Thesis advisor: Sara Cordes / Thesis advisor: Ellen Winner / Evidence suggests that humans and non-human animals have access to two distinct numerical representation systems: a precise "object-file" system used to visually track small quantities (<4) and an approximate, ratio-dependent analog magnitude system used to represent all natural numbers. Although many studies to date indicate that infants can discriminate exclusively small sets (e.g., 1 vs. 2, 2 vs. 3) or exclusively large sets (4 vs. 8, 8 vs. 16), a robust phenomenon exists whereby they fail to compare sets crossing this small-large boundary (2 vs. 4, 3 vs. 6) despite a seemingly favorable ratio of difference between the two set sizes. Despite these robust failures in infancy (up to 14 months), studies suggest that 3-year old children no longer encounter difficulties comparing small from large sets, yet little work has explored the development of this phenomenon between 14 months and 3 years of age. The present study investigates (1) when in development infants naturally overcome this inability to compare small vs. large sets, as well as (2) what factors may facilitate this ability: namely, perceptual variability and/or numerical language. Results from three cross-sectional studies indicate that infants begin to discriminate between small and large sets as early as 17 months of age. Furthermore, infants seemed to benefit from perceptual variability of the items in the set when making these discriminations. Moreover, although preliminary evidence suggests that a child's ability to verbally count may correlate with success on these discriminations, simply exposure to numerical language (in the form of adult modeling of labeling the cardinality and counting the set) does not affect performance. / Thesis (PhD) — Boston College, 2015. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Psychology.

Two studies investigated the effects of various motor constraints of reaching on infants’ search performance on the A-not-B task. These studies were motivated by the idea that motor memories for reaching lead to A-not-B errors. The 2 motor constraints that were evaluated included barriers that blocked the path of the hand and hand-use preferences. Each of these motor constraints was examined separately. In Experiment 1, infants (N = 40, 20 8-month olds, 20 16-month olds) were given the A-not-B task twice. One condition was analogous to the traditional A-not-B task (i.e., using 2 hiding locations) and the other was modified such that a barrier (i.e., an opaque screen) blocked the infants’ reaching path of location A on A trials only. On A trials, all infants searched correctly less often when a barrier was present, and younger infants searched correctly less often than older infants. On B trials, younger infants made more errors in the no barrier condition, whereas older infants did not show any significant difference in B trial performance across conditions. In Experiment 2, infants (N = 51) completed an adapted handedness test (Michel, Ovrut, & Harkins, 1985) followed by a modified A-not-B task. The test assessed infants’ hand-use preferences for reaching, which was used to group infants into their respective preference group (i.e., consistent or inconsistent). Infants with a consistent preference were randomly assigned to a hiding side group (i.e., A on preferred side or A on non-preferred side). Infants searched correctly more often when hiding side was congruent with their preferred reaching hand, and older infants searched correctly more often than younger infants. On the B trial, neither age nor hiding side affected the production of the A-not-B error. Collectively, these studies present data that address the theory that motor memories for reaching are the cause for the production of A-not-B error. These studies provide novel evidence that motor memories for reaching are present in infants aged 8- and 16-months, and that motor memories can influence the production of such errors in certain A-not-B contexts. Implications and directions for future research are also discussed.

Two studies investigated the effects of various motor constraints of reaching on infants’ search performance on the A-not-B task. These studies were motivated by the idea that motor memories for reaching lead to A-not-B errors. The 2 motor constraints that were evaluated included barriers that blocked the path of the hand and hand-use preferences. Each of these motor constraints was examined separately. In Experiment 1, infants (N = 40, 20 8-month olds, 20 16-month olds) were given the A-not-B task twice. One condition was analogous to the traditional A-not-B task (i.e., using 2 hiding locations) and the other was modified such that a barrier (i.e., an opaque screen) blocked the infants’ reaching path of location A on A trials only. On A trials, all infants searched correctly less often when a barrier was present, and younger infants searched correctly less often than older infants. On B trials, younger infants made more errors in the no barrier condition, whereas older infants did not show any significant difference in B trial performance across conditions. In Experiment 2, infants (N = 51) completed an adapted handedness test (Michel, Ovrut, & Harkins, 1985) followed by a modified A-not-B task. The test assessed infants’ hand-use preferences for reaching, which was used to group infants into their respective preference group (i.e., consistent or inconsistent). Infants with a consistent preference were randomly assigned to a hiding side group (i.e., A on preferred side or A on non-preferred side). Infants searched correctly more often when hiding side was congruent with their preferred reaching hand, and older infants searched correctly more often than younger infants. On the B trial, neither age nor hiding side affected the production of the A-not-B error. Collectively, these studies present data that address the theory that motor memories for reaching are the cause for the production of A-not-B error. These studies provide novel evidence that motor memories for reaching are present in infants aged 8- and 16-months, and that motor memories can influence the production of such errors in certain A-not-B contexts. Implications and directions for future research are also discussed.