10-Month-Old Infants Are Sensitive to the Time Course of Perceived Actions: Evidence From a Study Combining Eye-tracking and EEG

Research has shown that infants are able to track a moving target efficiently – even if it is transiently occluded from sight. This basic ability allows prediction of when and where events happen in everyday life. Yet, it is unclear whether, and how, infants internally represent the time course of ongoing movements to derive predictions. In this study, 10-month-old crawlers observed the video of a same-aged crawling baby that was transiently occluded and reappeared in either a temporally continuous or non-continuous manner (i.e., delayed by 500 ms vs. forwarded by 500 ms relative to the real-time movement). Eye movement and rhythmic neural brain activity (EEG) were measured simultaneously. Eye movement analyses showed that infants were sensitive to slight temporal shifts in movement continuation after occlusion. Furthermore, brain activity related to sensorimotor rather than mnemonic processing differed between observation of continuous and non-continuous movements. Early sensitivity to an action’s timing may hence be explained within the internal real-time simulation account of action observation. Overall, the results support the hypothesis that 10-month-old infants are well prepared for internal representation of the time course of observed movements that are within the infants’ current motor repertoire.

before averaging.

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The analysis focused on infants' gaze behavior in reaction to the moving stimulus. However, it is 2 9 8 difficult to quantitatively determine gaze relative to moving objects based on raw gaze positions.

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To relate gaze and stimulus position, the midpoint of the minimal and maximal x-value of the does not represent a linear movement (see black dotted line in Figure 3A). Following, at each For statistical analysis, within subjects, the mean distance as well as the variance in distance 3 1 1 between gaze and stimulus position were calculated for each trial across predefined 500 ms time 3 1 2 windows for each phase of the trial (i.e., the last 500 ms of the pre-occlusion, the 500 ms of the was thus taken to reflect tracking accuracy. Variance in distance represents the average 3 1 7 fluctuation in tracking behavior, and was thus taken to reflect tracking consistency (i.e., whether 3 1 8 tracking was rather consistent or random across infants). according to the 10-20-system (EASYCAP GmbH, Herrsching, Germany). During recording, the 3 2 4 right mastoid electrode served as reference and the left mastoid was recorded as an additional 3 2 5 channel. Ground was placed at location AFz. Impedances were kept below 20 kΩ during 3 2 6 preparation. The EEG was recorded with an analog pass-band of 0.1 to 250 Hz and digitized with 3 2 7 a sampling rate of 1000 Hz.

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Prior to EEG-preprocessing, based on behavioral coding of video-recordings, trials were 3 2 9 discarded if infants (a) did not attend to the total duration of stimulus presentation and (b) produced limb movement that could be seen as part of imitative crawling. The latter criterion was 3 3 1 chosen because we were interested in brain activity related to action observation but not to 3 3 2 imitation. Furthermore, using Vision Analyzer 2 (Brain Products) for visual inspection, EEG trials 3 3 3 were discarded which comprised broken channels or extreme/untypical artifacts (i.e., extensive 3 3 4 movements). To this end, remaining EEG data were segmented into 4700 ms long epochs (from - were conducted using the FieldTrip (developed at the F.C. Donders Centre for Cognitive Neuroimaging, Nijmegen, The Netherlands; http://www2.ru.nl/fcdonders/fieldtrip/, Oostenveld et  pass 1 Hz, low pass 100 Hz, 6 th -order Butterworth-filter), and subjected to an extended infomax 3 4 5 Significant effects were followed up by separate Bonferroni-corrected ANOVAs or t-tests. depicted a movement that started500 ms earlier/later in the movement sequence than in the conditions throughout the pre-occlusion phase (see Figure 2). Accordingly, gaze positions were 4 1 2 about 150 pixels further backward in forwarded (see gray dotted line in Figure 3A) and further adaptation to non-matching stimulus reappearance in repeated/block stimulus presentation. infants gazed opposite the movement direction (solid black line in Figure 3A). This was followed between the experimental groups: In the Delay group, continuous movement resulted in a comparably at about 50 pixels mean distance (i.e., at front parts of stimulus) 700 ms post 4 5 9 occlusion-offset, showing that infants quickly caught up with the actual stimulus movement. temporal shifts in the continuation of transiently occluded movements. To analyze the mean distance as a marker for tracking accuracy in 500 ms time windows before, 4 6 7 during, and following occlusion, a mixed effects repeated-measures ANOVA was performed. The To evaluate the (d) three-way interaction effect, a total of six paired-sample t-tests were  To analyze the variance in distance as a marker of tracking consistency in 500 ms time windows 4 9 1 before, during, and following occlusion, a mixed-effects repeated-measures ANOVA was Using paired-sample t-tests to follow up on the main effect of (a) Phase indicated that variance in To follow-up on the main effect of (b) Group, an unpaired t-test showed that variance in distance In sum, variance in distance increased due to transient occlusions. In addition, tracking was less altered by further processing (e.g., learned expectations across repeated presentations). 1.41; all p > .250). Figure 6 provides an overview of the EEG results. Hence, counter to 5 1 9 expectations, no differential activation of frontal theta activity was found, indicating that the 5 2 0 manipulation of the time course of ongoing movement did not elicit differential demands on 5 2 1 memory processes. To analyze contributions from sensorimotor simulation to time-course representations, a mixed effects were observed (all F < 2.14, all p > .123).

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As also implied by the small effect size, follow-up repeated measures ANOVAs separately per results as displayed in Figure 6 it may be concluded that, during post-occlusion, central alpha In fact, the tracking patterns of both experimental groups were found to be unexpectedly Though illustrating infants' remarkable sensitivity to an action's time course, these findings 6 4 4 cannot solely be explained in terms of internal real-time processing. We can, however, only 6 4 5 speculate as to which processes may have contributed to the pattern of results.

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First, the present findings suggest that delayed and forwarded time-shifts in observed human 6 4 7 action are not processed similarly (Bremner et al., 2005;Striano et al., 2006). This corresponds to Second, the present findings may indicate that continuous movements are not always perceived as such (see also Adler et al., 2008). An influence of the stimulus context on action perception may 6 5 9 be explained in accordance with priming effects (e.g., Pavlova and Sokolov, 2000). For example, 6 6 0 when adults first performed a seemingly unrelated motor task (e.g., arm movement) and later 6 6 1 occlusion suggest that infants did not learn that the stimulus' reappearance position was kept There was a considerable drop-out on the level of trials and participants in both eye and brain Wiley. Standardization of automated analyses of oculomotor fixation and saccadic behaviors. Marshall, P.J., Bar-Haim, Y., and Fox, N.A. (2002). Development of the EEG from 5 months to 4 8 4 7 years of age. Clin. Neurophysiol. 113, 1199-1208. doi: 10.1016/S1388-2457 (02) execution in 14-month-old infants: an event-related EEG desynchronization study. Dev. Mitrani, L., and Dimitrov, G. (1978). Pursuit eye movements of a disappearing moving target.