The Auditory Steady State Response (ASSR) is an objective electrophysiological test used to evaluate hearing. This test measures the brain’s electrical activity in response to sound, without requiring any conscious participation from the individual being tested. It is a non-invasive procedure designed to estimate a person’s hearing abilities, particularly their hearing thresholds. ASSR helps professionals understand how well someone hears different pitches and volumes by analyzing brain responses to specific auditory stimuli.
Understanding Auditory Steady State Response
ASSR operates by measuring the brain’s sustained electrical activity in response to rapid, repeated auditory stimuli. When the ear receives a sound, it converts vibrations into electrical signals that travel along the auditory nerve to various brain regions involved in hearing. If sounds are presented quickly and consistently, these brain regions generate a continuous, “steady state” electrical signal that mirrors the rhythm of the incoming sound.
Why ASSR is Used
ASSR is particularly useful for objectively assessing hearing in individuals who cannot provide reliable behavioral responses during traditional hearing tests. This includes infants, young children, individuals with developmental delays, or those who are sedated for medical procedures. The test can estimate hearing thresholds, which are the softest sounds a person can hear at different pitches.
An advantage of ASSR is its ability to provide frequency-specific information. It can assess hearing sensitivity at various frequencies, such as 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz, which is important for accurately diagnosing hearing loss and fitting hearing aids. This objective measurement helps guide treatment plans, especially in very young children, to support their speech and language development.
The ASSR Testing Process
During an ASSR test, the patient typically lies or sits comfortably in a quiet room, and it is beneficial for them to remain still or asleep for accurate results. Small electrodes are placed on the scalp, usually on the forehead and behind each ear, similar to an electroencephalogram (EEG). These electrodes are connected to an amplifier and a computer that records the brain’s electrical responses.
Soft earphones or insert earphones are then placed in the patient’s ears to deliver sounds. These sounds are continuous, modulated tones that vary in frequency and intensity. An advantage of ASSR is its ability to test multiple frequencies simultaneously in both ears, which can make the process more efficient, often taking about 20 to 25 minutes for eight frequency thresholds.
Interpreting ASSR Results
The brain responses measured during an ASSR test are processed by computer software using statistical detection algorithms to determine if a response is present at specific frequencies and intensities. This analysis helps to objectively identify the softest sounds that evoke a consistent brain response, effectively estimating the individual’s hearing thresholds. These estimated thresholds are often plotted on a graph similar to a traditional audiogram.
The results provide detailed information about the degree and type of hearing loss across different pitches. This objective data guides audiologists in diagnosing hearing impairment and developing appropriate treatment plans, such as selecting and fitting hearing aids. While ASSR-based audiograms generally align with behavioral audiograms, correction factors are sometimes applied to account for slight differences.
ASSR Compared to Other Hearing Tests
ASSR is an objective hearing test, similar in some ways to the Auditory Brainstem Response (ABR) test, as both measure electrophysiological responses to sound. Both tests use electrodes placed on the scalp and deliver acoustic stimuli through earphones. However, ASSR utilizes continuous modulated tones, whereas ABR typically uses brief clicks or tone bursts.
ASSR provides more frequency-specific information and can test multiple frequencies simultaneously in both ears. This makes ASSR efficient for comprehensive assessments, particularly for infants or others needing a quiet state. While ABR measures neural responses in the auditory nerve and brainstem, ASSR analyzes steady-state auditory stimuli in the frequency domain, offering precise information about hearing sensitivity across a wider range of hearing levels, including severe to profound hearing loss.