Orientation CP is the faster or more accurate discrimination of two orientations from
different categories (e.g., oblique1 and vertical1) compared to two orientations from
the same category (e.g., oblique1 and oblique2), even when the degree of difference
is equated across conditions. Here, we assess whether there are hemispheric asymmetries
in this effect for adults and 5-month-old infants. Experiment 1 identified the location
of the vertical-oblique category boundary. Experiment 2, using a visual search task
with oriented lines found that adult search was more accurate when the target and
distractors were from different orientation categories, compared to targets and distractors
of an equivalent physical difference taken from the same category. This effect was
stronger for targets lateralized to the left visual field (LVF) than the right visual
field (RVF), indicating a right hemisphere (RH) bias in adult orientation CP. Experiment
3, replicated the RH bias using different stimuli and also investigated the impact
of visual and verbal interference on the category effect. Experiment 4, using the
same visual search task, found that infant search was also faster when the target
and distractors were from different orientation categories than the same, yet this
category effect was stronger for RVF than LVF lateralized targets, indicating a LH
bias in orientation CP at 5 months. These findings are contrasted to equivalent studies
on the lateralization of color CP (e.g., Gilbert, Regier, Kay, & Ivry, 2005). The
implications for theories on the contribution of the left and right hemispheres of
the infant and adult brain to categorical computations are discussed.