The perception of head and neck angular displacement in normal and labyrinthine-defective subjects. A quantitative study using a 'remembered saccade' technique.

Abstract

The detection of horizontal angular displacement of the head or trunk was assessed in 13 normal subjects and three labyrinthine defective patients. Discrete rotational displacements to the whole body (vestibular stimulus, VS), the trunk whilst keeping the head earth-stationary (cervical stimulus, CS) or the head upon the stationary trunk (combined VS+CS in passive and active mode) were delivered. Amplitudes ranged from 10 degrees to 40 degrees with peak velocities from 5 degrees to 40 degrees/s. Response was quantified as saccadic gain (final position of the eye/amplitude of the rotational stimulus) in a 'remembered saccade' task. it was found that normal subjects show different accuracy according to the sensory channel investigated. If stimulus amplitude was limited (20-30 degrees), gain dropped as rotational velocity decreased during VS but not in conditions including cervical stimulation (CS or VS+CS). Responses combining cervical and vestibular stimuli (VS+CS) were closer to unity gain and showed less scatter than isolated VS and CS. In the labyrinthine-defective patients, VS yielded no structured response but all responses involving cervical stimulation were identical to those of normal subjects when analysed both as a function of stimulus amplitude and velocity. Neither in normal nor in labyrinthine-defective subjects were there significant differences in the ability to detect head angular displacement between passive or active head turns (passive or active VS+CS). The following conclusions can be made. (i) 'Remembered saccade' techniques can be used to investigate cervico-vestibular perception. (ii) The 'high pass' characteristic of the response during VS agrees with the dynamic sensitivity of the vestibular nerve signals. Cervical experiments confirmed that neck responses are position-driven (i.e. 'tonic'). (iii) The detection of head turns is only slightly more accurate when vestibular and cervical signals are combined, but the main input controlling this perception is provided by neck afferents. No specific function for 'efference copy' was apparent in these experiments. (iv) There is no significant change for detecting head turns (on the trunk) in labyrinthine-defective patients. (v) The lack of detectable changes in cervical tasks in labyrinthine-defective patients does not support the view that the potentiation of the cervico-ocular reflex (COR) observed in these patients is due to enhanced sensitivity of the neck afferent system.