Fig. 1.

Sliding condition: Familiarization events. At the beginning of each familiarization event, the actor sat on a wooden chair centered behind the window in the apparatus’s back wall; she wore a blue shirt and a white visor. A muslin curtain behind the actor hid the test room. In each familiarization event, an object stood on the apparatus floor 10 cm in front of the window; the center of the object was positioned 30.5 cm from the left wall. The object used in the first familiarization event was a plastic toy fish 7.5 cm high, 5 cm wide, and 9.5 cm long; it was bright pink and decorated with black stripes. The object used in the second familiarization event was a cardboard box 6.5 cm high, 8 cm wide, and 14 cm long; it was covered with green contact paper and its edges were outlined with yellow tape. The object used in the third familiarization event was a baby shoe 5 cm high, 6.5 cm wide, and 11.5 cm long; it was made of blue denim fabric and had a white shoelace and a white rubber sole. The actor’s bare right hand rested on the apparatus floor with the tip of the middle finger 4 cm behind the object. Each familiarization trial consisted of a 5-s pre-trial followed by a main-trial. During the pre-trial, there was a 4-s pause (to give the infants the opportunity to inspect the object), and then the actor reached for and grasped the object (1 s). During the main-trial, the actor repeated the following sequence of actions until the trial ended: she slid the object forward 15 cm (2 s), paused (1 s), slid the object back to its original position (2 s), and again paused (1 s). To help the actor adhere to the script just described, a metronome beat softly once per second.