The basis for using bone-conducted vibration or air-conducted sound to test otolithic function.

Extracellular single neuron recordings of primary vestibular neurons in Scarpa's ganglion in guinea pigs show that low-intensity 500 Hz bone-conducted vibration (BCV) or 500 Hz air-conducted sound (ACS) activate a high proportion of otolith irregular neurons from the utricular and saccular maculae but few semicircular canal neurons. In alert guinea pigs, and humans, 500 Hz BCV elicits otolith-evoked eye movements. In humans, it also elicits a myogenic potential on tensed sternocleidomastoid muscles. Although BCV and ACS activate both utricular and saccular maculae, it is possible to probe the functional status of these two sense organs separately because of their differential neural projections. Saccular neurons have a strong projection to neck muscles and a weak projection to the oculomotor system. Utricular afferents have a strong projection to eye muscles. So measuring oculomotor responses to ACS and BCV predominantly probes utricular function, while measuring neck muscle responses to these stimuli predominantly probes saccular function.

Influence of pitch tilts on the perception of gravity-referenced eye level in labyrinthine defective subjects.

We investigate the role of vestibular information in judging the gravity-referenced eye level (i.e., earth-referenced horizon or GREL) during sagittal body tilt whilst seated. Ten bilateral labyrinthine-defective subjects (LDS) and 10 age-matched controls set a luminous dot to their perception of GREL in darkness, with and without arm pointing. Although judgements were linearly influenced by the magnitude of whole-body tilt, results showed no significant difference between LDS and age-matched controls in the subjective GREL accuracy or in the intra-subject variability of judgement. However, LDS performance without arm pointing was related to the degree of vestibular compensation inferred from another postural study performed with the same patients. LDS did not utilize upper limb input during arm pointing movements as a source of graviceptive information to compensate for the vestibular loss. The data suggest that vestibular cues are not of prime importance in GREL estimates in static conditions. The absence of difference between controls and LDS GREL performance, and the correlation between the postural task and GREL accuracy, indicate that somatosensory input may convey as much graviceptive information required for GREL judgements as the vestibular system.

The spectral main sequence of human saccades.

Despite the many models of saccadic eye movements, little attention has been paid to the shape of saccade trajectories. Some investigators have argued that saccades are driven by a rectangular "bang-bang" neural control signal, whereas others have emphasized the similarity to fast arm movement trajectories, such as the "minimum jerk" profile. However, models have not been tested rigorously against empirical trajectories. We examined the Fourier transforms of saccades and compared them with theoretical models. Horizontal saccades were recorded from 10 healthy subjects. The Fourier transform of each saccade was accurately computed using a padded fast Fourier transform (FFT), and the frequencies of the first three minima (M1, M2, M3) in each energy spectrum were measured to a precision of 0.12 Hz. Each subject showed near-linear trends in the relationships among M1, M2, and M3 and the reciprocal of duration (1/T), which we call the "spectral main sequence." Extrapolation of plots did not pass through the origin, indicating a subtle departure from self-similarity. Bivariate confidence regions were established to allow for slope-intercept variability. The nonharmonic relationships seen cannot arise from a rectangular saccadic pulse driving a linear ocular plant. The relationships are also incompatible with minimum acceleration, minimum jerk, or higher-order minimum square derivative trajectories. The best fits were made by trajectories that minimize postmovement variance with signal-dependent noise (). It is concluded that the spectral main sequence is exquisitely sensitive to the saccade trajectory and should be used to test objectively all present and future models of saccades.

Saccadic strategies in children with hemianopia.

Multiple hypometric (undershooting) saccades are generally reported as a compensatory strategy in adults with homonymous hemianopia. However, hypermetric (overshooting) saccades have been reported to develop spontaneously as a beneficial strategy in response to predictable targets. We examined the saccades of 10 children (aged 5 to 16 years) with homonymous hemianopia to determine the type of compensatory eye-movement strategies employed 6 months to 16 years after hemianopia onset. Homonymous hemianopia was identified using perimetry and/or pattern visual evoked potentials and supported with results of neuroimaging. Eye movements were recorded using bitemporal electrooculography. Saccades were elicited to a red light source in a semipredictable paradigm. We found that hypermetria was not a consistent compensatory strategy in our patients. In spite of the predictability of our paradigm and the long follow-up period, multiple hypometric saccades into the blind field appeared to be the preferred strategy.

Familial congenital saccade initiation failure and isolated cerebellar vermis hypoplasia.

The underlying lesion in congenital saccade initiation failure (c-SIF) ('congenital ocular motor apraxia', 'Cogan's apraxia') is uncertain. Often no abnormality can be found, yet in others a midline cerebellar abnormality has often been reported. We examined this cerebellar association in a brother and sister. In addition to standard ophthalmological and neurological examinations, both siblings underwent ocular motor testing and neuroradiological investigations including CT and MRI. Both siblings exhibited the typical signs of c-SIF, including headthrusting, synkinetic blinking, missed-nystagmus quick phases, mild developmental delay, and speech difficulties. CT and MRI revealed cerebellar vermis hypoplasia in the brother, but appeared normal in the sister. No other neuroradiological abnormalities were detected. These cases highlight the wide variability in the association of vermis abnormalities with c-SIF, despite the inheritance and similar clinical manifestations. They show that either: (1) the vermis is causal in saccade triggering, but that c-SIF may result from very subtle damage that is beyond MRI resolution in some cases; or (2) that a vermis abnormality per se is not causative but only a marker of another subtle abnormality, either structural or possibly biochemical.