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S A Klein

Publications and source records attributed to S A Klein.

At least 91 records · Page 5Linked to original sources
PubMed1994

Amblyopic and peripheral vernier acuity: a test-pedestal approach.

This manuscript is concerned with three visual systems with degraded spatial vision: (i) anisometropic amblyopia; (ii) strabismic amblyopia; and (iii) the normal periphery. The question we ask here, is whether the poor positional acuity of each of these visual systems can be understood on the basis of reduced sensitivity to the local contrast information in the stimulus. To answer this question, we use a "test-pedestal" approach to position acuity. In the first experiment we measure our observers' thresholds for detecting both the pedestal stimuli (edges and lines) and the test or cue stimuli (lines and dipoles). This approach also provides an estimate of the size of the spatial pooling (integration) region for the local contrast cue. In experiments two and three, we measure line and edge vernier acuity as a function of contrast, and compare the losses to those found for the detection of the respective offset cues. The local contrast hypothesis predicts similar losses in vernier acuity and in "cue" detection in amblyopic or peripheral vision. Moreover, the precise form of the contrast response function can provide insights into the nature of the loss, and places constraints on the likely models for amblyopic or peripheral vision. Our results suggest that the loss in vernier acuity of our anisometropic amblyopes can be understood on the basis of the reduced local contrast sensitivity and by increased spatial pooling. In strabismic amblyopes and in the normal periphery, there appears to be an extra loss, which cannot be accounted for by either reduced local contrast sensitivity or by increased spatial pooling. Additional experiments and computational modeling suggest that the "extra" loss is not due to spatial undersampling or additive positional jitter, but rather results from positional noise at a "second" stage.

Adolescent↗
PubMed1994

Discrimination of position and contrast in amblyopic and peripheral vision.

Many computational models of normal vernier acuity make predictions based on the just-noticeable contrast difference. Recently, Hu, Klein and Carney [(1993) Vision Research, 33, 1241-1258] compared vernier acuity and contrast discrimination (jnd) in normal foveal viewing using cosine gratings. In the jnd stimulus the test grating was added in-phase to the (sinusoidal) pedestal, whereas in the vernier stimulus the same test grating was added with an approx. 90 deg phase shift to the pedestal. In the present experiments, we measured thresholds for discriminating changes in relative position and changes in relative contrast for abutting, horizontal cosine gratings in a group of amblyopes using the Hu et al., test-pedestal approach. The approach here is to ask whether the reduced vernier acuity of amblyopes can be understood on the basis of reduced contrast sensitivity or contrast discrimination. Our results show that (i) abutting cosine vernier acuity is strongly dependent on stimulus contrast. (ii) In both anisometropic and strabismic amblyopes, abutting cosine vernier discrimination thresholds are elevated at all contrast levels, even after accounting for reduced target visibility, or contrast discrimination. (iii) For both strabismic and anisometropic amblyopes, the vernier Weber fraction is markedly degraded, while the contrast Weber fraction is normal or nearly so. (iv) In anisometropic amblyopes the elevated vernier thresholds are consistent with the observers' reduced cutoff spatial frequency, i.e. the loss can be accounted for on the basis of a shift in spatial scale. (v) In strabismic amblyopes and in the normal periphery, there appears to be an extra loss, which can not be accounted for by either reduced contrast sensitivity and contrast discrimination or by a shift in spatial scale. (vi) This extra loss cannot be quantitatively mimicked by "undersampling" the stimulus. (vii) Surprisingly, in some strabismics, and in the periphery, at relatively high spatial frequencies, vernier thresholds appear to lose their contrast dependence, suggesting the possibility that there may be qualitative differences between the normal fovea and these degraded visual systems. (viii) This contrast saturation can be mimicked by "undersampling" the target, or by introducing strips of mean luminance between the two vernier gratings, thus mimicking a "scotoma". Taken together with the preceding paper, our results suggest that the extra loss in position acuity of strabismic amblyopes and the normal periphery may be a consequence of noise at a second stage of processing, which selectively degrades position but not contrast discrimination.

Amblyopia↗
PubMed1994

Evaluation of ultrasound as a tool in the follow-up of patients with breast implants: a preliminary, prospective study.

We prospectively reviewed clinical findings in 93 patients with 154 silicone breast implants (SBIs), as well as ultrasound findings in 57 patients with 94 SBIs. Ultrasound findings of implant wall redundancy, combined with abnormal silicone echogenicity, correlated well with predicted and surgically confirmed SBI rupture. Age-related implant findings (i.e., length of time since implant placement) included capsular contracture, a variety of abnormal ultrasound findings, subjective reports of joint pain or fatigue, and a greater probability of rupture. Prospective screening of otherwise satisfied patients with SBIs raises concerns about the need for removal when satisfactory alternatives for maintaining breast size are limited. The ability to predict impending rupture will increase this dilemma.

Adult↗
PubMed1994

Effect of interseed spacing, tissue perfusion, thermoseed temperatures and catheters in ferromagnetic hyperthermia: results from simulations using finite element models of thermoseeds and catheters.

Finite element heat-transfer models of ferromagnetic thermoseeds and catheters are developed for simulating ferromagnetic hyperthermia. These models are implemented into a general purpose, finite element computer program to solve the bioheat transfer equation. The seed and catheter models are unique in that they have fewer modeling constraints than other previously developed thermal models. Simulations are conducted with a 4 x 4 array of seeds in a multicompartment tissue model. The heat transfer model predicts that fractions of tumor greater than 43 degrees C are between 8 and 40% lower when seed temperatures depend on power versus models which assume a constant seed temperature. Fractions of tumor greater than 42 degrees C, in simulations using seed and catheter models, are between 3.3 and 25% lower than in simulations with bare seeds. It is demonstrated that an array of seeds with Curie points of 62.6 degrees C heats the tumor very well over nearly all blood perfusion cases studied. In summary, results herein suggest that thermal models simulating ferromagnetic hyperthermia should consider the power-temperature dependence of seeds and include explicit models of catheters.

Catheterization↗
PubMed1993

Inverting a Laplacian topography map.

There are some occasions when it is better to use the Laplacian of the scalp voltages (e.g., for topographic displays and dipole source localization) and other occasions when it is better to use the original voltages (e.g., for comparison to non-Laplacian topography maps). This paper presents a simple algorithm for going between these two representations of the data. The inverse Laplacian involves iterative feedback. An Excel spreadsheet implementation of the algorithm is presented.

Algorithms↗
PubMed1993

Can sinusoidal vernier acuity be predicted by contrast discrimination?

A test-pedestal approach, with a test grating superimposed on a masking pedestal, was used to compare sinusoidal grating vernier acuity and contrast discrimination thresholds. The goal is to develop a simple model for vernier acuity without assumptions about underlying mechanisms. In the contrast discrimination task, subjects were asked to detect contrast increments in the presence of a base pedestal. In the vernier task, a test grating shifted by 90 deg relative to the pedestal grating was added to one-half of the pedestal grating to produce a vernier offset. When expressed in the same contrast units and compared under optimal conditions, vernier and contrast discrimination thresholds agree well at spatial frequencies between 2 and 20 c/deg and at pedestal contrasts above 10 times detection threshold. Thus, under these conditions, contrast discrimination predicts grating vernier acuity. To account for the discrepancies between vernier thresholds and contrast just noticeable difference (JND) when conditions deviate from optimal, one needs to make assumptions about the underlying mechanisms.

Contrast Sensitivity↗
PubMed1993

Understanding the diffractive bifocal contact lens.

How a diffractive bifocal contact lens works is not intuitively obvious. This is because the diffractive bifocal operates by an unusual blend of refraction and interference. The present article seeks to clarify how it achieves its two focal powers. Most of this article, however, is devoted to the conceptually simpler case of diffraction gratings in order to develop an intuition for how a double focus is produced. Single slit, double slit, and multiple slit gratings are considered using two formalisms: Feynman's "arrow" method (also called phasors) and the Fourier method. When dealing with linear diffraction gratings we are interested in the light intensity in the focal plane. By a simple redefinition of variables, all of the mathematics for diffraction gratings can be applied to circular lenses. For this latter case instead of looking at the light intensity in the focal plane, we look at light intensity on the optic axis. The appearance of two focal points is a simple consequence of a combined interference and diffraction pattern. The goal of this article is to present an introduction to Fourier optics and an analysis of the bifocal contact lens in a reader-friendly manner.

Contact Lenses↗
PubMed1993

Evaluation of cavernous transformation of the portal vein by magnetic resonance imaging.

We describe two children who underwent magnetic resonance imaging (MRI) as part of the evaluation of isolated splenomegaly after Doppler ultrasound was unable to identify the portal vein. The diagnosis of cavernous transformation of the portal vein and the normalcy of the biliary tract and hepatic architecture were confirmed by MRI. Because of the risks associated with computed tomography (CT) and angiographic evaluation of such patients, the use of MRI, a noninvasive modality, in the evaluation of pediatric patients requires further consideration.

Angiography↗
PubMed1992

The role of local contrast in the visual deficits of humans with naturally occurring amblyopia.

We measured the positional acuity of amblyopic observers and their sensitivity to the local contrast information which provides the cue for the position judgement. Our results suggest that there exist fundamental differences in the neural losses in humans with strabismic and anisometropic amblyopia. The losses in positional acuity of anisometropic amblyopes may be accounted for on the basis of the reduced contrast sensitivity and increased neural pooling of the underlying visual filters; whereas strabismic amblyopes, like the normal periphery, show an extra loss, which may be accounted for on the basis of scrambling, or jitter in the topographic mapping of information from retina to cortex. Since neurons in the striate cortex of monkeys show precise positional coding, it would be of particular interest to examine the positional acuity and local contrast sensitivity in cortical neurons of monkeys with experimental amblyopia using the same stimuli to measure both.

Amblyopia↗
PubMed1992

"Weber's law" for position: the role of spatial frequency and contrast.

We used Gabor bars to measure the effects of spatial bandwidth, spatial scale, contrast and separation on three-line spatial interval discrimination (bisection). In the first experiment, we used stimuli that were well above threshold. Our results show that at all spatial scales, spatial interval discrimination (three-line bisection) thresholds are proportional to the separation of the Gabor patches (i.e. Weber's law) when the separation exceeds approximately 2.5 times the standard deviation (sigma) of the Gaussian envelope. The optimal threshold occurs when the separation is approx. 2-2.5 sigma, and for separations larger than the optimal, bisection thresholds are equal to a more or less constant Weber fraction (delta s/s) of approx. 0.02-0.04. These results are consistent with a number of previous studies. In the second experiment, we examined the effect of contrast. Our results show an interaction between separation and stimulus visibility. Reducing the stimulus contrast has a marked effect on spatial interval thresholds at small separations (e.g. separations less than about 3 sigma), and much less effect at larger separations. Thus, the Weber's law relationship appears to depend on the visibility of the stimuli, but does not depend on the spatial frequency or bandwidth of the stimuli. These results can be predicted by an ideal observer model of spatial interval discrimination.

Contrast Sensitivity↗
PubMed1992

Acetaminophen fails to inhibit ethanol-induced subjective effects in human volunteers.

In animals, ethanol causes some of its CNS effects by releasing prostaglandins (PG); this is demonstrated by reports that prostaglandin synthetase inhibitors (PGSIs) diminish ethanol-induced effects. However, use of animals in these studies has precluded testing for subjective effects. We studied the interaction of ethanol and acetaminophen, a PGSI, in a double-blind crossover experiment. Six adult males were given no drug or acetaminophen (0, 325, 650, 1300 or 1950 mg) 75 min before ethanol (total dose = 0.625 g/kg; five divided doses). Physiologic, subjective and performance measures were collected. Compared to the no drug condition, ethanol significantly increased ratings of drug "liking," "drunk," "sluggish" and "drug strength" and decreased ratings of "sober." Ethanol increased heart rate and acetaminophen did not diminish or enhance this effect. The failure to antagonize ethanol-induced subjective and physiologic effects by acetaminophen in humans may be due to species differences or inadequate dosage of the PGSI. It is also possible that subjective and certain physiologic effects of ethanol in humans are not mediated by prostaglandin-dependent neural processes. Nevertheless, the finding that at greater than typical analgesic doses, acetaminophen failed to prevent subjective effects of ethanol is of clinical significance.

Acetaminophen↗
PubMed1992

A corneal topography algorithm that produces continuous curvature.

A new method is developed for calculating corneal curvature based on the reflected image of a series of concentric rings (keratoscopy). Previous methods had the drawback that the calculated corneal power was discontinuous in that it changed abruptly at each ring. The present method fits the corneal curvature with a cubic polynomial and produces a continuous estimate of the local power of the cornea. The cubic function also provides a better fit to the corneal curvature than previous methods that used a circle to provide the local fit. The full computer program for the algorithm is presented. Four different definitions of the local dioptric power are discussed.

Algorithms↗
PubMed1990

Equivalent intrinsic blur in spatial vision.

We used Gaussian blurred stimuli to explore the effect of blur on three tasks: (i) 2-line "resolution"; (ii) line detection; and (iii) spatial interval discrimination, in both central and peripheral vision. The results of our experiments can be summarized as follows. (i) 2-Line "resolution": thresholds for pairs of unblurred, low contrast, stimuli are approx. 0.5 min arc in the fovea. When the stimulus blur is small, it has little effect upon 2-line "resolution"; however, when the stimulus blur, sigma, exceeds 0.5 min, thresholds are degraded. We operationally define this transition point as the equivalent intrinsic blur or Bi. When the standard deviation of the stimulus blur, sigma, is greater than Bi, then the "resolution" threshold is approximately equal to sigma. Both the unblurred "resolution" threshold, and the equivalent intrinsic blur, Bi, vary with eccentricity in a manner consistent with the variation of cone separation within the central 10 deg. When the stimulus blur exceeds the equivalent intrinsic blur, "resolution" in the periphery is the same as in the fovea. (ii) Line detection: when the standard deviation of the stimulus blur, sigma, is less than Bi, then the line detection threshold is approximately inversely proportional to sigma (it is approximately TdBi/sigma) i.e. it obeys Ricco's law. When the standard deviation of the stimulus blur, sigma, is greater than Bi, then the "resolution" threshold is approximately equal to sigma and the detection threshold is approximately a fixed contrast (to be referred to as Td). According to (i) and (ii), the equivalent intrinsic blur, Bi, plays a dual role in determining both the "resolution" threshold and the detection threshold, Bi corresponds to the "Ricco's diameter" for spatial summation in a detection task, and it also corresponds to the "resolution" threshold for thin lines. This connection between detection and "resolution" is somewhat surprising. (iii) Spatial interval discrimination: thresholds are proportional to the separation of the lines (i.e. Weber's law). At the optimal separation, the thresholds represent a "hyperacuity" (i.e. they are smaller than the "resolution" threshold). For unblurred lines, the optimal separation is approximately 2-3 times the "resolution" limit at all eccentricities, so the optimal separation varies with eccentricity at the same rate as the equivalent intrinsic blur, Bi. However, the optimal spatial interval threshold falls off with eccentricity about 3-4 times more rapidly, consistent with the rate of decline of other position acuity tasks.(ABSTRACT TRUNCATED AT 400 WORDS)

Fovea Centralis↗
PubMed1990

Equivalent intrinsic blur in amblyopia.

We used Gaussian blurred stimuli to explore the effect of blur on three tasks: (i) 2-line resolution; (ii) line detection; and (iii) spatial interval discrimination, in observers with amblyopia due to anisometropia, strabismus, or both. The results of our experiments can be summarized as follows. (i) 2-Line resolution: in normal foveal vision, thresholds for unblurred stimuli are approx. 0.5 min arc in the fovea. When the standard deviation (sigma) of the stimulus blur is less than 0.5 min, it has little effect upon 2-line resolution; however, thresholds are degraded when the stimulus blur, sigma, exceeds 0.5 min. We operationally define this transition point, as the equivalent intrinsic blur, or Bi. When the stimulus blur, sigma, is greater than Bi, then the resolution threshold is approximately equal to sigma. In all of the amblyopic eyes, 2-line resolution thresholds for unblurred stimuli were elevated, and the equivalent intrinsic blur was much larger. When the stimulus blur exceeds the equivalent intrinsic blur, resolution thresholds were similar in amblyopic and nonamblyopic eyes. (ii) Line detection: in both normal and amblyopic eyes, when the stimulus blur, sigma, is less than Bi, then the line detection threshold is approximately inversely proportional to sigma; i.e. (it obeys Ricco's law). When sigma is greater than Bi, the equivalent intrinsic blur, then the detection threshold is approximately a fixed contrast. All of the amblyopic eyes showed markedly elevated thresholds for detecting thin lines, but normal or near normal thresholds for detecting very blurred lines. Consequently, Ricco's diameter is larger in amblyopic than in normal eyes. (iii) Spatial interval discrimination: thresholds are proportional to the separation of the lines (i.e. Weber's law). At the optimal separation, spatial interval discrimination thresholds represent a "hyperacuity" (i.e. they are smaller than the resolution threshold). For unblurred lines, the optimal separation is approx. 2-3 times Bi. In the normal fovea, and in the amblyopic eyes of anisometropic amblyopes the optimal spatial interval discrimination threshold is about one-fifth of the resolution threshold (i.e. a hyperacuity); and over a wide range of separations, spatial interval discrimination thresholds begin to rise when the stimulus blur exceeds about one-third of the separation between the lines as long as the contrast is sufficiently high. In contrast, in strabismic amblyopes, like the normal periphery, the optimal spatial interval discrimination thresholds are worse (higher) than would be expected based upon the resolution limit of the strabismic amblyopic eye.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1990

The role of separation and eccentricity in encoding position.

We measured two- and three-line spatial interval and alignment thresholds for a wide range of separations and eccentricities. In order to avoid confounding the role of separation and eccentricity, the test lines were presented on iso-eccentric arcs, with radii between 0.625 and 10 deg. The iso-eccentric paradigm allows separation to be varied over a large range while holding eccentricity constant. Our main finding is that for all tasks, at all eccentricities, Weber's law fails at large separations. Our results are consistent with the hypothesis that distance judgements are limited by at least two factors: (1) when the separation of the iso-eccentric test lines is small with respect to the eccentricity position thresholds are proportional to the separation of the features, and show little dependence on eccentricity. This is Weber's law for position, and the threshold is approx. 0.03-0.05 times the stimulus separation; (2) when the separation of the iso-eccentric test lines is comparable in size to the eccentricity, position thresholds are proportional to the target eccentricity, and are essentially independent of separation. In this "eccentricity regime", position discrimination thresholds are = ECC * k where ECC is the stimulus eccentricity in degrees, and k is a fraction of the eccentricity (approximately 0.01-0.03). The observer uses whichever mechanism is more sensitive to the stimulus. Under the conditions of our experiments, the observers perform a Weber computation when the separation, S, between the pair of iso-eccentric test lines is less than about 0.5 * ECC, and apply a "cortical ruler" when S is greater than about 0.5 * ECC. If the angle subtended by the fixation point and the pair of iso-eccentric test lines is considered, then for both the two-line and the three-line tasks, when the angle is less than approx. 30 deg, thresholds are proportional to separation, and when it is between 30 and 180 deg, thresholds are proportional to eccentricity.

Discrimination, Psychological↗
PubMed1990

Vernier acuity as line and dipole detection.

The vernier judgment is commonly thought of as discriminating the displacement of a portion of a pattern. However, we have found it revealing to consider vernier stimuli in another light; as the composite of a test pattern superimposed on a masking pedestal. The pedestal is the pattern with zero spatial offset, and the test pattern is the luminance distribution which, when added to the pedestal, produces the offset. For example, a vernier offset of an edge can be generated by adding a thin line (the derivative of an edge) to one half of an edge pedestal, and a vernier offset of a line can be generated by adding a thin dipole (the derivative of a line) to one half of a line pedestal. Vernier thresholds for low contrast edge and line pedestals can be directly predicted from detection thresholds of thin lines and dipoles on uniform fields. A surprisingly simple relationship is also derived between vernier thresholds and the size of Ricco's integration zone. We have found this masking paradigm to be fruitful and believe it is relevant to all the hyperacuities, not just vernier.

Contrast Sensitivity↗
PubMed1990

Spatial interval discrimination with blurred lines: black and white are separate but not equal at multiple spatial scales.

We used Gaussian blurred lines of same- and opposite-polarity to measure the effects of blur on 3-line spatial interval discrimination (bisection). The results of our experiments can be summarized as follows. Spatial interval discrimination (3-line bisection) thresholds are proportional to the separation of the lines (i.e. Weber's law). At the optimal separation, spatial interval discrimination thresholds for same-polarity lines represent a "hyperacuity" as small as 2 sec arc. For same-polarity Gaussian blurred lines, over a wide range of the blur standard deviations (sigma), the optimal threshold occurs when the separation is approx. 2 sigma, and the optimal threshold is about 0.02 sigma, or a Weber fraction (delta s/s) of 0.01. For opposite-polarity lines, under conditions where same-polarity stimuli yield the best thresholds (at a separation approximately 2 sigma), spatial interval thresholds are an order of magnitude worse than that for same-polarity lines, suggesting that the localization of stimili of opposite-polarity is much worse than that of same-polarity stimuli over a wide range of spatial scales. At large separations, greater than about 5 sigma, spatial interval discrimination thresholds are more or less independent of both contrast and polarity. While hyperacuity is generally thought of in terms of the tiny spatial thresholds which are obtained at small separations with stimuli comprised of thin lines, the present results, and those of others, suggest that for same-polarity stimuli, hyperacuity thresholds are a general property of the visual system, occurring at many spatial scales. The present results also suggest that the poor localization of opposite-polarity lines occurs at multiple spatial scales, when the line separation is less than about five times the stimulus spread. We consider several models which can account for particular features of our data.

Contrast Sensitivity↗