The influence of antagonist muscle control strategies on the isometric frequency response of the cat's ankle joint.

This study investigated the effect of various strategies to control the interaction between agonist and antagonist muscles on the frequency response of the isometric cat ankle joint actuated by the tibialis anterior (TA) and soleus (SOL) muscles. Some strategies were based on the physiologic need for increasing joint stability during forceful contractions; with these strategies, the proportional rate of physiologic antagonist activity was termed antagonist gain. Other strategies were based on the electrical stimulation literature, which advocates co-contraction at low force levels. The range of crossover of antagonist activity to the agonist's domain was termed overlap. Strategies consisting of 0%, 10%, and 20% antagonist gain were combined with 0%, 50%, and 100% overlap for a total of nine strategies. These were applied to the TA and SOL using sinusoidal input signals varying in frequency from 0.4 to 6 Hz. Gain and phase Bode plots were constructed through the use of the fast Fourier transforms (FFT's); and analysis of variance determined the significance of differences in gain and phase across frequencies. Best-fit models consisting of four poles and two zeroes were used to fit the experimental data and compared against an analytical model of muscles acting independently across the joint. Harmonic distortion was calculated to evaluate signal quality. It was found that changing the overlap and the antagonist gain produces significant changes in the dynamic response of the two-muscle joint system. The analytical approach to modeling such a system tends to consistently overestimate gain. It is suggested that signal quality is optimal when a moderate amount of antagonist gain (10%) is engaged, with overlap of 50% to smooth transitions between opposing movements. It is expected that this type of strategy will achieve optimum signal quality while preserving the long-term integrity of the joint.

Neurological pupillary noise in narcolepsy.

Pupillometry has a long but inconclusive history as a means of measuring human alertness. Spontaneous pupillary oscillations in narcoleptics and the sleep deprived are a recognized but quantitatively elusive indication of alertness. Stimulation of the pupillary light reflex (PLR) has provided contradictory or confusion indications of alertness levels. Results from 10 diagnosed narcoleptics and 10 control subjects in which the PLR system was stimulated and a reliable (90%) discriminator derived for classifying narcoleptics and controls was reported. Random pupillary oscillations, which is called pupillary noise to distinguish these oscillations from spontaneous ones, were estimated from continuous pupil diameter recordings using a recursive least squares method applied to a subject-specific PLR system model. Pupillary noise sum of squares indicate that narcoleptics have significantly (P < 0.005) less PLR noise than controls. This difference was attributed to supranuclear inhibition of randomly active Edinger-Westphal neurons long hypothesized to be the source of random pupillary oscillations. This inhibition also has been suggested as a cause of PLR sensitivity to nocturnal sleep quality so it may be that these findings apply to the sleep deprived and not just specifically to narcoleptics.

Artifact management in pupillometry.

A detailed analysis of pupillometry data collection and handling procedures in the initial study uncovered a number of problems that threatened the integrity of the data, including improper procedures, lack of adherence to data collection rules, and inaccurate mathematical calculation of results. Substantial modifications in procedures were made to improve data collection and reduce artifact. With the increased sampling rate from 5 to 60 Hz and use of a videotape playback system, a more accurate and thorough method for removing artifact from pupillometry data was demonstrated in a subsequent study. The automated cleaning algorithm system proved to be efficient at detecting and removing artifact, as well as alerting users to artifact that might not be replaceable automatically. Additionally, this system provided another method of data storage, videotape, which was beneficial in reviewing the pupil behavior that was digitally recorded. Now that procedures for collecting pupil data and managing artifact have been objectively tested, steps can be taken towards establishing pupillometry as a reliable and valid screening tool for detecting excessive sleepiness.

The wind-elicited escape response of cockroaches (Periplaneta americana) is influenced by lesions rostral to the escape circuit.

When the escape response of the cockroach (Periplaneta americana) is triggered by wind, it is mediated by the cercal-to-giant interneuron pathway and leg motor circuitry, within the abdominal and thoracic portions of the ventral nerve cord. We have found that a lesion rostral to the thorax (transection of a cervical connective) produces specific changes in wind-evoked escape. Lesioned animals reliably displayed short-latency responses to wind. However, the orientation of the initial turning component of escape was altered and the duration of subsequent running was reduced. Preliminary physiological study suggests that changes in the orientation of escape reflect changes in the integration of wind-sensory signals by thoracic circuitry. These findings imply that rostral centers influence sensorimotor integration underlying wind-evoked escape.