I am a comparative psychologist interested in laterality and motor skill. My work has spanned every major taxonomic level in primates including black-and-white ruffed lemurs, spider monkeys, rhesus monkeys, and chimpanzees. In the human domain, my expertise is in early development from infancy through preschool age. My research has been funded by the National Institutes of Health (NIH/NICHD) and several intramural grants. In 2019, I was awarded the FIU CASE Faculty Award for Research.
Measurement matters for charting the development of handedness. My work emphasizes balancing motor challenge with a rapidly changing manual repertoire in measuring the development of handedness. For infants, hand preference is typically calculated from reaching. Once infants can grasp objects, the more complex skill role-differentiated bimanual manipulation (RDBM) emerges, which challenges children to use one hand to stabilize an object and the opposing hand to manipulate the object (scored as the preferred hand). I have developed two measures to index RDBM, one for toddlers and the other for preschoolers, which filled two gaps in the literature. While fluctuations in early hand use have been interpreted as evidence that handedness is unstable until later childhood, I have found that only a subset of children exhibit variable hand use trajectories; some children show consistent hand use preferences from infancy. Understanding the development of handedness requires longitudinal designs and robust sampling to correctly identify these different patterns.
Hand use patterns predict distal language outcomes in children. Children are variable in how they use their hands as early as 6 months of age, and these differences can be parsed into patterns using longitudinal designs. Children also differ in language as early as 2 years, and this word gap has a negative impact on later achievement. I have found that handedness trajectories account for over one-quarter of the variability seen in language skill at 2 years of age. Children who exhibited a consistent pattern of right preference from infancy through toddlerhood had the highest language scores compared to children who exhibited no hand preference as infants, but a clear pattern of right or left preference as toddlers. Consistency in toddler hand use continues to predict greater language skill at 3 and 5 years of age. Hand preference trajectories explain language outcomes beyond SES, and infant patterns are a stronger predictor relative to toddler patterns. These findings have important implications for characterizing motor-language cascades.
Spider monkeys exhibit lateralized behaviors across motor and social domains. The spider monkey is an excellent model for testing hypotheses regarding primate laterality because (1) their four-digit hand structure differs from most primates; (2) their prehensile tail functions like a fifth limb and is capable of manipulation; and (3) their social structure mirrors humans. I have shown that spider monkeys can isolate a single digit, which increases the expression of handedness. These data overturn the long-held claim that spider monkeys do not have independent digit control. Spider monkeys can solve goal-directed tasks with the tail, and skilled tail use is highly lateralized. Finally, there is a left bias in spider monkey embraces, but no side bias for grooming, suggesting that lateralization in spider monkeys' social behavior may be linked to differences in social risk.
Laterality is an organizing principle in primate manual control. A natural assumption regarding laterality in the motor system is that the dominant hand/limb is universally superior to the nondominant hand/limb. However, each hemisphere may instead be specialized for particular motor control processes that work together in a coordinated fashion, a hypothesis known as dynamic dominance. My work using 2D and 3D motion analysis has tested these two possibilities using various reach-to-grasp tasks. I have shown that there is no systematic pattern between hand preference and limb control, or hemispheric specialization and limb control, in human infants. In adults however, observed interlimb differences favored the right side regardless of handedness. Rhesus monkey infants exhibited a similar pattern to human adults, but observed interlimb differences favored the left side regardless of handedness. Spider monkeys perform reach-to-grasp movements almost exclusively with the preferred hand, and reach kinematics vary as a function of the grasp to be used and the object's size.