No abstract

Saccades were measured by magnetic search coil technique in 13 patients with Alzheimer's disease and compared with those of 11 age-matched control subjects. We analyzed responses to target motion of predictable and unpredictable timing or amplitude, and responses to briefly flashed targets. Latencies were significantly prolonged and peak velocities reduced only when the timing of target motion was unpredictable. Saccades to predictable amplitude targets were abnormally hypometric. In 8 patients, impersistence of gaze was manifested by large-amplitude saccadic intrusions away from the intended position of gaze. When instructed to make saccades away from a flashed target, patients made reflexive saccades toward the target. We attribute this visual grasp reflex to frontal lobe degeneration. These eye movement deficits can be used to quantify motor dysfunction in Alzheimer's disease.

The present study characterizes a previously reported adaptive phenomenon in a somatosensory-motor system involved in directional control of locomotor trajectory through foot contact with the floor. We call this the "podokinetic" (PK) system. Podokinetic adaptation was induced in six subjects by stepping in-place over the axis of a horizontally rotating disc over a range of disc angular velocities (11.25-90 degrees/s) and durations (7.5-60 min). After adaptation, subjects were blindfolded and attempted to step in-place on the floor without turning. Instead they all rotated relative to space. The rate of the "podokinetic afterrotation" (PKAR) was linearly related to stimulus amplitude up to 45 degrees/s, and the ratio of initial PKAR velocity to that of the adaptive stimulus was approximately 1:3. PKAR exhibited exponential decay, which was composed of "short-" and "long-term" components with "discharging" time constants on the order of 6-12 min and 1-2 h, respectively. The effect of stimulus duration on PKAR revealed a "charging" time constant that approximated that of the short-term component. A significant suppression of PKAR occurred during the 1 st min of the postadaptive response, suggesting functional interaction between the PK and vestibular systems during the period of vestibular stimulation. During PKAR subjects perceived no self-rotation, indicating that perception as well as locomotor control of spatial orientation were remodeled by adaptation of the PK system.