Dov Sagi | Weizmann Institute of Science (original) (raw)

Papers by Dov Sagi

Spatial Vision, 2000

Extinction is manifested in conditions of bilateral simultaneous stimulation, as a failure to det... more Extinction is manifested in conditions of bilateral simultaneous stimulation, as a failure to detect the stimulus contra-lateral to the side of a cerebral lesion, while the same stimulus is correctly detected there when presented in isolation. The phenomenon is usually interpreted in terms of impaired mobilization of attention from an attended to an unattended object. We have recently shown, using pairs of Gabor patches as stimuli, that pair detection is maximally improved in conditions where the two stimuli presented simultaneously to the two halves of the visual eld are co-oriented and co-axial and their location is not too eccentric. Here we add new information by showing that contrast isotropy of the stimulus pair is important in producing this orientation-similarity gain. The further advantage of co-oriented co-linear stimuli over co-oriented parallel (vertical) stimuli was shown exclusively with iso-contrast stimulus pairs, and was signi cantly enhanced when the contrast level of the stimulus pair was low. Stimulus properties producing reduced extinction seem to correlate with the selectivity pattern and contrast dependence of (a) spatial lateral facilitation observed in psychophysical studies with normal observers, and (b) long-range interactions observed in the primary visual cortex. Thus, two remote visual stimuli seem to be processed as a single object when the corresponding neuronal activities are linked via long-range lateral interactions. The present demonstration of contrast dependency in such processing, strengthens our previous conjecture that even in the presence of signi cant, extinction producing, parietal damage, the primary visual cortex preserves the capacity to encode, using long-range lateral interactions, an image description in which visual objects are already segregated from background.

Abstract Motion-induced blindness (MIB) is a visual phenomenon in which a salient static target s... more Abstract Motion-induced blindness (MIB) is a visual phenomenon in which a salient static target spontaneously fluctuates in and out of visual awareness when surrounded by a moving mask pattern. It has been hypothesized that MIB reflects an antagonistic interplay between cortical representations of the static target and moving mask. Here, we report evidence for such antagonism between human ventral and dorsal visual cortex during MIB.

Perceptual organization is a basic process of the visual system, and is responsible for binding r... more Perceptual organization is a basic process of the visual system, and is responsible for binding related features into coherent representations. However, in certain brain diseases, this process can become less efficient (Silverstein, in press). One such condition is schizophrenia, where over 25 studies have now demonstrated reduced perceptual organization (Uhlhaas & Silverstein, 2005).

Practicing sensory tasks could result in two main perceptual outcomes. The first, and more widely... more Practicing sensory tasks could result in two main perceptual outcomes. The first, and more widely documented, is perceptual learning referring to long-lasting improvement of perceptual thresholds. The second is perceptual deterioration, which is observed when the number of trials is increased within a training session or between closely spaced sessions.

Amblyopic vision is thought to be limited by abnormal long-range spatial interactions, but their ... more Amblyopic vision is thought to be limited by abnormal long-range spatial interactions, but their exact mode of action and relationship to the main amblyopic deficit in visual acuity is largely unknown.

Interactions between spatial frequency channels were tested in two ways: we measured the discrimi... more 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 (confrasf 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 (parfern 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 I 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.

Unilateral spatial neglect (USN) patients show reduced contrast sensitivity on their contralesion... more Unilateral spatial neglect (USN) patients show reduced contrast sensitivity on their contralesional side and often miss their non-salient stimuli. What their subjective experience is when successfully reporting a stimulus remains unclear. Here, we report that despite large contrast sensitivity differences between the sides, the relative attenuation in perceived contrast measured in a contrast-matching task was small.

Abstract Supra-threshold spatial integration was studied by testing the saliency of multi Gabor e... more Abstract Supra-threshold spatial integration was studied by testing the saliency of multi Gabor element con gurations in short duration binocular rivalry (dichoptic masking) conditions. Dichoptic presentations allows for a competition between spatially overlapping supra-threshold stimuli that involve non-overlapping receptive elds in the rst stage of visual ltering. Di erent spatial con gurations of Gabor patches (== 0: 12 deg) were presented to one eye (target) together with a bandpass noise presented to the other eye (mask).

Adults with amblyopia were recently shown to perform abnormally in tasks requiring integration of... more Adults with amblyopia were recently shown to perform abnormally in tasks requiring integration of local features into global percepts. Moreover, spatial interactions in amblyopic patients, though often found to be abnormal, showed marked variability. Here we measured collinear lateral interactions using Gabor patches in a large number of amblyopic (N= 75) and normal subjects (N= 25), testing four spatial frequencies (1.5, 3, 6, 9 cpd).

Introduction This review addresses several aspects of visual imagery, as an example for the inter... more Introduction This review addresses several aspects of visual imagery, as an example for the interactions between perception, memory and consciousness1. Visual imagery is the natural ability to invent or recreate an experience that resembles the experience of actually perceiving an object or an event, in the absence of retinal input. Visual imagery and perception share several functional properties, and apparently share common underlying brain structures.

The detection threshold of a centrally placed Gabor target is reduced in the presence of aligned ... more The detection threshold of a centrally placed Gabor target is reduced in the presence of aligned high-contrast Gabor patches that are optimally spaced from the target (Polat & Sagi, 1993). Here we determined whether threshold reduction is due to signal enhancement or to decreased signal response variability (internal noise), using a recently developed analysis for a Signal Detection Theory (SDT)-based contrast-identification paradigm (Katkov, Tsodyks, & Sagi, 2007a).

One of the fundamental functions performed in vision is perceptual grouping. Through grouping we ... more One of the fundamental functions performed in vision is perceptual grouping. Through grouping we are able to derive, from the relationships between local regions of a scene, a perception of the overall structure and also of the distinctiveness of the separate objects contained within it. A long standing debate in psychology concerns the extent to which perceptual processes are autonomous, operating independently of top-down attentional control1-5.

ABSTRACT In real life, individuals are faced with more than one perceptual event on which they ha... more ABSTRACT In real life, individuals are faced with more than one perceptual event on which they have to make distinct decisions. It is shown that for a range of such multistimulus environments, decision behavior departs from optimality in the sense that subjects do not set their decision criteria in accordance with the requirements of each individual event.

Abstract: Extinction is manifested in conditions of bilateral simultaneous stimulation, as a fail... more Abstract: Extinction is manifested in conditions of bilateral simultaneous stimulation, as a failure to detect the stimulus contra-lateral to the side of a cerebral lesion, while the same stimulus is correctly detected there when presented in isolation. The phenomenon is usually interpreted in terms of impaired mobilization of attention from an attended to an unattended object.

Klein [Klein, AS (2006). Separating transducer nonlinearities and multiplicative noise in contras... more Klein [Klein, AS (2006). Separating transducer nonlinearities and multiplicative noise in contrast discrimination. Vision Research, 46, 4279–4293] questions the existence of intrinsic singularities in two-alternative force-choice (2AFC) Signal Detection Theory (SDT) models, suggesting that the singularities found in Katkov et al.[Katkov, M., Tsodyks, M., & Sagi, D.(2006a). Singularities in the inverse modeling of 2AFC contrast discrimination data. Vision Research, 46, 259–266; Katkov, M., Tsodyks, M., & Sagi, D.(2006b).

We investigate binding within the framework of a model of excitatory and inhibitory cell assembli... more We investigate binding within the framework of a model of excitatory and inhibitory cell assemblies that form an oscillating neural network. Our model is composed of two such networks that are connected through their inhibitory neurons. The excitatory cell assemblies represent memory patterns. The latter have different meanings in the two networks, representing two different attributes of an. object, such as shape and color.

Abstract According to classical signal detection theory (SDT), in simple detection or discriminat... more Abstract According to classical signal detection theory (SDT), in simple detection or discrimination tasks, observers use a decision parameter based on their noisy internal response to set a boundary between “yes” and “no” responses. Experimental paradigms where performance is limited by internal noise cannot be used to provide an unambiguous measure of the decision criterion and its variability. Here, unidimensional external noise is used to estimate a criterion and its variability in stimulus space.

The visual system can adapt to optical blur, whereby the adapted image is perceived as sharp. Her... more The visual system can adapt to optical blur, whereby the adapted image is perceived as sharp. Here we show that adaptation reduces blur-induced biases in shape perception, with repeated adaptations (perceptual learning), leading to unbiased perception upon re-exposure to blur. Observers wore a cylindrical lens of+ 1.00 D on one eye, thus simulating monocular astigmatism. The other eye was either masked with a translucent blurred lens (monocular) or unmasked (dichoptic).

Abstract Using oriented Gabor patches, we found that for nearly cardinal target orientations, obl... more Abstract Using oriented Gabor patches, we found that for nearly cardinal target orientations, oblique flankers' orientations induced more interference than did cardinal flankers' orientations. This pattern was observed both at the local and global levels of flankers' orientation. With respect to the global orientation (flankers' global arrangement around the target), there was no difference between the effects of the two cardinal orientations, and both induced the same amount of interference.

Spatial Vision, 2000

Extinction is manifested in conditions of bilateral simultaneous stimulation, as a failure to det... more Extinction is manifested in conditions of bilateral simultaneous stimulation, as a failure to detect the stimulus contra-lateral to the side of a cerebral lesion, while the same stimulus is correctly detected there when presented in isolation. The phenomenon is usually interpreted in terms of impaired mobilization of attention from an attended to an unattended object. We have recently shown, using pairs of Gabor patches as stimuli, that pair detection is maximally improved in conditions where the two stimuli presented simultaneously to the two halves of the visual eld are co-oriented and co-axial and their location is not too eccentric. Here we add new information by showing that contrast isotropy of the stimulus pair is important in producing this orientation-similarity gain. The further advantage of co-oriented co-linear stimuli over co-oriented parallel (vertical) stimuli was shown exclusively with iso-contrast stimulus pairs, and was signi cantly enhanced when the contrast level of the stimulus pair was low. Stimulus properties producing reduced extinction seem to correlate with the selectivity pattern and contrast dependence of (a) spatial lateral facilitation observed in psychophysical studies with normal observers, and (b) long-range interactions observed in the primary visual cortex. Thus, two remote visual stimuli seem to be processed as a single object when the corresponding neuronal activities are linked via long-range lateral interactions. The present demonstration of contrast dependency in such processing, strengthens our previous conjecture that even in the presence of signi cant, extinction producing, parietal damage, the primary visual cortex preserves the capacity to encode, using long-range lateral interactions, an image description in which visual objects are already segregated from background.

Abstract Motion-induced blindness (MIB) is a visual phenomenon in which a salient static target s... more Abstract Motion-induced blindness (MIB) is a visual phenomenon in which a salient static target spontaneously fluctuates in and out of visual awareness when surrounded by a moving mask pattern. It has been hypothesized that MIB reflects an antagonistic interplay between cortical representations of the static target and moving mask. Here, we report evidence for such antagonism between human ventral and dorsal visual cortex during MIB.

Perceptual organization is a basic process of the visual system, and is responsible for binding r... more Perceptual organization is a basic process of the visual system, and is responsible for binding related features into coherent representations. However, in certain brain diseases, this process can become less efficient (Silverstein, in press). One such condition is schizophrenia, where over 25 studies have now demonstrated reduced perceptual organization (Uhlhaas & Silverstein, 2005).

Practicing sensory tasks could result in two main perceptual outcomes. The first, and more widely... more Practicing sensory tasks could result in two main perceptual outcomes. The first, and more widely documented, is perceptual learning referring to long-lasting improvement of perceptual thresholds. The second is perceptual deterioration, which is observed when the number of trials is increased within a training session or between closely spaced sessions.

Amblyopic vision is thought to be limited by abnormal long-range spatial interactions, but their ... more Amblyopic vision is thought to be limited by abnormal long-range spatial interactions, but their exact mode of action and relationship to the main amblyopic deficit in visual acuity is largely unknown.

Interactions between spatial frequency channels were tested in two ways: we measured the discrimi... more 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 (confrasf 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 (parfern 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 I 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.

Unilateral spatial neglect (USN) patients show reduced contrast sensitivity on their contralesion... more Unilateral spatial neglect (USN) patients show reduced contrast sensitivity on their contralesional side and often miss their non-salient stimuli. What their subjective experience is when successfully reporting a stimulus remains unclear. Here, we report that despite large contrast sensitivity differences between the sides, the relative attenuation in perceived contrast measured in a contrast-matching task was small.

Abstract Supra-threshold spatial integration was studied by testing the saliency of multi Gabor e... more Abstract Supra-threshold spatial integration was studied by testing the saliency of multi Gabor element con gurations in short duration binocular rivalry (dichoptic masking) conditions. Dichoptic presentations allows for a competition between spatially overlapping supra-threshold stimuli that involve non-overlapping receptive elds in the rst stage of visual ltering. Di erent spatial con gurations of Gabor patches (== 0: 12 deg) were presented to one eye (target) together with a bandpass noise presented to the other eye (mask).

Adults with amblyopia were recently shown to perform abnormally in tasks requiring integration of... more Adults with amblyopia were recently shown to perform abnormally in tasks requiring integration of local features into global percepts. Moreover, spatial interactions in amblyopic patients, though often found to be abnormal, showed marked variability. Here we measured collinear lateral interactions using Gabor patches in a large number of amblyopic (N= 75) and normal subjects (N= 25), testing four spatial frequencies (1.5, 3, 6, 9 cpd).

Introduction This review addresses several aspects of visual imagery, as an example for the inter... more Introduction This review addresses several aspects of visual imagery, as an example for the interactions between perception, memory and consciousness1. Visual imagery is the natural ability to invent or recreate an experience that resembles the experience of actually perceiving an object or an event, in the absence of retinal input. Visual imagery and perception share several functional properties, and apparently share common underlying brain structures.

The detection threshold of a centrally placed Gabor target is reduced in the presence of aligned ... more The detection threshold of a centrally placed Gabor target is reduced in the presence of aligned high-contrast Gabor patches that are optimally spaced from the target (Polat & Sagi, 1993). Here we determined whether threshold reduction is due to signal enhancement or to decreased signal response variability (internal noise), using a recently developed analysis for a Signal Detection Theory (SDT)-based contrast-identification paradigm (Katkov, Tsodyks, & Sagi, 2007a).

One of the fundamental functions performed in vision is perceptual grouping. Through grouping we ... more One of the fundamental functions performed in vision is perceptual grouping. Through grouping we are able to derive, from the relationships between local regions of a scene, a perception of the overall structure and also of the distinctiveness of the separate objects contained within it. A long standing debate in psychology concerns the extent to which perceptual processes are autonomous, operating independently of top-down attentional control1-5.

ABSTRACT In real life, individuals are faced with more than one perceptual event on which they ha... more ABSTRACT In real life, individuals are faced with more than one perceptual event on which they have to make distinct decisions. It is shown that for a range of such multistimulus environments, decision behavior departs from optimality in the sense that subjects do not set their decision criteria in accordance with the requirements of each individual event.

Abstract: Extinction is manifested in conditions of bilateral simultaneous stimulation, as a fail... more Abstract: Extinction is manifested in conditions of bilateral simultaneous stimulation, as a failure to detect the stimulus contra-lateral to the side of a cerebral lesion, while the same stimulus is correctly detected there when presented in isolation. The phenomenon is usually interpreted in terms of impaired mobilization of attention from an attended to an unattended object.

Klein [Klein, AS (2006). Separating transducer nonlinearities and multiplicative noise in contras... more Klein [Klein, AS (2006). Separating transducer nonlinearities and multiplicative noise in contrast discrimination. Vision Research, 46, 4279–4293] questions the existence of intrinsic singularities in two-alternative force-choice (2AFC) Signal Detection Theory (SDT) models, suggesting that the singularities found in Katkov et al.[Katkov, M., Tsodyks, M., & Sagi, D.(2006a). Singularities in the inverse modeling of 2AFC contrast discrimination data. Vision Research, 46, 259–266; Katkov, M., Tsodyks, M., & Sagi, D.(2006b).

We investigate binding within the framework of a model of excitatory and inhibitory cell assembli... more We investigate binding within the framework of a model of excitatory and inhibitory cell assemblies that form an oscillating neural network. Our model is composed of two such networks that are connected through their inhibitory neurons. The excitatory cell assemblies represent memory patterns. The latter have different meanings in the two networks, representing two different attributes of an. object, such as shape and color.

Abstract According to classical signal detection theory (SDT), in simple detection or discriminat... more Abstract According to classical signal detection theory (SDT), in simple detection or discrimination tasks, observers use a decision parameter based on their noisy internal response to set a boundary between “yes” and “no” responses. Experimental paradigms where performance is limited by internal noise cannot be used to provide an unambiguous measure of the decision criterion and its variability. Here, unidimensional external noise is used to estimate a criterion and its variability in stimulus space.

The visual system can adapt to optical blur, whereby the adapted image is perceived as sharp. Her... more The visual system can adapt to optical blur, whereby the adapted image is perceived as sharp. Here we show that adaptation reduces blur-induced biases in shape perception, with repeated adaptations (perceptual learning), leading to unbiased perception upon re-exposure to blur. Observers wore a cylindrical lens of+ 1.00 D on one eye, thus simulating monocular astigmatism. The other eye was either masked with a translucent blurred lens (monocular) or unmasked (dichoptic).

Abstract Using oriented Gabor patches, we found that for nearly cardinal target orientations, obl... more Abstract Using oriented Gabor patches, we found that for nearly cardinal target orientations, oblique flankers' orientations induced more interference than did cardinal flankers' orientations. This pattern was observed both at the local and global levels of flankers' orientation. With respect to the global orientation (flankers' global arrangement around the target), there was no difference between the effects of the two cardinal orientations, and both induced the same amount of interference.