Analysing the visual properties of a stimulus, such as its shape or its motion, involves a tempor... more Analysing the visual properties of a stimulus, such as its shape or its motion, involves a temporally constrained cascade of processes including sensory integration, attentional selection and perceptual decision. In natural conditions, these processing stages may temporally overlap. In this study, we used a time constrained paradigm designed to segregate these stages, and recorded MEG activity to characterize their cortical correlates. The experimental sequence comprised a structure-from-motion (SFM) target flanked by pre and post masks, which limited the stimulus-driven processes in time. The MEG responses were recorded in three perceptual tasks bearing on the presence or absence of the target SFM stimulus, on its direction of motion or on its 3D shape. Subjects’ responses were delayed to the end of the trials using a random stimulus-response mapping. The analysis of the evoked magnetic fields reveals temporally non-overlapping responses for stimulus-driven and task-related processes. Source reconstruction shows that the occipital, dorsal and ventral stimulus-related areas follow different temporal profiles, with the response in IPS slowly rising after the beginning of the activity in the lateral occipital cortex and falling back to the baseline prior the delayed motor response. Task dependent differences are mainly observed during the late component of these responses. Altogether, the results indicate that sensory integration and decisional processes may involve overlapping regions but at different time periods. The rising activity found in IPS may reflect accumulating evidence on the presented stimulus leading to a motor response, in line with the results from single cell studies in monkey LIP [1]. [1] Huk AC, Shadlen MN (2005) Neural activity in macaque parietal cortex reflects temporal integration of visual motion signals during perceptual decision making. The Journal of neuroscience, 25 (45): 10420-36.
This study investigates neural magneto-encephalographic (MEG) correlates of visual form and motio... more This study investigates neural magneto-encephalographic (MEG) correlates of visual form and motion binding. Steady-state visual evoked fields (SSVEF) were recorded in MEG while observers reported their bound or unbound perception of moving bars arranged in a square shape. By using pairs of oscillating vertical and horizontal bars, "frequency-tagged" at f1 and f2, we identified a region with enhanced sustained power at 2f1+2f2 intermodulation frequency correlated with perceptual reports. Intermodulation power is more important during perceptual form/motion integration than during the perceptual segmentation of the stimulus into individual component motions, indicating that intermodulation frequency power is a neuromarker of form/motion integration. Source reconstruction of cortical activities at the relevant frequencies further reveals well segregated activity in the occipital lobe at the fundamental of the stimulation, f1 and f2, widely spread activity at 2f1 and 2f2 and a focal activity in the medial part of the right precentral sulcus region at the intermodulation component, 2f1+2f2. The present findings indicate that motion tagging provides a powerful way of investigating the processes underlying visual form/motion binding non-invasively in humans.
Psychophysical and physiological studies suggest that long-range horizontal connections in primar... more Psychophysical and physiological studies suggest that long-range horizontal connections in primary visual cortex participate in spatial integration and contour processing. Until recently, little attention has been paid to their intrinsic temporal properties. Recent physiological studies indicate, however, that the propagation of activity through long-range horizontal connections is slow, with time scales comparable to the perceptual scales involved in motion processing.
Accurate interpretations of image require the segmentation of motion signals produced by differen... more Accurate interpretations of image require the segmentation of motion signals produced by different objects with the simultaneous integration of motion signals produced by the same object. We investigated a motion integration paradigm in which the direction of an object's motion could only be determined from an integration of motion signals across the disconnected object edges. In a series of experiments we show that observers' ability to determine object motion depends significantly upon stimulus duration, luminance contrast and edge width. These effects suggest that the visual system, after some delay, relies upon relatively thick, luminance defined contour discontinuities to segment moving images.
The integration of information from different sensory modalities has many advantages for human ob... more The integration of information from different sensory modalities has many advantages for human observers, including increase of salience, resolution of perceptual ambiguities, and unified perception of objects and surroundings. Several behavioral, electrophysiological and neuroimaging data collected in various tasks, including localization and detection of spatial events, crossmodal perception of object properties and scene analysis are reviewed here. All the results
Studying object recognition is central to fundamental and clinical research on cognitive function... more Studying object recognition is central to fundamental and clinical research on cognitive functions but suffers from the limitations of the available sets that cannot always be modified and adapted to meet the specific goals of each study. We here present a new set of 3D scans of real objects available on-line as ASCII files, OB3D. These files are lists of dots, each defined by a triplet of spatial coordinates and their normal that allow simple and highly versatile transformations and adaptations. We performed a web-based experiment to evaluate the minimal number of dots required for the denomination and categorization of these objects, thus providing a reference threshold. We further analyze several other variables derived from this data set, such as the correlations with object complexity. This new stimulus set, which was found to activate the Lower Occipital Complex (LOC) in another study, may be of interest for studies of cognitive functions in healthy participants and patients with cognitive impairments, including visual perception, language, memory, etc.
Analysing the visual properties of a stimulus, such as its shape or its motion, involves a tempor... more Analysing the visual properties of a stimulus, such as its shape or its motion, involves a temporally constrained cascade of processes including sensory integration, attentional selection and perceptual decision. In natural conditions, these processing stages may temporally overlap. In this study, we used a time constrained paradigm designed to segregate these stages, and recorded MEG activity to characterize their cortical correlates. The experimental sequence comprised a structure-from-motion (SFM) target flanked by pre and post masks, which limited the stimulus-driven processes in time. The MEG responses were recorded in three perceptual tasks bearing on the presence or absence of the target SFM stimulus, on its direction of motion or on its 3D shape. Subjects’ responses were delayed to the end of the trials using a random stimulus-response mapping. The analysis of the evoked magnetic fields reveals temporally non-overlapping responses for stimulus-driven and task-related processes. Source reconstruction shows that the occipital, dorsal and ventral stimulus-related areas follow different temporal profiles, with the response in IPS slowly rising after the beginning of the activity in the lateral occipital cortex and falling back to the baseline prior the delayed motor response. Task dependent differences are mainly observed during the late component of these responses. Altogether, the results indicate that sensory integration and decisional processes may involve overlapping regions but at different time periods. The rising activity found in IPS may reflect accumulating evidence on the presented stimulus leading to a motor response, in line with the results from single cell studies in monkey LIP [1]. [1] Huk AC, Shadlen MN (2005) Neural activity in macaque parietal cortex reflects temporal integration of visual motion signals during perceptual decision making. The Journal of neuroscience, 25 (45): 10420-36.
This study investigates neural magneto-encephalographic (MEG) correlates of visual form and motio... more This study investigates neural magneto-encephalographic (MEG) correlates of visual form and motion binding. Steady-state visual evoked fields (SSVEF) were recorded in MEG while observers reported their bound or unbound perception of moving bars arranged in a square shape. By using pairs of oscillating vertical and horizontal bars, "frequency-tagged" at f1 and f2, we identified a region with enhanced sustained power at 2f1+2f2 intermodulation frequency correlated with perceptual reports. Intermodulation power is more important during perceptual form/motion integration than during the perceptual segmentation of the stimulus into individual component motions, indicating that intermodulation frequency power is a neuromarker of form/motion integration. Source reconstruction of cortical activities at the relevant frequencies further reveals well segregated activity in the occipital lobe at the fundamental of the stimulation, f1 and f2, widely spread activity at 2f1 and 2f2 and a focal activity in the medial part of the right precentral sulcus region at the intermodulation component, 2f1+2f2. The present findings indicate that motion tagging provides a powerful way of investigating the processes underlying visual form/motion binding non-invasively in humans.
Psychophysical and physiological studies suggest that long-range horizontal connections in primar... more Psychophysical and physiological studies suggest that long-range horizontal connections in primary visual cortex participate in spatial integration and contour processing. Until recently, little attention has been paid to their intrinsic temporal properties. Recent physiological studies indicate, however, that the propagation of activity through long-range horizontal connections is slow, with time scales comparable to the perceptual scales involved in motion processing.
Accurate interpretations of image require the segmentation of motion signals produced by differen... more Accurate interpretations of image require the segmentation of motion signals produced by different objects with the simultaneous integration of motion signals produced by the same object. We investigated a motion integration paradigm in which the direction of an object's motion could only be determined from an integration of motion signals across the disconnected object edges. In a series of experiments we show that observers' ability to determine object motion depends significantly upon stimulus duration, luminance contrast and edge width. These effects suggest that the visual system, after some delay, relies upon relatively thick, luminance defined contour discontinuities to segment moving images.
The integration of information from different sensory modalities has many advantages for human ob... more The integration of information from different sensory modalities has many advantages for human observers, including increase of salience, resolution of perceptual ambiguities, and unified perception of objects and surroundings. Several behavioral, electrophysiological and neuroimaging data collected in various tasks, including localization and detection of spatial events, crossmodal perception of object properties and scene analysis are reviewed here. All the results
Studying object recognition is central to fundamental and clinical research on cognitive function... more Studying object recognition is central to fundamental and clinical research on cognitive functions but suffers from the limitations of the available sets that cannot always be modified and adapted to meet the specific goals of each study. We here present a new set of 3D scans of real objects available on-line as ASCII files, OB3D. These files are lists of dots, each defined by a triplet of spatial coordinates and their normal that allow simple and highly versatile transformations and adaptations. We performed a web-based experiment to evaluate the minimal number of dots required for the denomination and categorization of these objects, thus providing a reference threshold. We further analyze several other variables derived from this data set, such as the correlations with object complexity. This new stimulus set, which was found to activate the Lower Occipital Complex (LOC) in another study, may be of interest for studies of cognitive functions in healthy participants and patients with cognitive impairments, including visual perception, language, memory, etc.
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The experimental sequence comprised a structure-from-motion (SFM) target flanked by pre and post masks, which limited the stimulus-driven processes in time.
The MEG responses were recorded in three perceptual tasks bearing on the presence or absence of the target SFM stimulus, on its direction of motion or on its 3D shape.
Subjects’ responses were delayed to the end of the trials using a random stimulus-response mapping.
The analysis of the evoked magnetic fields reveals temporally non-overlapping responses for stimulus-driven and task-related processes. Source reconstruction shows that the occipital, dorsal and ventral stimulus-related areas follow different temporal profiles, with the response in IPS slowly rising after the beginning of the activity in the lateral occipital cortex and falling back to the baseline prior the delayed motor response.
Task dependent differences are mainly observed during the late component of these responses. Altogether, the results indicate that sensory integration and decisional processes may involve overlapping regions but at different time periods. The rising activity found in IPS may reflect accumulating evidence on the presented stimulus leading to a motor response, in line with the results from single cell studies in monkey LIP [1].
[1] Huk AC, Shadlen MN (2005) Neural activity in macaque parietal cortex reflects temporal integration of visual motion signals during perceptual decision making. The Journal of neuroscience, 25 (45): 10420-36.
The experimental sequence comprised a structure-from-motion (SFM) target flanked by pre and post masks, which limited the stimulus-driven processes in time.
The MEG responses were recorded in three perceptual tasks bearing on the presence or absence of the target SFM stimulus, on its direction of motion or on its 3D shape.
Subjects’ responses were delayed to the end of the trials using a random stimulus-response mapping.
The analysis of the evoked magnetic fields reveals temporally non-overlapping responses for stimulus-driven and task-related processes. Source reconstruction shows that the occipital, dorsal and ventral stimulus-related areas follow different temporal profiles, with the response in IPS slowly rising after the beginning of the activity in the lateral occipital cortex and falling back to the baseline prior the delayed motor response.
Task dependent differences are mainly observed during the late component of these responses. Altogether, the results indicate that sensory integration and decisional processes may involve overlapping regions but at different time periods. The rising activity found in IPS may reflect accumulating evidence on the presented stimulus leading to a motor response, in line with the results from single cell studies in monkey LIP [1].
[1] Huk AC, Shadlen MN (2005) Neural activity in macaque parietal cortex reflects temporal integration of visual motion signals during perceptual decision making. The Journal of neuroscience, 25 (45): 10420-36.