On the contribution of binocular disparity to the long-term memory for natural scenes

Binocular disparity is a fundamental dimension defining the input we receive from the visual world, along with luminance and chromaticity. In a memory task involving images of natural scenes we investigate whether binocular disparity enhances long-term visual memory. We found that forest images studied in the presence of disparity for relatively long times (7s) were remembered better as compared to 2D presentation. This enhancement was not evident for other categories of pictures, such as images containing cars and houses, which are mostly identified by the presence of distinctive artifacts rather than by their spatial layout. Evidence from a further experiment indicates that observers do not retain a trace of stereo presentation in long-term memory

The Role of Dopamine in Anticipatory Pursuit Eye Movements: Insights from Genetic Polymorphisms in Healthy Adults

There is a long history of eye movement research in patients with psychiatric diseases for which dysfunctions of neurotransmission are considered to be the major pathologic mechanism. However, neuromodulation of oculomotor control is still hardly understood. We aimed to investigate in particular the impact of dopamine on smooth pursuit eye movements. Systematic variability in dopaminergic transmission due to genetic polymorphisms in healthy subjects offers a noninvasive opportunity to determine functional associations. We measured smooth pursuit in 110 healthy subjects genotyped for two well-documented polymorphisms, the COMT Val158Met polymorphism and the SLC6A3 3´-UTR-VNTR polymorphism. Pursuit paradigms were chosen to particularly assess the ability of the pursuit system to initiate tracking when target motion onset is blanked, reflecting the impact of extraretinal signals. In contrast, when following a fully visible target sensory, retinal signals are available. Our results highlight the crucial functional role of dopamine for anticipatory, but not for sensory-driven, pursuit processes. We found the COMT Val158Met polymorphism specifically associated with anticipatory pursuit parameters, emphasizing the dominant impact of prefrontal dopamine activity on complex oculomotor control. In contrast, modulation of striatal dopamine activity by the SLC6A3 3´-UTR-VNTR polymorphism had no significant functional effect. Though often neglected so far, individual differences in healthy subjects provide a promising approach to uncovering functional mechanisms and can be used as a bridge to understanding deficits in patients

Thresholds for the identification of the direction of motion of plaid patterns defined by luminance or chromatic contrast

Contrast thresholds for identification of the direction of motion were determined for sinusoidal gratings and plaid patterns moving in eight possible directions. Since plaid patterns are the sum of two component gratings, a prediction of the thresholds for plaids can be made by assuming that the motions of both component gratings are independently identified (probability summation). In agreement with standard two-stage models of plaid perception, our results show that for stimuli defined by luminance contrast, plaid direction thresholds can be predicted well from the component thresholds. This also holds for fast-moving isoluminant plaid patterns, but for slowly moving (<4 Hz) isoluminant plaids, direction thresholds were substantially higher than the prediction from the components. In the latter case, subjects frequently were unable to identify the motion of the plaid in the pattern direction, even when the direction of motion of both components could be reliably identified. Different mechanisms might underlie the perception of luminance and isoluminant plaids at slow speeds

Orienting during gaze guidance in a letter-identification task

The idea of gaze guidance is to lead a viewer’s gaze through a visual display in order to facilitate the viewer’s search for specific information in a least-obtrusive manner. This study investigates saccadic orienting when a viewer is guided in a fast-paced, low-contrast letter identification task. Despite the task’s difficulty and although guiding cues were ad-justed to gaze eccentricity, observers preferred attentional over saccadic shifts to obtain a letter identification judgment; and if a saccade was carried out its saccadic constant error was 50%. From those results we derive a number of design recommendations for the process of gaze guidance

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

Chromatic Contrast Sensitivity

International audienc

Neuronal Processing Delays Are Compensated in the Sensorimotor Branch of the Visual System

AbstractMoving objects change their position until signals from the photoreceptors arrive in the visual cortex. Nonetheless, motor responses to moving objects are accurate and do not lag behind the real-world position [1]. The questions are how and where neural delays are compensated for. It was suggested that compensation is achieved within the visual system by extrapolating the position of moving objects [2]. A visual illusion supports this idea: when a briefly flashed object is presented in the same position as a moving object, it appears to lag behind [3, 4]. However, moving objects do not appear ahead of their final or reversal points [5–7]. We investigated a situation where participants localized the final position of a moving stimulus. Visual perception and short-term memory of the final target position were accurate, but reaching movements were directed toward future positions of the target beyond the vanishing point. Our results show that neuronal latencies are not compensated for at early stages of visual processing, but at a late stage when retinotopic information is transformed into egocentric space used for motor responses. The sensorimotor system extrapolates the position of moving targets to allow for precise localization of moving targets despite neuronal latencies

Spatial distortions and processing latencies in the onset repulsion and Fröhlich effects

AbstractIn the Fröhlich illusion, the first position of a moving target is mislocalized in the direction of motion. In the onset repulsion effect, the opposite error occurs. To reconcile these conflicting error patterns, we improved previous methods by using natural pointing movements and a large range of target velocities. Displacement was found to increase in the direction of motion, but the linear function relating velocity and displacement was shifted opposite to the direction of target motion. The results suggest that onset localization may be determined by two independent factors: first, an (attentional) delay that accounts for the increase of displacement in the direction of motion with increasing velocity. This delay is visible in motor and probe judgments and explains the Fröhlich illusion. Second, motor judgments are offset opposite to the direction of target motion. This bias is unique to motor judgments (pointing) and may be partially explained by attentional repulsion