Hemispheric Specialisation and Interference Effects in The Performance of a Dual-task
Christine Boyle
Edith Cowan University
Abstract
This experiment involved 241 participants in a within-subjects design, to examine the amount of balancing interference between the left and right hands, within silent and verbal conditions. Participants balanced a dowel rod on the index finger of each hand for as long as possible in each condition, followed by the completion of a handedness questionnaire. Dowel rod balancing times were recorded in seconds and balancing interference was measured as the difference between the silent and verbal balancing times. Results indicated that there was a significant difference between the left and right hands in balancing interference, where the interference was greatest for the right-hand and the sample showed an overall preference for right-handedness. It is believed that the higher balancing interference in the right-hand may have occurred due to left hemispheric specialisation in speech, causing the verbal stimulus to interrupt the balancing of the rod in the dual-task condition.
Hemispheric Specialisation and Interference Effects in The Performance of a Dual-task
Hemispheric asymmetry is a well known research area that gained its popularity from the modern split-brain studies conducted by R.W. Sperry and colleagues during the 1970’s (Gazzaniga, 2002). The split-brain studies examined surgically split or brain-injured patients who had their corpus callosum removed to reduce epileptic seizures (Rao, Jacob, Lin, Robey & Huber, 1992). The corpus callosum is a connecting communication link between the two cerebral hemispheres, the left and the right, of the human brain. It was believed that when it was removed, the transfer of information would be blocked between the two hemispheres and as a result communication between the two may cease, forcing them to operate independently (Rao et al., 1992). This identified that normal people in comparison appear to have functional hemispheric differences in everyday life (Rao et al., 1992) which prompted research into hemispheric specialisation that suggests particular functions are dominant in a specific hemisphere and could vary according to situational or attentional factors (Gazzaniga, 2002, 2005; Mohr et al., 2005). The most obvious functional hemispheric asymmetry in humans is the dominance of language and speech in the left hemisphere and the right hemisphere’s specialisation in visual and spatial processing (Gazzaniga, 2002, 2005).
Common methods of investigating hemispheric asymmetries in normal people have involved observing their performance while performing two unrelated tasks simultaneously are known as dual-task experiments (Kinsbourne & Cook, 1971). It is believed that performance tends to better in concurrent tasks if they are programmed by different hemispheres, rather than being controlled by the same hemisphere (Hicks, Provenzano & Rybstein, 1975; White & Kinsbourne, 1980). Furthermore, previous research suggests that muscles that manipulate hand movements can only be coordinated by their contra-lateral hemisphere in the brain, in other words, the right hemisphere should only control the left-hand and the left hemisphere should only control the right-hand (Gazzaniga, 2002).
Johnson and Kozma (1977) and Kinsbourne and Cook (1971) developed versions of the dual-task experiment that tested interactions between verbal and manual activities, both of which were thought to be lateralised in the left hemisphere. They sought to confirm if these activities would compete for resources if they were performed simultaneously, a process called hemispheric sharing of functions and if so would the process result in interference between the two tasks and thus decrease performance (Johnson & Kozma, 1977; Kinsbourne & Cook, 1971). They found that interference was greater for the right-hand than for the left-hand when subjects attempted to balance a dowel rod on the index finger of each hand while simultaneously verbalising a secondary stimulus (Johnson & Kozma, 1977; Kinsbourne & Cook, 1971). Therefore, in the dual-task studies interference between a manual and verbal task should arise when the main task is being performed by the right-hand, as it is controlled by the left hemisphere which specialises in both of the functions being carried out concurrently (Hiscock, Perachio & Inch, 2001; Kinsbourne & Cook, 1971). Whereas if the task is performed by the left-hand then the verbal task should still be coordinated by the left hemisphere but the right hemisphere would be left free to coordinate the manual task, therefore, little or no interference should occur between the two tasks when using the left-hand (Hiscock et al., 2001; Kinsbourne & Cook, 1971).
In addition, previous research has identified that hemispheric specialisation may vary between individuals due to their hand preference, intellectual ability and gender, suggesting that preferred handedness could be used to identify if subjects speech function was lateralised to the left hemisphere or the right, which could significantly alter the results of dual-task studies (McGowan & Duka, 2000). Also, it is believed that males tend to perform better at spatial processing tasks and females perform better on language and verbal tasks, therefore, these gender differences could influence the level of interference that may occur when performing concurrent verbal and manual tasks (Hiscock et al., 2001; McGowan & Duka, 2000). Moreover, Oldfield (1971) suggested that the incidence of left-handedness was genetically greater among males than females and vice versa, which further indicates that gender differences may be an important factor to examine in dual-task experiments.
Finally, it was also proposed by Gazzaniga (2002) that the level of interference observed in a dual-task would be dependent on the level of difficulty of the secondary task, suggesting that the harder the task was the more hemispheric resources that would be needed to perform the task. Also, the harder the concurrent tasks are the more that the primary hemisphere carrying out the functions might need to call on the other hemisphere for help to perform both tasks simultaneously (Gazzaniga, 2002). This implies that the harder the task was and the harder the one hemisphere had to work to carry out the task, the harder it would be for the other hemisphere to carry out another task simultaneously which would most likely result in interference (Gazzaniga, 2002). Also, this suggests that hemispheric specialisation of independent functions may not be to one specific hemisphere only as previous research has portrayed it to be and that people may use both hemispheres equally to participate in all activities (Rao et al., 1992).
This study used a dual-task procedure similar to Johnson and Kozma’s (1977) and Kinsbourne and Cook’s (1971) studies that examined whether interference would occur when a verbal task and a manual task are performed concurrently, and if it did, with which hand the interference would be the greatest. It was hypothesised that balancing interference would be greater in the verbal condition of speaking every second letter of the alphabet while balancing the dowel rod, than in the silent condition of balancing the dowel rod quietly. It was also hypothesised that interference would be greater for the right-hand than for the left-hand. This result was predicted based on the hemispheric sharing of functions notion that suggests performing a verbal and a right-hand manual task concurrently will require sharing of functional resources within the left hemisphere, which would result in a higher level of interference (McFarland & Geffen, 1982; McGowan & Duka, 2000).
Method
Participants
The participants comprised of 270 students from the Edith Cowan University, Joondalup Campus who had undertaken PSY3350 Biological Psychology during 2007 and 2010. There were 53 males and 217 females. The data analysis excluded three participants who balanced the dowel rod for 300 seconds or more. Note that as gender was not a primary variable under examination in this experiment; the analyses also did not include this variable.
Materials
A wooden dowel rod was used for participants to balance on the index finger of each hand, and measured 92cm in length and 1.25cm in width. A stopwatch was used to record balancing time in seconds for each trial and the times were recorded on a data record (see Appendix A). Lastly, a handedness questionnaire was used to obtain each participant’s preference for left or right handedness in a series of unilateral tasks, to identify if they are primarily left or right handed in everyday activities (see Appendix B).
Procedure
In a standing position, participants practised balancing a dowel rod on an index finger for 3 minutes, alternating between the left and right hands. They then proceeded with 8 test trials, twice in each of the following conditions; left-hand silent task, left-hand verbal task, right-hand silent task, and right-hand verbal task. The verbal condition required participants to repeat the alphabet aloud (in a reasonable loud voice), while skipping every second letter and balancing the dowel rod simultaneously. Participants were encouraged to move around and make any movements necessary to keep the rod balanced for as long as possible. The order of trials was randomly selected by each participant. Balancing times were recorded in seconds using a stopwatch and balancing times were stopped if the time reached 300 seconds (5-minutes) or if the dowel rod dropped to the ground or touched a part of the participant’s body, other than the hands. The balancing times for the silent and verbal conditions for each hand were recorded on a data record.
Design
This experiment used a 2 (hand: left vs. right) by 2 (condition: silent vs. verbal) within-subjects design and is a partial replication of Kinsbourne and Cook’s (1971) and Johnson and Kozma’s (1977) studies.
Results
A dependent t test was computed to compare the amount of left balancing interference with the amount of right balancing interference for each of the participants in this study. The balancing interference was calculated by subtracting the silent balancing time from the verbal balancing time for both the left and right hands.
The results indicated that there was a significant difference between the left and right hands in balancing interference, t(269) = 2.88, p =.004. This shows that when participants performed the verbal and right-hand manual tasks concurrently in the verbal condition, they experienced greater balancing interference (-4.68 secs) due to the sharing of functions within the left hemisphere, supporting hemispheric specialisation. Whereas when the left-hand was used within the verbal condition, less balancing interference (-1.62 secs) was experienced. This was because the verbal task did not need to share its function and thus neural resources with the manual task, as both tasks were simultaneously coordinated from separate hemispheres, where the manual task was controlled from the right hemisphere and the verbal task from the left hemisphere.
The mean balancing time for the silent condition was 10.29 (SD=14.22) for the left-hand and 17.42 (SD=26.35) for the right-hand. The mean balancing time for the verbal condition was 8.68 (SD=9.29) for the left-hand and 12.58 (SD=16.80) for the right-hand. The mean balancing times averaged across all participants in the silent and verbal conditions, for both the left and right hands are shown in Figure 1 below.
Figure 1. Mean balancing times (in seconds) averaged across all participants in the silent and verbal conditions, for both the left and right hands.
In addition, a handedness quotient was calculated from the Handedness Questionnaire using a scale where extreme left-handedness was represented by 0 and extreme right-handedness was represented by +1.0. The results indicate that the mean handedness quotient was 0.86 (SD=0.17), that shows the significance of right-handedness in this study, as well as the balancing times of right-hander’s, as determined by participant’s stated handedness and their handedness quotient.
Discussion
The results of this experiment were significant and supported the hypothesis that balancing interference would be greater in the dual-task verbal condition than in the silent condition, particularly when using the right-hand. This result occurred because the verbalisation during the dual-task involved the lateralisation of language and speech in the left hemisphere, as the concept of hemispheric specialisation suggests (Gazzaniga, 2002, 2005; Mohr et al., 2005). This resulted in both tasks being coordinated simultaneously within the left hemisphere, where they competed for functional resources causing the balancing interference effect (Hicks et al., 1975; Hiscock et al., 2001; Kinsbourne & Cook, 1971; McFarland & Geffen, 1982; McGowan & Duka, 2000).
In other words, the complex verbal task that was controlled from the left hemisphere competed with neural activity required for the manual balancing task when using the right-hand, because this hand’s movements were also controlled by the left hemisphere (Johnson & Kozma, 1977; Kinsbourne & Cook, 1971). Also, as the right hemisphere is presumed to be less involved in language functions, the verbal task did not need to compete with the manual balancing task for resources within the left hemisphere, because the left-hand manual task was controlled separately from the right hemisphere (Johnson & Kozma, 1977; Kinsbourne & Cook, 1971). This also indicates that right-hand performance involved greater balancing interference than the left-hand, which further supports the hypothesis of the current study and the notions of hemispheric specialisation and sharing of functions (Hiscock et al., 2001; Johnson & Kozma, 1977; Kinsbourne & Cook, 1971).
Furthermore, the results indicate that participants experienced some interference during the verbal condition when using the left-hand, which suggests that even when each task was coordinated from separate hemispheres, both the verbal and manual tasks still competed for attention, which caused a distraction effect in the other when performing both concurrently (Johnson & Kozma, 1977; Kinsbourne & Cook, 1971). Therefore, the overall result that participant’s ability to continue to balance the dowel rod would decline when they were required to simultaneously speak out every second letter of the alphabet, is consistent with previous research that verbal activity should interfere with concurrent manual activity to some degree and that asymmetric interference may be attributed to a difference between the efficiency of within-hemisphere and between-hemisphere sharing of functions and resources (Hiscock et al., 2001; McGowan & Duka, 2000).
Moreover, this experiment agrees with findings by Kinsbourne and Cook (1971) and White and Kinsbourne (1980) that the more difficult each concurrent task is to perform the more interference should occur between them. Similarly, the more difficult the two tasks are, the more difficult it should be for the left hemisphere to control them simultaneously within the same hemisphere, than it would be for the two hemispheres to control one task each (Kinsbourne & Cook, 1971; White & Kinsbourne, 1980). However, it has been suggested that in some instances if the capacity needed to perform both tasks concurrently exceeds the general operating capacity of the left hemisphere to control both tasks alone, then one task may be transferred to the right hemisphere to be controlled, in which case having both hemispheres coordinating tasks together may also increase interference (Gazzaniga, 2002; Hicks et al., 1975; White and Kinsbourne, 1980). This indicates that although it has been argued in prior research that the left hemisphere is dominant in dual-task procedures involving verbal and manual tasks, it may not exercise complete control and that both hemispheres may participate equally especially when the task is more complex. Therefore, referring to the left hemisphere as the language hemisphere dominant in verbal activities is an oversimplification of current and prior findings and may affect the validity of the results in this experiment (Gazzaniga, 2005; Mohr et al., 2005; Rao et al., 1992).
In this study, a possible confounding factor was that participant’s gender differences were not analysed with the data. Hiscock et al. (2001) found that although gender differences in manual performance may be irrelevant to gender differences in lateralisation, they may still influence the asymmetry of interference in the dual-task verbal condition, as males tend to perform better at manual tasks and females better at verbal tasks and left hemisphere verbal dominance tends to be more common in males. This could have skewed the results in this study as at least 80% of participants were female, therefore, future research should attempt to use a more balanced sample of both genders and should assess for significant differences in task performance between them (Hiscock et al., 2001; McGowan & Duka, 2000).
In addition, this experiment could identify participant’s primarily spoken language, which in this study was assumed to be English. As it is possible that some participants are bilingual and English may not be their primary or even secondary language, so if they were expected to perform the verbal condition component in English, this may have increased the difficulty of the task and thus interference observed (Hiscock et al., 2001; Vaid & Lambert, 1979). Also, previous research has found that increased phonetic difficulty in verbal conditions leads to greater interference, therefore, differing levels of phonetic difficulty as well as primary language could be examined (Hicks et al., 1975; McFarland & Geffen, 1982).
Overall, the large amount of research on hemispheric specialisation and hemispheric sharing of functions appears to support the results of this study that interference is greater between concurrent verbal and right-hand manual tasks as these functions are believed to be lateralised to the left hemisphere and hence compete for resources when performed simultaneously (Gazzaniga, 2002, 2005; Johnson & Kozma, 1977; Kinsbourne & Cook, 1971). However, future research should focus more on gender differences, language preference, hand preference and difficulty of the tasks to be performed, to more clearly identify the functional operations of hemispheric specialisation and when interference between-hemispheres and within-hemispheres actually occurs, rather than assuming a correlation effect based on theoretical study that oversimplifies the processes discussed in this experiment (Gazzaniga, 2005; Hiscock et al., 2001; McGowan & Duka, 2000; Mohr et al., 2005; Rao et al., 1992). ReferencesGazzaniga, M.S. (2002). The split-brain revisited. Scientific American: The hidden mind, 12, 26-31.Gazzaniga, M.S. (2005). Forty-five years of split-brain research and still going strong. Neuroscience, 6, 653-659. Retrieved from http://0-proquest.umi.com.library.ecu.edu.au/pqdweb?index=4&did=1160527201&SrchMode=2&sid=4&Fmt=6&VInst=PROD&VType=PQD&RQT=309&VName=PQD&TS=1285393622&clientId=7582Hicks, R.E., Provenzano, F.J., & Rybstein, E.D. (1975). Generalised and lateralised effects of concurrent verbal rehearsal upon performance of sequential movements of the fingers by the left and right hands. Acta Psychologia, 39, 119-130. Doi: 10.1016/0001-6918(75)90004-9Hiscock, M., Perachio, N., & Inch, R. (2001). Is there a sex difference in human laterality IV. An exhaustive survey of dual-task interference studies from six Neuropsychology journals. Journal of Clinical and Experimental Neuropsychology, 23, 137-148. Doi: 10.1076/jcen.23.2.137.1206Johnson, O., & Kozma, A. (1977). Effects of concurrent verbal and musical tasks on a unimanual skill. Cortex, 13, 11-16. Kinsbourne, M., & Cook, J. (1971). Generalised and lateralised effects of concurrent verbalization on a unimanual skill. Quarterly Journal of Experimental Psychology, 23, 341-345. Doi: 10.1080/14640746908401828McFarland, K., & Geffen, G. (1982). Speech lateralisation assessed by concurrent task performance. Neuropsychologia, 20, 383-390. Doi: 10.1016/0028-3932(82)90038-0McGowan, J.F., & Duka, T. (2000). Hemispheric lateralisation in a manual-verbal task combination: The role of modality and gender. Neuropsychologia, 38, 1018-1027. Doi: 10.1016/S0028-3932(99)00155-4Mohr, C., Michel, C.M., Lantz, G., Ortigue, S., Vlaud-Delmon, I., & Landis, T. (2005). Brain state-dependent functional hemispheric specialization in men but not in women. Cerebral Cortex, 15, 1451-1458. Doi: 10.1093/cercor/bh1025Oldfield, R.C. (1971). The assessment and analysis of handedness: The Edinburgh inventory. Neuropsychologia, 9, 97-113. Doi: 10.1016/0028-3932(71)90067-4Rao, H.R., Jacob, V.S., Lin, F., Robey, D., & Huber, G.P. (1992). Hemispheric specialization, cognitive differences and their implications for the design of decision support systems. MIS Quarterly, 16, 145-151. Retrieved from http://0-proquest.umi.com.library.ecu.edu.au/pqdweb?index=0&sid=5&srchmode=2&vinst=PROD&fmt=6&startpage=-1&clientid=7582&vname=PQD&RQT=309&did=616744&scaling=FULL&ts=1285393777&vtype=PQD&rqt=309&TS=1285393793&clientId=7582Vaid, J., & Lambert, W.E. (1979). Differential cerebral involvement in the cognitive functioning of bilinguals. Brain and Language, 8, 92-110. Doi: 10.1016/0093-934X(79)90043-9White, N., & Kinsbourne, M. (1980). Does speech output control lateralise over time? Evidence from verbal-manual time-sharing tasks. Brain and Language, 10, 215-223. Doi: 10.1016/0278-2626(82)90010-0
