Temporal Processing
Temporal processing refers to the rate at which an individual’s auditory system can process, analyze, and interpret acoustic stimuli across a period of time. This ability includes not only the awareness of stimuli, but also the perception of sound modifications (Shinn, 2003). In order for an individual to develop adequate speech, language, and listening skills, temporal processing is critical. Understanding speech in both quiet and noise backgrounds demands temporal processing skills because the signals fluctuate over time (Geffner & Ross-Swain, 2013).
A variety of aspects fall within the domain of temporal processing, including temporal ordering/sequencing, temporal resolution/discrimination, temporal integration/summation, temporal masking, temporal asynchrony, binaural temporal processing, and more. The first aspect of temporal processing is temporal ordering/sequencing, which is one’s ability to process a sequence of stimuli in the correct order. This is affected by the amount and manner of stimuli presented, among other things. Temporal resolution/discrimination is another aspect of temporal processing. This is the shortest interval during which an individual can differentiate between two separate acoustic signals. For normal listeners detecting brief stimuli, this interval is generally 2 to 3 msec (Shinn, 2003); for individuals with compromised temporal processing abilities, gaps of 8 ms or longer are often necessary for detection (Lister, Roberts, Shackelford & Rogers, 2006). Temporal resolution is often assessed using gap detection threshold (GDT) tests, which assess one’s ability to perceive a silent interval, or gap, between two stimuli (Tyler, Summerfield, Wood, & Fernandes, 1982). Temporal resolution and gap detection threshold have been widely investigated and will be further discussed later in this paper.
A third aspect of temporal processing is temporal integration, which is the “summation of neuronal activity” or improved detection that occurs after a stimulus is presented for a longer duration (Shinn, 2003, p. 52). Temporal masking, which is a fourth aspect of temporal processing, occurs when a threshold of one sound is shifted when in the presence of another stimuli. Temporal difference limen and gap difference limen are two other aspects of temporal processing. Temporal difference limen refers to the amount of change necessary for an individual to detect a durational change of a signal. Similarly, gap difference limen is the amount of change required to detect a difference in the duration of a silent period, or gap, that occurs between two stimuli (Tyler et al., 1982). When functioning properly, all of these aspects, among others, collaborate to allow individuals to process auditory stimuli and understand speech effortlessly.
Temporal Resolution & Gap Detection
As previously stated, temporal resolution is the shortest time during which the human auditory system can detect a separation between two signals. Lister et al. (2006) describe temporal resolution as “the ability to follow and resolve rapid fluctuations in intensity and frequency over time,” (p. 133). Temporal resolution is a key aspect involved in comprehension of degraded speech, including speech in background noise. It is frequently measured through gap detection tests, but also through temporal modulation detection and gap discrimination (Geffner & Ross-Swain, 2013). GDT is essentially the minimum detectable duration of silence that occurs between two stimuli in the presence of noise (Tyler et al., 1982). During GDT tests, subjects are generally asked to indicate when a period of silence is detected within the noise (Shinn, 2003). Interestingly, the closer a stimuli is presented to an individual’s auditory threshold, the longer duration necessary for gap detection. As a stimulus is presented at louder levels, the duration of gap detection decreases (Geffner & Ross-Swain, 2013).
How Temporal Processing Occurs
Kopp-Scheinplug & Tempel (2015) discuss the how temporal processing and encoding is accomplished primarily by auditory brainstem neurons. Other cells are also involved, including spherical bushy cells and principal cells located in the ventral cochlear nucleus (VCN) and medial nucleus of the trapezoid body (MTB), respectively. These cells are able to process incoming signals quickly and accurately in order to produce the correct outgoing signals. Hall, Johnsruddel, Haggard, Palmer, Akeroyd, & Summerfield (2002) suggest that these regions are more widely activated when incoming stimuli are temporally complex. Additionally, as signals become more complicated, stimulation spreads to areas beyond auditory regions.
Effects of Inadequate Temporal Processing
Insufficient temporal processing abilities, specifically temporal resolution, can cause difficulty separating transient or rapidly presented sounds, as found by Hill, Hartley, Glasberg, Moore & Moore (2004). Additionally, some researchers suggest that temporal analysis (as measured by psychoacoustical tasks) is poorer in listeners who have hearing impairments that are cochlear in nature (Tyler et al., 1982). Overall, Geffner & Ross-Swain (2013) summarize that speech perception abilities, both in quiet and in noise, are degraded in individuals with temporal processing problems.
Hill et al. (2004) indicated that children have a broader auditory temporal window than adults, often causing inefficient temporal processing in children. This can explain why children do not perform as well as adults on gap detection and other temporal tasks, and why auditory processing disorders are detected predominantly in children (although they also occur in adults).
Means of Assessing Temporal Processing
A variety of tests are in the battery for assessing temporal processing. Shinn et al. (2003) state that Frequency and Duration Pattern Tests, which mainly assess temporal ordering abilities, are the most commonly utilized means of assessing temporal processing clinically. Another test is the Random Gap Detection Test (RGDT), which uses broadband noise to assess gap detection abilities. Lister et al. (2006) suggest that the Adaptive Test of Temporal Resolution (ATTR) is a more precise clinical tool than the RGDT. Other tests that can assess temporal processing are the Gaps-in-Noise (GIN) Test and Time-Compressed Speech. Despite a variety of clinical tests available, it should be noted that there is not currently a “gold standard” in assessment of temporal processing (Domitz & Schow, 2000).
It is of utmost importance for temporal processing difficulties to be detected and treated promptly, especially in pediatric populations. Timely intervention for children with temporal processing deficits helps curtail possible speech and language delays, and also improves auditory processing in noise and other difficult listening situations (Geffner & Ross-Swain, 2013).
Auditory Processing Disorders
An auditory processing disorder (APD) is diagnosed when an individual exhibits normal pure tone hearing sensitivity, but poor auditory performance in other areas. Kopp-Scheinpflug & Tempel (2015) state that individuals with APDs “encapsulate a variety of impediments that influence the way complex acoustic signals are processed by the brain,” (p. 214). APD is not the result of other language, cognitive, or social issues. It should be noted that although auditory processing disorders are most often discussed in relation to pediatric populations, they occur in people of all ages and adults are also at risk for auditory processing deficits. Interestingly, according to Shinn (2012), APDs occur in only about 2% of pediatric populations but in about 70% of elderly people, especially those who have neurological defects.
Some auditory difficulties faced by patients with auditory processing disorders include localization, sound discrimination, speech comprehension, temporal processing, and following directions, particularly in noisy or reverberant environments. Specifically, children with APD may experience difficulty developing speech, language, and literacy, while adults with APD may have difficulty understanding speech (Geffner & Ross-Swain, 2013). Current means of defining, identifying, evaluating, and diagnosing APD are lacking, and there are presently no universal treatment standards (Dawes, Sirimanna, Burton, Vanniasegaram, Tweedy, & Bishop, 2009).
Causes of Auditory Processing Disorders
It is largely unknown what causes auditory processing disorders. According to Dawes et al. (2009), a possible etiology of APD is an interruption of sound processing that occurs due to irregularities in the central auditory nervous system (CANS). A more controversial explanation is that APD is the result of attentional, learning, or language problems rather than a disorder in itself. Some research also suggests that prematurity, chronic otitis media, or head trauma may also contribute to auditory processing disorders.
Diagnostic Criteria
Clinicians currently lack a “gold standard” for assessing and diagnosing auditory processing disorders, although certain test batteries, including the SCAN-C and SCAN-A (intended for children and adults, respectively), are widely used in the U.S. APD is diagnosed when an individual presents with a normal audiogram, unexplained poor listening abilities, and atypical performance on auditory processing tests (Dawes et al., 2009). Auditory brainstem responses (ABR) can also be used as an objective means of assessing the auditory system’s response to speech stimuli (Kopp-Scheinpflug & Tempel, 2015). Significant research is needed to develop a thorough and standardized method of identifying and evaluating APD.
Temporal Processing Abilities in Individuals with Auditory Processing Disorders
A clear link has been found between auditory processing disorders and temporal processing abilities. One of the key auditory features affected in individuals with auditory processing disorders is temporal processing (Lister et al., 2006). Shinn et al. (2003) explain that although the exact processes involved in temporal processing disorders remain unknown, it is widely accepted that temporal skills are an essential and fundamental aspect of nearly all auditory processing abilities. Speech understanding, language acquisition, and reading development may be negatively impacted in children, and adults with temporal processing deficiencies will also undergo difficulties with speech perception and amplification (Geffner & Ross-Swain, 2013). Specifically, Dawes et al. (2009) suggest that an underlying cause of APD may be insufficient temporal coding.
In individuals with normal processing abilities, auditory processing occurs at incredibly rapid rates. However, when an individual with auditory processing deficits has difficulty with temporal resolution, temporal summation, or temporal sequencing, temporal processing as a whole becomes significantly delayed and inefficient (Kopp-Scheinpflug & Tempel, 2015). Other difficulties involved in APD, including figure-ground testing, or listening in noise, and auditory memory, are also rooted in temporal processing abilities (Dawes et al., 2009). All of these factors help explain why a patient who presents with temporal insufficiencies is often suspected of having an APD (Jerger, 1998). Any impairment or disruption in temporal processing can result in auditory processing deficits of varying degrees.
The Means By Which Temporal Processing Underlies Auditory Processing Disorders
As previously stated, adequate temporal processing is essential for appropriate auditory processing abilities (Shinn, 2003). Kopp-Scheinplfug & Tempel (2015) discuss the mechanisms of how the cochlea and CANS process sounds temporally. When the cochlea processes complex stimuli, sounds are characterized by frequency and location of basilar membrane activation. The CANS, specifically the ascending pathway, is then able to rapidly process and interpret the acoustic environment, which is essential for accurate temporal processing abilities. Because the neurons of the ascending pathway have wider diameters and thicker myelin than other neurons, they are able to ensure precise processing of temporal cues by maintaining the firing patterns of action potentials. The timing of these firing patterns are critical for temporal processing, and “any degradation of action potential timing within and/or past the cochlea is likely to impair auditory processing,” (Kopp-Scheinpflug & Tempel, 2015, p. 214). In other words, even a minor disruption of action potentials can have significant effects on temporal processing, and possible interruptions may contribute to auditory processing disorders.
Aside from neuronal firing patterns, certain genes may also be a contributing factor for irregular temporal processing in individuals with auditory processing disorders. Proper functioning of ion channels and neurotransmitter release requires coding by specific genes. If any of these related genes are mutated or not functioning properly, temporality, speech perception, and localization can be diminished. Atypical action potential firing rate and genetic mutations are possible explanations of how temporal processing deficits underlie auditory processing disorders. Additionally, when analyzing at the cellular level, latency and jitter of auditory nuclei firing “are ideal markers to detect auditory processing dysfunction,” (Kopp-Scheinpflug & Tempel, 2015, p. 215). This is a significant finding, which can help researchers understand the micromechanisms of temporal processing in individuals with auditory processing disorders.
Assessing Temporal Processing in Individuals with Auditory Processing Disorders
When evaluating auditory processing disorders, researchers have often utilized linguistic tests, including the SCAN-C and SCAN-A, to assess auditory processing in general. Nonlinguistic measures should also be used as a supplement. Means of analyzing temporal processing specifically include Gap Detection Tests, Duration Pattern Tests, Frequency Pattern Tests, and the Adaptive Test of Temporal Resolution (Dawes et al., 2009). Lister et al. (2006) discuss the importance of APD screening batteries employing temporal gap detection measures. The authors suggest use of broadband noise as opposed to other stimuli, and also promote use of the Adaptive Test of Temporal Resolution (ATTR), which was a new clinical test at time of publication. A decade ago, the authors discussed that the ATTR “promises to be a clinically feasible addition to the APD test battery,” (p. 139), especially for assessing temporal processing. Today, this statement holds true, as the ATTR is now a widely used clinical tool for assessing temporal processing abilities in individuals with APD. Shinn (2012) notably specifies that different temporal tests each only assess one aspect of temporal processing, and there is not yet one comprehensive test to assess all four sub-components of temporal processing.
Conclusions and Future Considerations
Auditory processing disorders occur when an individual present with impaired auditory processing and perception, including diminished localization, speech comprehension, and temporal processing abilities, in conjunction with normal pure tone hearing sensitivity. Temporal processing refers to the central auditory nervous system’s ability to process and decipher auditory stimuli over time. It is widely accepted that temporal processing is a critical, underlying aspect of auditory processing abilities, and that individuals with APD often have impaired temporal processing abilities. Temporal processing is so critical for auditory processing because it provides the foundation for appropriate speech, language, listening, academic, and social skills.
Early identification and treatment of auditory processing disorders is of the utmost importance in order to prevent language and academic delays. However, clinicians face a major obstacle due to the fact that there is no “gold standard” for the assessment and intervention of APD. Additionally, diagnosis is further complicated by the fact that APD often presents very similarly to specific language impairment (SLI), attention deficit hyperactivity disorder (ADHD), and dyslexia (Dawes et al., 2009). Since APD in itself is not entirely understood by professionals, accurate diagnosis and treatment becomes problematic. Similarly, although various clinical tests for assessing temporal processing do exist, there is no universal or standardized testing battery to analyze patients’ temporal abilities at this time.
In terms of intervention, there are not many evidence-based treatments for APD presently. Auditory training, speech therapy, sound discrimination exercises, and assistive listening devices to improve signal-to-noise ratio are often the utilized for management of APD. Newer literature suggests that using mobile application (apps) focusing on temporal processing skills may be effective platform of service delivery supplemental to treatments by an audiologist or speech-language pathologist (Geffner & Ross-Swain, 2013).
There are clear gaps in the literature involving both auditory processing disorders and temporal processing, and further investigation is certainly needed. A more thorough differential diagnosis must be developed to distinguish APD from and SLI, ADHD, or dyslexia, and a “gold standard” diagnostic test battery must be developed. In terms of temporal processing, further investigation is needed into the exact genes and neuronal firing patterns that contribute to atypical temporal processing. Individuals with auditory processing disorders, which are rooted in temporal processing deficiencies, will likely have a better prognosis once the exact mechanisms of temporal processing are better understood.