Biomedical subjects

L O Harvey

Publications and source records attributed to L O Harvey.

9 recordsLinked to original sources

PubMed1991

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 degrees eccentricity. Beyond 6 degrees, 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 Foester's original observation of a qualitative difference between foveal and peripheral vision.

Adult↗

PubMed1990

Visual masking at different polar angles in the two-dimensional Fourier plane.

Human contrast thresholds were measured at 86 points in the two-dimensional Fourier plane with and without masks. The test stimuli were sinusoidal gratings in a 2.55-deg circular field. The superimposed masks were sinusoidal gratings having a polar spatial frequency of 8 cycles/deg, a contrast of 0.31, and one of five polar angles: 90, 105, 120, 135, and 180 deg. The test grating contrast for 75.5% correct detection in a three-alternative, forced-choice paradigm was determined by a maximum-likelihood adaptive psychophysical procedure. Three other observers were tested on subsets of these conditions. The threshold elevation surfaces produced by the masks lead to the following conclusions: spatial-frequency bandwidth is independent of mask orientation and has a value of approximately 2 octaves, orientation bandwidth is wider for oblique masks than for horizontal and vertical masks, the principle of spectral polar separability is violated, and two-dimensional Gabor functions in the frequency domain account for the masking effects. Individual differences were found among the observers.

Adult↗

PubMed1989

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 (beta) 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 degrees, 1 degree, and 2 degrees 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 used.

Adult↗

PubMed1985