Assessment of mazindol for abuse liability.
Explore the source record for details and available documents.
Dose-Response Relationship, Drug↗
Biomedical subjects
S A Klein
Publications and source records attributed to S A Klein.
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↗
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↗
"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↗
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↗
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↗
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↗
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↗
Peripheral hyperacuity: three-dot bisection scales to a single factor from 0 to 10 degrees.
Performance of three-dot bisection was determined as a function of the feature separation at eccentricities of 0-10 deg along the inferior vertical field meridian by using dot stimuli scaled in size to compensate for eccentricity. For these briefly flashed dot stimuli, the entire function of three-dot bisection acuity against dot separation worsens away from the fovea with a single scaling factor that is compatible with the change of the cortical magnification factor in area V1. When the presentation duration of the stimulus was lengthened from 150 msec to 1 sec, the improvement was much greater in the fovea than at 10-deg eccentricity for closely separated stimuli. We attribute this difference to a luminance cue (detection of a brightness change) that is present for a long stimulus duration at small separations in the fovea, but not in the periphery.
Fovea Centralis↗
Peripheral hyperacuity: isoeccentric bisection is better than radial bisection.
Performance of three-dot bisection was determined as a function of orientation for a variety of feature separations and field meridians at eccentricities of 0-10 deg for two observers. The dot stimuli and separations were scaled in size to compensate for eccentricity. The precision of three-dot bisection was found to depend on the direction of test-feature offset. In the fovea, horizontal and vertical bisections were better than oblique bisections, while at eccentricities of 5-20 deg, isoeccentric (on a tangent to a circle of a given eccentricity) bisection was better than radial bisection. The direction of offset was more important than the orientation of the stimulus. Large separations showed a stronger effect than small separations. The anisotropy of bisection appears different from the meridional effect for resolution and is unlikely to be simply related to a local anisotropy of the cortical magnification factor.
Discrimination, Psychological↗
Maintenance of type A behavior: support for a type-similarity hypothesis.
Explore the source record for details and available documents.
Adolescent↗
Hypercalcemia in association with mesoblastic nephroma: report of a case and review of the literature.
Hypercalcemia, often associated with certain types of adult tumors, has also been described in pediatric neoplasms. In childhood, the more common associations include lymphoma, leukemia, rhabdomyosarcoma and rarely neuroblastoma. However, recently, several infants with hypercalcemia were described having renal tumors without bone metastases. The following is a case report of a 2-month-old infant who presented with severe hypercalcemia and a large right-sided abdominal mass, which at surgery was diagnosed as a cellular mesoblastic nephroma.
Female↗
Phase discrimination of compound gratings: generalized autocorrelation analysis.
The Julesz paradigm involving statistical constraints for the study of texture discrimination is extended to continuous-contrast repetitive patterns by using nth-order autocorrelation functions rather than Julesz's nth-order statistics. Second-order autocorrelations specify the power spectrum of the pattern, and the higher-order autocorrelations specify different levels of phase relationships in the pattern. A method is presented for generating patterns with statistical constraints of any order, in one and two dimensions. We show that, without scrutiny by foveal attention, discrimination of continuous textures fails at about the level of fourth-order constraints. An explanation for this failure based on the bandwidth of spatial-frequency-tuned mechanisms is excluded. The autocorrelation approach therefore may provide a general metric for the description of phase discrimination of repetitive textures.
Biometry↗
A three alternative tracking paradigm to measure vernier acuity of older infants.
Vernier acuity was measured in infants 1 to 14 months of age using a dynamic three alternative tracking paradigm. The location of the vernier offset would move randomly to one of three screen locations. An observer, unaware of the stimulus location, viewed the infant from behind a screen and guessed the position of the vernier offset. The magnitude of the vernier offset was controlled by a staircase and the results were analyzed using a broken line psychometric function. These thresholds were compared to thresholds obtained from the same infants during the same session with a two alternative forced choice preferential looking paradigm. Differences in the results of these two procedures and those reported previously in the literature are discussed in terms of the differences in the nature of the visual stimuli, one containing motion and one without motion.
Aging↗
Selectivity of the evoked potential for vernier offset.
Hyperacuity thresholds of a few arc seconds can be achieved psychophysically for a variety of spatial localization tasks. The present experiments show that evoked potentials can be elicited in response to the introduction of vernier offsets, but not by the introduction of other cues to hyperacuity such as bisection or relative pattern motion, although each of these cues is equally salient psychophysically. Moreover, vernier acuity measurements and the evoked potentials elicited in response to vernier offsets are strongly degraded by the introduction of flanking stimuli 2-4 min from the vernier target. This suggests that the hyperacuity VEP is a cortical correlate of a very specific type of hyperacuity, that produced by vernier offsets (colinearity failure).
Adult↗
Vernier acuity, crowding and cortical magnification.
When a vernier target is flanked by optimally positioned lines, foveal vernier discrimination is strongly degraded (Westheimer and Hauske, 1975). We confirmed this observation (Experiment I) and have mapped out a 2 dimensional "perceptive field" for crowding in the fovea using a 2 dot target (Experiment II). Crowding was also measured in peripheral vision, using either small flanking dots as masks (Experiment III), or using repetitive vernier gratings (Experiment IV). The results showed that when scaled in proportion to recent estimates of the cortical magnification factor, vernier acuity is as good in the periphery as it is centrally. Both centrally and peripherally, there appears to be a psychophysical processing module which we term a "perceptive hypercolumn". At all eccentricities vernier thresholds were found to be approximately 1/40 of the size of a perceptive hypercolumn and were elevated if interfering contours are present in the same (or adjacent) hypercolumns.
Form Perception↗
Vernier acuity, crowding and amblyopia.
When a vernier target is flanked by a pair of optimally positioned flanks, offset discrimination is strongly degraded. Spatial interference with vernier acuity was studied in each eye of observers with unilateral amblyopia associated with strabismus, anisometropia or both, and were compared to the functions obtained in the normal periphery (Levi et al., 1985). The results showed that: (1) For both strabismic and anisometropic amblyopes, as in normal central and peripheral vision, the extent of spatial interference was proportional to the unflanked vernier threshold. (2) For anisometropic amblyopes, grating and vernier acuity are affected similarly. (3) For strabismic amblyopes, like the normal periphery, vernier and grating acuity are decoupled, with vernier falling off faster than grating acuity. (4) The preferred eyes of strabismic but not anisometropic amblyopes have poorer vernier acuity than the normal controls. A conceptual framework for amblyopia based upon spatial filtering and spatial sampling is discussed.
Adult↗
Lactobacillemia in pregnancy.
A pregnant woman allergic to penicillin was successfully treated with erythromycin for pyelonephritis and bacteremia caused by Lactobacillus.
Adult↗