Spatiotemporal interpolation and quality of apparent motion.

We investigate the conditions under which stimuli in apparent (sampled) motion are indistinguishable from those in smooth motion and compare this discrimination with the precision achieved by the visual system in interpolating apparent motion. In an initial experiment, observers were required to discriminate smooth from apparent motion, at variable step sizes, contrasts, velocities, and stimulus types (broadband line or bar stimuli and grating patches of different spatial frequency). Thresholds for discriminating smooth from sampled motion were approximately 40 arc min under optimal conditions, corresponding to the diameter of foveal photoreceptors. The tolerated step size between stations increased with velocity, more so for low- than for high-spatial-frequency stimuli. Tolerated step size decreased with presentation duration and with stimulus contrast. A separate experiment examined precision of interpolation. Vernier offsets were produced through temporal delays along the trajectory of an apparent motion, and thresholds for the discrimination of direction of offset were measured as a function of speed of motion and of distance between stations of apparent motion. Perfect interpolation was achieved for distances between stations of approximately 2 arc min. A model based on spatiotemporal filtering at an early stage of processing accounts well for the results of both types of experiments.

Markers of bone metabolism during short-term administration of thyroxine in healthy volunteers.

We investigated the influence of L-thyroxine (L-T4) treatment over 3 weeks on biochemical markers of bone turnover in 12 healthy young men (age 25.6 +/- 1.4 years, BMI: 22.6 +/- 2.5 kg/m2). Serum parameters indicating bone formation [bone Gla protein (BGP), carboxyterminal propeptide of type I procollagen (PICP), and bone-specific alkaline phosphatase (BAP)] and bone resorption [cross-linked carboxyterminal telopeptide of type I collagen (ICTP) and the urinary excretion of pyridinoline (Pyr) and deoxypyridinoline (D-Pyr)] were measured before and after three weeks of treatment with 300 micrograms L-T4/d. T3 and T4 significantly increased and TSH decreased to almost undetectable levels even when measured with a third generation TSH assay. Markers of bone formation showed variable responses with a small but significant increase in BGP but not in PICP or BAP. In contrast, all parameters of bone resorption increased significantly with a good correlation between D-Pyr excretion and the serum parameter ICTP (r = 0.78, p < 0.0001). These changes in bone-turnover markers were not necessarily paralleled by comparable increments of other markers of tissue thyrotoxicosis (SHBG, pulse rate, VO2), suggesting a variability in tissue sensitivity. These rapid responding parameters, especially in the easily obtainable serum parameter ICTP, might be valuable tools in the evaluation of several states of thyroxine excess.

Thyroid hormones influence perceptual processing in a visual search paradigm.

The mechanisms that mediate the effects of thyroid hormones on higher cognitive processes are not well understood. In the present experiments, event-related potentials (ERPs) and behavioral measurements were recorded in a visual search paradigm where a target item had to be detected in a number of distractor items. 10 healthy subjects were given a daily dose 300 micrograms thyroxine (T4) p.o. and recordings were made before and after 3 weeks of treatment. All subjects developed hyperthyroidism with a completely suppressed TSH (1.6 +/- 0.5 mU/l vs. non detectable), a significant increase in T3 (1.5 +/- 0.2 vs. 1.9 +/- 0.2 ng/ml). T4 (6.6 +/- 1.6 micrograms/dl vs 11.9 +/- 2.7 micrograms/dl) and sex-hormone binding globulin (2.9 +/- 1.1 vs. 3.8 +/- 1.3 micrograms/ml; all p less than 0.0005). Compared to the control group (n = 15), thyroid hormones improved controlled serial visual processing, as indexed by changes of the late positive ERP component and target detection hit rate (in both cases p less than 0.01), whereas parallel detection of salient features remained unchanged. The data suggest a differential impact of thyroid hormones on visual information processing.