Head and Eye Movements in Free Visual Search: Effects of Restricted Field of View and Night Vision Goggle Imagery

Combined head and eye movements were measured as observers searched for simple grating targets (2 degrees or 6 degrees dia.) super-imposed on each of two levels of background detail, using each of three instantaneous fields of view (IFOV) (IFOV = 10, 20 or 40 degrees) The relevant data were the pattern of head movements and the magnitude, direction, and fixation duration of gaze saccades. In addition, a separate set of observers performed the same search task using real night vision goggles (NVGs) (IFOV = 38 degrees). It was found that increasing the IFOV resulted in a decrease in the magnitude of head movements and a concomitant increase in the magnitude of eye movements. This apparent tradeoff suggests that only about 10 degrees of the visual periphery was effectively used in the visual search, Head scans and gaze magnitudes were independent of either the size of the test target or the level of background detail. Fixation duration was dependent on target size but only at the smallest IFOV. Saccadic duration was also plotted as a function of saccade magnitude (i.e., the main sequence), and we found that a power function with an exponent of about 0.35 gave the best fit to these data. Finally, the similarity of the data obtained in the IFOV =40 degrees condition and those obtained using real NVGs (IFOV =38 degrees) suggests that the conclusions reached here may be generalized to visual search performed with real NVGs under the stimulus conditions tested here.