This study investigated the pattern of interference in dual task performance. The aim of the study was to examine whether handedness and language interfered with motor performance. The study hypothesized that there would be a significant difference in performance between right and left hand motor activity and right hand performance would interfere more with dual task performance as both tasks require resources from the left hemisphere. A 2 (left and right) x 2 (language and motor) repeated measures design was employed. Thirty students from the University of Central Lancashire completed a language task (repeating a sentence) and right and left handed motor tasks in single and dual conditions. Results revealed that right hand motor activity performance significantly declined when concurrently performed with the speech task whereas the left hand performance increased. There was a significant interaction between handedness and task. These findings therefore suggest that when two tasks are performed concurrently requiring the same hemispheric resources interference is more likely to occur resulting in poor performance.
The human brain is a paired organ composed of two hemispheres. The two hemispheres appear to be mirror images of each other. The most distinctive feature of the human brain is the asymmetric distribution of cognitive functions between the two hemispheres - this is called lateralization. There are several functional differences between the two hemispheres, the right hemisphere is specializes in spatial and emotional skill and the left hemisphere specializes in language, mathematical and musical skills. The differences in the hemispheres have been linked to the development of language. The left hemisphere controls the movement and sensations of the right side of the body and vice versa. The evidence for the specialization of the left hemisphere has been derived by the anatomical work of Dax (1836), Broca (1864) and Wernicke (1876) (cited in Springer & Deutsch 1993).
Dax (1836) (cited in Springer & Deutsch 1993) was the first to identify lateralization of function; he found that not one of his forty patients with speech problems displayed brain damage which was restricted to the right hemisphere. From his findings he concluded each hemisphere controlled different functions and proposed that language primarily resides in the left hemisphere. However, his work was quickly dismissed and forgotten. The most convincing evidence for language lateralization was provided almost thirty years later when Broca (1864) (cited in Springer & Deutsch 1993) reported the results of a postmortem examination of two aphasic patients, patients with deficits in the use of language attributed to damage to the brain. He discovered both patients had damage to the inferior prefrontal cortex in the left hemisphere. This became to be known as the Broca's area, the area that is associated with speech and grammar production. Damage to the Broca's area primarily results in Broca's aphasia characterized by speech production impairment; speech slows down and becomes telegraphic, despite this speech remains moderately good.
Shortly after Broca's findings, Wernicke (1876) (cited in Springer & Deutsch 1993) discovered another part of the left hemisphere concerned with language comprehension which was consequently named Wernickes's area. He found patients with damage to the Wernicke's area could speak fluently but had problems with syntax and lexical words making it difficult to both understand and produce spontaneous speech, this is known as Wernicke's aphasia.
It was concluded from both Broca (1864) and Wernickes (1876) (cited in Springer & Deutsch 1993) findings that language primarily resides in the left hemisphere, as it became knowledge that individuals who had damage to the left side of the brain often suffered speech disorders such as aphasia, along with difficulties with the sensory motor control of the right side of the body. The frontal lobe contains the areas associated with motor movement - called the motor strip. Motor movement is contra laterally controlled, the motor strip located in the left hemisphere is in control for the movements of the right side of the body and the motor strip located in the right hemisphere is in control for the movement in the left side of the body. Therefore if the motor strip is damaged the brain will find it difficult to control one half of the bodies movement, for example as a result of a stroke in the left hemisphere the right side of the body will stop working and vice versa. Therefore it was concluded that each cerebral hemisphere is contra lateral as it receives information primarily from the opposite side of the body.
A relationship between language and handedness was proposed by Broca (1861) (cited in Springer & Deutsch 1993) as the cerebral hemispheres are contra lateral. He proposed that right handed individuals had a dominant left hemisphere which controlled speech and motor activities whereas left handed individuals had the right hemisphere dominant. This assumption has been supported by Rasmussen & Milner's (1977) study (cited in Wickens 2005) in which the Wada technique was adopted in order to study localisation of language - this is anaesthetising one hemisphere of the brain. The study's findings revealed that 96% of right hand dominants and an estimated 70 % of left hand dominants had language lateralised in the left hemisphere. Another 15 % of left hand dominants were found to have Right hemisphere language dominance and 5% indicated mixed hemisphere dominance. These findings have been contradicted by Anon (2000) who found 75% of right handed individuals had language situated in the right hemisphere.
Smith & Sugar (1965) (cited in Wickens 2005) case study of a 5 yr old boy had his left hemisphere removed. He was capable of comprehending speech but had great difficulty producing speech. The boy's progression was monitored over time he was found to have better language abilities along with a normal IQ indicating damage to the left hemisphere can result in poor language development but the right hemisphere can substitute as a replacement if damage occurs in early life as time allows successful incorporation of language to the right hemisphere over time. These findings have been supported by Sperry (1974) split brain studies.
The corpus callosum is the pathway of communication connecting the two hemispheres. The corpus callosum consists of two types of fibers, one with a lager diameter which controls the sensory -motor co-ordination and the other with a smaller diameter which connects the association areas. In patients with severe epilepsy the corpus callosum can be detached in order to prevent seizures these patients are referred to as split brain patients. This procedure also disrupts the communication of perceptual and cognitive information between the two hemispheres; despite this patients have no problem with everyday language, as auditory and visual information is distributed through both hemispheres. Sperry (1974) tested the functional specialization of the left and right hemispheres in epileptic patients who had undergone a surgical cutting of the corpus callosum in order to prevent epileptic seizures spreading through the brain.
Sperry's (1974) (cited in Springer & Deutsch 1993) study involved presenting split brain patients images either to the right or left visual field. He found patients who were presented an image to the left visual field could not say what the object was as the information was analyzed in the right hemisphere, patients could not process the words but were able to point to a matching item with their right hand - indicating the right hemisphere plays a role in speech production, as it is capable of interoperating stimuli but lacks the verbal ability to produce speech. Whereas patients who were presented an image to the right visual field could say what the object was as the information was analyzed in the left hemisphere where words could be processed..This phenomenon would not occur in normal individuals as the two hemispheres could communicate through the corpus callosum. The studies findings support the assumption that language is lateralized in the left hemisphere.
Speech is the primary form of communication for humans; speech is almost always accompanied with other tasks such as watching television and talking, driving and talking. Norman & Bobrow (1975) said when two tasks are performed simultaneously; they may interfere with each other. This is called divided attention it occurs when an individual is concentrating on two tasks at the same time. Divided attention is known to differ in normal individuals and split- brain individuals, those who have had a cut in the corpus callosum.
Dual tasks divide the attention given to each task as the resources required have to be divided in order for the task to be performed - leading to an increase in cognitive demands. As individuals we have limited cognitive resources at our disposal inevitably leading to a decrease in performance. This decline in performance of dual task compared to single task is known as the dual - task decrement. Dual task studies of verbal and motor tasks have found that right handed individuals show greater decrement than left handed individuals. This performance pattern is known as the lateralized dual task decrement, this can be seen as evidence for left-hemisphere lateralization for language.
Kinsbourne & Hicks (1978) proposed the functional distance model, in order to explain the dual task interference. The model suggest that performing a dual task will interfere with the activity of one hand more than the other, if the task demands resources primarily from one hemisphere. Thus the interference that occurs is attributed to the distance between the areas of the brain which are activated for the task. For example a dual task which is related to separate brain areas, such as the left and right hemisphere would result in little decrement in performance than when a dual task requires the same brain area such as the left hemisphere resulting in greater decrement in performance as a result of divided attention.
In line with the functional distance model, differences between right and left sided motor performance may be found in a dual task with language as both the left hemisphere motor strip and language area are anatomically close to each other. The hypothesized interference is drawn from hand dominance and language localization in the brain. Therefore right handed individuals who have language localized in the left hemisphere, will perform significantly better with their right hand than with their left hand in a single motor task but when performing a dual-task of language-motor there will be an evident decrease in performance as both tasks require the use of specialized areas in the left hemisphere thus dividing the attention.
Evidence for the lateralized dual-task decrement was provided by Chang & Hammond (1987) study in which participants were required to repeat monosyllabic stimuli whilst tapping their fingers in a cyclic manner. A decline in performance was found when either hand was used, it was noted that participants dominating their right hand showed a greater decline in performance than participants dominating their left hand. These findings were supported by Ashton & Mcfarland (1989) study in which participants were required to repeat short stories either silently or verbally whilst tapping their fingers.
Additional support was provided by Simon and Sussman (1987) study in which 260 participants were divided in groups based on gender and handedness. The dual task consisted of a language and motor task - finger tapping, the tapping was recorded by itself and then with the language task. They found there was a greater decline in performance for the dominant hand despite of whether the participant was left or right handed. These findings were further supported by Seth- Smith et al (1989) who conducted a study in order to determine whether the difference in task performance could be attributed to difference in language lateralization in males and females. A dual-task of finger tapping and language task was employed. A significant decline in performance was found when the tasks were done at the same than when alone. It was concluded that the motor task relied upon the same processing resources as the movement of the right hand which resulted in interference.
Kinsbourne & Cook (1971) used lateralized motor responses to examine the effect of balancing a dowel on either the right or left index finger and speaking , the results showed a greater interference when dual- tasks required resources from the same hemisphere. The balancing times were found to be longer in the verbal condition when the left hand was used than when the right hand was used. This is consistent with the functional distance model.
One the whole it can be concluded that when an individual performs a dual task requiring separate hemispheres there would be very little interference than when a dual task requires the use of the same hemisphere, resulting in more obvious decrement in performance.
The present study intended to examine the pattern of interference in dual task performance. This was done by employing a dual task of language and motor task in which hand dominance was manipulated in order to see the affect it had upon performance. The aim of the study was to examine whether handedness and language interfered with motor performance.
The study hypothesized that "there would be a significant difference in performance between right and left hand motor activity." The study also hypothesized that "right hand performance on the dual task would decline as both the tasks require resources from the left hemisphere and that the left hand would remain unaffected by the dual task demands."
The aim of the study was to examine whether handedness and language interfered with motor performance. For this experiment a repeated measures design was employed this meant that each participant was tested in each of the four conditions. The four experimental conditions were being manipulated and the subject's performance of picking up peas was being measured. The four experimental conditions were; condition one- left hand motor activity, condition two- right hand motor activity, condition three - dual left -speech task and condition four- dual right -speech task.
A sample of 30 individuals from the University of Central Lancashire was recruited to participate in the study. Participants were recruited by visiting the University canteens, before recruiting participants handedness was distinguished by asking three questions, " What hand would you hold a match in?' 'Which hand would you use to throw a ball?' and 'Which hand would you hold a hammer?' If right handedness was dominant the individual was recruited. The researchers briefly outlined the research and what participation entailed and provided contact information should the participant be interested. The researchers stressed confidentiality and reminded the participants that they had the right to withdraw as participation was completely voluntary but withdrawal would not be possible once data had been collected due to anonymity. In this study no note was made of gender or age. (Appendix A)
In this experiment a pack of dried chick peas was used and two bowls. One bowl was large and flat and contained the chick peas which were to be transferred into another bowl which was smaller but taller - to ensure the peas could not be thrown in or bounce out. A timing device was also used to time the experiment. A computer was used to type up the raw data (appendix C). The SPSS statistic program was also used to calculate and analyze the raw data. (Appendix B).
Participants were approached in the university canteens and asked if they would kindly participate in a study on language lateralization. Once consent was received the participant's handedness was distinguished by asking three questions - if the participant was right hand dominant they were recruited if not they were thanked for their willingness. The participant was then given a brief overview of the research and what participation entailed. Once each researcher had recruited five participants each they were escorted to the Darwin building laboratory. The participants were briefed and told to complete the task as fast as they could without making mistakes using their precision grip - index finger and thumb (Appendix D) Each participant was seated at a table were two bowls were placed at 30cm distances from each other across the bodyline in an outward position so that the movement would be identical for both the left and right hand. Each participant was tested in four conditions - counter balancing was used to determine the order. Each participant was given a practice motor trail to understand what they were expected to do. Condition one - the left hand motor task required the participant to use their left hand precision grip (thumb and finger) to transfer one pea from the pea bowl to the empty bowl within thirty seconds. Condition two - the right hand motor task required the participant to use their right hand precision grip (thumb and finger) to transfer one pea from the pea bowl to the empty bowl within thirty seconds. Condition three- dual left - speech task required the participant to use their left hand precision grip (thumb and finger) to transfer one pea from the pea bowl to the empty bowl whilst repeating the sentence 'Shelly went to the park' within thirty seconds. Condition four- dual right- speech task required the participant to use their right hand precision grip (thumb and finger) to transfer one pea from the pea bowl to the empty bowl whilst repeating the sentence 'Shelly went to the park' within thirty seconds. After each condition the amount of peas transferred to the empty bowl were counted and recorded. (Appendix B). Once each participant had completed all four conditions the participants were debriefed in which they were told the aim of the study and were reminded they had the right to withdraw there and then and withdrawal would not be possible at a later date due to anonymity. (Appendix E). Once all the raw data had been collected all the data was entered in to the SPSS statistic program to be analyzed.(Appendix A and B).
On completion of the experiment the results were collated for each of the conditions to make the raw data.
The raw data (appendix B) obtained consisted of the mean frequency scores for the amount of peas collected in the two tasks, motor and dual for each hand, left and right. Table 1 shows the overall means and standard deviation scores for the mean frequency of the amount of peas collected across the four conditions. The data presented in table 1 shows the mean and standard deviations for the amount of peas collected across the four conditions, left motor task, right motor task, left dual task and right dual task. It shows the highest mean score was for left dual task, 19.00 and the lowest was for left motor task, 17.40. This shows that performance was significantly better for left dual task than left motor task. In the motor task condition the right hand, 18.83, performance was slightly better than the left motor, 17.40. The standard deviation scores across the four conditions are more or less similar; the lowest score was for the right dual task, 2.39 implying the performances in this task were closer to the average than any other condition.
In order to see if the results were significant, the data was analyzed using a 2 x 2 repeated measures ANOVA as the assumptions were met (Appendix B). The results for the ANOVA indicated a significant effect for handedness [ F(1,29) = 1.85, p <.05, Eta 2 = .060 (60%)] showing the performance was better for the left hand than right hand, this is further supported by the means in Table 1. The analysis also showed a non significant effect of task (motor and dual) [ F (1,29) = 1.16 , p > 0.5 = Eta2 = .038 (38%) ]. A significant interaction between handedness (left and right) and task (motor and dual) [ F (1,29) = 5.97, p <0.5 =.17 (17%).] A interaction graph was produced to see the amount of peas collected across the four conditions more clearly Figure 1supports the mean findings in Table 1. It can be seen there is an evident interaction between handedness and task. The interaction graph shows that the performance in the motor task condition was poorer when the left hand was dominated compared to right hand domination. It can also be drawn that the left dual task performance was significantly better than right hand dual task. There is a significant difference in left hand performance showing motor task performance was poorer compared to dual task. In addition it can be seen right hand motor task was performed significantly better compared to the dual right task. Post hoc comparisons using paired samples t-tests (adjusted p < .025) revealed that performance was significantly better with the left hand on the tasks t(29)=3.4, p < .005 than the right hand t(29) = 6.4, p>.05. (Appendix C)
The aim of the study was to examine whether handedness and language interfere with motor performance. Right and left handed performance was measured in order to determine whether there would be support for the functional distance hypothesis proposed by Kinsbourne & Hicks (1978). The present study hypothesized that 'there would be a significant difference in performance between right and left hand motor activity.' This was supported by the ANOVA results as a significant difference for handedness was found showing performance was better for the left than the right hand, implying the interference for the right hand performance was greater than the interference for the left hand. Further analysis showed that there was no significant main effect between motor task and dual task. However, the ANOVA results provided support for the hypothesis that 'right hand performance on the dual task would decline as both the tasks require resources from the left hemisphere and that the left hand would remain unaffected by the dual task demands', as a significant interaction was found between handedness and task implying interference is more likely to occur when using a hand requiring the same hemispheric resources as the other task. These results are in accordance with those proposed by the functional distance hypothesis.
Table 1 show the means and standard deviations for the amount of peas collected across the four conditions. The data shows that participants performed poorer in the left motor activity than right motor activity. The difference in motor task performance may be due to the fact that the dominant hand may have already established the resources required for motor activities therefore have the ability to adjust to the motor task with ease, where as the left hand may not have established an motor pattern therefore heavily reliant upon neural resources in order to complete the task - resulting in poor performance. It can also be seen that left motor task performance is significantly poorer than left dual performance, a decline in performance would have been expected, but a significant rise in performance occurred this can be explained due to practice effects to an extent but as the task conditions were counterbalanced this is fairly unlikely. Alternatively this can be explained by the neural resources required for the motor task relied upon the right hemisphere and the speech task relied upon the left hemisphere therefore the tasks demand were divided between the two hemispheres consequently resulting in less interference and better performance. The right hand motor activity was significantly better than left hand motor activity - this was expected as all participants were predominantly right handed. Figure 1 shows there is an evident decrement in performance in right dual task this decline is in line with functional distance hypothesis as both tasks required resources from the left hemisphere which were anatomically close resulting in interference.
Therefore, from the results it can be safely concluded that interferences is more likely and obvious when doing a dual task which requires resources from the same hemisphere. The current study's findings support Kinsbourne & Cook (1971) findings that there was greater interference when dual tasks required resources from the same hemisphere. The study's findings can also be linked to Simon and Sussman (1987) findings as a decline in performance for the dominant hand was found. Furthermore the results support Chang & Hammond (1987) that participants dominating their right hand showed a greater decline in performance than participants dominating their left hand. Additionally supporting Seth - smith et al (1989) findings that motor tasks relied upon the same processing resources as the movement of the right hand resulting in interference. The study's finding of an significant interaction between handedness and task, can be interpreted as support for Broca (1861) belief that the cerebral hemispheres are contra lateral and that language is lateralised in the left hemisphere for right hand dominants, as right hand dual task resulted in interference implying the participants had dominant left hemispheres which controlled both speech and motor control activities.
The researchers acknowledged a few limitations of the study. One limitation of the study is that only thirty participant's from the University of Central Lancashire participated. The participants were all right hand dominant and participated in all four conditions. This is a relatively small sample size therefore it may have caused the results to be unrepresentative and biased and inevitably reduced the statistical power of analysis. In future replication of the study should be conducted with a larger sample to increase generalization and accuracy of results. A flaw within the selection of participants is that no note of age or gender was made; mature participants may have performed the task with much more ease due to advanced motor skills in comparison to younger participants. Furthermore the participants were voluntary; this may have produced a sampling bias because participants were willing to volunteer their time may differ in attitude to those who decline.
The methodology involved the completion of a dual task of speech and motor. The motor task involved transferring peas from one bowl to another a moderately easy task. The fatigue effect could have occurred leading to the loss of interest and resulting in poor performance or alternatively the learning effect may have occurred as the motor task was repeated in all conditions which may have resulted in better performance. In a future study a more difficult motor task could be used in order to see whether the interference was greater than that of a simple task. A more difficult task could be a finger tapping task which requires the participants to trace their finger in various shapes such as a triangle. The speech task in the current study was fairly simple, in future a much more difficult sentence could be employed to allow researchers to determine a more obvious interference between the hemispheric functions activated to carry out the task.
In the current study hand dominance was determined by asking three questions, although the participants answered that their right hand was dominant they may not always dominate their right hand in other tasks. Therefore in future hand dominance could be determined by getting the participants to complete several tasks prior to conducting the study.
In a future replication of the present study, changes can be made, such as a more detailed measurement to be taken of motor activity by measuring the rate at which peas are transferred from one bowl to another. It would also be beneficial to make sure that both the motor and speech task are difficult enough so that the participants are not able to perform the tasks automatically with little effort or allow the learning effect to occur as quickly. A replication of the present study using left dominant participants may able to support and evaluate any difference found in the population allowing understanding of whether the interference found was based upon hand dominance or processing demands in the hemispheres. This research could be extended to examine the affects of dual task demands on individuals with speech disorders such as aphasia, providing invaluable information on how the disorder could be treated.
The findings of the current study propose the need for future research into dual task interference. Future studies into dual task performance could look at how tasks are prioritized by looking at task preference to see if performance is affected. This could be done by conducting a study with three different conditions such as motor condition, dual condition and dual with a task preference - in which the researcher tells the participants to focus on one task more than the other. The dual condition would allow the researcher to determine the initial interference between the two tasks and the task preference condition would allow the researcher to verify whether task preference resulted in significant performance changes.
In conclusion the study supported the hypothesis; 'right hand performance on the dual task would decline as both the tasks require resources from the left hemisphere and that the left hand would remain unaffected by the dual task demands', as the results obtained showed an evident decline in right hand dual performance. The results further support the hypothesis there would be a significant difference in performance between right and left hand motor activity. Therefore the study's findings support the belief that language is lateralized in the left hemisphere as the right hand dual task showed evident interference. The lateralization of language is valuable to surgeons who treat brain damaged patients; this knowledge will help limit damage to speech by ensuring the Wernickes and Brocas area in the left hemisphere are not damaged during surgery. Moreover the study's findings can be extended and applied to people with communication disorders such aphasia in order to help treat their disorder and help suffers cope with everyday tasks, which often require dual task performance.
1. Anon (2000). Language lateralization in healthy right-handers. Brain, 123(1), 74-81.
2. Chang, P., & Hammond, G. R. (1987). Mutual interactions between speech and finger movements. Journal of Motor Behavior, 19, 265-274.
3. Kinsbourne, M., & Hicks, R. E. (1978). Functional cerebral space: A model for overflow, transfer and interference effects in human performance: A tutorial review. In J. Requin (Ed.), Attention and Performance VII (pp. 345-362). Hillsdale, NJ: Laurence Erlbaum Associates.
4. Norman, D. A., & Bobrow, D. G. (1975). On data-limited and resource-limited processes.Cognitive Psychology, 7, 44-64.
5. Seth-Smith, M., Ashton, R., & McFarland, K., (1989). A dual-task study of sex differences in language reception and production. Cortex, 25, 425-431.
6. Simon, T. J., & Sussman, H. M. (1987). The dual task paradigm: Speech dominance or manual dominance? Neuropsychologia, 25, 559-569.
7. Springer, S. & Deutsh, G. (1947) Left Brain, Right Brain. 4th edition. New York. W.H Freeman
8. Wickens, A (2005) Foundations of Biopsychology. 2nd Edition. Harlow: Prentice Hall