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To explain this I have to step back and show you something interesting about adults when they perform, and when they observe actions.

Performing actions (e.g. stacking blocks): you don't look at your hand but at the block it will pick up, or, when holding a block, at the location where it will place a block. That is, in acting the eyes move just ahead of the action.

‘Positive relative times of the arrival of gaze at the goal area indicate that gaze precedes the agent’s arrival (predictive); negative values indicate gaze arrival after agent arrival (non-predictive). Each age group is n = 12. Error bars are s.e.m.’

GH : grasping hand; BH : back of hand; MC : mechanical claw

Kanakogi & Itakuar, 2011 show that abilities to grasp objects are correlated with abilities to track the goal of a grasping action (as measured by proactive gaze).

x-axis is alpha, grasping angle. ‘An α angle value from 90 to 180° indicates that the infant is engaged in a one-handed grasping action.’

‘The angle α is an index of the development of the one­handed grasping action and was calculated by measuring the angle of a straight line de ned by the infant’s two hands (the apex of the junction of the thumb and index nger) when crossed by an imaginary line projecting frontally from the infant (Fig. 2b). If infants grasped for the objects with their le hand, we reversed the red right­angled triangle from one side to the other side and calculated the angle α in the same way. e angle α value of 90° corresponded to a perfect alignment of the hands in a two­handed reach. erefore, the angle α value deviates from 90° towards 180°, and bigger angle α value indicates more mature one­handed grasping. If the angle α was over 90°, the infant was considered to be engaged in a one­handed grasping action.’

Further support for a link between action performance and goal tracking comes from a developmental study by Ambrosini et al which studied whether proactive gaze in infants is influenced by pre-shaping of the hand, and, in particular, whether it is influenced by precision grips.

By using no shaping (a fist), Ambrosini et al could treat sensitivity to whole-hand grasp and precision grip separately.

‘infants’ ability to perform specific grasping actions with fewer fingers directly predicted the degree with which they took advantage of the availability of corresponding pre-shape motor information in shifting their gaze towards the goal of others’ actions’ (Ambrosini et al., 2013, p. 6).

Further, changing from a bodily action to the operation of a mechanical claw (say) undermines the goal tracking effect.

So Kanakogi & Itakuar, 2011 make two points: (i) goal-tracking depends on action capabilities; and (ii) only works for events involving biomechanically similar affectors

Needham et al, 2002 showed that putting ‘sticky mittens’ on 3-month-old infants (for 10-14 play sessions of 10 minutes each) resulted in their spending more time visually and manually exporing novel objects.

The observation was based on this study, which we saw earlier

The results show that infants who played wearing the mittens first were more attentive to the goal.

But one potential objection to this study concerns observation vs performance. The infants who played wearing sticky mittens first had spent longer observing actions by the time it came to the violation of expectations trial. Could it be observation of action (including one’s own) rather than performance that matters?

To address this issue, Sommerville et al. (2008) did a study in which one group had observation while the other group had performance. The participants were 10-month-old infants this time.

The materials were a bit different: so that training vs observation could be as similar as possible with respect to the causal structure exposed, there was a hook to get an object.

The results show that infants with the training paid attention to the distal goal (choice of toy) whereas those without paid attention to the choice of cane.

... or IMPAIR INDIRECTLY ...

Experiment 1 : shows that 6-month-old infants can distinguish a phonetic contrast they have never heard before (one that occurs in Hindi but not their linguistic environments.) (The contrast used was the Hindi dental /d/̪ versus retroflex /ɖ/ distinction.)

These graphs show a difference in mean looking time between cases in which phonemes are alternated and cases in which they are not. (Iff infants distinguish, they should find the alternating phonemes more interesting.)

Experiment 2: but not when they have a tongue-controlling dummy in their mouths

Experiment 3: but yes when they have a dummy which leaves the tongue free.