Spontaneous eye movements in goldfish: oculomotor integrator performance, plasticity, and dependence on visual feedback.

To quantify performance of the goldfish oculomotor neural integrator and determine its dependence on visual feedback, we measured the relationship between eye drift-velocity and position during spontaneous gaze fixations in the light and in the dark. In the light, drift-velocities were typically less than 1 deg/s, similar to those observed in humans. During brief periods in darkness, drift-velocities were only slightly larger, but showed greater variance. One hour in darkness degraded fixation-holding performance. These findings suggest that while visual feedback is not essential for online fixation stability, it may be used to tune the mechanism of persistent neural activity in the oculomotor integrator.

Distortion-product emissions in rabbit: II. Prediction of chronic-noise effects by brief pure-tone exposures.

In an attempt to predict the susceptibility of the cochlea to the harmful influences of excessive sound, the effects of initially exposing the same rabbits to brief pure-tones were related to the subsequent effects of octave-band noise (OBN) exposure using measures of distortion-product otoacoustic emissions (DPOAEs). The pure-tone exposure paradigm consisted of determining the rate at which a 100-dB SPL, low-frequency tone reduced the amplitude of a 1.5-kHz DPOAE, elicited by 50-dB SPL primaries. To establish the stability of the rate-reduction index, the tonal-exposure protocol was repeated on three separate occasions for each subject. Subsequently, the same rabbits were exposed chronically to a 95-dB SPL OBN, centered at 1-kHz, until DPOAE amplitudes between 1-5 kHz were diminished to noise-floor levels, i.e., by 10-30 dB, in response to 45-dB SPL primaries. The results revealed a visually apparent relation between the slope of the tonal-induced DPOAE-loss functions and the number of days required to reach the criterion decrement in emission level during chronic exposure to noise. Analysis of the frequency extent of the noise-induced changes revealed significant correlations between the previously measured rate of pure-tone induced reductions of DPOAE amplitude and the subsequent amount of decrement produced by OBN exposure. Thus, rabbits exhibiting slow rates of tonal-induced decrements in low-frequency DPOAEs were resistant to the amplitude-reducing effects of a subsequent chronic OBN exposure in that it typically took more than four days to achieve the targeted amount of DPOAE loss.(ABSTRACT TRUNCATED AT 250 WORDS)

Distortion-product emissions in rabbit: I. Altered susceptibility to repeated pure-tone exposures.

An important issue in understanding the development of noise-induced hearing loss is whether prior acoustic overstimulation alters the susceptibility of the cochlea to further damage. The present work was designed to establish a model of activity-dependent changes in the susceptibility of the cochlea to acoustic overstimulation by regularly exposing the ear to a low-frequency pure tone. As a quantitative index of cochlear function, 2f1-f2 distortion-product otoacoustic emissions (DPOAEs) were monitored systematically over time in three groups of rabbits, with each group experiencing a unique paradigm that incorporated repeated exposure to the low-frequency tone. Common to each rabbit's exposure protocol was that a given experimental session consisted of two exposure episodes, separated by a 40-min period. Experimental sessions were repeated three times, with 2- to 3-day recovery periods interposed between sessions. The rate of decrement in DPOAE amplitude over a prescribed time period was utilized as a measure of susceptibility to the acoustic trauma. The overall results indicated that ears were more susceptible to exposure 40 mins after the first exposure of a session than they were initially. A series of control experiments indicated that the robustness of the acoustic middle-ear reflex (AMR) did not change between the exposure episodes. Consequently, changes in the AMR could not account for the increased susceptibility seen following the first exposure. However, in awake rabbits with stronger AMRs, higher pure-tone exposure levels were needed to produce increased susceptibility to the second exposure. After 2-3 days of intersession recovery, susceptibility to the effects of excessive sound returned close to its original baseline level. The outcome of these studies demonstrated a reduced capacity for the ear to resist the harmful effects of exposure to a moderately intense tone, which was repeated twice over a brief 40-min period, but little change in susceptibility when identical exposures were repeated over longer intersession intervals of several days.

Technetium-99m labeled micro aerosol "Pertechnegas". A new agent for ventilation imaging in suspected pulmonary emboli.

In this preliminary study, four patients suspected of having pulmonary emboli underwent ventilation imaging after inhaling approximately 1 mCi (37 MBq) of Tc-99m Pertechnegas (micro aerosol carbon particles labeled with Tc-99m) in five breaths or less. Planar images in multiple projections were recorded for preset counts. A final posterior image was acquired to evaluate residual lung background activity. Immediately following ventilation imaging, perfusion imaging in the identical projections was performed using 4 mCi (148 MBq) of Tc-99m MAA. Two of the four patients demonstrated matched uniform ventilation and perfusion and two showed segmental ventilation-perfusion "mismatched" defects consistent with pulmonary emboli. In each case, residual Pertechnegas background lung activity (count rate) at the end of the last ventilation image was less than 10% of the initial Tc-99m MAA counts (count rate). The advantages of Pertechnegas, in comparison to aerosolized DTPA, include: less residual ventilation image activity superimposed on the MAA perfusion images, reduced demand for patient cooperation due to fewer required breaths (five or less), and lack of deposition of activity in the central airways. The authors conclude that Pertechnegas has properties that make it ideally suited for routine lung ventilation imaging.