Abstract
Keywords
Sensory integration (SI) therapy is a treatment technique widely used by occupational therapists in the management of sensory processing dysfunction (SPD) as seen in children with autism spectrum disorder (ASD; Lane & Schaaf, 2010). Demonstrating the effectiveness of SI treatment with behavioral measures has produced inconsistent results that are difficult to interpret (Hoehn & Baumeister, 1994; Miller, Coll, & Schoen, 2007; Schaaf & Miller, 2005). Reviews of SI treatment effectiveness occurring between 1980 and 2006 reveal a lack of meaningful outcome measures, homogeneous samples, adequate power to detect change, and methodology that includes randomization and replication of treatment (Baranek, 2002; Hoehn & Baumeister, 1994; Leong & Carter, 2008). Therefore, more systematic research with increased specificity of participant variables (Baranek, 2002) is needed to help distinguish behavioral, motor, and physiological responses to treatment. The need for these studies is further supported by the scientific community through agencies and research groups involved in treatment planning (American Academy of Pediatrics, 2012) or systematic reviews and replication of outcome studies (National Autism Center, 2015).
To ensure continued progress in measurement related to SI treatment effectiveness in children with sensory processing deficits, occupational therapy researchers are using physiological markers to objectively measure children’s response to sensory stimulation. This measure is increasingly being used in studies by researchers who are evaluating the efficacy of SI therapy by occupational therapists.
In the field of occupational therapy, the assessment of physiological response to sensation was first initiated in 1999 with a laboratory paradigm called the Sensory Challenge Protocol (SCP; McIntosh, Miller, Shyu, & Hagerman, 1999; Miller et al., 1999). This protocol is used to measure electrodermal activity (EDA), which refers to changes in electrical conductance of the skin associated with eccrine sweat gland activity innervated by the sympathetic branch of the autonomic nervous system (Fowles, 1986; McIntosh et al., 1999). The two components of EDA measured during the administration of the SCP are skin conductance level (SCL) at rest in the absence of specific stimuli and skin conductance response (SCR) to a specific stimulus. Greater increases in SCR infer heightened arousal in response to sensation (Critchley, 2002; Fowles, 1986; Vetrugno, Liguori, Cortelli, & Montagna, 2003). Although it is known that the SCP yields physiological markers that quantify electrical conductance of the skin at rest and in response to sensations of movement, touch, sound, light, taste, and smell, it is still not known whether these measures are reliable.
Although there has been an apparent increase in use of physiological sensory processing measures as markers to differentiate groups on the basis of response to sensation (Mangeot et al., 2001), to confirm results from behavioral screens for SPD (Su, Wu, Yang, Chen-Sea, & Hwang, 2010), or to show treatment effectiveness (Miller et al., 2007), the reliability of these measures is at best limited. Researchers who assessed reliability of physiological markers (EDA measurement) used small subsets of samples (Miller et al., 1999), measures of association and not agreement (McIntosh et al., 1999), or small sample and no control group (Schoen, Miller, Brett-Green, & Hepburn, 2008), resulting in a wide range of reliability indices (McIntosh et al., 1999; Miller et al., 1999; Schoen et al., 2008). Thus, further critical evaluation and analysis of the test–retest reliability of physiological markers are needed before these markers can be used as indicators of SI treatment effectiveness.
The purpose of this study was to establish the test–retest reliability of EDA to measure sensory processing with the SCP in typically developing (TD) children and children with ASD. Accordingly, one question directed this methodological research investigation: Is the test–retest measure of EDA a reliable measure of physiological sensory processing in children with and without ASD?
Method
Participants
A convenience sample of boys between the ages of 4 and 11 yr with and without a diagnosis of ASD was recruited for the study. Children who had visual or hearing problems, who were unable to follow simple commands, or who were unable to sit for a minimum of 30 min were excluded from the study because these skills are required to participate in the protocol. Ability to sit for 30 min was predetermined via parent or teacher report. Children having additional medical problems or taking medications that affected arousal were excluded to avoid confounding variables that may have affected response to sensation. The primary investigator contacted parents interested in having their children participate in the study. The investigator screened the potential participant during a telephone interview by asking a series of questions regarding type of school attended, medical history, medications, participation in any therapies, and sensitivity to sensations. The study was approved by the institutional review board. Participants signed an assent form, and parents signed a consent form, before participating in the study.
Instrumentation
The SCP, a laboratory procedure designed by Lucy Miller (Miller et al., 1999), was used to measure response to sensation with EDA to assess changes in electrical conductance of the skin on the basis of eccrine sweat gland activity (McIntosh et al., 1999). The primary investigator participated in specialized training at the Sensory Processing Disorder Foundation under the direction of Lucy Miller to run the SCP and to perform data reduction, analysis, and interpretation of EDA measures produced during the laboratory procedure. This integrated laboratory system (PsyLab System; Contact Precision Instruments, Cambridge, MA) along with the measurement of skin conductance procedures recommended by Martin and Venables (1966) and Fowles et al. (1981) were used to collect the data. Details of the data collection process with the PsyLab System have been previously described (Miller et al., 1999). A software program—namely, SAM.EXE (Contact Precision Instruments, Cambridge, MA)—was used to run the SCP. The data thus obtained were reduced with PsyLab 7 (Contact Precision Instruments, Cambridge, MA). Physiological waveform data collected during the testing sessions were converted to numeric lists that were then exported to Excel (Microsoft Corp., Redmond, WA) for analysis.
Procedure
Participants were tested twice within a period of 2–6 wk on the basis of Schoen et al.’s (2008) procedures. The researcher explained the procedures involved in the experiment using lay terminology. The child was introduced to the testing area, a laboratory setting that was designed to look like the inside of a spaceship. Ambient lighting in the room was set to a low level (lux level: 8.0–10.0) throughout the procedure. The child was invited to sit in a sturdy chair with a spaceship control panel in front of him. A video clip of the movie Apollo 13 (Grazer & Howard, 1995) was displayed showing the astronauts as they are hooked up with electrode placement before launch into space. The researcher explained to the child that he too would be hooked up with stickers just like the astronauts before beginning the procedure. As the child watched the video, three electrodes were placed on the child’s chest in a triangular pattern at the base and center of the rib cage. Two smaller electrodes were placed on the left thenar and hypothenar eminences of the left hand (only the hand electrodes were used in relation to the variables presented in this study); self-adherent wrap that was 2 in. (5.08 cm) wide was used to further secure lead placement. When electrode placement was complete, the child was instructed to sit still like a robot, to keep his feet flat on floor, and to keep his left hand palm up resting on the armrest. EDA was recorded continuously during eight conditions (six conditions with stimuli presentations and two conditions with no stimuli presentations) presented in the following order: baseline, tone, visual, siren, olfactory, tactile, movement, and recovery as per the SCP protocol (McIntosh et al., 1999; Miller et al., 1999). The signals were sampled at 1000 Hz, digitized, stored on a computer, and later reduced with PsyLab. The entire laboratory protocol took about 45 min to complete.
Two types of EDA measures were collected and analyzed: tonic and phasic. The two tonic variables were (1) SCL at rest and (2) nonspecific skin conductance response (NSR) in the absence of a specific stimulus. The four phasic variables were (1) SCR to a specific stimulus, (2) mean magnitude of SCR including zero response, (3) mean amplitude of SCR for all nonzero responses, and (4) habituation of the slope of SCL across eight trials (Schoen et al., 2008). Raw data were reduced with standardized PsyLab 7 procedures. Artifact behaviors, such as movements, deep breaths, sneezes, or coughs, were recorded and then filtered from the data.
Statistical Analysis
Using descriptive statistics, we summarized EDA dependent variable data with SCR magnitude, SCR amplitude, SCL, NSR, and habituation. An intraclass correlation coefficient (ICC) for each dependent variable was used to assess test–retest measures of EDA scores under each condition for the total group and for each group separately. The following criteria were used to interpret ICC values: (1) Values greater than .90 indicated excellent reliability, (2) values between .75 and .89 indicated good reliability, (3) values between .40 and .74 indicated moderate reliability, and (4) values less than .40 indicated poor reliability (Portney & Watkins, 2008; Schoen et al., 2008).
Results
Participants
Data from 32 participants (TD children, n = 18; children with ASD, n = 14) were analyzed from a total pool of 49 participants who met the study inclusion criteria. Seventeen participants were excluded from analysis for the following reasons: 6 participants were excluded because of excessive artifacts, 5 participants were excluded because of technical difficulty, 5 participants were excluded because they were unable to tolerate the test, and 1 participant was excluded for not showing up for the second test.
Reliability of Electrodermal Activity
Tonic Variables.
SCLs during baseline and recovery revealed moderate ICC reliability for total (N = 32) and individual (TD children, n = 18; children with ASD, n = 14) groups. ICC test–retest reliability during baseline was .66 for the total participant pool, .65 for the TD group, and .65 for the group with ASD. During the recovery condition, ICC reliability was .71 for the total participant pool, .68 for the TD group, and .73 for the group with ASD. Additionally, tonic nonspecific response (NSR) during phasic and SCL recovery domains revealed moderate to excellent reliability for total (N = 32) and individual (TD children, n = 18; children with ASD, n = 14) groups. ICCs for the total group ranged from .71 to .93, ICCs for the TD group ranged from .51 to .84, and ICCs for the group with ASD ranged from .62 to .92.
Phasic Variables.
As shown in Table 1, the test–retest reliability indices of the total participant pool (N = 32) for mean amplitude of SCR were moderate to good, with ICCs ranging from .60 to .81. The TD group reliability was moderate to good, with ICCs ranging from .48 to .82. Reliability for the group with ASD was also moderate to good, with ICCs ranging from .42 to .83. Similarly, as shown in Table 2, the reliability indices of the total participant pool for mean magnitude of SCR were moderate to good, with ICCs ranging from .50 to .75. The TD group reliability was moderate to good for five of six domains, ranging from .51 to .83. The reliability for the group with ASD was also moderate to good for five of six domains, ranging from .56 to .87. In addition, as shown in Tables 1 and 2, the TD group had higher mean amplitude and magnitude of response as well as greater standard deviations than the children with ASD.
Test–Retest Reliability for Amplitude of Skin Conductance Response
Note. N = 32. Amplitude does not include zero response. ASD = autism spectrum disorder; ICC = intraclass correlation coefficient; M = mean; SCP = Sensory Challenge Protocol; SD = standard deviation; TD = typically developing.
Test–Retest Reliability for Magnitude of Skin Conductance Response
Note. N = 32. Magnitude includes zero response. ASD = autism spectrum disorder; ICC = intraclass correlation coefficient; M = mean; SCP = Sensory Challenge Protocol; SD = standard deviation; TD = typically developing.
Discussion
An important outcome of this study is that the phasic and tonic variables (EDA) in response to the SCP were reliable in children with and without ASD. The inclusion of a TD comparison group and phasic amplitude response further extends the findings of Schoen et al. (2008), who found moderate to good reliability for phasic magnitude of response for five of six SCP domains and moderate reliability for tonic SCL and NSR in children with ASD. In the current study, TD children had higher mean amplitude and magnitude of response as well as greater standard deviations than the children with ASD. In addition, responses to the tactile and vestibular domains were not as reliable for the group with ASD as they were for the TD group. The lower reactivity and ICC reliability in the group with ASD may indicate a less organized neural response. The vestibular and tactile systems as prime neural organizers that register sensory information from the skin, muscles, joints, and vestibular system to organize a response are central to Sensory Integration Theory (SIT; Ayres, 1979). SIT posits that development of a neuronal model of self in space allows interaction with the world. On the basis of the higher means and standard deviations of the TD group for both amplitude and magnitude, it is reasonable to suggest that TD children have a greater range of variability of response than children with ASD. Decreased physiological flexibility of response in children with ASD may be the foundation for decreased behavioral flexibility and adaption to change. This proposal should be tested in future studies.
The inclusion of psychophysiology of EDA and its use in the SCP in doctoral occupational therapy curricula will increase opportunities to collect more reliable data, generate rational science-driven conclusions, and advance evidence-based practices dealing with SPD. Reliable EDA measures can facilitate alternate methods of data analysis to analyze differences in response among and within groups with ASD as well as the existence of response patterns. Improving the understanding of response patterns in children with ASD may provide the ability to describe the level of sympathetic response and may answer the following clinically important question: Is there more than one pattern of response to sensation among children with ASD?
Conclusion
As with any research, there are several limitations that must be noted with our study. First, generalizability of our study results is limited because we used a convenience sample of children recruited from New Jersey schools and therapy clinics. Moreover, the participants in the clinical group were primarily high-functioning children with ASD, and therefore the results are not generalizable to the full spectrum of ASD functioning. Finally, the size of our sample was small, and the control group was not age-matched.
In summary, there have been few studies published to date in which researchers have examined the reliability of EDA as an indicator of sensory processing in children with ASD compared with TD children. Researchers using the SCP have reported outcomes that quantify a link between sensory processing and EDA without first establishing the reliability of the tool (Mangeot et al., 2001; Su et al., 2010). The results of our study support the use of EDA with the SCP as a reliable tool. Moreover, these findings improve the utility and power of outcomes already reported in the literature that link sensory processing with EDA, thus supporting SIT and the use of this theoretical frame by clinicians in the management of ASD.
Implications for Occupational Therapy Practice
The findings from this reliability study on EDA clearly support the American Occupational Therapy Association’s (2007) Centennial Vision, which emphasizes that occupational therapy as a health care profession be driven by evidence-based science practices. As a direct result of establishing EDA’s reliability, two implications for occupational therapy practice are noted:
First, previous work reported in the literature in which researchers used EDA to measure responses to the SCP can now be reevaluated by clinicians and researchers to evaluate the power of outcomes reported. The establishment of reliable quantifiable responses to sensation in children with ASD was a necessary step toward accurately measuring SI treatment effectiveness for engagement in habilitative and functional tasks during occupational therapy.
Second, occupational therapy researchers should use more physiological measures in their research to ascertain the mechanism by which the functional changes occur, and then that mechanism and its proof should be taught in occupational therapy curricula.
