Auditory Steady State Response
Non-behavioral tests became in use in the 1970’s with the development of the auditory brainstem response (ABR). The testing method became popular in the 1980’s and revolutionized audiometric testing methods. The method however had its weaknesses: it is often difficult to record and record near threshold levels, it provides limited information regarding the nature of hearing loss, it requires extensive multiple testing and interpretation of response are subjective (Cone-Wesson et al, 2002).
Another testing method that became popular was through otoacoustic emissions (OAE’s). In the 1990’s, OAE’s were utilized to asses frequency-specific responses of outer hair cells but the test can not be utilized to establish the degree of hearing loss (Batterjee and Dutt, 2005, p. 2).However, the test has proven its use in hearing screening for infants and its still is used extensively today due primarily to its high degree of objectivity in testing cochlear function. To address the limitations of ABR and OAE, a more comprehensive testing methodology was evolved and the result was the development of ASSR.
Though auditory steady-state response (ASSR) testing is still in its infancy, it has already shown itself to be a promising audiometric diagnostic and clinical tool. Acoustic reflexes or behavioral measures in audiometric testing may be problematic with patients who are unable to provide deliberate response to stimuli such as infants, mentally disabled and otherwise incapacitated, whether deliberate or not (Batterjee and Dutt, 2005). In these situations, there is a need for a dependable audiogram to reveal true hearing thresholds across various frequencies to facilitate diagnosis and treatment.
Delineating of Auditory Steady State Response
Auditory Evoked Potential (AEP) stimulates the brain to react to auditory stimuli that can either be transient or steady-sate in nature. Transient AEP’s such as ABR’s respond to stimuli momentarily and infrequently. The frequency of responses may also vary during testing with repetition. Steady-state AEP is a reaction to recurring stimuli that are sustained by a continuous amplitude and phase during testing. The result is a response whose waveform from the brain is identical to the waveform of the stimulus creating a cycle of stimulus and response explicit to the testing frequency or a steady-sate evoked potential (SSEP) known as ASSR (Hall, 2000).
ASSR traces its development from the 40Hz event related potential (ERP) studies conducted in the 1980’s is a steady. 40Hz ERP is an example of steady-state response produced through stimulation at the rate of 40 times per second (Batterjee and Dutt, 2005, p. 4). The method promised to be a dependable estimation tool to define low frequency hearing thresholds which was a limitation of ABR. The drawback to it however was its vulnerability to sleep and attention preconditions and undependable application to newborn testing (Hall, 2000).
Researches in the 1990’s suggested the use of higher modulated audiometric systems. One of the main objectives was to develop steady-state response tools that can estimate frequency-specific hearing in infants which were estimated to be over 60 Hz (Batterjee and Dutt, 2005). Also, there were evolving studies on how to utilize multiple stimuli concurrently to both ears to streamline testing and assessment methods (John, 2003). These efforts led to the foundation of ASSR.
The main benefit of ASSR system is its capacity to make a distinction of levels of hearing loss. Knowledge of hearing loss profundity is essential in treatment selection as well as technical aspects related to implants and other hearing aids. It can provide information for amplification needs of individuals in relation to their hearing capacity accurately (Picton et al, 2002).
Bringing together ASSR evaluations and behavioral analysis or methods can also facilitate the effectiveness of therapy or rehabilitation of patients ensuring them maximization of existing hearing capacity or applied tools. Efficiency in testing is also achieved since ASSR requires less time than ABR testing (Batterjee and Dutt, 2005, pp. 5-8). It also been applied in testing babies at birth hastening testing and treatment for infants improving chances for auditory function (Luts, Desloovere and Wouters, 2006).
Furthermore, according to Batterjee and Dutt, ASSR does away with subjective interpretation of responses as well as satisfy the prescribed requirement of the Joint Commission on Infant Hearing (JCIH) and the American Speech and Hearing Association (ASHA) (2005, p.1-9).
Recent studies have shown that ASSR’s are not as accurate as they have claimed to be and may differ significantly form actual behavioral thresholds (Pastor et al, 2003). It Ted Venema’s studies have also shown that ASSR’s for several carrier frequencies, in particular those at 30 dBHL, can be missing in individuals with normal-hearing (2004).
ASSR still needs further studies to define its use and implication to the field. Further studies should be done in particular with higher frequencies as well as studying external factors that can influence responses.
Multiple Stimuli Application
As previously mentioned, multiple stimuli application became a focus of researches in 1990’s consequent to studies to explore steady-state response. Studies in multiple stimuli application aim to enhance threshold estimation (Lins & Picton, 1995).
One of the challenges in dealing with multiple application stimulation or synchronized ASSR and behavioral threshold testing is the difficulty in identifying and observing different generators without signal-to-noise and multiple electrode use (Picton, 2002, pp. 25-26.)
According to a study of auditory steady-state responses to multiple simultaneous done by Lins and Picton, “steady-state responses can follow multiple simultaneous auditory stimuli” (1995, pp. 430-432). If the stimuli used is modulated at varying rates, exclusive responses can be exhibited and can de identified within frequency domain of the spectral component that is consistent or associated with the rate of modulation. Considering this, it points out that steady-state response can still be monitored even if there are multiple stimuli modulations (pp.425-428).
In another study focusing on multiple auditory steady-state responses to amplitude-modulated (AM) and frequency-modulated (FM) stimuli showed that reaction to AM occur later than FM responses. The study also experimented on different levels of modulation to study changes in response. It showed that resulting mixed modulation (MM) elicit independent response and that the most significant MM response was evident when the maximum frequency of MM is used right after the use of the maximum amplitude MM (John et al, 2001)
Multiple stimuli testing can show relationship of not just stimuli but responses to advance cognitive studies as well as provide applications for improvement The application of multiple ASSR is an indication of the motivation to comprehensively study auditory responses not only as a clinical study but also to recognize the reality of people’s hearing experience. The understanding of the effects of multiple stimuli also expands current methods of audiometry and allows for the development of studies that can lead to improving methods today.
Observation and Recording
Steady-state responses observation has allowed scientist the opportunity to study responses better than ABR. This has led to a better understanding of responses and stimuli and the requirementents for treatment of auditory conditions.
Recording of the responses may vary subtly between single stimuli and multiple stimuli but the same basic methods are used. There is no significant difficulty in recording multiple responses since they can be tracked by their specific variations.
Recording parameters of ASSR have not yet been established but generally follow standards and principles previously utilized in ABR and OAE recording. Recording is done through the use of electrodes, differential amplification, signal averaging and filtering set lower than those used for ABR (Venema, 2004). The left hand side of Figure 1 shows that ASSR’s are elicited; it is characterized by strong responses and strong clustered vectors as well as uniform phases. On the other hand, if there is no ASSR response, as shown by the right hand side of Figure 1, the vectors are short and arbitrarily spread out.
Increasing response using a differential amplifier can be done through the placement of electrodes that transmit varying voltages. Actual ASSR waveforms are measured in nanovolts which ease their observation as well allow for multiple methods for recording and presentation. ASSR’s utilizes single-channel vertical electrode montage that allows the testing of both ears separately without necessitating adjustment of electrodes on test subjects (Venema, 2004; Dimitrijevic, 2004).
Methods for Multiple Auditory Steady-State Response
The multiple auditory steady-state response (MASTER) techniques makes available the concurrent observation and assessment of four varying tonal frequencies in each ear (John & Picton, 2000). It makes use of 1 Hz-300 Hz filters to coincide with ASSPR spectral energy. The methodology allows for detailed evaluation of specific hearing thresholds of infants who failed newborn hearing screening test.
The stimuli specific responses are mapped individually and are used to identify specific responses in multiple auditory steady-state tests. The modulation, frequency and spectral components are used to consider and evaluate responses. A factor that can enlarge multiple-response recording time is the minute size of responses recorded at 500 and 4000 Hz but increasing intensities of stimuli can augment responses without influencing other responses (John et al, 2002).
Uses Auditory Steady State Response
ASSR has a lot of potential and application in evaluating aided and unaided hearing thresholds. It promises to be an important tool for professionals in testing and allows for various applications to several fields of study (Venema, 2004). Integration and standardization of ASSR will enhance its addition to customary audiologic procedures (Batterjee and Dutt, 2005). But most of all, continuous studies and application of ASSR and applications will establish its importance in the field of Audiology.
Infant Hearing Screenings
Studies done to compare ASSR and click-evoked ABR and behavioral hearing thresholds showed that broadness of the range in infants. Dichotic multiple-stimulus ASSR technique provided an important diagnostic tool to asses the hearing-impaired children and provided implicit details for rehabilitation after neonatal hearing assessment (Luts, Desloovere and Wouters, 2006). In 1994, the UNHS stipulated that all infants diagnosed with impaired hearing must be ascertained before they reach the age of three months and to receive treatment by their sixth month. This is to highlight the need for opportune treatment and rehabilitation (Batterjee and Dutt, 2005).
Compared to ABR and OAE, ASSR is able to meet JCIH and ASHA standards regarding hearing assessment (Hall, 2000). ASSR has allowed for the development of specific rehabilitation programs for infants due to proper evaluation of degrees of hearing loss. Though ASSR may become unreliable and difficult to observe with low modulation rates, it still is the most suited in testing infants and also with adolescents and children with hearing and hearing aids (Lins & Picton, 1995; Picton et al, 2002)
ASSR has been utilized to study auditory steady state evoked potential (SSEP) both for natural and sedated sleep (Hall, 2000). The study showed that amplitude diminished considerably during sleep but consequent phase variability did not change considerably (Jerger et al, 1986). It suggested the use of phase variability of SSEP as a method to assess and estimate threshold sensitivity of patients.
According to Ted Venema (2004), ASSR’s can actually be more extensivelsy observed during sleep. He attributed this to the individual’s relaxed state that enhances ASSR documentation.
Schizophrenia Related Studies
ASSR has also been used to schizophrenia, schizotypal personalities and similar disorders. Amplitude-modulated tones were utilized to assess steady state AEP use with individuals diagnosed with schizophrenia, schizoaffective disorders (Picton, 2002).
The consequent analysis of evoked magnetic fields showed the deficiency of steady-state responses to recurring stimuli which revealed neural synchronization irregularities that can affect perception and cognitive assimilation in patients (Lindsay & Norman, 1972; Picton, 2002).
ASSR does not involve voluntary response and has become a promising tool in monitoring patient health during surgery. It also has a promising application in monitoring brain function in comatose patients, the mentally disabled or in assessing anyone unable or unwilling to reveal actual hearing capacities (Venema, 2004).
ASSR monitoring acts as a safeguard to ensure proper anesthetic is applied to patients. ASSR checks ensure that patients are not in any distress even when they are not exhibiting any sign of consciousness.
ASSR can also be utilized to study effects of extreme conditions to brain function or to ASSR itself. An evaluation of ASSR changes due to manipulated conditions can serve as reference for overall neural response changes.
For example, a study conducted by Lucertini, Verde and De Santis tried to evaluate ASSR variations after exposure to hypobaric hypoxia. Their study showed that exposure resulted to a phase shift in ASSR sinus wave after two exposures. Their study pointed out to the vulnerability of the central acoustic pathway to hypoxia and it was concluded that the auditory system organ functions can be significantly impaired by repeated exposure to hypoxic conditions (2002, pp.110-113). Further studies are being done today to study effects on other sensory preceptor organs as well as the effect of other extreme conditions.
Rehabilitation and Treatment
Because of the greater specificity in hearing impairment that ASSR affords during assessment, treatment and rehabilitation can be made more specific for patients. This is critical not only to afford care but also to prevent further hearing loss of debilitation.
ASSR has significantly contributed to the level of amplification in developing hearing aids (Picton et al, 2002) and understanding of sensory response systems during schizophrenic and psychotic attacks (Picton, 2002; Pastor et al, 2003)
Auditory steady-state response has opened up new possibilities for the filed of Audiology as well as incorporation of principles in other fields of study. ASSR combined with established techniques and application of ABR and OAE can further develop the filed audiometric techniques and applications.
ASSR method flexibility should also be tempered with the need to create standards and procedures for its use so that it can be utilized as a universal tool. One of the strengths of using ASSR is the variety of methodologies for its use however the prevailing methods should take in consideration diagnostic requirements and information for use in rehabilitation and treatment. ASSR has proven itself to be definitive but at the same time a flexible diagnostic and research tool.
The insight into human perception that ASSR affords us today is essential in developing a comprehensive understanding of not only human function but also experience as a whole. It has been shown that ASSR as an AEP evolved to be more responsive to clinical and research studies than ABR and OAE. The increasing application of ASSR not only expands Audiology but also our understanding of ASSR into everyday life. ASSR provides a new method of understanding response mechanisms as well as furthering the neurological studies that can enhance function and performance and ability.
ASSR’s objectivity, frequency specificity and accuracy clinical assessment of pediatric, disabled and other special circumstanced patients has allowed for care and rehabilitation despite difficulties associated with these patients. It has allowed for the development of better treatment and rehabilitation programs to suit their specific conditions that would otherwise been impossible. It has allowed preventive protective care for the hearing impaired especially in infants and young children who could still recover hearing capacities. Therefore, ASSR should be studied and developed because it has already shown that it is a useful tool to improve quality of life.
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