Contrast sensitivity norms for the Mentor B-VAT II-SG video acuity tester.

The Mentor B-VAT II-SG video acuity tester is a high-resolution video system designed to present many of the targets used in standard vision examinations (Snellen optotypes, clock charts, etc.). The SG model also presents sinusoidal gratings at various spatial frequencies and contrasts, making it possible to measure contrast sensitivity functions (CSF's). Using this system, we measured CSF's in a group of 69 young observers with well corrected vision. Contrast sensitivity scores were obtained at each of five spatial frequencies. A total of 5710 trials was presented. The mean contrast sensitivity scores of our group agree well with norms of other available CSF measurement systems, provided that the low spatial frequency truncation effects of each system are taken into account. A random subset of the group was retested 1 month later; no significant differences were found, demonstrating the reliability of the measurements. The data of this study should aid the clinician in identifying visual disorders. The data may be used to determine quantitative deviations from the norms or as a rapid screening test.

Effect of luminance level on contrast sensitivity in myopia.

High myopia results in stretching of the retina. We expected this stretching to impair visual function. Accordingly, we tested static and dynamic spatial contrast sensitivity and temporal modulation sensitivity at photopic, mesopic, and scotopic luminance levels. None of these functions differed significantly for high myopes (6 to 10 D) relative to age-matched normals. We conclude that retinal stretching need not induce visual deficits.

The shape of the high frequency flicker sensitivity curve.

For a light flickering with a modulation amplitude delta L, there is a single frequency f above which the light appears steady (fused). The relationship between delta L and f has been measured often under a variety of conditions, but its mathematical form is disputed. Three candidate functions have been proposed: (1) log delta L alpha f [Ferry-Porter law], (2) log delta L alpha f1/2 [diffusion model], and (3) log delta L alpha log f [cascaded integrator model]. Although all three functions roughly fit flicker fusion data (after appropriate linear transformation), they differ in curvature. We compared these functions using the general expression (4) log delta L alpha f lambda, where lambda denotes a curvature parameter. Functions (1)-(3) are special cases of (4) with lambda = 1, 0.5, and 0 respectively. When applied to 35 sets of flicker fusion data, the mean values of the best fitting lambda for each data-set was 0.919 (95% confidence interval 1.027 to 0.812). We conclude that the Ferry-Porter law describes most data better than a diffusion model, and that an integrator model deviates consistently in every case.

High myopia does not affect contrast sensitivity.

The axial elongation of high myopia is known to induce tangential stretching forces on the retina. Such forces might be expected to compromise retinal circuitry and thus affect the temporal and spatial contrast sensitivity function. In fact, we find that simple high myopes have normal contrast sensitivity for stationary gratings, moving gratings, and uniform field flicker. This suggests that, unlike many other retinal disease processes, the high myope's retina retains its normal integrity until the outer retina is compromised.

Three clinical tests of the spatial contrast sensitivity function: a comparison.

The purpose of this study was to establish a practical basis of comparison among three clinical tests designed to measure an observer's spatial contrast sensitivity function (CSF): the AO contrast sensitivity test plates (Arden grating test), the Vistech VCTS 6500 chart, and the Nicolet CS-2000 contrast sensitivity testing system (Optronics system). Each test was administered under standard conditions to a randomly selected subset of 71 normal observers. Scores were converted to absolute contrast and spatial frequency units for comparison. Results were similar for all three tests at intermediate and high spatial frequencies, but at low spatial frequencies the Vistech and Optronics tests yielded significantly lower sensitivity scores than the AO plates. This discrepancy is probably due to area truncation.

The aesthetics of the preauricular graft in facial reconstruction.

In the past six years we have used 55 preauricular full-thickness skin grafts to reconstruct defects of the nose, medial canthal area, forehead, and lip. Results have been good to excellent in all cases. The donor incision is the same one used for a rhytidectomy in which the sole reason for operation is cosmetic. Hence the donor scar in this area must certainly be considered cosmetically acceptable.

The Doppler and its use in axial flaps.

The Doppler ultrasonic flowmeter has been used as an aid in locating blood vessels that supply axial flaps in several different clinical situations. It has also been of value in microvascular surgery in which axial vessels of island flaps can be located and continuous patency monitored after their transfer and anastomosis.

Effects of texture on visual noise masking.

Visual noise masks, composed of letter fragments scattered randomly over the visual field, disrupt the recognition of briefly viewed words when presented immediately before and/or after these words. According to the widely held feature-similarity assumption, maximal disruption is predicted when the mask's letter fragments are equal in size and shape to the target letters. This assumption is not supported by the present experiment, which measured the masking effectiveness of visual noise patterns varying in texture (achieved by photographic reduction or enlargement). For target letters with stroke widths subtending 7.4' or less, the optimally disruptive mask did not vary with the stroke width of the target letters, but rather, was invariably composed of letter fragments with stroke widths subtending about 7'. For larger target words, however, this mask was not optimally disruptive. These results suggest that recognition of letters less that about 7' is mediated by a single size-tuned mechanism.

Word length and visual-noise texture in backward masking.

Visual-noise masks composed of randomly scattered letter fragments disrupt the recognition of briefly viewed letter arrays when the masks are presented immediately after these arrays. According to the feature-similarity assumption, maximal disruption should occur when the target letters and the letter fragments comprising the mask share a common stroke width. This assumption was disconfirmed in the present experiment. Two-, four-, and six-letter arrays, whose stroke width subtended a visual angle of 3-8', were viewed tachistoscopically in conjunction with visual-noise masks whose letter fragments subtended visual angles of 1-2', 1-9', 3-8', 6-6', or 9-7'. The most effective mask for all three letter-array lengths was the one composed of letter fragments subtending 6-6'. The results, together with those of previous experiments, are accounted for in terms of a neural line-detecting mechansim.