Auditory steady-state responses.

BACKGROUND: The auditory steady state response (ASSR) is an auditory evoked potential (AEP) that can be used to objectively estimate hearing sensitivity in individuals with normal hearing sensitivity and with various degrees and configurations of sensorineural hearing loss (SNHL). For this reason, many audiologists want to learn more about the stimulus and recording parameters used to successfully acquire this response, as well as information regarding how accurately this response predicts behavioral thresholds across various clinical populations. PURPOSE: The scientific goal is to create a tutorial on the ASSR for doctor of audiology (Au.D.) students and audiologists with limited (1-5 yr) clinical experience with AEPs. This tutorial is needed because the ASSR is unique when compared to other AEPs with regard to the type of terminology used to describe this response, the types of stimuli used to record this response, how these stimuli are delivered, the methods of objectively analyzing the response, and techniques used to calibrate the stimuli. A second goal is to provide audiologists with an understanding of the accuracy with which the ASSR is able to estimate pure tone thresholds in a variety of adult and pediatric clinical populations. DESIGN: This tutorial has been organized into various sections including the history of the ASSR, unique terminology associated with this response, the types of stimuli used to elicit the response, two common stimulation methods, methods of objectively analyzing the response, technical parameters for recording the ASSR, and the accuracy of ASSR threshold prediction in the adult and pediatric populations. In each section of the manuscript, key terminology/concepts associated with the ASSR are bolded in the text and are also briefly defined in a glossary found in the appendix. The tutorial contains numerous figures that are designed to walk the reader through the key concepts associated with this response. In addition, several summary tables have been included that discuss various topics such as the effects of single versus multifrequency stimulation techniques on the accuracy of estimating behavioral thresholds via the ASSR; differences, if any, in monaural versus binaural ASSR thresholds; the influence of degree and configuration of SNHL on ASSR thresholds; test-retest reliability of the ASSR; the influence of neuro-maturation on ASSR thresholds; and the influence of various technical factors (i.e., oscillator placement, coupling force, and the number of recording channels) that affect bone conducted ASSRs. CONCLUSION: Most researchers agree that, in the future, ASSR testing will play an important role in clinical audiology. Therefore, it is important for clinical audiologists and Au.D. students to have a good basic understanding of the technical concepts associated with the ASSR, a knowledge of optimal stimulus and recording parameters used to accurately record this response, and an appreciation of the current role and/or limitations of using the ASSR to estimate behavioral thresholds in infants with various degrees and configurations of hearing loss.

Escherichia coli RecBC helicase has two translocase activities controlled by a single ATPase motor.

E. coli RecBCD is a DNA helicase with two ATPase motors (RecB, a 3'→5' translocase, and RecD, a 5'→3' translocase) that function in repair of double-stranded DNA breaks. The RecBC heterodimer, with only the RecB motor, remains a processive helicase. Here we examined RecBC translocation along single-stranded DNA (ssDNA). Notably, we found RecBC to have two translocase activities: the primary translocase moves 3'→5', whereas the secondary translocase moves RecBC along the opposite strand of a forked DNA at a similar rate. The secondary translocase is insensitive to the ssDNA backbone polarity, and we propose that it may fuel RecBCD translocation along double-stranded DNA ahead of the unwinding fork and ensure that the unwound single strands move through RecBCD at the same rate after interaction with a crossover hot-spot indicator (Chi) sequence.