Measuring threshold and latency of motion perception on a swinging bed

Abstract

Introduction: Our objective was to develop and to evaluate a system to measure latency and threshold of pendular motion perception based on a swinging bed.

Materials and methods: This prospective study included 30 healthy adults (age: 32 ± 12 years). All subjects were tested twice with a 10 min. interval. A second trial was conducted 2 to 15 days after. A rehabilitation swinging bed was connected to an electronic device emitting a beep at the beginning of each oscillation phase with an adjustable time lag. Subjects were blindfolded and auditory cues other than the beep were minimized. The acceleration threshold was measured by letting the bed oscillate freely until a natural break and asking the patient when he did not perceive any motion. The perception latency was determined by asking the patient to indicate whether the beep and the peak of each oscillation were synchronous. The time lag between sound and peak of the head position was swept from -750 to +750 ms by 50 ms increments.

Results: The mean acceleration threshold was 9.2±4.60 cm/s2. The range width of the synchronous perception interval was estimated as 535±190 ms. The point of subjective synchronicity defined as the center of this interval was -195±106 ms (n = 30). The test-retest evaluation in the same trial showed an acceptable reproducibility for the acceleration threshold and good to excellent for all parameters related to sound-movement latency.

Conclusion: Swinging bed combined to sound stimulation can provide reproducible information on movement perception in a simple and non-invasive manner with highly reproducible results.

Fig 1. Experimental setup.

The patient is installed on a swinging bed with eyes blinded by a mask. An electronic device (A) detects the bed’s movements and emits a beep at the beginning of each oscillation period. The movement detector is composed of an infrared emitter (A), a deflector (B) and a cover (C) placed under the bed. The cover edge was also used to measure deviation from the equilibrium position in acceleration threshold measurements. The speaker emitting the beep was placed inside the emitter box approximately 1.5 meters away from the subject’s ears.

Fig 2. Relation between sound stimuli and bed oscillation.

During the swinging bed-oscillations a beep was generated by the electronic device with an adjustable time lag. The zero was defined as the peak of the oscillation (head at its maximal height). The time lag was modified from -750 ms to +750 ms with 50 ms increments. Subjects were asked to indicate whether the sound and the peak are synchronous. The synchronous perception interval is depicted in gray. The upper and lower borders were measured. The middle of the range was defined as subjective synchronicity delay.

Fig 3. Dispersion of sound-peak (SP) and peak-sound (PS) thresholds, synchronous perception intervals, point of subjective synchronicity (PSS) and acceleration thresholds.

Open circles represent individual values (n = 30). Each value is the mean of 2 or 4 replicates. Horizontal bars represent mean and the error bars depict standard deviation. For the delays, the zero was defined by the peak of the bed oscillation (head at its maximal height). The point of subjective synchronicity which represents the middle of the synchronous perception interval had a negative value in all cases.