There has been interest in the importance of fusion threshold as a measure of temporal resolution for a number of years. Temporal resolution, an element of temporal processing, is the ability of our auditory system to follow the rapid changes that happen in sound in speech, music and other environmental sounds. Essentially, if we are unable to track these changes in sound we would be said to have a temporal processing deficit. This would usually link closely with challenges in the normal development of listening, reading and language skills.
Robert Keith, Ph.D*, developer of a number of auditory processing assessments including SCAN-3 discusses the importance of measuring auditory fusion as one of the methods for assessing temporal processing disorders.
Auditory fusion is measured by listening through headphones to two tones that are presented very closely together. The gap between the two tones is known as the Interstimulus Interval or ISI and is measured in milliseconds. As the ISI reduces there comes a point when we perceive the two separate sounds as being only one sound. The two sounds have been fused in our auditory perception. This is the auditory fusion threshold. Ideally, according to Dr. Keith, anyone with normal temporal resolution abilities over about 8 years of age will be able to perceive two separate sounds even with a gap as small as 8 milliseconds. If we hear two sounds that are, say 15 milliseconds apart, as one sound, this can affect how we process sounds in everyday life.
In speech, a ‘b’ and a ‘d’ can sound very similar and if we have difficulties with temporal processing, we may not be able to hear the difference between the sounds in speech spoken at a normal pace. In a stream of speech we may only hear the end of the sound as it becomes fused with other sound heard before or after. You can understand why such challenges will lead to problems with listening, literacy development and strong speech and language skills.
Auditory fusion is one of a number of important temporal processing measures that should be screened if we wish to understand how well our basic auditory processing is performing. Other measures include temporal order, duration pattern and rhythm abilities. Time is the key to the processing of sound as sound only exists within the framework of time.
TAVS, the Test of Auditory and Visual Skills assesses auditory fusion and, along with other subtests in TAVS, can really help you to understand the unseen auditory processing skills of the children and adults you may be working with. As TAVS also offers a visual fusion screening test and many other areas of auditory and visual screening, it is a fundamental tool to be used with any children and adults with literacy, listening, speech and other sensory challenges.
*Keith, R. Online resource http://www.audiologyonline.com/articles/auditory-fusion-test-revised-1232