In this study, different functions which are known to be lateralized to either the right or left hemisphere were used during a Dichotic Listening test, dominant recall from concerned hemispheres was intended. The processing of nouns as a linguistic function is lateralized within the left hemisphere for the majority of people, just as the recognition of melodies as a musical ability is lateralized within the right hemisphere for most people. Left hemisphere involvement in language functioning has been suggested by fMRI, Dual Tasks, Dichotic Listening tasks and PET studies which indicate a left hemisphere dominance for noun processing and recall (__). Must as melody recognition requires a certain level of musical ability and it's functioning is shown to reside in the right hemisphere (__).
In this study it was predicted that for the noun condition a left hemisphere advantage would be displayed and for the music condition a right hemisphere advantage would be displayed.
The main findings of this study confirmed hypothesis one. The findings here are consistent with the evidence regarding lateralization of language functioning; as a significant REA for noun recall was found; signifying left hemisphere dominance for noun processing.
A number of studies support the left hemispheric dominance for language processing and comprehension, (Hughdahl, 2005; Knecht, 2004; Sakai et al, 2005; Whitehouse, 2008) especially noun recall (Kimura (1961); Knecht, 2000; Gonzalez, 2009). For example Asbjørnsen and Helland (2006) described how Dichotic Listening tasks, display left hemisphere localisation for language functions. It appears that when opposing verbal stimuli are presented to both ears, "the stimuli arriving at the ear opposite the dominant hemisphere are more effectively acknowledged" (Kimura 1961). This support the views put forth by Kimura (1961) and others that it is the crossed auditory pathways that hold an advantage when dichotically presented stimuli. Under normal circumstances there is no competition between the pathways to both hemispheres as verbal stimulus enters both ears (__). Normal individuals have efficient pathways and it is only when material is presented simultaneously that this competing of pathways occurs; resulting in contralateral pathways gaining an advantage. I.e. a left hemisphere dominance for noun recall and language processing was displayed as stimuli presented to the left ear has an advantage (Kolb)
In terms of the neuroanatomical representaion, the REA found within this study also supports findings of functional lateralization. Left hemisphere dominance for language processing, displayed by the REA found within this study, can be explained by the known participation of the left temporal cortex and paretial cortex in the comprehension of language (Longstaff, 2005). As stated earlier, the presence of Wernicke's area, known to be the sensory area involved in the understanding of speech (Wernicke, 1995), tends to be larger in the left side of the brain in a majority of people; also explain the REA found within this study for noun recall (Hugdahl, 2005; Steinmetz, 1996; grey book). As the left hemisphere is critical to the understanding of human language; producing higher levels of noun recall (Purves, Augustine, Fitpatrick et al, 1997). The presence of Broca's area in the left side of the brain can also help explain the REA found within this study; as this is the area known for the processing of language functions (Broca, 1864); backing up a left hemisphere dominance for language functioning. However significant results within this study can only put forth the theory of left hemisphere dominance of noun recall and cannot specify regions of the brain involved in the process.
The significant REA for the processing of nouns within this study supports the growing evidence that language functioning resides within the left hemisphere (__). However, Dichotic Listening performance had been found to be influenced by a number of factors. Dichotic Listening does not recognise the contribution of the structure of auditory pathways
testing for noun recall and language functioning has been known to produce a REA bias.
A theoretical model for
DL performance must recognise the contribution of the structure of the
auditory pathways, the dynamic changes in performance and perception
caused by attention and emotional states, and the unique contributions of
both cerebral hemispheres in the processing of the given stimulus, in order to
predict dichotic listening performance
Asbjørnsen and Turid Helland 2006
Low levels of noun recall displayed by some participants can be accounted for by the high demand of unrelated cognitive functions such as attention and memory. As information is absorbed it enters the sensory memory store, which holds incoming sensory information for a brief amount of time before it is either lost or passed into the short term memory store for quick retrieval (Atkinson & Shiffrin 1968; Sperling, 1960). The short term memory is known to have a capacity of 7 + or - 2 and information is thought to be held for a maximum of 30 minutes (__). Short term memory allows for a recall period of several seconds to a minute without rehersal (__). Participants tended to remember words either played immediately or last, during the Dichotic Listening task, displaying classic signs of retrieval before decay. (__). As the Dichotic Listening test involved a lot of competing words it can be thought that participants could only pass minimal information into the short memory store before decay took place (Miller, 1956. wiki); producing average levels of noun recall from the left ear of around __ words. Levels of noun recall may depend on individual memory capacity, however this does not dispute the REA displayed for language comprehension, only the low levels of noun recall demonstrated by some participants.
Some participants did however display LEA for noun recall as previously stated by (__)
This may be accounted for by
Hypothesis two was confirmed as Dichotically presented musical stimuli yielded a significant left ear advantage (LEA) for the majority of participants (N=24); signifying a right hemisphere advantage for musical processing. Some participants, however, displayed a right ear advantage (REA); signifying left hemisphere dominance.
A number of studies support the right hemispheric dominance for musical ability, (Kimura, 1964; Boucher & Bryden, 1997; Nevlan, 2009) especially melody recognition (__). For example __ (__) described how Dichotic Listening tasks, display right hemisphere localisation for musical functioning. The LEA advantage found for melody recognition within this study, can also be accounted for through the same process as described for the REA of noun recall. During the Dichotic Listening task, when opposing musical auditory stimuli are presented to both ears, the contralateral pathway holds an advantage; resulting in a LEA, supporting a right hemisphere advantage for musical ability (__).
In terms of the neuroanatomical representaion, the LEA found within this study also supports findings of functional lateralization. Right hemisphere dominance for melody recognition, displayed by the LEA found within this study, can be explained by the known participation of the auditory cortex, right prefrontal cortex and the cerebellum in the processing of music; all of which are larger in the right hemisphere (Longstaff, 2005??)
LEA found within this study may also be a result of the greater activation level found in the cerebral cortex during musical memory (Strickland, 2001), as stated earlier. Also, the involvement of the right auditory cortex in the perception of harmony, and melody perception (Tramo, 2001). The LEA displayed within this study adds to the belief that areas in the prefrontal cortex, the cerebellum and the superior temporal gyri, which tend to be skewed towards the right hemisphere, play a major role in the perception of melodies (Snyder and Large, 2005). However significant results within this study can only again put forth the theory of right hemisphere dominance of musical functioning and cannot specify regions of the brain involved in the process.
It an also be argued that this LEA displayed could be attributed to a contour-based type of melody recognition supported by the right hemisphere (Bever and Chiarello, 1974). These results do not contradict these arguments.
These results seem to contrast with previous reports of right hemisphere dominance for melody recognition in both musicians and non musicians. (__). However these studies used a more complex set of musical material such as excerpts from musical pieces or melodies consisting of several tones. (__)
Explanations for a REA as displayed by a small proportion of participants within this study (N=5) may be accounted to the possibility of a musical advantage known to reside in musically trained individuals (Davidson & Schwartz, 1977; Sergent, Zuck, Terriah, & MacDonald, 1992; Vollmer-Haase, Finke, Hartje, & Bullahellwig, 1998). Other forms of testing such as the Wada procedure display bilateral characteristics of musical functioning (Gordon & Bogen, 1974). Zatorre (1984) also found this to be the case in brain damage individuals concluding there may not be cerebral dominance for musical ability but bilateral characteristics depending on characteristics of musical processing (Springer, 1997). As did research into individuals with amusia (Edgren, 1895; Henschen, 1920; Kleist, 1928; all cited in Benton, 1977).
It an be thought that it could not only be the neurological make up of an individual that determines laterality but also the learning capacity.(Limm et al, 2001). No advanced form of musical training was reported by any participants, however individual neurological characteristics, such as more developed left planum temporales found in musicians (__), may produced results of a REA for music recognition compared to the universal thoughts of LEA.
Inconsistent results may also be a consequence of Dichotic listening bias. Right hemisphere dominance in dichotic listening.
Bilateral functioning of melody recognition for musicians in dual task.
Puts into question whether there is an actual laterality of functions or a bias portrayed depending on method used.
The significant LEA for the processing of music found within this study supports the growing evidence that musical ability lies within the right hemisphere (Matteis et al., 1997; Zatorre, 1984 MORE!). However, the task did pose a low demand on other cognitive functions, such as attention and also memory. Possible explanations for subjects who did not show a LEA for musical processing could lie in these other functions. Participants displaying REA may have held stronger memories for the musical tune played into that ear.
One of the challenges of this research is that all people have different memory functions (__) and this was not taken into account when results were analysed. This may corrupt the findings; demonstrating memory functions rather than hemisphere dominance of cognitive functions.
Handedness was also not taken into consideration; left handed individuals sometimes show cerebral dominance of the right hemisphere not left hemisphere for overall lateralisation of cognitive functions, Kneckt (2000)
Limitations also included sample group size; more valid and reliable results could have been obtained if more people were tested.
The results of the present study contribute to theorising of brain laterality and hemisphere dominance of noun comprehension and melody recognition.