RESPIRATORY SINUS ARRHYTHMIA AS A FUNCTION OF COGNITIVE ATTENTION By RACHEL AYN IVES

RSA was assessed from electrocardiographic recordings from 95 individuals between the ages of 18 and 22 at resting, and while they were completing three tasks. The tasks were computerized performance tasks that provided increasing levels of difficulty and memory load. Subjects were asked to respond when the same letter repeated itself either one back, two back, or three back, depending on the task. The number of true positives declined as the tasks became increasingly difficult, and the number of false positives increased between the one back task and the two back task, but decreased between the two back task and the three back task. RSA suppression was greater for individuals with a higher resting RSA. RSA was enhanced during the one back task and suppressed during the two and three back tasks. No correlation was found between resting RSA and RSA suppression versus true or false positive responses during each task. No effect was found between resting RSA and RSA suppression versus positive or negative affect at onset of task. These results suggest that although there is a relationship between resting RSA suppression and cognitive attention during the tasks, it was not exactly as expected. The data also suggest that as the task becomes more difficult, RSA is suppressed to a higher extent. Introduction Respiratory Sinus Arrhythmia, or the naturally occurring change in heart rate due to respiration, is an index of parasympathetic activity. It has been used in many studies in the past two decades to quantify vagal control (Allen, Chambers, & Towers, 2007). This stems from the Polyvagal Theory introduced by Porges in 1995. Porges claims that RSA may be used to quantify vagal control, because the phasic cycle of inhibitory influence on the sino-atrial node is equivalent to the phasic cycle of inhibitory influence on respiratory rhythm. Therefore, the RSA can be used to quantify nucleus ambiguus influence on the vagus nerve, or vagal control (Porges, 1995). However, this theory does have critics that believe that RSA cannot be used to quantify vagal control because respiration is not a controlled variable (Grossman & Taylor, 2007). Normally, acceleration of the heart rate is linked to inspiration and deceleration of the heart rate is linked to expiration. However, Porges explains that in the mammalian brain the vagus nerve, also known as the 10 th cranial nerve, has two branches that originate in two different brain stem nuclei, the dorsal motor nucleus (DMNX) and the nucleus ambiguus. Vagal fibers that originate in both nuclei terminate on the sino-atiral node of the heart. The dorsal motor nucleus contains ummyelinated axons and the nucleus ambiguus contains myelinated axons, meaning that the nucleus ambiguus has much faster propagation of an electrical impulse to the heart. The nucleus ambiguus has been found to solely regulate RSA without aid of the DMNX. The DMNX inhibits the heart, which causes the heart rate to be slower. The NA acts to mobilize the organism in order to adapt to its environment, such as a fight or flight response. This includes the promotion of attention, communication, and emotional expression. It does this by removing the inhibitory influence on the heart, causing the heart rate to speed up. Once the stressful event is over, it inhibits the heart again causing the heart rate to return back to a resting state (Porges, 2005). Porges claims that attention is ergotropic, meaning that it disrupts homeostasis of the organism. The requirement of attention would coexist with a decrease in vagal tone, or RSA. The polyvagal theory explains that when a mammal is forced into a stressful state or needs to complete a cognitive task, RSA is suppressed during the task but returns to its resting state immediately upon task completion (Porges, 1992). In a study conducted by Porges and his colleagues, children with Attention Deficit Hyperactivity Disorder were treated with Ritalin (methylphenidate) and completed a sustained attention task. During the experiment which included sixteen children with diagnosed hyperactivity, the subjects completed a task in which they must push a button in order to start a toy car on a race track. The conditions were such that they were told that they were competing against another experimenter and that they would win the matchbox car if they win all of the races. Ten races were completed with each subject, but in reality the other matchbox car was dependent on the subjects’ response time. This was in order to keep the child’s attention because they were under the belief that they were winning the game. Attention was quantified by physiological data and response time. The data were divided into three sections; pretrial, phasic, and tonic. Pretrial consisted of five seconds before the task begins, phasic is the first five seconds, and tonic is the second five seconds during the task. The data were then also separated into two categories; slow responders and fast responders. Attention improved with the children that had been exposed to the treatment, and suppression of heart rate variability was found during completion of the attention task. The slow responders had a large increase in response time after medication, whereas the fast responder group had a small decrease in response time, although insignificant. This supports the previous hypothesis that individual response to methylphenidate is dependent on the amount of arousal. This means that if a subject has extremely low arousal, they will have a more significant reaction to the drug and higher changes in RSA and attention compared to those with normal arousal. Another finding of this study was the subjects who showed increased social function correlated with a decrease in response time. Subjects who exhibited low social performance were also mostly included in the group of slow responders. Limitations of this experiment include that the subject pool of sixteen children was much too small and that the task at hand depended a lot on motivation of the subject. I think that in further studies it would be better to not include a task in which the subject is motivated to win something, such as the toy car, because motivation can influence attention ultimately affecting the data (Porges, Walter, Korb, & Sprague, 1975). Another study showed that when infants were shown a visual stimulus on a screen with an interrupting stimulus, infants with a lower heart rate compared to their resting heart rate were less easily distracted. The amount that the heart rate decreases had a positive correlation with age when using infants at 14, 20, or 26 weeks of age. However, age had no distinct correlation with heart rate during the heart acceleration trials. This shows that attention increases as the infant develops. This study was done by having infants watch a complex image on a television monitor and introducing another visual stimulus on a screen within their field of vision. The stimulus was presented at three different intervals: one when the heart rate was in the resting heart rate range, one during a decreased heart rate, and one during an increased heart rate. The physiological response of the subjects differed between the primary stimulus and the secondary stimulus. This supports Richards’ theory that there are separate phases of attention that influence visual preference in infants. During the heart rate acceleration phase when the subjects’ heart rate was returned to resting it showed a dip below resting for a brief period. This could suggest a short refractory period of the heart rate before it returns to the original resting state. The amount of attention was quantified by using the head turning model. Infants with higher baseline RSA also had a faster heart rate deceleration during the attention task. One of the limitations of this study was that several of the infants looked away before the second stimulus was able to be presented, which could influence the validity of the data collected (Richards, 1987). Continuous performance tasks have been used for many years in order to evaluate sustained attention over a longer period of time. They have been used more recently to diagnose children with Attention Deficit Hyperactivity Disorder. Computerized continuous performance tasks have been shown to have the same correlation with inattention, impulsivity, and hyperactivity as many other measures previously used. The task has also been found to have more sensitivity than the widely used Connors Teacher Rating Scale (Klee, S.H. & Garfinkel, B.D., 1983). Porges also collaborated on a study in which school age children were given a continuous performance task during electrocardiographic recording. The relationships between resting RSA, change in RSA, and measures of attention were examined. He proposes three hypotheses’ as to why this change happens: higher RSA is associated with higher attentional capacity; resting RSA is related to the amplitude of physiological responses; and higher change in RSA is correlated with higher mental effort. The results showed that children with higher RSA and a larger change in RSA performed better on the first three minutes of the attention task, but this did not continue for the rest of the task. This may be because the task is too lengthy. This would affect the data because the difference can be easily seen between subjects with higher RSA and lower RSA during the first three minutes. However, after three minutes subjects with lower RSA could have similar results to subjects with higher RSA because the first three minutes enabled them to learn and perform better on the rest of the task, also known as practice effects. An alternate idea is that subjects with higher RSA and attention during the first three minutes of the task got tired of the task, which lowered their overall attention. This could be why the attention of both groups was quite similar. Both groups of subjects in this study showed a suppression of RSA during the task, supporting the first and third hypotheses’. Both of the groups also showed a heart rate increase during RSA suppression during the task. This supports Porges’ theory on the biological mechanism of the nucleus ambiguus withdrawing inhibitory contact, causing the heart rate to speed up during a task. This increase in heart rate shows that the subjects’ were putting mental effort towards the task they were given (Suess, Porges, & Plude, 1994). In 1972, Porges performed an experiment which is extremely similar to my current proposal. Male college students from an introductory level psychology class were split into two groups; one control group and two experimental groups. The first experimental group completed the task with a fixed amount of time between stimuli and the second experimental group completed the task with a variable amount of time between stimuli. The experimental groups completed a task in which they were instructed to press the spacebar once a light in front of them changed from green to red for several minutes. The control group was submitted to the same conditions but was never instructed to push anything. Both groups had physiological data recorded during this task. The duration of the task was sixteen seconds and ten trials were performed with each subject. The data were quantified by separating the physiological data into four segments, which each lasted eight seconds: pre-period, phasic period, tonic period, and response period. The mean heart rate was shown to decelerate through the tonic period, but increased during the response period for the experimental group with the variable time stimuli. However, the experimental group with the fixed time stimuli did not show any of the same significant results that the variable time experimental group did. The control groups showed an equivalent heart rate throughout all four phases. This supports the theory that heart rate is accelerated due to attention because the control group had no changes. A test for simple effects was done and showed that there was a significant change in heart rate variability for all four of the phases in the experimental fixed groups. It also showed that the heart rate was the most different during the tonic phase of the experiment. The reaction time did not differ between the fixed and variable group, but showed to be a function of trial time. This means that with more trials and more practice, the reaction time increased due to a learning curve. This data supports a hypothesis of attention with two components. First, a phasic response is dependent on the change in stimulus, and then a tonic response correlates with a decrease in heart rate variability (Porges, 1972). While promising, these prior studies have not fully examined the relationship between RSA and sustained attention in healthy, young adults. Experiments that have been done on adults include reaction time and tracking tasks, rather than a sustained attention task such as a continuous performance task. I will be studying the correlation between RSA and sustained attention by use of a computerized continuous performance task. A Positive Affect and Negative Affect Scale will be employed in order to see if affect influences performance and physiological responding. Subjects will complete a three separate 90 second continuous performance tasks while an electrocardiograph is recorded. During the task, the subject will be asked to respond to a simple stimulus (the repeated letter) by pressing the space bar on the computer. Then, they will be asked to respond to the same stimulus only within a certain condition (the letter one back, two back, and three back). Respiratory sinus arrhythmia will be quantified using the software program CMetX. Attention during the continuous performance task will be quantified by the number of correct responses and commission errors during the task. The primary research question is whether RSA will be correlated with sustained attention in healthy adults in the same way that it was correlated in infants and school-aged children. I hypothesize that a higher baseline RSA and a larger change in RSA between resting and the task will be correlated with higher performance on a sustained attention task. These hypotheses’ originate from Porges’ theory that a task that requires attention is ergotropic. Autonomic adaptive regulation causes the nucleus ambiguus to withdraw inhibition of the vagus nerve, speeding up the heart rate, and promoting attention. This will result in a decrease in vagal control during the sustained attention task due to the lack of inhibition of the vagus nerve by the nucleus ambiguus. A decrease in vagal control will correlate with a decrease in RSA due to the equivalent phasic cycles of inhibitory influence (Porges, 1995). If there is a larger change in RSA between resting and the task, then the subject will demonstrate higher attention and improved performance. Also, if the resting RSA is higher, then the subject will have more attention capacity to allocate during a task. This is supported by the theory that higher levels of resting RSA are associated with physiological flexibility. This means that those with higher levels of resting RSA will also have a larger change in RSA during the continuous performance task, resulting in more attention during the task. The theory suggests that the evolutionary function of the nucleus ambiguus, the phylogenetically newer branch of the vagus nerve that is responsible for RSA, is to enable flexibility that is associated with social behavior in mammals. Therefore, a higher resting RSA enables physiological flexibility which aids individuals in acquiring more attention when necessary.