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Three experiments examined the use of vibrotactile cues to guide an operator toward a target. Vibrotactile stimulation on the hand can provide spatially stabilizing cues for feedback of subtle changes in position. When such feedback is present, a deviation from the point of origin results in tactile stimulation indicating the direction and magnitude of the positional error. Likewise, spatial deviation from a desired position displayed tactually can provide robust position guidance and stabilization sufficient to improve the acquisition time and accuracy of fine cursor control. A major advantage of this mode of information representation is that it can be present at the same time as visual cues with minimal cross-modal interference. Our findings suggest that performance is actually enhanced when both tactile and visual cues are present. Although previous studies have suggested that various forms of tactile feedback can provide position guidance and stabilization, to our knowledge, this work is the first that details the effect of tactile feedback on target acquisition directly.

We present an improved video-based computer system for on-line tracking of small markers moving in a plane. The system consists of a CCD camera, a video monitor, a dedicated 80386/20 microcomputer, a video frame grabber, and custom software. Up to four markers can be tracked at the 30-Hz video frame rate using a two-step, correlation-based search procedure. We discuss the requisite hardware and software requirements and illustrate how this tracking system can be used to collect strain data during biaxial stretching tests on planar soft tissues. Operating at 30 Hz, this system is an improvement over those previously reported, which are either slower or yield less information.

Regional ventricular wall stress is a critical determinant of cardiac function. There are, however, no validated methods for accurately estimating this stress. We have shown in the isolated ventricular septum that, during steady-state indentations, the transverse stiffness (the ratio of indentation stress [pressure acting on indenter face] to indentation strain [amount of indentation/nonindented thickness]) can be used as an estimate of the in-plane wall stress. Because of the long acquisition time for those transverse stiffness determinations, it was not possible to follow changes in wall stress over a single contraction. We recently developed a dynamic indentation system that can determine transverse stiffness in as little as 10 ms, allowing estimation of wall stress over a single contraction cycle. The apparatus consists of an indentation probe coupled to a linear motor. This indentation system was tested on two beating canine ventricular septa that were mounted in a biaxial system the could apply strains in the plane of the septa and measure the resulting in-plane stresses. The probe indented the septa with peak displacements of 0.1-0.5 mm at frequencies of 20 and 50 Hz. The transverse stiffness was calculated as the slope of the relation between the indentation stress and indentation strain during each high-frequency indentation. Consistent with earlier studies, the transverse stiffness was related to the inplane stress. In contrast to earlier studies, however, these dynamic transverse stiffness determinations could be made during a single contraction. Thus, dynamic transverse stiffness determinations allow estimation of wall stress in the isolated septa by minimal surface contact, and may lead to methods for estimating wall stress in the intact heart.

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