DEPENDENCY OF SPATIOTEMPORAL CHARACTERISTICS OF HEAD STABILIZATION ON VISUAL AND INERTIAL STIMULATION Mobin Rastgar Agah, Kurosh Darvish, W. Geoffrey Wright, and Emily Keshner Temple University, Philadelphia, PA, USA email:
[email protected] web: www.temple.edu/engineering/research/labs/tbl INTRODUCTION Falls are a leading cause of injury in older adults. Interaction of visual, vestibular and somatosensory input controls the spatial orientation of the body and a mis-match among these inputs can lead to a faulty perception of body position and motion and cause postural instability. The role of these sensory systems in head control makes the head a good prototype of the whole body to study the effect of conflicting sensory inputs on postural stability [1]. The visual system is more sensitive to retinal slip velocity than acceleration while the vestibular system is more sensitive to accelerations. These neurophysiological properties were employed to investigate the effect of conflicting inputs to these sensory systems in order to determine if both postural control (i.e. head stabilization) and spatial perception are similarly affected by a conflict in these sensory systems [2]. Since the dominance of these sensory systems depends on the velocity and acceleration of the motion, the amplitude and frequency of a periodic passive motion together with changes in the visual depth of field were manipulated. Thus, our goal was to study the spatiotemporal effect of discordant visual and vestibular inputs on head stabilization and visual perception during dynamic visual-inertial stimulation. The relationship between visual to inertial motion directions were manipulated using realistic visual virtual environments. METHODS For detecting the movement of the head in three directions of roll, pitch and yaw, three uni-axial gyroscopes (ADXR623 and ADXR624, Analog Device, Inc.) were attached in an orthogonal array on a light-weight helmet. A mini-camera (203CA-1, Pine Computer, CA) was also attached on top of the
helmet to provide the visual input for the subjects through a head mounted display (HMD) (I-glasses HR920-3D, 920,000 Pixels per LCD, i-O Display Systems, Sacramento, CA). Twelve subjects (3 females, 9 males, age 22 to 32 yrs) participated in this study. The lightweight helmet was firmly secured on the head of subjects seated on a race car seat (Corbeau, Sandy, UT). Using a 5-point harness system the trunk was comfortably fixed while the head was free to move in all directions. The seat was mounted on a sled allowing passive anterior-posterior (A-P) translation of the subject on a track (WIESELTM SPEEDLine®WH120). The movement of the sled was controlled by a programmable modular drive system (MDS) (Control Techniques Drives Inc.). Sinusoidal signals at 4 different frequencies of 0.1, 0.2, 0.5 and 1.1 Hz were generated and used as the input signal of the MDS to move the subjects. Each trial included 5 cycles and the amplitudes of sinusoidal input at different frequencies were chosen for overlapping peak acceleration or overlapping peak velocity as shown in Table 1. For each frequency, subjects were exposed to 4 different visual conditions in random sequence: 1- Backward (BW), camera was pointed in the posterior direction, 2- Eyes closed (EC), no input to the HMD, 3- Eyes open (EO), camera was pointed in the anterior direction, 4- Sideways (SW), camera was pointed in the lateral direction parallel to the subject’s interaural axis. To test the effect of depth of field on the stability of head in the SW condition, half of the subjects were randomly chosen to see a deep field of view while the rest experienced a shallow field of view. The test was conducted 3 times for each frequency-visual condition and gyroscope signals were processed and recorded at a sampling rate of 1000 Hz. As a first approximation the head movement was considered a linear response to the inertial
input. To obtain the linear response, the first and last cycles were discarded and a least square sine fit regression was used to evaluate the amplitude and phase of the angular velocities of the head in roll, pitch and yaw axes. Typical curve fits are shown in Figure 1. Table 1 – Conditions of sled inertial input Freq (Hz) 0.1 0.2 0.5 1.1
Amp. (m) 1.500 0.750 0.120 0.024
amax (g) 0.06 0.12 0.12 0.12
vmax (m/s) 0.94 0.94 0.38 0.17
discordance. Such discordance, however, is not unusual, in that it occurs for any passenger whose visual frame of reference is fixed, limited, or obscured from seeing outside the vehicle, such as in the backseat of a car, on a bus, airplane, tank, or below deck on a boat. With a clear relevance to real-world behavior, we plan to use dynamic visualinertial inputs during tasks performed in a virtual environment to gain a greater understanding of the dissociable and overlapping processes for sensorimotor adaptation underlying both postural and upper extremity motor control.
RESULTS AND DISCUSSION REFERENCES There was a main effect of inertial condition on amplitude for all axes of head motion (p
