Postural control is an important factor for early motor development; however, compared with adults, little is known about how infants control their unperturbed upright posture. This lack of knowledge, particularly with respect to spatial and temporal characteristics of infants' unperturbed independent standing, represents a significant gap in the understanding of human postural control and its development. Therefore, our first analysis offers a thorough longitudinal characterization of infants' quiet stance through the 9 months following the onset of independent walking. Second, we examined the influence of sensory-mechanical context, light touch contact, on infants' postural control. Nine typically developing infants were tested monthly as they stood on a small pedestal either independently or with the right hand lightly touching a stationary contact surface. In addition to the longitudinal study design, an age-constant sample was analyzed to verify the influence of walking experience in infant postural development without the confounding effect of chronological age. Center of pressure excursions were recorded and characterized by distance-related, velocity, and frequency domain measures. The results indicated that, with increasing experience in the upright, as indexed by walk age, infants' postural sway exhibited shifts in rate-related characteristics toward lower frequency and slower, less variable velocity oscillations without changing the spatial characteristics of sway. Additional touch contact stabilized infants' postural sway as revealed by decrease in sway position variance, amplitude, and area as well as lower frequency and velocity. These results were confirmed by the age-constant analysis. Taken together, our findings suggest that instead of progressively reducing the sway magnitude, infants sway differently with increasing upright experience or with additional somatosensory information. These differences suggest that early development of upright stance, particularly as it relates to increasing postural and locomotor experience, involves a refinement of sensorimotor dynamics that enhances estimation of self-motion for controlling upright stance.

No abstract

We have designed and built a low noise, broadband transimpedance amplifier that allows for sensing both the amplitude and phase of an oscillating quartz tuning fork for a cryogenic atomic force microscope. The circuit uses a global feedback scheme, where the input stage, located next to the quartz tuning fork at low temperature, is followed by an operational amplifier stage operated at room temperature. At 4.2 K, with a 1 MΩ metal film feedback resistor, the amplifier yields an output noise floor of 2×10−7 V/Hz and a bandwidth of 200 kHz. When it is used with a commercial 32 kHz quartz tuning fork, the calibrated sensitivity at 4.2 K is determined to be 30.8 μV/pm.

We have designed and fabricated a cryogenic scanning tunneling microscope for probing lithography defined nanometer-scale devices. The piezoelectric double tube is capable of scanning an area up to 22 mϫ22 m, while maintaining atomic resolution. In addition, the sample mount has a 5 mmϫ4 mm traveling range. Most importantly, the system is compact and, as a result, it can be inserted into the bore of a superconducting magnet. In this work, we demonstrate a unique application of scanning tunneling system, i.e., the scanning tip is in direct contact with the sample. The spectroscopic information therefore reflects the true characteristics of the devices under test, unlike the typical case where the tunneling barrier through vacuum imposes a large series resistance, on the order of 10 9 ⍀. The design as well as the operation of this compact scanning tunneling microscope is described.

Articles you may be interested inCoulomb-modified equilibrium and nonequilibrium properties in a double quantum dot Aharonov-Bohm-Fano interference device