Lewis O. Harvey

Contrast thresholds for identification of numeric characters in direct and eccentric view

Aubert and Foerster (1857) are frequently cited for having shown that the lower visual acuity of peripheral vision can be compensated for by increasing stimulus size. This result is seemingly consistent with the concept of cortical magnification, and it has been confirmed by many subsequent authors. Yet it is rarely noted that Aubert and Foerster also observed a loss of the “quality of form.” We have studied the recognition of numeric characters in foveal and eccentric vision by determining the contrast required for 67% correct identification. At each eccentricity, the lowest contrast threshold is achieved with a specific stimulus size. But the contrast thresholds for these optimal stimuli are not independent of retinal eccentricity as cortical magnification scaling would predict. With high-contrast targets, however, threshold target sizes were consistent with cortical magnification out to 6° eccentricity. Beyond 6°, threshold target sizes were larger than cortical magnification predicted. We also investigated recognition performance in the presence of neighboring characters (crowding phenomenon). Target character size, distance of flanking characters, and precision of focusing of attention all affect recognition. The influence of these parameters is different in the fovea and in the periphery. Our findings confirm Aubert and Foerster’s original observation of a qualitative difference between foveal and peripheral vision.

Efficient estimation of sensory thresholds

Laboratory computers permit detection and discrimination thresholds to be measured rapidly, efficiently, and accurately. In this paper, the general natures of psychometric functions and of thresholds are reviewed, and various methods for estimating sensory thresholds are summarized. The most efficient method, in principle, using maximum-likelihood threshold estimations, is examined in detail. Four techniques are discussed that minimize the reported problems found with the maximum-likelihood method. A package of FORTRAN subroutines, ML-TEST, which implements the maximum-likelihood method, is described. These subroutines are available on request from the author.

Senescent changes in scotopic contrast sensitivity

Scotopic contrast sensitivity functions (CSFs) were measured for 50 observers between the ages of 20 and 88 years. Using a maximum-likelihood, 2-alternative, temporal forced-choice threshold-estimation algorithm, scotopic CSFs were measured at 7 spatial frequencies ranging from 0.2 to 3.0 cpd, with mean retinal illuminance equated for observers at -0.85 log scotopic Trolands. For each stimulus condition, eight cycles of a horizontal sinusoidal grating were presented within +/- 1 S.D. of a 2-D Gaussian-spatial envelope and within a 1-s Gaussian-temporal envelope. Stimuli were centered on the nasal retina along the horizontal meridian 6 degrees from the fovea. Scotopic CSFs were found to be low-pass. Statistically significant age-related declines in contrast sensitivities were found for spatial frequencies at or below 1.2 cpd. There was also a statistically significant decrease in the high frequency cut-off with age (P < 0.01). An explanation of these results in terms of optical factors is rejected, while the results are consistent with age-related changes in the magnocellular pathway.

Light-difference threshold and subjective brightness in the periphery of the visual field

SummaryLight-difference thresholds were measured in the center and periphery of the visual field at photopic and scotopic levels. Under photopic conditions the fovea has the lowest light-difference threshold. From the fovea to 10 degrees eccentricity threshold gradually increases. It remains constant up to approximately 35 degrees eccentricity in the temporal visual field (nasal retina). Beyond the edge of this plateau of constant light-difference threshold, it again increases to the limit of the visual field. Under scotopic conditions the extent of the plateau of constant light-difference threshold remains the same as under photopic conditions. The fovea itself, however, and its immediate environment are less sensitive than the plateau area.Subjective brightness of a supra-threshold target is not dependent on its position in the visual field. A target with a given luminance will elicit the same brightness sensation at all retinal positions. As a consequence of this brightness constancy throughout the visual field, peripheral targets at threshold appear brighter than foveal targets at threshold because a peripheral target at threshold has more luminance than a foveal target at threshold.ZusammenfassungDie Inkrementalschwelle wurde in der Fovea und in der Peripherie des Gesichtsfeldes bei photopischen und skotopischen Adaptationsbedingungen gemessen. Bei photopischen Bedingungen ist die Schwelle in der Fovea am geringsten (größte Sensitivität). Von der Fovea bis etwa 10 Grad in die Peripherie nimmt die Inkrementalschwelle allmählich zu. Die Schwelle bleibt dann konstant bis etwa 35 Grad im temporalen Gesichtsfeld (nasale Retina) und bis etwa 20 Grad im nasalen Gesichtsfeld (temporale Retina). Jenseits dieses Plateaus konstanter Sensitivität nimmt die Schwelle wieder zu, bis schließlich das Ende des Gesichtsfeldes erreicht wird. Bei skotopischen Adaptationsbedingungen wurde dieselbe Ausdehnung des Plateaus konstanter Schwelle im temporalen und nasalen Gesichtsfeld beobachtet. Die Fovea und die unmittelbare Umgebung der Fovea haben bei skotopischen Bedingungen eine geringere Sensitivität.Die subjektive Helligkeit überschwelliger Reize ist nicht abhängig von der Lage des Lichtreizes im Gesichtsfeld. Ein überschwelliger Reiz mit gegebener Intensität hat überall im Gesichtsfeld dieselbe subjektive Helligkeit. Als Konsequenz der Konstanz der Helligkeit im Gesichtsfeld erscheinen Schwellenreize in der Peripherie heller als Schwellenreize im Fovea-nahen Bereich, da die Lichtintensität für Schwellenreize in der Peripherie größer ist als im Fovea-nahen Bereich.

Cortical Magnification Theory Fails to Predict Visual Recognition

The sense of form is poor in indirect view. Yet the cortical magnification theory asserts that the disadvantage can be made up by scaling the image size according to the spatial variation in the mapping of the retina onto the cortex. It is thus assumed that all visual information passes through a functionally homogeneous neural circuitry, with the spatial sampling of input signals varying across the visual field. We challenge this notion by showing that character recognition in the visual field cannot be accommodated by any concept of sole size scaling but requires increasing both size and contrast of the target being viewed. This finding is formalized into a hyperbolic law which states that target size multiplied by log contrast is constant across the visual field. We conclude that the scalar cortical magnification theory fails for character recognition since the latter depends on multidimensional pattern representations in higher, i.e. striate and prestriate, cortical areas.

Fast and accurate measurement of taste and smell thresholds using a maximum-likelihood adaptive staircase procedure.

This paper evaluates the use of a maximum-likelihood adaptive staircase psychophysical procedure (ML-PEST), originally developed in vision and audition, for measuring detection thresholds in gustation and olfaction. The basis for the psychophysical measurement of thresholds with the ML-PEST procedure is developed. Then, two experiments and four simulations are reported. In the first experiment, ML-PEST was compared with the Wetherill and Levitt up-down staircase method and with the Cain ascending method of limits in the measurement of butyl alcohol thresholds. The four Monte Carlo simulations compared the three psychophysical procedures. In the second experiment, the test-retest reliability of ML-PEST for measuring NaCl and butyl alcohol thresholds was assessed. The results indicate that the ML-PEST method gives reliable and precise threshold measurements. Its ability to detect malingerers shows considerable promise. It is recommended for use in clinical testing.

Efficient estimation of sensory thresholds with ML-PEST.

A set of C and C+2 routines are described that allow the efficient estimation of sensory thresholds in psychophysical experiments using a maximum-likelihood staircase procedure. They have been used effectively in visual, auditory, gustatory, and olfactory psychophysics.

Spatial frequencies and the cerebral hemispheres: contrast sensitivity, visible persistence, and letter classification.

The hypothesis that the two cerebral hemispheres are specialized for processing different visual spatial frequencies was investigated in three experiments. No differences between the left and right visual fields were found for: (1) contrast-sensitivity functions measured binocularly with vertical gratings ranging from 0.5 to 12 cycles per degree (cpd); (2) visible persistence durations for 1- and 10-cpd gratings measured with a stimulus alternation method; and (3) accuracy (d’) and reaction times to correctly identify digitally filtered letters as targets (L or H) or nontargets (T or F). One significant difference, however, was found: In Experiment 3, a higher decision criterion (ß) was used when filtered letters were identified in the right visual field than when they were identified in the left. The letters were filtered with annular, 1-octave band-pass filters with center spatial frequencies of 1,2,4,8, and 16 cpd. Combining four center frequencies with three letter sizes (0.5°, 1°, and 2° high) made some stimuli equivalent in distal spatial frequency (cycles per object) and some equivalent in proximal spatial frequency (cycles per degree). The effective stimulus in the third experiment seemed to be proximal spatial frequency (cycles per degree) not distal (cycles per object). We conclude that each cerebral hemisphere processes visual spatial frequency information with equal accuracy but that different decision rules are use

Visual texture perception and Fourier analysis.

The relationship between the Fourier spectra of visual textures (represented by four hypothetical visual channels sensitive to spatial frequencies) and the perceptual appearance of the textures was investigated. Thirty textures were synthesized by combining various spatial frequencies of different amplitudes. Twenty subjects grouped the textures into 2, 3, 4, and 5 groups based on the similarity of their appearance. The groupings were analyzed by means of linear discriminant analysis using the activity of the four channels as predictor variables. The groupings were also examined by multidimensional scaling, and the resulting stimulus configuration was canonically correlated with the channel activity. The results of both analyses indicate a strong relationship between the perceptual appearance of the textures and their Fourier spectra. These findings suport a multiple-channel spatial-frequency model of perception.

Making Better Use of Scientific Knowledge: Separating Truth From Justice

One major reason that scientific information is not often effectively applied in the formation of social policy is that available methods for coping with the uncertainty in scientific judgments are overlooked. Application of such methods is as necessary as reducing uncertainty through the acquisition of more knowledge for these methods result in the more effective use of knowledge already at hand. Furthermore, failure to cope with uncertainty through explicit, analytical methods results in failure to separate fact (truth) from value (justice), which in turn results in wasted knowledge and unnecessary dispute. We present four concrete examples of how such circumstances can be avoided.