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S A Klein

Publications and source records attributed to S A Klein.

At least 19 recordsLinked to original sources
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↗
PubMed1989

Peripheral positional acuity: retinal and cortical constraints on 2-dot separation discrimination under photopic and scotopic conditions.

The precision of discriminating the separation of two dots was measured as a function of separation for eccentricities of 0-10 deg under photopic and scotopic conditions. At each eccentricity, the 2-dot separation discrimination thresholds showed a V-shaped dependence on separation. For separations less than approximately twice the resolution threshold, performance deteriorated from photopic to scotopic conditions and appeared to be limited by ganglion cell receptive field size or spacing. For separations smaller than 10% of the effective eccentricity (eccentricity + 0.6 deg), the photopic 2-dot separation discrimination thresholds were significantly better than 3-dot bisection thresholds previously measured under similar experimental conditions, supporting the hypothesis that 3-dot bisection suffers from spatial interference for these separations. Interestingly, under scotopic conditions, 2-dot separation discrimination thresholds were better than resolution for a range of separations at each eccentricity, implying that cone input was not necessary for hyperacuity performance. 2-dot separation discrimination thresholds for large separations were little changed from photopic to scotopic luminance conditions.

Humans↗
PubMed1989

Both separation and eccentricity can limit precise position judgements: a reply to Morgan and Watt.

Weber's law is ubiquitous in position judgements. In a variety of position acuity tasks, the position threshold is proportional to the separation of the reference features. Recently, we (Klein & Levi, 1987; Levi, Klein & Yap, 1988) suggested that two sensory processes may serve to limit position acuity, and thus contribute to Weber's law for position. One is the target separation, and the other is the target eccentricity. In order to test this idea, we pitted separation against eccentricity by measuring spatial interval discrimination thresholds on an iso-eccentric arc (Levi et al., 1988). Over a 5-fold range of separations, we found that thresholds were independent of separation, and concluded that at large separations, eccentricity can limit precise position judgements. In the preceding article, Morgan and Watt (1989) have questioned this conclusion, and have shown that the effects of eccentricity are small in an arc length discrimination task. In the present article, we: (i) address the objections raised by Morgan and Watt; (ii) show that our data and 2-mechanism model are consistent with many previous studies; and (iii) show that Morgan and Watt's task is inherently difficult, so that rather than tapping the sensory limits imposed by the target eccentricity, performance on the arc length task is constrained by the cognitive demands of the task, or by the difficulty of reconstruction. In contrast, the measurement of chord length (spatial interval discrimination on an iso-eccentric arc) can be simply done by calculating the distance between the endpoints. Thus, at large separations, thresholds for the chord length judgements are much lower than those of Morgan and Watt, and are proportional to the eccentricity of the targets.

Humans↗
PubMed1988

"Weber's law" for position: unconfounding the role of separation and eccentricity.

Bisection thresholds are approximately proportional to the separation/eccentricity of the targets. This "Weber's law" for position has been invoked over the past century. It is the separation of the reference targets, or their eccentricity which determines the threshold? In previous studies separation and eccentricity are confounded. In the present report we have pitted separation against eccentricity. Bisection thresholds were measured for stimuli presented on an isoeccentric arc, so that separation could be varied while holding the eccentricity of the test lines constant. We used a 5-fold range of separations from 2-10 deg. In this regime, the present results provide strong evidence against Weber's law. When separation is varied but eccentricity held constant, there is no Weber's law. Rather the thresholds are approximately constant. Our results suggest that the judgement of the separation of widely separated objects is similar to a distance measurement using a ruler on the cortex, in that the error of measurement is independent of the separation between objects. The results imply that when we attempt to gauge the distance between widely separated objects it is unlikely that we do so on the basis of the outputs of large spatial filters; rather it appears that we make such judgements by estimating the cortical distance which separates the targets of interest.

Adult↗
PubMed1987

Positional uncertainty in peripheral and amblyopic vision.

Three experiments were performed to examine positional acuity and the role of spatial sampling in central, peripheral and amblyopic vision. In the first experiment, 3-line bisection acuity was compared to grating acuity. In normal foveal vision bisection acuity represents a hyperacuity. In anisometropic amblyopes, bisection acuity is reduced in rough proportion to their grating acuity. In strabismic amblyopes, and in the normal periphery, bisection acuity is reduced to a greater extent than grating acuity. This result implies that reduced contrast sensitivity of the spatial filters is not sufficient to account for the increased positional uncertainty found in peripheral vision and in strabismic amblyopia. In order to test the hypothesis that the high degree of positional uncertainty evident in these visual systems is a consequence of sparse spatial sampling, bisection thresholds and width discrimination thresholds were measured with stimuli comprised of discrete samples. The results showed that normal foveal vision and the vision of anisometropic amblyopes show little benefit from adding discrete samples to the stimulus. In contrast, the normal periphery, and the central field of strabismic amblyopes demonstrate marked positional uncertainty which can be efficiently reduced in proportion to the square root of the number of samples (up to about 10) comprising the stimulus in the direction orthogonal to the discrimination cue. In aggregate the results suggest that anisometropic and strabismic amblyopia are fundamentally different. The positional uncertainty in anisometropic amblyopia is consistent with the reduced sensitivity of the spatial filters. The data of the normal periphery and of the central field of strabismic amblyopes suggest that the cortical sampling grain imposes a fundamental limit upon their positional acuity.

Amblyopia↗
PubMed1987

Diagnostic amniocentesis and bacteraemia.

Thirty antenatal patients with intact membranes were studied to determine the incidence of bacteraemia induced by transabdominal amniocentesis. No bacteraemias were detected following the procedure. Antibiotic prophylaxis is probably not warranted for immunocompetent hospitalized patients undergoing amniocentesis.

Adult↗
PubMed1987

Nonlinear directionally selective subunits in complex cells of cat striate cortex.

1. We have analyzed receptive fields (RFs) of directionally selective (DS) complex cells in the striate cortex of the cat. We determined the extent to which the DS of a complex cell depends on spatially identifiable subunits within the RF by studying responses to an optimally oriented, three-luminance-valued, gratinglike stimulus that was spatiotemporally randomized. 2. We identified subunits by testing for nonlinear spatial RF interactions. To do this, we calculated Wiener-like kernels in a spatial superposition test that depended on two RF positions at a time. The spatial and temporal separation of light and dark bars at these two positions varied over a spatial range of 8 degrees and a temporal range of +/- 112 ms in increments of 0.5 degree and 16 ms, respectively. 3. DS responses in complex cells cannot be explained by their responses to single light or dark bars because any linear superposition of responses whose time course is uniform across space shows no directional preference. 4. Nonlinear interactions between a flashed reference bar that is fixed in position and a second bar that is flashed at surrounding positions help explain DS by showing multiplicative-type facilitation for bar pairs that mimic motion in the preferred direction and suppression for bar pairs that mimic motion in the null direction. Interactions in the preferred direction have an optimal space/time ratio (velocity), exhibited by elongated, obliquely oriented positive domains in a space-time coordinate frame. This relationship is inseparable in space-time. The slope of the long axis specifies the preferred speed, and its negative agrees with the most strongly suppressed speed in the opposite direction. 5. When the reference bar position is moved across the RF, the spatiotemporal interaction moves with it. This suggests the existence of a family of nearly uniform subunits distributed across the RF. We call the subunit interaction, as averaged across the RF, the "motion kernel" because its spatial and temporal variables are those necessary to specify the velocity, the only parameter that distinguishes a moving image from a temporally modulated stationary image. The nonlinear interaction shows a spatial periodicity, which suggests a mechanism of velocity selectivity for moving extended images.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

Position sense of the peripheral retina.

Position acuity was measured over a wide range of eccentricities, from 3 min to 10 deg in the horizontal meridian, by using both a three-dot bisection task and a three-dot vernier task. A foveal fixation dot appeared for 1 sec before an outer pair of test dots was flashed for 200 msec. Bisection and vernier tasks were used to measure position acuity in the radial and tangential directions, respectively. The vernier data were well fitted by a straight line on linear axes of offset threshold versus eccentricity. The bisection data, on the other hand, were poorly fitted by a single straight line. However, a double-line fit worked very well. The line segment at large eccentricities (greater than 0.5 deg) had an x intercept of about 0.6 deg, in good agreement with previous estimates based on cortical magnification and on hyperacuity in the presence of flanks. These results imply that three-dot vernier thresholds are set by a single orientation mechanism at all eccentricities and that three-dot bisection thresholds are set by a pair of mechanisms. For eccentricities less than 15 min, thresholds are in good agreement with calculations based on spatial-frequency filters. For larger eccentricities, the bisection thresholds agree with scaled anatomical modules that are presumed to exist in the human visual cortex. The thresholds for position acuity in the tangential direction are as low as 0.005 times the eccentricity. In the radial direction, thresholds are poorer, implying that additional cortical factors may further constrain performance.

Humans↗