The Mechanisms and Consequences of Cerebral Lateralization
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There is a clearly established division of functional processing between left and right hemispheres of the brain, with the pattern showing robust consistency across individuals. The finding of functional differences between the hemispheres of the brain raises two important questions: First, what mechanisms gave rise to the lateralized biases in processing function? Second, what are the consequences of functional asymmetry of cognitive processing in the human brain on our everyday behaviour? Examining the mechanisms that give rise to cerebral lateralization, Experiments 1 and 2 tested the assumption that there is a causal relationship in the degree and direction of lateralization between left- and right-hemisphere dominant tasks. In experiment 1, this relationship between left-hemisphere processing of speech sounds and right-hemisphere processing of emotional vocalizations was examined using dichotic listening tasks. An overall complementary pattern of lateralization was observed across participants, but no significant relationship was found for degree of lateralization of speech and emotional vocalization processing within individuals. These results support the view that functions in the left and right hemispheres are independently lateralized. In Experiment 2 we examined the relationship pattern in degree of lateralization between linguistic processing and melody recognition using dichotic-listening tasks. The expected left-hemisphere advantage was observed for the linguistic processing task, but the expected right-hemisphere advantage was not observed for the melody recognition task, precluding an informative assessment of complementarity between the two tasks. The division of processing between the two hemispheres of the brain has been shown to result in lateralized performance advantages and behavioural biases. Examining these consequences of lateralization, Experiments 3 through 6 explored the influence of lateral biases on everyday behaviour. Experiments 3 and 4 examined the influence of asymmetries in facial attractiveness on posing biases. Despite evidence suggesting that the right side of the face is found to be more attractive, professional modeling photographs examined in Experiment 3 revealed a leftward posing bias suggesting that asymmetries in facial attractiveness are not dominant in influencing posing behaviour, even when the purpose of the image is to highlight attractiveness. Experiment 4 controlled for image selection biases by examining posing behaviour directly and revealed a rightward posing bias when participants were asked to emphasize their attractiveness. Experiments 5 and 6 examined the influence of lateralized cognitive processing demands on seating preferences. Experiment 5 investigated the real-world seating patterns of theatre patrons during actual film screenings. It was found that, when processing expectations relied on right-hemisphere dominant processes, such as emotional, facial, or visuospatial processing, people were more likely to choose a seat to the right side of the room. Experiment 6 was designed to test two competing theories that have attempted to explain seating biases: one posits that expectation of processing demand drives the bias; the other posits that basic motor asymmetries drive the bias. Through naturalistic observation, I recorded classroom-seating choices of university students using photographs. When processing expectations relied on left-hemisphere dominant processes, such as linguistic processing, people were more likely to choose seats on the left side of the classroom; this finding contrasts the right side bias observed in theatre seating studies, providing evidence that expectation of processing demands influences the seating bias. Addressing the mechanisms that guide the evolution of lateralization, no support for the assumption of a causal relationship between complementary left-and right-lateralized cognitive functions was found. Additionally, examination of asymmetries in everyday behaviours such as seating and posing provide evidence that the lateralization of cognitive functions has a direct influence on human behaviour and interaction with the environment.
DegreeDoctor of Philosophy (Ph.D.)
SupervisorElias, Lorin J.
CommitteeBorowsky, Ron; Farthing, Jon; Vrbancic, Mirna
Copyright DateJune 2015