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D Sagi

Publications and source records attributed to D Sagi.

54 records · Page 3Linked to original sources
PubMed1992

Visual attention and perceptual grouping.

Perceptual organization is thought to involve an analysis of both textural discontinuities and perceptual grouping. In earlier work, we found that textural discontinuities were detected normally even when visual attention was engaged elsewhere. Here we report how perceptual grouping is affected when visual attention is engaged by a concurrent visual task. To elicit perceptual grouping, we used the Gestalt demonstrations of grouping on the basis of proximity and of similarity. Four tasks were investigated, some requiring the observer to discriminate between horizontal and vertical grouping, and some requiring the observer to merely detect the presence or absence of grouping. Visual attention was engaged at the center of the display by a form identification task. The detection of a textural discontinuity served as a control task. Concurrent form identification conflicted with all four grouping tasks, resulting in a significant reduction of grouping performance in each case. No performance reduction was observed when either form identification or grouping discrimination was combined with the detection of a textural discontinuity. These results suggest that perceptual grouping and form identification compete for visual attention, whereas the detection of a textural discontinuity does not.

Attention↗
PubMed1991

Where practice makes perfect in texture discrimination: evidence for primary visual cortex plasticity.

In terms of functional anatomy, where does learning occur when, for a basic visual discrimination task, performance improves with practice (perceptual learning)? We report remarkable long-term learning in a simple texture discrimination task where learning is specific for retinal input. This learning is (i) local (in a retinotopic sense), (ii) orientation specific but asymmetric (it is specific for background but not for target-element orientation), and (iii) strongly monocular (there is little interocular transfer of learning). Our results suggest that learning involves experience-dependent changes at a level of the visual system where monocularity and the retinotopic organization of the visual input are still retained and where different orientations are processed separately. These results can be interpreted in terms of local plasticity induced by retinal input in early visual processing in human adults, presumably at the level of orientation-gradient sensitive cells in primary visual cortex.

Discrimination, Psychological↗
PubMed1991

Short- and long-range processes in structure-from-motion.

Human ability to detect 3-D structure in an array of 2-D moving dots was tested. Under limited exposure time, we found high detection rates only when the 2-D motion was restricted to the spatio-temporal region of short-range motion. Long-range moving dots failed to produce a strong impression of 3-D structure and yielded only weak detection rates. This result is consistent with the view that the processing of long-range motion is more serial than that of short-range motion.

Depth Perception↗
PubMed1991

Texture-based tasks are little affected by second tasks requiring peripheral or central attentive fixation.

Experiments are described in which observers attempted to perform concurrently two separate visual tasks. Two types of tasks were used: the identification of a T-shaped or L-shaped letter target, and the detection or localization of a texture element of unique orientation (texture target) within a dense texture. Combining these tasks to form various task pairs, performance as a function of stimulus onset asynchrony (SOA) was established separately for each task in a pair. In addition, performance was measured when each task was carried out by itself. When paired, two identification tasks (T or L) on (two) letter targets required a significantly larger SOA than either identification task by itself. This outcome suggests the involvement of serial performance and competition for a limited resource, confirming that letter identification requires attentive fixation. However, when the identification of a central letter target was paired with the localization (upper or lower hemifield) of an eccentric texture target, performance in the pair was comparable to performance of each task by itself. This suggests parallel performance and a lack of conflict over resources. The outcome was similar when the identification of an eccentric letter target was paired with the detection (present or absent) of an eccentric texture target. These results are consistent with the possibility that localization and detection of a textural singularity do not require attentive fixation.

Attention↗
PubMed1990

Detection of an orientation singularity in Gabor textures: effect of signal density and spatial-frequency.

This work presents evidence for a second stage of spatial filtering in early vision. This second stage operates on the output of the well known linear spatial filters and integrates their thresholded responses with a center-surround weighting function. The evidence for the existence of this second stage comes from experiments where observers have to detect a Gabor signal with known parameters (target) among a varied number of other Gabor signals having orthogonal orientation (distractors). Detection performance on this task depends on the number of distractors and the distance between them: when the number of distractors is small performance deteriorates with an increasing number of distractors; however, when the number of distractors becomes larger performance improves with an increasing number of distractors. This improvement depends on the spatial-frequency of the signals and their spatial separation. Best performance is achieved when the spatial separation between signals is larger than three times their center wavelength but smaller than nine times their wavelength, implying a second stage filtering with a center size of six wavelengths and a total size of 18 wavelengths. This second stage of filtering may underlie our ability to detect certain texture boundaries preatentively.

Discrimination, Psychological↗
PubMed1990

Spatial variability as a limiting factor in texture-discrimination tasks: implications for performance asymmetries.

Texture-discrimination tasks reveal a pronounced performance asymmetry depending on which texture represents the foreground region (small area) and which represents the ground (large area). This asymmetry implies that some global processes are involved in the segmentation process. We examined this problem within the context of the texture-segmentation algorithm, assuming two filtering stages. The first stage uses spatial frequency and orientation-selective (Gabor) filters, whereas the second stage is formed by low-resolution edge-detection filters. The presence and location of texture borders are indicated by significant responses in the second stage. Spurious texture borders may occur owing to textural local variabilities (such as orientation randomization), which are enhanced by the first stage. We suggest that these spurious borders act as background noise and thus limit performance in texture-discrimination tasks. The noise level depends on which texture occupies the ground in the display. We tested this model on numerous pairs of textures and found remarkably good correlation with human performance. A prediction of the model, namely, that discrimination asymmetry will be reduced when textural elements have identical orientation, was tested psychophysically and confirmed.

Algorithms↗
PubMed1990

Vision outside the focus of attention.

We investigated the relationship between focal attention and a feature-gradient detection that is performed in a parallel manner. We found that a feature gradient can be detected without measurable impairment of performance even while a concurrent form-recognition task is carried out. In spite of the fact that the form-recognition task engages focal attention and thus removes attentive resources from the vicinity of the feature gradient. This outcome suggests strongly that certain perceptions concerning salient boundaries and singularities in a visual scene can be accomplished without the aid of resource-limited processes, such as focal attention, and, by implication, that there may exist two distinct perceptual faculties (one attentive, the other not) that are able to bring complementary kinds of visual information simultaneously to our awareness.

Attention↗
PubMed1987

Parallel and serial processes in motion detection.

Apparent motion was used to explore humans' ability to perceive the direction of motion in the visual field. A marked qualitative difference in this ability was found between short- and long-range motion. For short-range motion, the detection of the direction of motion is characterized by parallel operation over a wide visual field (that is, detection performance is independent of the number of objects in an array). When the positional displacement is large relative to an object's size, the direction of motion is detected in a serial manner. The process of detection is limited in this case by the ability to detect other events, such as appearance and disappearance of an object, and the ability to compute their spatio-temporal relations. The results are consistent with a previously suggested division of the motion detection system into short- and long-range processes. The direction of short-range motion can be perceived in parallel (preattentively), whereas long-range motion is attentive and requires more complicated computations. It seems that the detection of long-range motion is a conjunction task, combining the detection of disappearance and appearance.

Humans↗
PubMed1987

Short-range limitation on detection of feature differences.

We studied the ability of observers to detect the presence of a clearly visible line segment against a background of line segments of different orientation. As we increase the number (density) of these background lines, we find that detectability does not behave monotonically. Adding a small number of background lines decreases detectability but if adjacent line segments are permitted to fall in close range, a further increase of background lines improves performance which eventually reaches a constant level. This suggests that detection of feature differences involves a short-range process. The range of this process is about two degrees or twice the length of the line segments used. Thus texture-gradients between different elements are only formed if the distance between these elements is not much larger than the average element size.

Form Perception↗
PubMed1985

"Where" and "what" in vision.

The mixture of a few horizontal and vertical line segments embedded in an aggregate of diagonal line segments can be rapidly counted and their positions rapidly determined by a parallel (preattentive) process. However, the discrimination between horizontal and vertical orientation (that is, discrimination of a single conspicuous feature) requires serial search by focal attention. Under recent theories of attention, focal attention has been assumed to be required for the recognition of different combinations of features. According to the findings of this experiment, knowing "what" even a single feature is requires time-consuming search by focal attention. Only knowing "where" a target it is mediated by a parallel process.

Attention↗
PubMed1985

Fast noninertial shifts of attention.

It was suggested that some discrimination tasks (e.g. discrimination between the letters T and L) require serial search by scrutinizing each letter (target) with a small aperture of focal attention. Here we examine the effect of intertarget distance on discrimination performance, using two targets. We find reduction in performance at short distances, in agreement with masking studies, but constant performance independent of distance outside this masking region. This constant performance is still lower than expected from masking effects and might reflect attentive process. Sequential presentation of the targets with delays up to 30-40 ms, while reducing available processing time, does not cause reduction in performance, thus supporting the suggestion that discrimination of the two targets is a serial process. The independence of performance on distance suggests fast noninertial shifts of attention.

Attention↗
PubMed1984

The contrast dependence of spatial frequency channel interactions.

Interactions between spatial frequency channels were tested in two ways: we measured the discriminability by the visual system between two compound spatial-frequency gratings, of components with spatial frequencies in the ratio 1:3, when the difference between the two gratings was an increase (or decrease) in contrast of both components of the compound grating (contrast discrimination), or when the difference between the two gratings was an increase in contrast of one component and a decrease in contrast of the other component (pattern discrimination). We found that the contrast: pattern discriminability ratio differs significantly from unity in most conditions. Furthermore, this ratio is generally greater when the components of the grating are in peaks-add relative phase than when in peaks-subtract phase. On the other hand, the ratio was close to unity for grating components of spatial frequencies 1 and 9 cycles/deg. These results suggest that the human visual system contains spatial frequency channels with bandwidths of between 1.6 and 3.2 octaves and that these relatively broad channels have peaks-add spatial profiles. The channels appear linear at intermediate contrasts and spatial frequencies, but super-linear at high spatial frequencies and contrasts. Contrast and spatial frequency may be interchangeable for the determination of the linearity of the visual system.

Discrimination, Psychological↗
PubMed1984

Detection versus discrimination of visual orientation.

The role of focused attention in vision is examined. Recent theories of attention hypothesize that serial search by focal attention is required for discrimination between different combinations of features. Experiments are reported which show that the mixture of a few (less than five) horizontal and vertical line segments embedded in an aggregate of diagonal line segments can be rapidly counted (also called 'subitizing') by a parallel (preattentive) process, while the discrimination between horizontal and vertical orientation requires serial search by shifting focal attention to each line segment. Thus detecting and counting targets that differ in orientation can be done in parallel by a preattentive process, whereas knowing 'what' the orientation of a target is (horizontal or vertical, ie of a single conspicuous feature) requires a serial search by focal attention.

Attention↗
PubMed1983

Discriminability of suprathreshold compound spatial frequency gratings.

We have investigated the detectability of small contrast changes in compound suprathreshold gratings, containing two sinusoidal components. We examined the relationship between the detectability of changes in each of the components alone and the detectability of changes in the two components together. It was found that for the pairs of spatial frequencies 1 and 9 c/deg there is a square law summation, but for spatial frequency pairs omega and 3 omega there is a greater summation which depends on the relative phases of the gratings' components. The results indicate the existence of broadband phase selective spatial frequency channels, of 2 octaves bandwidth.

Discrimination, Psychological↗
PubMedDate not supplied

Production and molecular characterization of clinical phase i anti-melanoma mouse IgG3 monoclonal antibody R24.

R24 is a mouse IgG3 monoclonal antibody (mab) that reacts with the ganglioside GD3 expressed by cells of neuroectodermal origin. The anti-tumor activity of R24 has been demonstrated in initial phase I and pilot trials in patients suffering from metastatic melanoma. The purpose of this study was to investigate the biotechnological production and particularly the glycosylation of this clinically important antibody. Growth, metabolism, and IgG production of R24 secreting hybridoma cells were analyzed on 1 L bioreactor bench scale using repeated-batch mode. The amount of 57 mg of pure mab was obtained from 1.6 L crude supernatant by protein A chromatography. Western blot binding assays with sugar-specific lectins revealed glycosylation of the heavy chains, whereas no carbohydrates were detectable on the light chains. Because glycosylation is essential for antibody effector functions in vivo (such as complement fixation or binding to macrophage Fc receptors), mab R24 was subjected to both enzymatic deglycosylation using PNGase F and chemical deglycosylation by hydrazinolysis. Released glycans were structurally characterized by high pH anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD), matrix assisted laser desorption ionization time-of-flight (MALDI-TOF), and electrospray ionization quadrupole time-of-flight (ESI-QTOF) mass spectrometry. Six major biantennary chains of the complex glycosylation phenotype were found with variations in galactosylation and core fucosylation. The predominant N-linked structure, indicating the high degree of agalactosyl glycoforms, was the agalacto biantennary chain with a relative percentage of 57% (51% core-fucosylated, 6% nonfucosylated). The second most abundant oligosaccharide was the monogalacto biantennary chain amounting to 30% (26% core- and 4% nonfucosylated). The antibody contained 0.46 microg sialic acid per mg protein, which splits into 0.243 microg Neu5Gc and 0.217 microg Neu5Ac, corresponding to a Neu5Ac:Neu5Gc ratio of 1:1.06. Furthermore, the antigen specificity of R24 was determined by immunodetection of GD3 on thin-layer chromatograms, and real time GD3-antibody binding interactions were measured with an optical biosensor (BIAcore). From the structural data obtained in this study it is concluded that glycosylation of the antibody may be important in the clinical outcome of targeted anti-cancer immunotherapy.

Animals↗
PubMedDate not supplied

Enhanced detection in the aperture of focal attention during simple discrimination tasks.

There is increasing evidence that it is possible to shift an aperture of focal attention to a position in visual space independent of fixation and that this can be done much faster than the eyes are able to move. Recently, we showed that such serial scrutiny by the aperture of focal attention is required before an observer is able to tell what a target is (for example, to know whether the orientation of a line segment is horizontal or vertical). Here we considered whether attention directed towards a specific position in the visual field for an orientation discrimination task improves performance on a simple detection task in the area to which attention is directed. We found that a small test flash could be detected when it was positioned near a peripheral line target presented briefly, if the orientation of the target had to be identified. The test flash could not be detected when presented at some distance from the same target or when another target had to be identified. This enhancement implies that even simple identification tasks such as orientation discrimination are not performed passively by the visual system.

Discrimination, Psychological↗