Visual working memory contents bias ambiguous structure from motion perception

The way we perceive the visual world depends crucially on the state of the observer. In the present study we show that what we are holding in working memory (WM) can bias the way we perceive ambiguous structure from motion stimuli. Holding in memory the percept of an unambiguously rotating sphere influenced the perceived direction of motion of an ambiguously rotating sphere presented shortly thereafter. In particular, we found a systematic difference between congruent dominance periods where the perceived direction of the ambiguous stimulus corresponded to the direction of the unambiguous one and incongruent dominance periods. Congruent dominance periods were more frequent when participants memorized the speed of the unambiguous sphere for delayed discrimination than when they performed an immediate judgment on a change in its speed. The analysis of dominance time-course showed that a sustained tendency to perceive the same direction of motion as the prior stimulus emerged only in the WM condition, whereas in the attention condition perceptual dominance dropped to chance levels at the end of the trial. The results are explained in terms of a direct involvement of early visual areas in the active representation of visual motion in WM

When geometry constrains vision : systematic misperceptions within geometrical configurations

International audienceHow accurate are we in reproducing a point within a simple shape? This is the empirical question we addressed in this work. Participants were presented with a tiny disk embedded in an empty circle (Experiment 1 and 3) or in a square (Experiment 2). Shortly afterwards the disk vanished and they had to reproduce the previously seen disk position within the empty shape by means of the mouse cursor, as accurately as possible. Several loci inside each shape were tested. We found that the space delimited by a circle and by a square is not homogeneous and the observed distortion appears to be consistent across observers and specific for the two tested shapes. However, a common pattern can be identified when reproducing geometrical loci enclosed in a shape: errors are shifted toward the periphery in the region around the center and toward the center in the region nearby the edges. The error absolute value declines progressively as we approach an equilibrium contour line between the center and the outline of the shape where the error is null. These results suggest that enclosing an empty space within a shape imposes an organization to it and warps its metrics: not only the perceived loci inside a shape are not the same as the geometrical loci, but they are misperceived in a systematic way that is functional to the correct identification of the center of the shape. Eye movements recordings (Experiment 3) are consistent with this interpretation of the data

The haptic and the visual flash-lag effect and the role of flash characteristics.

When a short flash occurs in spatial alignment with a moving object, the moving object is seen ahead the stationary one. Similar to this visual "flash-lag effect" (FLE) it has been recently observed for the haptic sense that participants judge a moving hand to be ahead a stationary hand when judged at the moment of a short vibration ("haptic flash") that is applied when the two hands are spatially aligned. We further investigated the haptic FLE. First, we compared participants' performance in two isosensory visual or haptic conditions, in which moving object and flash were presented only in a single modality (visual: sphere and short color change, haptic: hand and vibration), and two bisensory conditions, in which the moving object was presented in both modalities (hand aligned with visible sphere), but the flash was presented only visually or only haptically. The experiment aimed to disentangle contributions of the flash's and the objects' modalities to the FLEs in haptics versus vision. We observed a FLE when the flash was visually displayed, both when the moving object was visual and visuo-haptic. Because the position of a visual flash, but not of an analogue haptic flash, is misjudged relative to a same visuo-haptic moving object, the difference between visual and haptic conditions can be fully attributed to characteristics of the flash. The second experiment confirmed that a haptic FLE can be observed depending on flash characteristics: the FLE increases with decreasing intensity of the flash (slightly modulated by flash duration), which had been previously observed for vision. These findings underline the high relevance of flash characteristics in different senses, and thus fit well with the temporal-sampling framework, where the flash triggers a high-level, supra-modal process of position judgement, the time point of which further depends on the processing time of the flash

Top-down influences on ambiguous perception: the role of stable and transient states of the observer

The world as it appears to the viewer is the result of a complex process of inference performed by the brain. The validity of this apparently counter-intuitive assertion becomes evident whenever we face noisy, feeble or ambiguous visual stimulation: in these conditions, the state of the observer may play a decisive role in determining what is currently perceived. On this background, ambiguous perception and its amenability to top-down influences can be employed as an empirical paradigm to explore the principles of perception. Here we offer an overview of both classical and recent contributions on how stable and transient states of the observer can impact ambiguous perception. As to the influence of the stable states of the observer, we show that what is currently perceived can be influenced (1) by cognitive and affective aspects, such as meaning, prior knowledge, motivation, and emotional content and (2) by individual differences, such as gender, handedness, genetic inheritance, clinical conditions, and personality traits and by (3) learning and conditioning. As to the impact of transient states of the observer, we outline the effects of (4) attention and (5) voluntary control, which have attracted much empirical work along the history of ambiguous perception. In the huge literature on the topic we trace a difference between the observer's ability to control dominance (i.e., the maintenance of a specific percept in visual awareness) and reversal rate (i.e., the switching between two alternative percepts). Other transient states of the observer that have more recently drawn researchers' attention regard (6) the effects of imagery and visual working memory. (7) Furthermore, we describe the transient effects of prior history of perceptual dominance. (8) Finally, we address the currently available computational models of ambiguous perception and how they can take into account the crucial share played by the state of the observer in perceiving ambiguous displays

Trial sequence.

<p>Observers viewed the stimuli through a black 680x1000 mm reduction screen with a circular hole (280 mm in diameter) cut out at its center and fixed at 6 cm in front of the computer monitor. Each trial began with a homogenous black screen lasting for 1200–1800 ms (meaning that the actual duration was randomly chosen in the interval). Then the circle outline (diameter: 16.3 degrees of visual angle) was presented (panel 1) and 300–500 ms later the small target disk was added to the display (panel 2). The whole stimulus (circle plus target) remained visible for 1500 ms. It was followed by a black screen for 500 ms (panel 3), and, then, by a masking screen for 1000 ms (panel 4). The screen went black again for 500 ms (panel 5), and then only the circle was displayed with a position jitter (with coordinates randomly chosen in an interval of ±3 degrees of visual angle independently on the X and Y axes). Afterwards, a crosshair cursor was displayed behind the reduction screen in a screen quadrant that did not contain the target. Participants were required to move the cursor inside the visible area in order to reproduce the previously seen target position: they clicked the mouse once they had achieved the exact match (panel 6). Stimuli are not drawn to scale.</p

Spatial alignment and response hand in geometric and motion illusions

Perception of visual illusions is susceptible to manipulation of their spatial properties. Further, illusions can sometimes affect visually guided actions, especially the movement planning phase. Remarkably, visual properties of objects related to actions, such as affordances, can prime more accurate perceptual judgements. In spite of the amount of knowledge available on affordances and on the influence of illusions on actions (or lack of thereof), virtually nothing is known about the reverse: the influence of action-related parameters on the perception of visual illusions. Here, we tested a hypothesis that the response mode (that can be linked to action-relevant features) can affect perception of the Poggendorff (geometric) and of the Vanishing Point (motion) illusion. We explored the role of hand dominance (right dominant versus left non-dominant hand) and its interaction with stimulus spatial alignment (i.e., congruency between visual stimulus and the hand used for responses). Seventeen right-handed participants performed our tasks with their right and left hands, and the stimuli were presented in regular and mirror-reversed views. It turned out that the regular version of the Poggendorff display generates a stronger illusion compared to the mirror version, and that participants are less accurate and show more variability when they use their left hand in responding to the Vanishing Point. In summary, our results show that there is a marginal effect of hand precision in motion related illusions, which is absent for geometrical illusions. In the latter, attentional anisometry seems to play a greater role in generating the illusory effect. Taken together, our findings suggest that changes in the response mode (here: manual action-related parameters) do not necessarily affect illusion perception. Therefore, although intuitively speaking there should be at least unidirectional effects of perception on action, and possible interactions between the two systems, this simple study still suggests their relative independence, except for the case when the less skilled (non-dominant) hand and arguably more deliberate responses are used

Repulsive serial effects in visual numerosity judgments

<p>Data pertaining to the study entitled "Repulsive serial effects in visual numerosity judgments". A description of the data format is contained in the Datadescription.txt file.</p

Results of Experiment 1: Modulation of radial and tangential errors.

<p>Average Constant Errors (CE) made when reproducing points locations (7 points by 8 radii, see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0151488#pone.0151488.g001" target="_blank">Fig 1A</a>), expressed in pixels. Black curves represent the radial component of the CE (i.e. the displacement along the direction of the corresponding radius). Positive values indicate displacement toward the periphery, negative toward the center. Black dotted lines and empty squares illustrate the expected null radial error at points placed on the circumference and at the center (respectively after 7 and before 1). Grey curves represent the tangential component of the CE (i.e. the lateral deviation from the radius direction, orthogonal to it). Positive values refer to clockwise deviations. Error bars represent the standard error of the mean.</p

Results, second experiment.

<p>Average point of subjectively equal position of the moving to the non-moving, stationary object (PSE) computed from flash onset position as a function of the intensity and duration of a haptic flash. A significant negative PSE (asterisk) indicates a flash-lag effect. Error bars represents standard errors of the means.</p

Experiment 2: Experimental design, expected and observed results.

<p><i>Panel A</i> illustrates the design used in Experiment 2, which is identical to that used in Experiment 1: 57 points belonging to the inner area of a square (side length: 16.3 degrees of visual angle) were presented. 56 points were distributed along 8 radii (from 0° to 315°) at 7 equally-spaced locations for each radius (labels 1 to 7). The remaining last point coincided with the center of the square. <i>Panel B</i>. The experimental procedure was the same as in Experiment 1. As in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0151488#pone.0151488.g001" target="_blank">Fig 1B</a>, expected results are illustrated by the curve <b>C</b> which represents the hypothetical modulation of estimation error between the center and the periphery. The black curve represents the empirical contour line of null error actually estimated by participants in Experiment 2.</p