Perceptual Science Series

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Anatoliy Kats and Vicky Froyen

Monday, October 25, 2010, 12:00pm - 07:00pm

Computer Science Department, Rutgers University and Psychology Department, Rutgers University

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Anatoliy Kats  (Computer Science Department, Rutgers University)

"Medial Part Abstraction and Grouping from Images"

Medial axis-based representations represent a powerful class of symmetry-based shape decomposition that supports effective shape categorization. However, their applicability to real scenes has been severely limited due to their restrictive assumption that the object's silhouette is available.  We present a novel framework for recovering a set of symmetric parts from a cluttered scene, and grouping them to form an approximation to an object's medial axis transform. Starting with a set of extracted image contours, one of the key contributions is a transformation of the symmetry-based perceptual grouping problem from the space of image contours to the dual space of medial contours. Given the graph of hypothesized medial axis fragments, we introduce a weight function that captures the nonaccidental relations between pairs of these fragments, and use it to group together medial fragments that belong to the same part, or medial branch. Defining a graph over the recovered parts, we introduce a second weight function that drives the grouping of parts into objects, effectively assembling the medial branches to yield the final medial axis approximation. The ability to recover a medial axis approximation of an object from a cluttered image will allow a rich community of skeleton-based recognition algorithms access to real-world imaging conditions.


Vicky Froyen (Psychology Department, Rutgers University)

"Rhythmic Timing and Movement Trajectories in Musical Expertise"

Timing of motor behavior is an important aspect of everybody's behavior, but crucial for musicians. Previous research has shown that musicians are  better in timing tapping movements than non-musicians. We take this one step further by studying the movement trajectories underlying rhythmic timing in experts and novices. Our guiding hypotheses were:  (a) experts adapt their trajectories more to the task constraints (target duration, rhythm complexity) imposed by the goal to minimize timing variability than novices do, and (b) along with lower timing variability experts are also more consistent in the movement trajectories they implement to realize their timing goals. As part of a larger fMRI study, 12 professional musicians (experts) and 12 non-musicians performed three isochronous tapping tasks (target durations 400, 800, and 1200 ms) and two rhythms which were composed of the same target durations. We measured angular movement trajectories at the wrist joint by means of a non-ferromagnetic shaft encoder attached to a lower-arm orthosis.  Performance was assessed after extended laboratory practice for the novices (8 sessions) and more limited training (3 sessions) for the experts. Movement trajectories for individual intertap intervals were parsed into three segments: dwell time after previous tap, final extension-flexion towards the tap ending the interval, and potential movements subdividing periods between the first and the final phase. Results showed that non-musicians typically had short dwelltimes and hardly ever performed subdividing movements. In contrast, half of the expert participants systematically applied subdivisions after short dwelltimes. The adaptability of this approach was amply demonstrated by
the finding that frequency of usage of this strategy increased as a function of interval duration and rhythmic complexity. The other half of musician participants showed very long dwelltimes before initiating their final extension-flexion movement towards the next tap. These findings provide support for our first hypotheses. Data analyses related to the second hypothesis showed that musicians do not have a higher consistency in their trajectory variables than non-musicians. In contrast their timing variability was found to be less. In summary we showed that expertise is not only related to the timing-level coordination but also to the trajectory-level coordination of rhythmic tapping.

Anatoliy Kats and Vicky Froyen