The use of a stereoscopic HMD may result in improved efficiency and safety in both endoscopic and open microsurgical procedures. We have verified that the HMD is comfortable during the course of a surgical procedure, is reliable, and allows for accessibility to the operative field with an excellent field of view and three-dimensional perception. Positioning and dexterity within the operative field are also enhanced. Additional uses relate to surgical training, multimodal information display, and operative rehearsals.

Vaughan and Graefe (1977) proposed that visual fixation duration measures dunng visual search are confounded by two factors, cognitive processing ti.~e and time required by the oculomotor system to trntiate the next eye movement, the oculomotor latency. It was concluded that, under some conditions, the fixation duration is independent of the visual information available to the eye during the fixation. That is the oculomotor latency exceeds the information p;ocessing time. This independe~~e was~ound. to occur in a relatively simple recogrution task involving one display item per fixation. In a visual search involving relatively high information .display it~ms.' or where several items are included m each fixation, the oculomotor latency would not be expected to determine the fixation duration.This proved to be the case in a study by Mackworth (1976) in which several items were included in each fixation. Subjects searched a strip of several rows of circles for a square target. Display density increased with the number of circles in the strip. By counting the number of eye movements made on each strip, the number of circles per fixation was estimated. Above four circles per fixation, both the fixation duration and the number of circles per fixation tended to increase with display density. Mackworth attributed the increase in fixation duration to the extra cognitive demand added by the need to process more nontarget circles during each fixation.An alternate method of estimating the number of display items per fixation is to construct a display that controls this factor. Mocharnuk (1978) had subjects search a display consisting of clusters of letters. It was likely that each fixation would accommodate one cluster. As expected, fixation duration increased with the number of letters per cluster. A fixation duration effect was also found by Moffitt (Note I), who varied the information content of each display item while controlling the number of items per fixation. Subjects scanned back and forth be~ween two changing display items, located 10 deg of visual angle apart, searching for a member of the memory. set. Given the size of each display item, . angle, and the distance between items, it is .unlikely that more than one item was processed during each fixation. The major finding was that the fixation duration increased as a function of memory set size. It was concluded that the fixation duration reflects the amount of information being processed.The studies cited demonstrate that the fixation duration will increase as a result either of increasing the number of items per fixation while holding the information value of each item constant or of increasing the information value of each item while holding the number of items per fixation constant. The case where the information value per item is varied and the number of items processed during each fixation is not controlled for would set the stage for a potential confounding. Consider two displays, one with simple and one with complex items. It is likely that the search r...

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