Whole - body motion - detection tasks can yield much lower thresholds 1 than direction - recognition tasks : implications for the role of vibration 2 3

47 Earlier spatial orientation studies used both motion-detection (e.g., did I move?) and direction48 recognition paradigms (e.g., did I move left/right?). The purpose of our study was to compare 49 thresholds measured using motion-detection and direction-recognition tasks on a standard Moog 50 motion platform to see if a substantial fraction of the reported threshold variation might be 51 explained by the use of different discrimination tasks in the presence of vibrations that vary with 52 motion. Thresholds for the perception of yaw rotation about an earth-vertical axis and for inter53 aural translation in an earth-horizontal plane were determined for four healthy subjects using 54 standard detection and recognition paradigms. For yaw rotation, two-interval detection 55 thresholds were, on average, 56 times smaller than two-interval recognition thresholds, and for 56 inter-aural translation, two-interval detection thresholds were, on average, 31 times smaller than 57 two-interval recognition thresholds. This substantive difference between recognition thresholds 58 and detection thresholds is one of our primary findings. For motions near our measured detection 59 threshold, we measured vibrations that matched previously established vibration thresholds. This 60 suggests that vibrations contribute to whole-body motion detection. We also recorded yaw 61 rotation thresholds on a second motion device with lower vibration and found direction62 recognition and motion-detection thresholds that were not significantly different from one 63 another or from the direction-recognition thresholds recorded on our Moog platform. Taken 64 together, these various findings show that yaw rotation recognition thresholds are relatively 65 unaffected by vibration when moderate (up to circa 0.08 m/s) vibration cues are present. 66 67