Measuring Integration Processes in Visual Symmetry with Frequency-Tagged EEG.

Symmetry is a highly salient feature of the natural world which requires integration of visual features over space. The aim of the current work is to isolate dynamic neural correlates of symmetry-specific integration processes. We measured steady-state visual evoked potentials (SSVEP) as participants viewed symmetric patterns comprised of distinct spatial regions presented at two different frequencies (f1 and f2). We measured intermodulation components, shown to reflect non-linear processing at the neural level, indicating integration of spatially separated parts of the pattern. We generated a wallpaper pattern containing two reflection symmetry axes by tiling the plane with a two-fold reflection symmetric unit-pattern and split each unit-pattern diagonally into separate parts which could be presented at different frequencies. We compared SSVEPs measured for wallpapers and control patterns for which both images were equal in terms of translation and rotation symmetry but reflection symmetry could only emerge for the wallpaper pattern through integration of the image-pairs. We found that low-frequency intermodulation components differed between the wallpaper and control stimuli, indicating the presence of integration mechanisms specific to reflection symmetry. These results showed that spatial integration specific to symmetry perception can be isolated through a combination of stimulus design and the frequency tagging approach.

The global slowdown effect: why does perceptual grouping reduce perceived speed?

The percept of four rotating dot pairs is bistable. The "local percept" is of four pairs of dots rotating independently. The "global percept" is of two large squares translating over one another (Anstis & Kim 2011). We have previously demonstrated (Kohler, Caplovitz, & Tse 2009) that the global percept appears to move more slowly than the local percept. Here, we investigate and rule out several hypotheses for why this may be the case. First, we demonstrate that the global slowdown effect does not occur because the global percept is of larger objects than the local percept. Second, we show that the global slowdown effect is not related to rotation-specific detectors that may be more active in the local than in the global percept. Third, we find that the effect is also not due to a reduction of image elements during grouping and can occur with a stimulus very different from the one used previously. This suggests that the effect may reflect a general property of perceptual grouping. Having ruled out these possibilities, we suggest that the global slowdown effect may arise from emergent motion signals that are generated by the moving dots, which are interpreted as the ends of "barbell bars" in the local percept or the corners of the illusory squares in the global percept. Alternatively, the effect could be the result of noisy sources of motion information that arise from perceptual grouping that, in turn, increase the influence of Bayesian priors toward slow motion (Weiss, Simoncelli, & Adelson 2002).

Associations between auditory pitch and visual elevation do not depend on language: evidence from a remote population.

Associations between auditory pitch and visual elevation are widespread in many languages, and behavioral associations have been extensively documented between height and pitch among speakers of those languages. However, it remains unclear whether perceptual correspondences between auditory pitch and visual elevation inform these linguistic associations, or merely reflect them. We probed this cross-modal mapping in members of a remote Kreung hill tribe in northeastern Cambodia who do not use spatial language to describe pitch. Participants viewed shapes rising or falling in space while hearing sounds either rising or falling in pitch, and reported on the auditory change. Associations between pitch and vertical position in the Kreung were similar to those demonstrated in populations where pitch is described in terms of spatial height. These results suggest that associations between visual elevation and auditory pitch can arise independently of language. Thus, widespread linguistic associations between pitch and elevation may reflect universally predisposed perceptual correspondences.

Therapeutic effects of a restraint procedure on posttraumatic place learning in fimbria-fornix transected rats.

Restraint procedures have been shown to influence the neural processes in the brain (dendritic changes or changes in the expression of neurotrophines, etc.) as well as to alter the behavioural performance. While many report deleterious effects of this procedure in normal animals, there are also indications of positive effects in the context of brain injury. In order to address the issue from the perspective of functional posttraumatic recovery, we studied 6 experimental groups of rats--3 groups undergoing a fimbria-fornix transection, and 3 groups remaining neurally intact. Within the lesioned and intact groups, respectively, one group of animals was subjected to an 8-day long restraint procedure (2 h daily) that ended immediately prior to the infliction of trauma; another group was subjected to the same procedure starting immediately after the infliction of trauma; and one group was not subjected to the restraint procedure at all. After a brief period of postoperative pause, the animals were tested on their acquisition of an 8-arm radial maze based place learning task and the effects of the restraint procedure on the task acquisition were evaluated. The results show that within the neurally intact groups, the administration of this procedure had no effect at all. However, the lesioned groups that were subjected to the restraint procedure showed significantly improved acquisition of the studied task compared to the lesioned animals that did not undergo the restraint procedure. The improved task performance suggests a therapeutic effect of this manipulation on the functional recovery after a mechanical trauma.