Origins and Functions of Laterality: Interactions of Motoric Systems

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Abstract
To evaluate lateral motoric bias in response systems at different levels of the neuraxis and assess the extent of interaction between these levels in the smalleared bushbaby (Otolemur garnettii), 27 animals were tested for lateral bias in hand use and whole-body turn bias in two postural conditions. Subjects retrieved mealworms quadrupedally by reaching downwards into glass jars and bipedally by reaching upwards to baited straws. Eye bias was assessed separately. Behaviours were scored from videotape. Two subgroups were identified: SHIFTERS changed hand preference with posture and had correlations of hand/eye bias in quadrupedal posture and of hand/turn bias, with more bimanual reaching, in bipedal posture; NONSHIFTERS were consistent in hand preference and more strongly lateralised in reach and turn than SHIFTERS. Subgroups did not differ in reach efficiency. Results are interpreted to support the value of the analysis of motoric levels and their interactions in the study of the evolution of laterality. Assuming natural selection for coordinated and targeted behaviours to be the source of lateralisation, several proposals in support of a motoric theory of laterality origins and functions are advanced.