Biomedical subjects

Steven L Bell

Publications and source records attributed to Steven L Bell.

5 recordsLinked to original sources

PubMed2006

Objective detection of evoked potentials using a bootstrap technique.

Evoked potentials are usually evaluated subjectively, by visual inspection, and considerable differences between interpretations can occur. Objective, automated methods are normally based on calculating one (or more) parameters from the data, but only some of these techniques can provide statistical significance (p-values) for the presence of a response. In this work, we propose a bootstrap technique to provide such p-values, which can be applied to a wide variety of parameters. The bootstrap method is based on randomly resampling (with replacement) the original data and gives an estimate of the probability that the response obtained is due to random variation in the data rather than a physiological response. The method is illustrated using auditory brainstem responses (ABRs) to detecting hearing thresholds. The flexibility of the approach is illustrated, showing how it can be used with different parameters, numbers of stimuli and with user-defined false-positive rates. The bootstrap method provides a new, simple and yet powerful means of detecting evoked potentials, which is very flexible and readily adapted to a wide variety of signal parameters.

Adolescent↗

PubMed2006

The auditory middle latency response, evoked using maximum length sequences and chirps, as an indicator of adequacy of anesthesia.

The auditory evoked potential known as the middle latency response (MLR), evoked with regular click stimulation at around 5 Hz, has been suggested as an indicator of adequacy of anesthesia. The MLR is a very small signal embedded in high levels of background noise, so it can take a long time to acquire. However, using a stimulus paradigm of chirps presented in a maximum length sequence, the acquisition of the MLR can be improved compared to using conventional click stimulation. In this pilot study, we investigated this new technique in a clinical environment. Significant changes in MLR amplitude, but not latency, were measured for six of seven subjects in association with changes in responsiveness to command using the isolated forearm technique. The absence of any latency shift differs from other studies of the MLR during anesthesia and highlights the limited understanding of the relationship between anesthesia and the MLR.

Acoustic Stimulation↗

PubMed2004

Expa: a program for calculating extreme pathways in biochemical reaction networks.

UNLABELLED: The set of extreme pathways, a generating set for all possible steady-state flux maps in a biochemical reaction network, can be computed from the stoichiometric matrix, an incidence-like matrix reflecting the network topology. Here, we describe the implementation of a well-known algorithm to compute these pathways and give a summary of the features of the available software. AVAILABILITY: The C-code, along with a Windows executable and sample network reaction files, are available at http://systemsbiology.ucsd.edu CONTACT: palsson@ucsd.edu.

Animals↗

PubMed2002

Optimizing the acquisition time of the middle latency response using maximum length sequences and chirps.

The middle latency response (MLR) may be used as an indicator of anesthetic depth but has been criticized due to its long acquisition time. This study explores methods for optimizing recording of the MLR to maximize signal-to-noise ratio (SNR) and hence reduce acquisition time. The first experiment investigates the effects of increasing stimulation rate beyond conventional values and also using higher rates by means of maximum length sequences (MLS). The second experiment compares the use of click and chirp stimuli to elicit the MLR, both at conventional and MLS stimulation rates. For all conditions total recording duration is fixed at 185 s and stimulation level is fixed at 60 dB SL. It was found that SNR increases progressively with rate using conventional click stimulation until the theoretical rate limit is reached at the reciprocal of the response duration. The SNR improvement is equivalent to increasing test speed by a factor of 3. Using MLS stimulation, the SNR increases further until a maximum is reached at a rate of 167 clicks/s, equivalent to a fivefold test speed improvement relative to a conventional recording at 5 clicks/s. The use of chirp stimuli designed to compensate for the frequency dependent cochlear traveling wave delay produces an increase in wave V-Na amplitude at all recording rates. For the later latency waves of the response an increase in amplitude is seen for MLS, but not for conventional chirp trains. The optimum SNR was obtained using chirp stimuli at a MLS rate of 167 opportunities/s. It is concluded that the combination of chirps and MLS can reduce acquisition time to less than one-tenth of that required for conventional stimulation at 5 clicks/s for the same SNR. This would confer material benefits for estimating anesthetic depth using MLR.

Adult↗

PubMed2002

An investigation of the use of band-limited chirp stimuli to obtain the auditory brainstem response.

Auditory brainstem responses (ABRs) using clicks enable global objective estimation of hearing threshold. Recently, it has been suggested that a chirp stimulus may produce a synchronous response from a larger portion of the basilar membrane than a click, and that a chirp will produce a greater amplitude response than a click at the same sensation level. Various workers have modified the stimulus to achieve frequency specificity (e.g. using tone-bursts). The present investigation used band-limited chirp stimuli having the same frequency-delay characteristics as the chirp mentioned above that compensate for frequency-dependent cochlear delays. The intention was to generate highly synchronized neural responses across parts of the nerve fibre array. Stimuli were presented at sensation levels between 10 and 50 dB to 10 adult subjects. Wave V was consistently identifiable even for low-frequency stimuli. Wave V amplitude increased and latency decreased as stimulus frequency increased. The latency decrease is consistent with high-frequency responses arising from basal regions of the cochlea. ABR thresholds were defined by objective estimation and visual inspection. Average ABR thresholds were 16 dB higher than behavioural thresholds for high-frequency chirps (3000-6000 Hz), increasing to 25 dB for low-frequency chirps (375-750 Hz). These ABR thresholds are closer to behavioural thresholds and have a smaller variance than reported for tone-burst stimuli without masking. However, they are not as close as those reported for tone-bursts in notched noise. The disadvantage of the band-limited chirps is that they have a wider spectral spread than tone-bursts and hence may elicit a response from unwanted frequency regions of the basilar membrane.

Acoustic Stimulation↗