Pupil dynamics during bistable motion perception

Pupil size not only varies to changes in illumination but is also modulated by several cognitive factors, making it a potentially versatile physiological marker of cortical states. We recorded pupil dynamics while subjects continuously reported their bistable perception of ambiguous moving stimuli, plaids, and partially occluded rotating diamonds. We observed small (about 5% of surface change on average) but reliable pupil dilation around (-300 ms to 1.5 s) the button presses indicating the changes of percepts. We found that 70% of pupil dilation could be accounted for by the motor response. The remaining perceptual component was similar for spontaneously occurring transitions and transitions triggered by physical stimulus manipulations. Moreover, the amplitude of pupil modulation in the spontaneous condition was unrelated to the duration of each perceptual state. It is therefore unlikely that the mechanisms of endogenous perceptual bistability reflect in the pupil. In addition, we measured a clear constriction of the pupil after blinks (about 8% of surface change on average). As pupil changes have the potential to entail retino-cortical activity, their monitoring in studies of visual processing could prove worthwhile.

Shape and motion interactions at perceptual and attentional levels during processing of structure from motion stimuli

This study uses a rapid-serial-visual-presentation (RSVP) paradigm to test the extent to which shape and motion direction can be independently accessed and processed during the perception of structure-from-motion (SFM) stimuli. Subjects reported the number of occurrences of shape or motion direction during RSVP sequences of 3D-SFM stimuli. Overall, performance was better for motion than shape. In the motion task, observers were less accurate when the motion direction was repeated revealing a repetition blindness (RB) effect. In addition, the repetition of shape, although irrelevant to the motion task, resulted in increased performance, without change in RB rate. In contrast, there was no RB at the group level in the shape task and the repetition of the irrelevant motion direction had no effect on the performance. A closer look at the data showed that observers fall in two statistically distinct groups for the shape task. Some observers (N = 6) show a repetition advantage (RA) while the others (N = 5) show a repetition blindness (RB) effect. No behavioral differences between groups could be found for the motion task. The implications of these results for models of SFM processing are discussed in the light of the type/token theory (N. Kanwisher, 2001).

Superposition catastrophe and form-motion binding

Numerous studies indicate that perceiving global object motion results from the integration of local component motions across space and time. Less attention has been paid to the issue of motion selection, necessary to avoid spurious associations of component motions belonging to different objects and to solve the so-called "superposition catastrophe problem" (F. Rosenblatt, 1961). We address this issue using outlines of geometrical shapes moving behind apertures that concealed their vertices such that recovering their global motion requires the selection and integration of some, but not all, component motions. Depending on which local motions are selected for motion integration, these stimuli yield the perception of either expansion/contraction, of global translation, or of segments moving independently. We show that the selection process depends on local and global stimulus parameters, including the local direction of figure's line-endings or the spatial configuration of component motions. In contrast, motion selection depends less on the width-i.e., spatial frequency content-or polarity of the edges. Finally, synchronous temporal modulation of component motions in the gamma range has little effect on motion selection. These results indicate that selecting component motions for motion integration is primarily determined by form constraints. As a consequence, current models assuming that mutually consistent component motions are bounded in a velocity space-lacking spatial organization should be revised to account for the present data. Alternately, interactions between visual areas selectively processing form and motion could be introduced in order to account for the perceptual binding of moving objects.

Directional shifts in the barber pole illusion: Effects of spatial frequency, spatial adaptation, and lateral masking

We report the results of psychophysical experiments with the so-called barber pole stimulus providing new insights on the neuronal processes underlying the analysis of moving features such as terminators or line-endings. In experiment 1, we show that the perceived direction of a barber pole stimulus, induced by line-ending motion, is highly dependent on the spatial frequency and contrast of the grating stimulus: perceived direction is shifted away from the barber pole illusion at high spatial frequency in a contrast dependent way, suggesting that line-ends are not processed at high spatial scales. In subsequent experiments, we use a contrast adaptation paradigm and a masking paradigm in an attempt to assess the spatial structure and location of the receptive fields that process line-endings. We show that the adapting stimulus that weakens most the barber pole illusion is localized within the barber pole stimulus and not at line-endings' locations. Current models of line-endings' motion processing are discussed in the light of these psychophysical results.

Contour interactions between pairs of Gabors engaged in binocular rivalry reveal a map of the association field

A psychophysical study was conducted to investigate contour interactions (the 'association field'). Two Gabor patches were presented to one eye, with random-dot patches in corresponding locations of the other eye so as to produce binocular rivalry. Perceptual alternations of the two rivalry processes were monitored continuously by observers and the two time series were cross-correlated. The Gabors were oriented collinearly, obliquely, or orthogonally, and spatial separation was varied. A parallel condition was also included. Correlation between the rivalry processes strongly depended on separation and relative orientation. Correlations between adjacent collinear Gabors was near-perfect and reduced with spatial separation and as relative orientation departed from collinear. Importantly, variations in cross-correlation did not alter the rivalry processes (average dominance duration, and therefore alternation rate, was constant across conditions). Instead, synchronisation of rivalry oscillations accounts for the correlation variations: rivalry alternations were highly synchronised when contour interactions were strong and were poorly synchronised when contour interactions were weak. The level of synchrony between these two stochastic processes, in depending on separation and relative orientation, effectively reveals a map of the association field. These association fields are not greatly affected by contrast, and can be demonstrated between contours that are presented to separate hemispheres.

Dynamic competition between contour integration and contour segmentation probed with moving stimuli

Line-ends, corners and junctions are important singularities for form analysis, object recognition, depth ordering or motion processing. In this study, we investigate the extent to which processing the motion of line ends depends on the spatial configuration of their immediate surround. To that aim, we used two vertical collinear line segments, translating clockwise or anti-clockwise along a circular path, together with a direction discrimination task. Direction discrimination was measured independently for outer line-ends--at both segments extremities--and inner line-ends--in between collinear segments--using line segments partially occluded by invisible masks such that the direction of either inner or outer line-ends' motion was restricted to a sinusoidal translation along a horizontal axis, and thus irrelevant for the motion task. Under these conditions, access to the direction of inner line-ends is longer and more difficult than it is for outer line-ends. Subsequent experiments show that these effects depend on the degree of collinearity between line segments. Similar experiments were performed after volunteers took a dose of Lorazepam, a benzodiazepine that facilitates the fixation of GABA on GABAA receptors. The results show that the differences between the processing of inner and outer line-ends is reduced, suggesting that the effect of the surround is modulated by inhibitory mechanisms. Using a simple model, we propose that this effect can be explained by a competition between a segmentation process based on surround suppression and contour integration through long-range horizontal connections, at or prior to motion processing stages.

The "silent" surround of V1 receptive fields: theory and experiments

The spiking response of a primary visual cortical cell to a stimulus placed within its receptive field can be up- and down-regulated by the simultaneous presentation of objects or scenes placed in the "silent" regions which surround the receptive field. We here review recent progresses that have been made both at the experimental and theoretical levels in the description of these so-called "Center/Surround" modulations and in the understanding of their neural basis. Without denying the role of a modulatory feedback from higher cortical areas, recent results support the view that some of these phenomena result from the dynamic interplay between feedforward projections and horizontal intracortical connectivity in V1. Uncovering the functional role of the contextual periphery of cortical receptive fields has become an area of active investigation. The detailed comparison of electrophysiological and psychophysical data reveals strong correlations between the integrative behavior of V1 cells and some aspects of "low-level" and "mid-level" conscious perception. These suggest that as early as the V1 stage, the visual system is able to make use of contextual cues to recover local visual scene properties or correct their interpretation. Promising ideas have emerged on the importance of such a strategy for the coding of visual scenes, and the processing of static and moving objects.

Geometry and the visual brain

Receptive fields structure of neurons in primary visual cortex suggests that they process visual stimuli in the frequency domain, in a way similar to the frequency analysis performed in the auditory system. As a consequence, both psychophysicists and electrophysiologists have long probed the visual system using extended sine wave gratings that are well localized in the frequency domain but poorly defined in visual space. Meanwhile, how the brain processes the geometrical properties and the spatial and temporal relationships between stimulus parts has received less attention. Recent progress in visual neuroscience that uncovered long-range horizontal connections between cortical neurons and revealed the complex architecture of primary visual cortex and feedback connectivity led to new insights concerned with the processing of geometrical properties of visual stimuli in V1. This paper presents a short historical perspective of the emergence of new issues related to the cortical architecture and its functional consequences on the processing of geometrical properties.

Crossmodal integration for perception and action

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 highlight the multiple faces of crossmodal interactions and provide converging evidence that the brain takes advantages of spatial and temporal coincidence between spatial events in the crossmodal binding of spatial features gathered through different modalities. Furthermore, the elaboration of a multimodal percept appears to be based on an adaptive combination of the contribution of each modality, according to the intrinsic reliability of sensory cue, which itself depends on the task at hand and the kind of perceptual cues involved in sensory processing. Computational models based on bayesian sensory estimation provide valuable explanations of the way perceptual system could perform such crossmodal integration. Recent anatomical evidence suggest that crossmodal interactions affect early stages of sensory processing, and could be mediated through a dynamic recurrent network involving backprojections from multimodal areas as well as lateral connections that can modulate the activity of primary sensory cortices, though future behavioral and neurophysiological studies should allow a better understanding of the underlying mechanisms.

Orientation dependent modulation of apparent speed: a model based on the dynamics of feed-forward and horizontal connectivity in V1 cortex

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. Using a simple model of V1 connectivity, we explore some of the implications of this slow dynamics. The model predicts that V1 responses to a stimulus in the receptive field can be modulated by a previous stimulation, a few milliseconds to a few tens of milliseconds before, in the surround. We analyze this phenomenon and its possible consequences on speed perception, as a function of the spatio-temporal configuration of the visual inputs (relative orientation, spatial separation, temporal interval between the elements, sequence speed). We show that the dynamical interactions between feed-forward and horizontal signals in V1 can explain why the perceived speed of fast apparent motion sequences strongly depends on the orientation of their elements relative to the motion axis and can account for the range of speed for which this perceptual effect occurs (Georges, Seriès, Frégnac and Lorenceau, this issue).

Orientation dependent modulation of apparent speed: psychophysical evidence

We report several experiments showing that a Gabor patch moving in apparent motion sequences appears much faster when its orientation is aligned with the motion path than when it is at an angle to it. This effect is very large and peaks at high speeds (64 degrees /s), decreases for higher and lower speeds and disappears at low speeds (4 degrees /s). This speed bias decreases as the angle between the motion axis and the orientation of the Gabor patch increases, but remains high for curvilinear paths, provided that element orientation is kept tangential to the motion trajectory. It is not accounted for by decision strategies relying on the overall length and duration of the motion sequence or the gap size (or spatial jump) between successive frames. We propose a simple explanation, thoroughly developed as a computational model in a companion paper (Seriès, Georges, Lorenceau & Frégnac: "Orientation dependent modulation of apparent speed: a model based on the dynamics of feedforward and horizontal connectivity in V1 cortex", this issue), according to which long-range horizontal connections in V1 elicit differential latency modulations in response to apparent motion sequences, whose read-out at an MT stage results in a perceptual speed bias. The consequences of these findings are discussed.

Perceptual grouping in the Ternus display: evidence for an 'association field' in apparent motion

We present psychophysical experiments designed to reveal the role of facilitative contour interactions (the so-called 'association field') in apparent motion. We use the Ternus display (a trio of horizontally aligned elements oscillating in apparent motion). This display is perceived in 'element' motion when interframe intervals (IFIs) are short, and in 'group' motion when IFIs are long. Using Gabor elements arranged collinearly or in parallel, IFI is varied to find group motion thresholds. Consistent with a role for collinearity in perceptual grouping, thresholds are lower for collinear displays. The collinear vs. parallel comparison is made while manipulating contrast, spatial frequency, eccentricity, phase, orientation jitter and element separation. Results show a clear effect of contrast not observed in lateral masking paradigms or in 'pathfinder' stimuli, with higher contrast promoting within-frame grouping, and evidence of facilitatory interactions among parallel elements (although over a smaller scale). The tendency for collinear displays to group more than parallel displays declined with eccentricity with no clear difference evident at 12 deg. These changes in group motion thresholds indicate changing association strengths among the elements and is accounted for in terms of an association field. Alternative accounts in terms of second-order collector units or visible persistence are considered but are not supported by the data.