric, and had approximately invariant half-maximum frequencies that lie 3/4 octave below and 1 octave above the signal frequency. The corresponding low contrast tuning functions exhibited peak threshold reduction at the signal frequency, with half-minimum frequencies at roughly +/- 0.25 octaves. For the narrow fields, the masking tuning functions were much broader at both low and high masking contrasts. A masking model is presented that encompasses contrast detection, discrimination, and masking phenomena. Central constructs of the model include a linear spatial frequency filter, a nonlinear transducer, and a process of spatial pooling that acts at low contrasts only.

Psychophysics of reading. XX. Linking letter recognition to reading speed in central and peripheral vision.

Our goal is to link spatial and temporal properties of letter recognition to reading speed for text viewed centrally or in peripheral vision. We propose that the size of the visual span - the number of letters recognizable in a glance - imposes a fundamental limit on reading speed, and that shrinkage of the visual span in peripheral vision accounts for slower peripheral reading. In Experiment 1, we estimated the size of the visual span in the lower visual field by measuring RSVP (rapid serial visual presentation) reading times as a function of word length. The size of the visual span decreased from at least 10 letters in central vision to 1.7 letters at 15 degrees eccentricity, in good agreement with the corresponding reduction of reading speed measured by Chung and coworkers (Chung, S. T. L., Mansfield, J. S., & Legge, G. E. (1998). Psychophysics of reading. XVIII. The effect of print size on reading speed in normal peripheral vision. Vision Research, 38, 2949-2962). In Exp. 2, we measured letter recognition for trigrams (random strings of three letters) as a function of their position on horizontal lines passing through fixation (central vision) or displaced downward into the lower visual field (5, 10 and 20 degrees ). We also varied trigram presentation time. We used these data to construct visual-span profiles of letter accuracy versus letter position. These profiles were used as input to a parameter-free model whose output was RSVP reading speed. A version of this model containing a simple lexical-matching rule accounted for RSVP reading speed in central vision. Failure of this version of the model in peripheral vision indicated that people rely more on lexical inference to support peripheral reading. We conclude that spatiotemporal characteristics of the visual span limit RSVP reading speed in central vision, and that shrinkage of the visual span results in slower reading in peripheral vision.

Psychophysics of reading-I. Normal vision

This paper is about the visual requirements for reading with normal vision. It is the first in a series devoted to the psychophysics of reading with normal and low vision. We have measured reading rates for text scanned across the face of a TV monitor while varying parameters that are important in current theories of pattern vision. Our results provide estimates of the stimulus parameters required for optimal reading of scanned text. We have found that maximum reading rates are achieved for characters subtending 0.3 degree to 2 degrees. Contrast polarity (black-on-white vs white-on-black text) has no effect. Reading rate increases with field size, but only up to 4 characters, independent of character size. When text is low-pass spatial-frequency filtered, reading rate increases with bandwidth, but only up to two cycles/character, independent of character size. When text is matrix sampled, reading rate increases with sample density, but only up to a critical sample density which depends on character size. The critical sample density increases from about 4 X 4 samples/character for 0.1 degree characters to more than 20 X 20 samples/character for 24 degrees characters. We suggest that one spatial-frequency channel suffices for reading.

Psychophysics of reading--II. Low vision.

Very little is known about the effects of visual impairment on reading. We used psychophysical methods to study reading by 16 low-vision observers. Reading rates were measured for text scanned across the face of a TV monitor while varying parameters that are likely to be important in low vision: angular character size, number of characters in the field, number of dots composing each character, contrast polarity (white-on-black vs black-on-white text), and character spacing. Despite diverse pathologies and degrees of vision loss in our sample, several major generalizations emerged. There is a wide variation in peak reading rates among low-vision observers, but 64% of the variance can be accounted for by two major distinctions: intact central fields vs central-field loss and cloudy vs clear ocular media. Peak reading rates for observers with central-field loss were very low (median 25 words/minute), while peak reading rates for observers with intact central fields were at least 90 words/minute (median 130 words/minute). Most low-vision readers require magnification to obtain characters of optimal size. Sloan M acuity was a better predictor of optimal character size than Snellen acuity, accounting for 72% of the variance. Low-vision reading is similar to normal reading in several respects. For example, both show the same dependence on the number of characters in the field. Our results provide estimates of the best reading performance to be expected from low-vision observers with characteristic forms of vision loss, and the stimulus parameters necessary for optimal performance. These results will be useful in the development of clinical tests of low vision, and in the design of low-vision reading aids.

Contrast detection and near-threshold discrimination in human vision

Abstract Forced-choice psychometric functions were determined for the detection of sinewave gratings and contrast discrimination of near-threshold gratings at spatial frequencies of 0.5, 2 and 8 c/deg. Detection psychometric functions all had the same S-shaped form. Discrimination functions were almost linear except at the upper end. Both sets of data can be described well by a detection model with a positively accelerating relation between contrast and mean decision variable and a differencing decision rule. Results of a paired comparisons experiment were consistent with the model and indicate that decision variable variance is nearly constant over the range of contrasts used in these experiments. The implications of these results for several models of contrast detection and discrimination are considered.

Psychophysics of reading—V. The role of contrast in normal vision

How does contrast affect reading rate? What is the role of contrast sensitivity? We measured reading rate as a function of the contrast and character size of text for subjects with normal vision. Reading rates were highest (about 350 words/min) for letters ranging in size from 0.25 degree to 2 degrees. Within this range, reading was very tolerant to contrast reduction--for 1 degree letters, reading rate decreased by less than a factor of two for a tenfold reduction in contrast. The results were very similar for white-on-black and black-on-white text. Reading rate declined more rapidly for very small (less than 0.25 degree) and very large (greater than 2 degrees) letters. People with low vision usually require large characters to read, so high contrast is particularly important for them. Taking 35 words/min to be a threshold for reading, we constructed a contrast-sensitivity function (CSF) for reading. We were able to relate the shape of this CSF to the shape of sine-wave grating CSFs.

A power law for contrast discrimination

Abstract Contrast increment thresholds were measured as a function of the background contrast of suprathreshold sine wave gratings at 2 and 8 c/deg. The resulting contrast discrimination functions obey power laws with exponents near 0.6 at 2 c/deg, and 0.7 at 8 c/deg. These exponents are influenced only slightly by pattern adaptation, gated vs continuous background gratings, and the psychophysical method. Webers Law does not hold for contrast discrimination under any of the conditions studied.

Psychophysics of reading. VIII The Minnesota low-vision reading test

This is the eighth in a series of papers dealing with the role of vision in reading. In previous papers, we have evaluated the effects of stimulus and subject variables on reading rate using a drifting-text procedure. In this paper, we describe a new test of reading rate that uses static text, called the Minnesota Low-Vision Reading Test (MNread). It is microcomputer-based, and more easily set up and administered than the drifting-text procedure. It is of potential value as a standardized psychophysical test of reading and should be useful in research, clinical, and educational applications. Some types of low-vision aids rely on drifting text and others on static text. Is reading performance different for these two modes of text presentation? We measured reading rate as a function of angular character size for normal and low-vision subjects with drifting and static text. Although reading rates were highly correlated for the two modes of text presentation, normal subjects usually read static text more rapidly. The reverse was true for low-vision subjects; their reading rates for drifting text were slightly higher (average 15%) than for static text.

Stereopsis and contrast

We have measured threshold disparity as a function of the spatial frequency (0.25-20 c/deg) and contrast (0.02-0.75) of sine-wave gratings. In forced-choice trials, subjects indicated whether a target grating had crossed or uncrossed disparity relative to a reference grating. Thresholds were lowest near 3 c/deg and rose in proportion to spatial period at lower frequencies. Above 3 c/deg, there were marked individual differences. Across the range of spatial frequencies, disparity sensitivity and contrast sensitivity were correlated (r = 0.84). Threshold disparity was inversely proportional to the square root of contrast. When the contrast seen by one eye was reduced producing unequal monocular contrasts, threshold disparity rose more than when the contrast seen by the two eyes was reduced by the same amount. Our results have implications for stereo models that use zero crossings, peaks and troughs, or centroids as matching primitives. These models can account for the decline in disparity sensitivity at low sapatial frequencies but only the peak model satisfactorily accounts for the effect of contrast. If the limiting sources of noise in the two eyes are highly correlated, the effect of unequal monocular contrast can be accounted for using a differential-amplifier principle.

Spatial-frequency and contrast properties of crowding

Crowding, the difficulty in recognizing a letter flanked by other letters, has been explained as a lateral masking effect. The purpose of this study was to examine the spatial-frequency and contrast dependencies of crowding, and to compare them with the properties of pattern masking. In experiment 1, we measured contrast thresholds for identifying the middle letters in strings of three randomly chosen lower-case letters (trigrams), for a range of letter spacings. Letters were digitally filtered using a set of bandpass filters, with peak object spatial frequencies ranging from 0.63 to 10 c/letter. Bandwidth of the filters was 1 octave. Frequencies of the target and flanking letters were the same, or differed by up to 2 octaves. Contrast of the flanking letters was fixed at the maximum value. Testing was conducted at the fovea and 5 degrees eccentricity. We found that crowding exhibits spatial-tuning functions like masking, but with generally broader bandwidths than those for masking. The spatial extent of crowding was found to be about 0.5 deg at the fovea and 2 deg at 5 degrees eccentricity, independent of target letter frequency. In experiment 2, we measured the contrast thresholds for identifying the middle target letters in trigrams for a range of flanking letter contrasts at 5 degrees eccentricity. At low flanker contrast, crowding does not show a facilitatory region, unlike pattern masking. At high flanker contrast, threshold rises with contrast with an exponent of 0.13-0.3, lower than corresponding exponents for pattern masking. In experiment 3, we varied the contrast ratio between the flanking letters and the target letters, and found that the magnitude of crowding increases monotonically with contrast ratio. This finding contradicts a prediction based on a grouping explanation for crowding. Our results are consistent with the postulation that crowding and masking may share the same first stage linear filtering process, and perhaps a similar second-stage process, with the additional property that the second-stage process in crowding pools information over a spatial extent that varies with eccentricity.