BackgroundPrevious findings have shown that humans can learn to localize with altered auditory space cues. Here we analyze such learning processes and their effects up to one month on both localization accuracy and sound externalization. Subjects were trained and retested, focusing on the effects of stimulus type in learning, stimulus type in localization, stimulus position, previous experience, externalization levels, and time.MethodWe trained listeners in azimuth and elevation discrimination in two experiments. Half participated in the azimuth experiment first and half in the elevation first. In each experiment, half were trained in speech sounds and half in white noise. Retests were performed at several time intervals: just after training and one hour, one day, one week and one month later. In a control condition, we tested the effect of systematic retesting over time with post-tests only after training and either one day, one week, or one month later.ResultsWith training all participants lowered their localization errors. This benefit was still present one month after training. Participants were more accurate in the second training phase, revealing an effect of previous experience on a different task. Training with white noise led to better results than training with speech sounds. Moreover, the training benefit generalized to untrained stimulus-position pairs. Throughout the post-tests externalization levels increased. In the control condition the long-term localization improvement was not lower without additional contact with the trained sounds, but externalization levels were lower.ConclusionOur findings suggest that humans adapt easily to altered auditory space cues and that such adaptation spreads to untrained positions and sound types. We propose that such learning depends on all available cues, but each cue type might be learned and retrieved differently. The process of localization learning is global, not limited to stimulus-position pairs, and it differs from externalization processes.

We studied the relationship of certain fetal and maternal hormones to indicators of lung maturation in 12 fetal lambs delivered at gestational ages (GA) of 123-149 days. Maternal estrogen, maternal progesterone, and fetal prolactin did not correlate with GA or the indicators of fetal lung maturation. Fetal cortisol (range 4-165 ng X ml-1) increased with advancing GA (r = 0.747, P less than 0.01). All of the following showed a wide range of late gestation and showed a significant positive correlation with fetal cortisol: lung volumes at 40 cmH2O and 10 cmH2O on the deflation during air pressure-volume studies; saturated phosphatidylcholine (SPC) in lung tissue and in lavage fluid expressed both as mg X g-1 of wet lung and as percent of total phospholipids (%PL); total SPC (lung tissue plus lavage fluid): and SPC in lavage fluid as percent of total SPC. Lung DNA correlated inversely with GA and cortisol. All variables (except lavage fluid SPC as %PL) correlated more closely with cortisol than GA. Morphological development of lung was also related more closely to cortisol than GA. These results suggest that functional lung maturity is attained late in gestation and that endogenous cortisol is an important physiological factor in control of fetal lung maturation.

The effects of prolonged intravenous infusions of cholic acid into fetal lambs are described in this study. The ewes (n = 10, 11 fetuses) were operated on at 114 days of gestation (term = 150 days) by placing plastic catheters in maternal and fetal vessels and in the amniotic cavity. Gestational ages were confirmed after delivery by radiographic examination of the ossification centers of the fetal legs. Infusions of cholic acid (1.6 mumoles/min-1) started 8 to 10 days after surgery in 5 fetuses (including one twin). The remaining 6 fetuses (also including one twin) were infused with 5% dextrose in water. Total plasma bile acids at the beginning of the experiment were similar in both groups (23.8 +/- 6.6 vs. 24.3 +/- 5.7 microM). No significant changes in fetal heart rate, blood pressure, blood gases or pH were detected during the infusion. Meconium-stained amniotic fluid was observed during the third day of infusion in all the fetuses infused with cholic acid and in one control fetus. Fetuses infused with cholic acid were delivered alive 19-26 days before term. The concentration of plasma bile acids in the experimental group at delivery was 829 +/- 305 microM, i.e. significantly higher than that of the control group (24.4 +/- 5.7 microM). Control fetuses (except one twin) were delivered at term. We concluded that cholic acid, even at the high dose infused, is neither lethal nor severely harmful for the fetus. Meconium passage of the fetuses infused with cholic acid, in our experiment, appeared to be related to the stimulatory effect of cholic acid on fetal colonic motility rather than to fetal hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)

Room acoustic simulation using digital waveguide modeling requires three-dimensional waveguide meshes in order to represent fully the acoustic properties of the space. This paper presents a systematic analysis of four mesh topologies suggested in the literature: rectilinear, tetrahedral, cubic close-packed and octahedral. These mesh structures are compared from the standpoint of computational efficiency, bearing in mind specific issues that are important for room acoustic simulation. Each mesh topology offers a different compromise between spatial resolution, bandwidth, dispersion characteristics (including suitability for the application of dispersion-compensation techniques), computation time, memory requirements and implementation complexity.