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1.1 The problems
Auditory Brainstem Response (ABR) (Hall, 1992) is one of the auditory evoked potentials obtained from the brain electrical activity through stimulation by an acoustic stimulation. It represents the neural activity from several anatomical structures within the peripheral and central auditory nervous system. ABR is composed of several voltage deflections occurring within the first 15 ms after stimulus onset (Katz, 2002) and consist of 5 to 7 peaks or waves that labeled using Roman numerals (Hood, 1998). ABR test offer several advantages such as assisting in identifying neurological abnormalities in the eight cranial nerve and auditory pathways of brainstem. Besides, ABR is also useful to estimate the hearing sensitivity of patients who cannot give valid or reliable hearing threshold using behavioral methods (Hood, 1998). Despite its advantages, the ABR has one obvious limitation. The ABR is a time consuming procedure because several number of ABR signals need to be averaged and collected in order to eliminate noise from other related activities. The process is called signal averaging function to improve the ABR signal to noise ratio (SNR) thus improve the likehood of ABR detection.
One of the possible solutions to reduce ABR acquisition time is by using high stimulus repetition rate. High stimulus rate cause time between presentations of stimuli equals to limit of ABR conventional averaging responses about 20 ms. However at rates faster than limit, conventional averaging ABR response will overlap and interfere to one another and causing distorted and useless final averaged (Eysholdt & Schreiner, 1982). Hence, Maximum Length Sequences (MLS) paradigm was introduced in order to increase the stimulus repetition rate that cannot be achieved using conventional averaging. MLS is a pseudorandom binary sequence that allow for each subsequent stimulus to be presented before the response of previous stimulus has completed. It works by process of deconvolution where response recorded using MLS stimulation can be extracted from the multiply overlapped combination of responses that will be generated by the stimulus sequence. Previous studies showed that high stimulus repetition rate through MLS generally can improve the ABR testing time compared to the conventional stimulus rate. However, by increasing the stimulus rate neural fatigue may cause poorer signal to noise ratio (SNR). High stimulus repetition rate with poorer SNR may lead to difficulty for Audiologist to detect ABR waveform with poor morphology. Thus, presenting the stimulus very quickly does not turn out to be that efficient as the clinician need to present more stimulus to get better SNR (Lasky R.E., Shi Y., & Hecox K.E., 1992; Marsh R.R., 1992; Bell S.L., Allen R., & Lutman M.E., 2000). However most of the literatures used an ideal SNR as a baseline instead of minimal SNR to conclude that the high stimulus repetition rates through MLS paradigm is not a beneficial tool to improve ABR testing time. In reality, Audiologist detects the
ABR waves based on its visibility rather than looking at the SNR value. Therefore there is a possibility that MLS can improve the ABR testing time by looking to other perspective such as Audiologist detection instead of looking at SNR. 1.2 Contributions of the study to the body of knowledge This study contributed to the body of knowledge in audiology field specifically in auditory electrophysiology part involving ABR test using MLS technique. Currently, there is no study was conducted concerning from linear and non linear MLS with subjective detection. This study is the first report on the following areas: 1. In measuring the improvement of MLS testing time in infants. 2. Improvement in testing time provided by new non linear algorithm for infant.
2.1 Description of Auditory Brainstem Response
Auditory Brainstem Response (ABR) is an evoked responses occur within the first 15 ms after stimulus acoustical or electrical stimulation consist of a series of 5 to 7 peaks or waves that represent neural activity at several anatomical sites (Hood,1998). The ABR response is recorded by the placement of the electrode that is attached on specific part of the head. ABR waves series are labeled based on the Roman numeral from I till VII. In general, wave I represent the neural activity of distal or peripheral portion of auditory nerve (Hood, 1998; Hall, 1992).Wave II is generated by the neural activity from the proximal eight nerves as it enters the brainstem (Hood, 1998). However due to the factor of age, wave II maybe absent in recording children as shorter eight nerve length (Hall, 1992). Meanwhile, wave III is contributed by the neurons in the cochlear nucleus and possibly other fibers that entering the cochlear nucleus (Hood, 1998). For wave IV, studies suggest that the third-order neurons likely involved the superior olivary complex. Other contributions of wave IV are the fibers at the area of cochlear nucleus and the nucleus of the lateral lemniscus (Hood, 1998). Wave V representing the neural activity in the lateral lemniscus and/or inferior colliculus (Hood, 1998).
2.2 Stimulus factor Both latency and amplitude of the early evoked potentials can be affected by manipulating the stimulus. Thus, it is necessary to obtain the best possible response and correctly interpret test result by using an optimal stimulus parameter.
2.2.1 Type of stimulus There are various types of stimulus used in ABR recording. ABR is best elicited by stimulus with brief onset due to its high dependency to the neural synchrony. Therefore, clicks are favored as a stimulus because it nature of abrupt onset and broadband spectrum that can elicit good neural synchrony at broader region of frequencies in basilar membrane (BM) and make it produce a robust response when measuring from the scalp (Katz, 2002). The neural synchrony occur in BM is based on the cochlear onset neuron in auditory nerve. Onset chopper (Oc) neurons have very specialized membrane properties and precise temporal processing. Hence, Oc neurons exhibit a wide dynamic range and robust firing to broadband stimuli and it suggested that they may have a role involved in signal processing in noise and in the detection of spectral cues related to sound localization (Mulders et al., 2007). Tone burst is another type of stimulus with brief onset that has specific frequency where it activates the restricted part of the basilar membrane in the cochlea by using certain stimulus envelope. However, due to its fast rise and fall time it has high tendency to have spectral splatter effect especially in low frequency tone burst that causing of unwanted contribution from high frequency region of the BM (Hall, 1992).
2.2.2 Stimulus polarity Polarity is referred to the direction or movement of transducer diaphragm in relation to the pressure wave generating at tympanic membrane. Condensation, rarefaction and alternating are three categories of stimulus polarity. Condensation polarity occurs when movement of transducers diaphragm is towards the tympanic membrane producing positive direction of sound wave, whereas movement of transducer away from tympanic membrane is called rarefaction polarity (negative direction). Alternating polarity is switching mode between both two polarities (Hall, 1992). Garga et al. (1991) reported that polarity will affect latencies for stimulus conditions response dominated by low frequency energy whereas at the high frequency energy there are no such affect were observed. These finding are completely consistent with behavior of individual hair cells and neurons within the auditory pathway true for normal hearing. According to Fowler et al. (2002), rarefaction clicks are expected to produce shorter latencies and greater amplitude for ABR compared to condensation. In contrast, Stockard et al. (1979) noted that 15% to 30% of normal subjects may show the opposite polarity pattern where shorter latency values for condensation than for rarefaction clicks (Hall, 1992). Even though there are arguments whether to use rarefaction or condensation stimuli. Study showed wave V amplitude tends to be larger in response to condensation stimuli but there is no significant latency difference in wave V latency to rarefaction or condensation stimuli in normal hearing individuals (Hood, 1998). This study used
condensation stimuli. It is based on presence or absence of wave V with subjective detection to identify the time. 2.2.3 Stimulus intensity The site of ABR generation along the basilar membrane is related to intensity. In general principle, ABR latency decreases and amplitude increases with greater stimulus intensity. There are two reasons behind it. Firstly, it is due to progressively faster rising generator potential within the cochlea causing similarly faster development of excitatory postsynaptic potentials (EPSPs). Besides, shorter travel time from the oval window to basal end occur if use high stimulus level resulting shorter latency of ABR waveform (Hall, 1992). This was supported by Picton et al. (1981) where high stimulus level between 75 dBnHL to 95 dBnHL may activate the basal part and then moves progressively toward apex for lower intensity level when decrease from 70 to 80 dBnHL (Hall, 1992). Only wave V is clearly visible whereas the earlier component tends to become indistinguishable at 35 dBnHL (Hood, 1998).
2.2.4 Stimulus rate Stimulus rate is defined as number of stimulus presented in one seconds. Rate need to be presented more than the duration of ABR which are more than 15 milliseconds (Hall, 1992). In general principle, stimulus repetition rates up to approximately 20 clicks per seconds have little effect on the ABR. However, above that rate the latenc