Motion transparency provides a challenging test case for our understanding of how visual motion, and other attributes, are computed and represented in the brain. However, previous studies of visual transparency have used subjective criteria which do not confirm the existence of independent representations of the superimposed motions. We have developed measures of performance in motion transparency that require observers to extract information about two motions jointly, and therefore test the information that is simultaneously represented for each motion. Observers judged whether two motions were at 90 degrees to one another; the base direction was randomized so that neither motion taken alone was informative. The precision of performance was determined by the standard deviations (S.D.s) of probit functions fitted to the data. Observers also made judgments of orthogonal directions between a single motion stream and a line, for one of two transparent motions against a line and for two spatially segregated motions. The data show that direction judgments with transparency can be made with comparable accuracy to segregated (non-transparent) conditions, supporting the idea that transparency involves the equivalent representation of two global motions in the same region. The precision of this joint direction judgment is, however, 2-3 times poorer than that for a single motion stream. The precision in directional judgment for a single stream is reduced only by a factor of about 1.5 by superimposing a second stream. The major effect in performance, therefore, appears to be associated with the need to compute and compare two global representations of motion, rather than with interference between the dot streams per se. Experiment 2 tested the transparency of motions separated by a range of angles from 5 degrees to 180 degrees by requiring subjects to set a line matching the perceived direction of each motion. The S.D.s of these settings demonstrated that directions of transparent motions were represented independently for separations over 20 degrees. Increasing dot speeds from 1 to 10 deg/s improved directional performance but had no effect on transparency perception. Transparency was also unaffected by variations of density between 0.1 and 19 dots/deg(2)

Area/Discipline: evolutionary < psychology, social < psychology Keywords: laughter, simulation, evolution, gender Abstract:This review addresses gender differences in laughter and smiling from an evolutionary perspective. Laughter and smiling can be responses to successful display behavior or signals of affiliation amongst conversational partners-differing social and evolutionary agendas mean there are different motivations when interpreting these signals. Two experiments assess perceptions of genuine and simulated male and female laughter and amusement social signals. Results show male simulation can always be distinguished. Female simulation is more complicated as males can distinguish cues of simulation yet judge simulated signals to be genuine. Females judge other female's genuine signals to have higher levels of simulation. Results highlight the importance of laughter and smiling in human interactions, use of dynamic stimuli, and using multiple methodologies to assess perception.

The processing of motion information by the visual system can be decomposed into two general stages; point-by-point local motion extraction, followed by global motion extraction through the pooling of the local motion signals. The direction aftereffect (DAE) is a well known phenomenon in which prior adaptation to a unidirectional moving pattern results in an exaggerated perceived direction difference between the adapted direction and a subsequently viewed stimulus moving in a different direction. The experiments in this paper sought to identify where the adaptation underlying the DAE occurs within the motion processing hierarchy. We found that the DAE exhibits interocular transfer, thus demonstrating that the underlying adapted neural mechanisms are binocularly driven and must, therefore, reside in the visual cortex. The remaining experiments measured the speed tuning of the DAE, and used the derived function to test a number of local and global models of the phenomenon. Our data provide compelling evidence that the DAE is driven by the adaptation of motion-sensitive neurons at the local-processing stage of motion encoding. This is in contrast to earlier research showing that direction repulsion, which can be viewed as a simultaneous presentation counterpart to the DAE, is a global motion process. This leads us to conclude that the DAE and direction repulsion reflect interactions between motion-sensitive neural mechanisms at different levels of the motion-processing hierarchy.

The marten subspecies on the island of Newfoundland, Martes americana atrata, is threatened. Survey data suggest that most of the extant marten population lives in old uncut balsam fir (Abies balsamea) forests, but a very few live in adjacent 40-to 60-year-old second-growth stands of balsam fir. We compared habitat structure and composition and prey abundance in old forest and second-growth stands to test the hypotheses that either food abundance or habitat quality, or both, limit use of the 40-to 60-year-old forests by marten. Snowshoe hares (Lepus americana) were most abundant in 40-year-old forests and also occurred in old forests, but field voles (Microtus pennsylvanicus) were not found in second-growth stands. A multivariate discriminant model indicated that older, uncut forests contained more structure than younger forests at ground level, because there was more woody debris, more young balsam fir, less litter, more mosses, and more low shrubs. Canopy cover was similar in all forest types, and subnivean access did not differ among the three age-classes when snow was about 1 m deep. We suggest that marten did not use 40-or 60-year-old forest stands because of the lack of the meadow voles that form a necessary part of their diet. Meadow voles likely respond to ground-level forest structure in selecting habitat, and this structure is unavailable in young forests. We recommend a management strategy for marten that would preserve current old forests as long as possible and allow sufficient second-growth balsam fir forest to become old forest with the required characteristics to maintain a viable marten population.

When viewing two superimposed, translating sets of dots moving in different directions, one overestimates direction difference. This phenomenon of direction repulsion is thought to be driven by inhibitory interactions between directionally tuned motion detectors. However, there is disagreement on where this occurs-at early stages of motion processing, when local motions are extracted; or at the later, global motion-processing stage following "pooling" of these local measures. These two stages of motion processing have been identified as occurring in area V1 and the human homolog of macaque MT/V5, respectively. We designed experiments in which local and global predictions of repulsion are pitted against one another. Our stimuli contained a target set of dots, moving at a uniform speed, superimposed on a "mixed-speed" distractor set. Because the perceived speed of a mixed-speed stimulus is equal to the dots' average speed, a global-processing account of direction repulsion predicts that repulsion magnitude induced by a mixed-speed distractor will be indistinguishable from that induced by a single-speed distractor moving at the same mean speed. This is exactly what we found. These results provide compelling evidence that global-motion interactions play a major role in driving direction repulsion.

We investigated whether infants from 8-22 weeks of age were sensitive to the illusory contour created by aligned line terminators. Previous reports of illusory-contour detection in infants under 4 months old could be due to infants' preference for the presence of terminators rather than their configuration. We generated preferential-looking stimuli containing sinusoidal lines whose oscillating, abutting terminators give a strong illusory contour in adult perception. Our experiments demonstrated a preference in infants 8 weeks old and above for an oscillating illusory contour compared with a stimulus containing equal terminator density and movement. Control experiments excluded local line density, or attention to alignment in general, as the basis for this result. In the youngest age group (8-10 weeks) stimulus velocity appears to be critical in determining the visibility of illusory contours, which is consistent with other data on motion processing at this age. We conclude that, by 2 months of age, the infant's visual system contains the nonlinear mechanisms necessary to extract an illusory contour from aligned terminators.

No abstract

Despite being a pan-cultural phenomenon, laughter is arguably the least understood behaviour deployed in social interaction. As well as being a response to humour, it has other important functions including promoting social affiliation, developing cooperation and regulating competitive behaviours. This multi-functional feature of laughter marks it as an adaptive behaviour central to facilitating social cohesion. However, it is not clear how laughter achieves this social cohesion. We consider two approaches to understanding how laughter facilitates social cohesion – the ‘representational’ approach and the ‘affect-induction’ approach. The representational approach suggests that laughter conveys information about the expresser’s emotional state, and the listener decodes this information to gain knowledge about the laugher’s felt state. The affect-induction approach views laughter as a tool to influence the affective state of listeners. We describe a modified version of the affect-induction approach, in which laughter is combined with additional factors – including social context, verbal information, other social signals and knowledge of the listener’s emotional state – to influence an interaction partner. This view asserts that laughter by itself is ambiguous: the same laughter may induce positive or negative affect in a listener, with the outcome determined by the combination of these additional factors. Here we describe two experiments exploring which of these approaches accurately describes laughter. Participants judged the genuineness of audio–video recordings of social interactions containing laughter. Unknown to the participants the recordings contained either the original laughter or replacement laughter from a different part of the interaction. When replacement laughter was matched for intensity, genuineness judgements were similar to judgements of the original unmodified recordings. When replacement laughter was not matched for intensity, genuineness judgements were generally significantly lower. These results support the affect-induction view of laughter by suggesting that laughter is inherently underdetermined and ambiguous, and that its interpretation is determined by the context in which it occurs.