During binocular-rivalry suppression, an ordinarily visible stimulus is erased from awareness, bu... more During binocular-rivalry suppression, an ordinarily visible stimulus is erased from awareness, but how is the sensory representation of that stimulus affected? Although it is established that rivalry suppression attenuates signal strength, the influence of suppression on signal fidelity remains unknown. Here, we show that noise plays a hitherto undiscovered role in the degradation of the percept under suppression. In Experiment 1, we measured psychometric functions for a stimulus presented under dominance and suppression, and found that the slope of these functions was shallower under suppression-a result suggesting that the signal representation was rendered noisier. Experiment 2 then revealed the source of this noise: An examination of the influence of suppression on the orientation bandwidth of noise masking showed that tuning bandwidth is significantly broadened under suppression. Thus, the discriminability of a suppressed stimulus is weakened not only by a general decrease in signal strength, but also by broader orientation tuning that introduces more noise in the neural representation of the suppressed stimulus.
Orientation selectivity is a fundamental, emergent property of neurons in early visual cortex, an... more Orientation selectivity is a fundamental, emergent property of neurons in early visual cortex, and discovery of that property [1, 2] dramatically shaped how we conceptualize visual processing [3-6]. However, much remains unknown about the neural substrates of these basic building blocks of perception, and what is known primarily stems from animal physiology studies. To probe the neural concomitants of orientation processing in humans, we employed repetitive transcranial magnetic stimulation (rTMS) to attenuate neural responses evoked by stimuli presented within a local region of the visual field. Previous physiological studies have shown that rTMS can significantly suppress the neuronal spiking activity, hemodynamic responses, and local field potentials within a focused cortical region [7, 8]. By suppressing neural activity with rTMS, we were able to dissociate components of the neural circuitry underlying two distinct aspects of orientation processing: selectivity and contextual effects. Orientation selectivity gauged by masking was unchanged by rTMS, whereas an otherwise robust orientation repulsion illusion was weakened following rTMS. This dissociation implies that orientation processing relies on distinct mechanisms, only one of which was impacted by rTMS. These results are consistent with models positing that orientation selectivity is largely governed by the patterns of convergence of thalamic afferents onto cortical neurons, with intracortical activity then shaping population responses contained within those orientation-selective cortical neurons.
When dissimilar images are presented to the two eyes, the human visual system lapses into binocul... more When dissimilar images are presented to the two eyes, the human visual system lapses into binocular rivalry, a unique perceptual state characterized by stochastic alternations in dominance of one of the two source images over the other. Probe targets delivered to an eye during suppression phases are more difficult to detect than probes delivered during dominance phases. Nearly all probe studies have involved presenting new stimulation (e.g., a spot of light) either superimposed on or replacing the suppressed stimulus. Here, we ask whether observers can detect a reduction in the contrast of the suppressed stimulus itself. In other words, can observers detect a probe that should make an already invisible stimulus even weaker? Specifically, we compared observers' ability to detect contrast increments and contrast decrements introduced within a rival pattern during dominance and suppression. Contrast increment thresholds were elevated across all pedestal contrasts when the increment was introduced during suppression compared to during dominance, replicating previous results. Contrast decrement thresholds measured during suppression were elevated to an even greater extent, but the fact that they were obtained at all establishes that observers were able to detect probes that should make an already invisible target even more difficult to perceive. In a second experiment, we found a similar pattern of results for contrast change detection in complex images of faces as well. Based on the resulting threshold-vs.-contrast functions, we suggest that, regardless of the complexity of the image, rivalry suppression modulates the neural contrast response function through a mixture of reduced overall response gain and a shift in the contrast gain.
We investigated the mechanisms underlying the effects of sustained and transient covert attention... more We investigated the mechanisms underlying the effects of sustained and transient covert attention on contrast sensitivity. The aim of this study was twofold: (1) Using a zero-noise display, we assessed whether sustained (endogenous) attention enhances contrast sensitivity via signal enhancement, and compared the magnitude of the effect with that of transient (exogenous) attention.
Human psychophysics and monkey physiology studies have shown that attention modulates early visio... more Human psychophysics and monkey physiology studies have shown that attention modulates early vision -contrast sensitivity and processing. But how can we bridge the effects of attention on perceptual performance to their neural underpinnings? Here we implement a population-coding model that estimates attentional effects on population contrast response given psychophysical data. Model results show that whereas endogenous (sustained, voluntary) attention changes population contrast-response via contrast gain, exogenous (transient, involuntary) attention changes population contrast-response via response gain.
How does attention optimize our visual system for the task at hand? Two mechanisms have been prop... more How does attention optimize our visual system for the task at hand? Two mechanisms have been proposed for how attention improves signal processing: gain and tuning. To distinguish between these two mechanisms we use the equivalent-noise paradigm, which measures performance as a function of external noise. In the present study we explored how spatial and feature-based attention affect performance by assessing their threshold-vs-noise (TvN) curves with regard to the signature behavioral effects of gain and tuning. Furthermore, we link our psychophysical results to neurophysiology by implementing a simple, biologically-plausible model to show that attention affects the gain and tuning of population responses differentially, depending on the type of attention being deployed: Whereas spatial attention operates by boosting the gain of the population response, feature-based attention operates by both boosting the gain and sharpening the tuning of the population response.
At any given moment, our visual system is confronted with far more information than it can proces... more At any given moment, our visual system is confronted with far more information than it can process effectively. The high energy cost of neuronal activity involved in cortical computation severely limits our capacity to process this information 1 . Visual attention serves as a mediating mechanism, enabling us to selectively grant priority of processing to certain aspects of the visual scene. One means of granting priority is to direct one's gaze towards the relevant location. However, many situations call for one to attend to an area in the periphery without actually directing gaze toward it. For example, when driving it is generally best to keep your eyes on the road ahead while covertly monitoring the periphery for cars, pedestrians and potential road hazards. The impact of covert attention 2 on visual performance is well-documented across a range of perceptual tasks, such as visual search 3-6 , letter identification 7,8 , contrast sensitivity 9-12 and spatial resolution . Several studies that used single-cell recording 17-22 , event-related potentials 23,24 and functional magnetic resonance imaging (fMRI) indicate that attentional modulation occurs as early as striate and extrastriate visual cortex.
We recently demonstrated that transient covert attention increases the apparent contrast of a sti... more We recently demonstrated that transient covert attention increases the apparent contrast of a stimulus (Carrasco, Ling, & Read, 2004). Schneider (2006) proposes that the observed increase in apparent contrast is largely due to sensory interactions occurring between the precue and stimulus, rather than to attention. Specifically, he reports that cuing effects only occur at contrasts near detection threshold, and that there are confounding sensory interactions between the cue and stimulus at suprathreshold detection contrasts. Our response is twofold. First, we outline the key methodological differences between our original study and Schneider’s that are likely to account for the different results, and explain how we had ruled out the sensory interaction explanation of the cue. Second, we directly test the prediction put forth by Schneider: If the effects were due to sensory interactions, reversing the luminance polarity of the precue in our paradigm should lead to differential cuing effects. We replicate one of the experiments of our original study and add a condition in which the cue luminance is either black or white. Our results replicated our previous findings—they showed an increase in apparent contrast to a high-contrast stimulus when it was precued. Moreover, we found that the black cue and the white cue had the same effect, thus ruling out the alternative explanation proposed by Schneider. Transient attentiondoes alter contrast appearance.
Does emotion affect how people see? We investigated the effects of emotion and attention, as well... more Does emotion affect how people see? We investigated the effects of emotion and attention, as well as their conjoint effect, on contrast sensitivity, a dimension of early vision. We manipulated the emotional valence and the attentional distribution of cues preceding a target stimulus and asked observers to judge the orientation of the target as contrast varied. This study provides the first behavioral evidence that (a) emotion enhances contrast sensitivity irrespective of attention and (b) emotion potentiates the effect of attention on contrast sensitivity.
We investigated the mechanisms underlying the effects of sustained and transient covert attention... more We investigated the mechanisms underlying the effects of sustained and transient covert attention on contrast sensitivity. The aim of this study was twofold: (1) Using a zero-noise display, we assessed whether sustained (endogenous) attention enhances contrast sensitivity via signal enhancement, and compared the magnitude of the effect with that of transient (exogenous) attention.
During binocular-rivalry suppression, an ordinarily visible stimulus is erased from awareness, bu... more During binocular-rivalry suppression, an ordinarily visible stimulus is erased from awareness, but how is the sensory representation of that stimulus affected? Although it is established that rivalry suppression attenuates signal strength, the influence of suppression on signal fidelity remains unknown. Here, we show that noise plays a hitherto undiscovered role in the degradation of the percept under suppression. In Experiment 1, we measured psychometric functions for a stimulus presented under dominance and suppression, and found that the slope of these functions was shallower under suppression-a result suggesting that the signal representation was rendered noisier. Experiment 2 then revealed the source of this noise: An examination of the influence of suppression on the orientation bandwidth of noise masking showed that tuning bandwidth is significantly broadened under suppression. Thus, the discriminability of a suppressed stimulus is weakened not only by a general decrease in signal strength, but also by broader orientation tuning that introduces more noise in the neural representation of the suppressed stimulus.
Orientation selectivity is a fundamental, emergent property of neurons in early visual cortex, an... more Orientation selectivity is a fundamental, emergent property of neurons in early visual cortex, and discovery of that property [1, 2] dramatically shaped how we conceptualize visual processing [3-6]. However, much remains unknown about the neural substrates of these basic building blocks of perception, and what is known primarily stems from animal physiology studies. To probe the neural concomitants of orientation processing in humans, we employed repetitive transcranial magnetic stimulation (rTMS) to attenuate neural responses evoked by stimuli presented within a local region of the visual field. Previous physiological studies have shown that rTMS can significantly suppress the neuronal spiking activity, hemodynamic responses, and local field potentials within a focused cortical region [7, 8]. By suppressing neural activity with rTMS, we were able to dissociate components of the neural circuitry underlying two distinct aspects of orientation processing: selectivity and contextual effects. Orientation selectivity gauged by masking was unchanged by rTMS, whereas an otherwise robust orientation repulsion illusion was weakened following rTMS. This dissociation implies that orientation processing relies on distinct mechanisms, only one of which was impacted by rTMS. These results are consistent with models positing that orientation selectivity is largely governed by the patterns of convergence of thalamic afferents onto cortical neurons, with intracortical activity then shaping population responses contained within those orientation-selective cortical neurons.
When dissimilar images are presented to the two eyes, the human visual system lapses into binocul... more When dissimilar images are presented to the two eyes, the human visual system lapses into binocular rivalry, a unique perceptual state characterized by stochastic alternations in dominance of one of the two source images over the other. Probe targets delivered to an eye during suppression phases are more difficult to detect than probes delivered during dominance phases. Nearly all probe studies have involved presenting new stimulation (e.g., a spot of light) either superimposed on or replacing the suppressed stimulus. Here, we ask whether observers can detect a reduction in the contrast of the suppressed stimulus itself. In other words, can observers detect a probe that should make an already invisible stimulus even weaker? Specifically, we compared observers' ability to detect contrast increments and contrast decrements introduced within a rival pattern during dominance and suppression. Contrast increment thresholds were elevated across all pedestal contrasts when the increment was introduced during suppression compared to during dominance, replicating previous results. Contrast decrement thresholds measured during suppression were elevated to an even greater extent, but the fact that they were obtained at all establishes that observers were able to detect probes that should make an already invisible target even more difficult to perceive. In a second experiment, we found a similar pattern of results for contrast change detection in complex images of faces as well. Based on the resulting threshold-vs.-contrast functions, we suggest that, regardless of the complexity of the image, rivalry suppression modulates the neural contrast response function through a mixture of reduced overall response gain and a shift in the contrast gain.
We investigated the mechanisms underlying the effects of sustained and transient covert attention... more We investigated the mechanisms underlying the effects of sustained and transient covert attention on contrast sensitivity. The aim of this study was twofold: (1) Using a zero-noise display, we assessed whether sustained (endogenous) attention enhances contrast sensitivity via signal enhancement, and compared the magnitude of the effect with that of transient (exogenous) attention.
Human psychophysics and monkey physiology studies have shown that attention modulates early visio... more Human psychophysics and monkey physiology studies have shown that attention modulates early vision -contrast sensitivity and processing. But how can we bridge the effects of attention on perceptual performance to their neural underpinnings? Here we implement a population-coding model that estimates attentional effects on population contrast response given psychophysical data. Model results show that whereas endogenous (sustained, voluntary) attention changes population contrast-response via contrast gain, exogenous (transient, involuntary) attention changes population contrast-response via response gain.
How does attention optimize our visual system for the task at hand? Two mechanisms have been prop... more How does attention optimize our visual system for the task at hand? Two mechanisms have been proposed for how attention improves signal processing: gain and tuning. To distinguish between these two mechanisms we use the equivalent-noise paradigm, which measures performance as a function of external noise. In the present study we explored how spatial and feature-based attention affect performance by assessing their threshold-vs-noise (TvN) curves with regard to the signature behavioral effects of gain and tuning. Furthermore, we link our psychophysical results to neurophysiology by implementing a simple, biologically-plausible model to show that attention affects the gain and tuning of population responses differentially, depending on the type of attention being deployed: Whereas spatial attention operates by boosting the gain of the population response, feature-based attention operates by both boosting the gain and sharpening the tuning of the population response.
At any given moment, our visual system is confronted with far more information than it can proces... more At any given moment, our visual system is confronted with far more information than it can process effectively. The high energy cost of neuronal activity involved in cortical computation severely limits our capacity to process this information 1 . Visual attention serves as a mediating mechanism, enabling us to selectively grant priority of processing to certain aspects of the visual scene. One means of granting priority is to direct one's gaze towards the relevant location. However, many situations call for one to attend to an area in the periphery without actually directing gaze toward it. For example, when driving it is generally best to keep your eyes on the road ahead while covertly monitoring the periphery for cars, pedestrians and potential road hazards. The impact of covert attention 2 on visual performance is well-documented across a range of perceptual tasks, such as visual search 3-6 , letter identification 7,8 , contrast sensitivity 9-12 and spatial resolution . Several studies that used single-cell recording 17-22 , event-related potentials 23,24 and functional magnetic resonance imaging (fMRI) indicate that attentional modulation occurs as early as striate and extrastriate visual cortex.
We recently demonstrated that transient covert attention increases the apparent contrast of a sti... more We recently demonstrated that transient covert attention increases the apparent contrast of a stimulus (Carrasco, Ling, & Read, 2004). Schneider (2006) proposes that the observed increase in apparent contrast is largely due to sensory interactions occurring between the precue and stimulus, rather than to attention. Specifically, he reports that cuing effects only occur at contrasts near detection threshold, and that there are confounding sensory interactions between the cue and stimulus at suprathreshold detection contrasts. Our response is twofold. First, we outline the key methodological differences between our original study and Schneider’s that are likely to account for the different results, and explain how we had ruled out the sensory interaction explanation of the cue. Second, we directly test the prediction put forth by Schneider: If the effects were due to sensory interactions, reversing the luminance polarity of the precue in our paradigm should lead to differential cuing effects. We replicate one of the experiments of our original study and add a condition in which the cue luminance is either black or white. Our results replicated our previous findings—they showed an increase in apparent contrast to a high-contrast stimulus when it was precued. Moreover, we found that the black cue and the white cue had the same effect, thus ruling out the alternative explanation proposed by Schneider. Transient attentiondoes alter contrast appearance.
Does emotion affect how people see? We investigated the effects of emotion and attention, as well... more Does emotion affect how people see? We investigated the effects of emotion and attention, as well as their conjoint effect, on contrast sensitivity, a dimension of early vision. We manipulated the emotional valence and the attentional distribution of cues preceding a target stimulus and asked observers to judge the orientation of the target as contrast varied. This study provides the first behavioral evidence that (a) emotion enhances contrast sensitivity irrespective of attention and (b) emotion potentiates the effect of attention on contrast sensitivity.
We investigated the mechanisms underlying the effects of sustained and transient covert attention... more We investigated the mechanisms underlying the effects of sustained and transient covert attention on contrast sensitivity. The aim of this study was twofold: (1) Using a zero-noise display, we assessed whether sustained (endogenous) attention enhances contrast sensitivity via signal enhancement, and compared the magnitude of the effect with that of transient (exogenous) attention.
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