Understanding the brain stem.

This article highlights features of brain anatomy that are important to know in interpreting magnetic resonance images. This article concentrates on the names of some brain stem structures, the three-dimensional appearance of six important tracts, and the location of cranial nerve nuclei.

Isolated sensory impairment of the thumb due to an intraneural ganglion cyst in the median nerve.

We describe an unusual case of digital neuropathy of the thumb due to compression of the median nerve in the distal forearm by an extruded ganglion cyst originating from the radio-scaphoid joint of the wrist. Electrophysiologically, both conduction slowing and block of median motor and sensory axons were present across the cyst. These changes improved after surgery.

Lateral deviation of the eyes on forced lid closure in patients with cerebral lesions.

We examined 35 patients with unilateral cerebral lesions to determine the incidence of lateral deviation of the eyes under forcefully closed lids and the reliability of this sign in predicting the side of the lesion. Only patients with radiologically confirmed unilateral lesions were studied. Over 70% of patients had contralateral ocular deviation (Cogan's "spasticity of conjugate gaze"), 20% had ipsilateral deviation, and less than 9% of the patients had no deviation. Lateral ocular deviation was as sensitive, but not as specific, as a unilateral Babinski plantar response in determining the side with the lesion. Contralateral deviation was more common with parietotemporal localization, suggesting that the phenomenon reflects an underlying disturbance of attentional mechanisms.

Acquired pendular nystagmus in toluene addiction.

We studied the ocular motor abnormalities in 4 patients chronically addicted to sniffing glue containing toluene. They showed acquired pendular nystagmus with horizontal and vertical components. One patient also showed saccadic oscillations. The pendular nystagmus may be a manifestation of a disturbance in brainstem-cerebellar connections secondary to the toxic effect of toluene on white matter.

Behavior of human horizontal vestibulo-ocular reflex in response to high-acceleration stimuli.

We studied the horizontal vestibulo-ocular reflex (VOR) during transient, high-acceleration (1900-7100 degrees/s2) head rotations in 4 human subjects. Such stimuli perturbed the angle of gaze and caused illusory movement of a viewed target (oscillopsia). The disturbance of gaze could be attributed to the latency of the VOR (which ranged from 6-15 ms) and inadequate compensatory eye rotations (median VOR gain ranged from 0.61-0.83).

Visual cancellation of the torsional vestibulo-ocular reflex in humans.

Using the eye-coil/magnetic field method, we measured the torsional vestibulo-ocular reflex (VOR) in ten subjects during active head rotations in roll at about 0.5 Hz. In the dark, regardless of instructions or mental effort, the gains (eye velocity/head velocity) had a mean value of around 0.61. When they viewed a visual display that was stationary, gains rose to 0.72. When viewing a visual display that moved in roll with their heads, subjects could decrease their gains to a mean of 0.46. Separate experiments showed that, as expected at this frequency, the optokinetic system made only a weak contribution. It has been proposed that the horizontal VOR is cancelled by the smooth pursuit system. Since there is no torsional pursuit system, some other mechanism must be used to augment or partially cancel the torsional VOR. Attempts to show that imagination could change this gain showed only weak effects. When asked to imagine an earth-fixed scene, gains were around 0.63; when asked to imagine a subject-fixed scene, gains decreased to only 0.60. When allowed to use a tactile contribution to aid the imagination in cancelling the VOR, the gain dropped further but only to 0.57. We conclude that mental effort in the dark has little influence on the torsional VOR but vision does by a mechanism that is not optokinetic or pursuit.