Mark Vergeer

Nonretinotopic Exogenous Attention

Attention is crucial for visual perception because it allows the visual system to effectively use its limited resources by selecting behaviorally and cognitively relevant stimuli from the large amount of information impinging on the eyes. Reflexive, stimulus-driven attention is essential for successful interactions with the environment because it can, for example, speed up responses to life-threatening events. It is commonly believed that exogenous attention operates in the retinotopic coordinates of the early visual system. Here, using a novel experimental paradigm [1], we show that a nonretinotopic cue improves both accuracy and reaction times in a visual search task. Furthermore, the influence of the cue is limited both in space and time, a characteristic typical of exogenous cueing. These and other recent findings show that many more aspects of vision are processed nonretinotopically than previously thought.

Looking at Two Paintings at Once: Luminance Edges Can Gate Colors

Two paintings, O1 and O2, were split into their luminance (grayscale) components L1, L2 and their color components C1, C2. The two color components, C1, C2, were transparently superimposed. Adding the grayscale of the first painting (= C1 + C2 + L1) looked like the original O1, while adding the grayscale of the second painting (= C1 + C2 + L2) looked like the original O2. Conclusion: the luminance contours selected or gated the congruent color contours and ignored non-congruent colors from the other painting.

Flexible color perception depending on the shape and positioning of achromatic contours

In this study, we present several demonstrations of color averaging between luminance boundaries. In each of the demonstrations, different black outlines are superimposed on one and the same colored surface. Whereas perception without these outlines comprises a blurry colored gradient, superimposing the outlines leads to a much clearer binary color percept, with different colors perceived on each side of the boundary. These demonstrations show that the color of the perceived surfaces is flexible, depending on the exact shape of the outlines that define the surface, and that different positioning of the outlines can lead to different, distinct color percepts. We argue that the principle of color averaging described here is crucial for the brain in building a useful model of the distal world, in which differences within object surfaces are perceptually minimized, while differences between surfaces are perceptually enhanced.

Training of binocular rivalry suppression suggests stimulus-specific plasticity in monocular and binocular visual areas

The plasticity of the human brain, as shown in perceptual learning, is generally reflected by improved task performance after training. Here, we show that perceptual suppression can be increased through training. In the first experiment, binocular rivalry suppression of a specific orientation was trained, leading to a relative reduction in sensitivity to the trained orientation. In a second experiment, two orthogonal orientations were suppressed in alternating training blocks, in the left and right eye, respectively. This double-training procedure lead to reduced sensitivity for the orientation that was suppression-trained in each specific eye, implying that training of feature suppression is specific for the eye in which the oriented grating was presented during training. Results of a control experiment indicate that the obtained effects are indeed due to suppression during training, instead of being merely due to the repetitive presentation of the oriented gratings. Visual plasticity is essential for a person’s visual development. The finding that plasticity can result in increased perceptual suppression reported here may prove to be significant in understanding human visual development. It emphasizes that for stable vision, not only the enhancement of relevant signals is crucial, but also the reliable and stable suppression of (task) irrelevant signals.

Binocular suppression occurs in object-centered coordinates

In binocular rivalry, only one image is perceived consciously when different, incompatible images are presented to the left and right eye, respectively. The other image is suppressed. Binocular suppression is generally assumed to occur within retinotopic coordinates. However, the world is continuously shifting on our retina because of the movements of the eyes and the objects themselves. Therefore, the visual system needs a mechanism to create binocular perceptual stability despite continuous changes in the retinal images. To investigate retinotopic versus object-centered binocular suppression, we combined the Ternus-Pikler paradigm with a binocular selective suppression paradigm. We presented a Ternus-Pikler display (TPD) in which three disks shifted by one position from frame 1 to frame 2. We presented a low-contrast grating in the central disk of the TPD to one eye and a high contrast bull’s eye at the same location to the other eye. The bull’s eye fully suppressed the percept of the grating. In the second frame, in half of the trials, a grating was presented on the central disk. In the other half, no grating was presented. The orientation of the grating (when presented) was the same as or orthogonal to the orientation of the first grating. No bull’s eye was presented in the second frame. Sensitivity to gratings with the previously suppressed orientation was reduced compared to orthogonal gratings, even though the gratings in both frames were presented at different retinotopic locations. These results are evidence for feature suppression in non-retinotopic, object-centered coordinates, giving rise to a new view on binocular rivalry where stimulus-selective, non-retinotopic inhibition is crucial in maintaining perceptual stability.

Visible and invisible stimulus parts integrate into global object representations as revealed by combining monocular and binocular rivalry

Our visual system faces the challenging task to construct integrated visual representations from the visual input projected on our retinae. Previous research has provided mixed evidence as to whether visual awareness of the stimulus parts is required for such integration to occur. Here, we address this issue by taking a novel approach in which we combine a monocular rivalry stimulus (i.e., a bistable rotating cylinder) with binocular rivalry. The results of Experiment 1 show that in a rivalry condition, where one half of the cylinder is perceptually suppressed, significantly more perceptual switches occur that are consistent with visual integration of the whole cylinder than occur in a control condition, where only half of the cylinder is presented at a time and the presentation of the two images is physically alternated. In Experiment 2, stimulation in the observers dominant eye was kept dominant by presenting the half cylinder in this eye at higher contrast and by surrounding it with a flickering context. Results show that the strong convexity bias that was found in a control condition, where no stimulus was presented in the suppressed eye, almost completely disappears when the unseen half is presented in the suppressed eye, indicating that both halves visually integrate and, subsequently, compete for convexity. These findings provide evidence that unseen visual information is biased towards a representation that is congruent with the current visible representation and, hence, that principles of perceptual organization also apply to parts of the visual input that remain unseen by the observer.

Binocular suppression learning reveals inhibitory plasticity in early vision.

In visual perceptual learning, the ability to respond to visible stimuli is improved through practice. The visual input on our retina, however, is intrinsically ambiguous, supporting a multitude of valid representations. The brain selects one out of many possible visual interpretations, which is neurally enhanced, while alternative interpretations remain perceptually suppressed. Here, we show that not only visibility can improve through training, but that suppression can also be trained. Throughout training (in total 2560 trial), an oriented grating presented to one eye was constantly suppressed by a high-contrast expanding bulls eye presented to the other eye. This suppression-trained grating was always presented to the same eye, always with the same orientation within an observer. Pre- and post training detection thresholds were measured for target gratings with the suppression-trained orientation and for gratings with the orthogonal orientation, in the trained and untrained eye, independently, using an adaptive Quest procedure. The target gratings competed with a low-contrast expanding bulls eye presented to the opposite eye. Performance showed a stronger improvement after training compared to before training for gratings presented to the eye that was dominant during training (where the bulls eye was presented), indicating eye-based learning. Most interestingly, we found a stimulus-specific effect of suppression learning, where improvement was significantly worse for detecting the trained orientation, relative to detection of the orthogonal orientation. Hereby, we show stimulus selectivity in binocular suppression, and that observers can be trained to suppress a certain stimulus. These findings are indicative of the plasticity of inhibitory networks responsible for perceptual suppression. Meeting abstract presented at VSS 2015.