An object-identity probability cueing paradigm during grasping observation: the facilitating effect is present only when the observed kinematics is suitable for the cued object

Abstract

Electrophysiological and psychophysical data indicate that grasping observation automatically orients attention toward the incoming interactions between the actor's hand and the object. The aim of the present study was to clarify if this effect facilitates the detection of a graspable object with the observed action as compared to an ungraspable one. We submitted participants to an object-identity probability cueing experiment in which the two possible targets were of the same dimensions but one of them presented sharp tips at one extreme while the other presented flat faces. At the beginning of each trial the most probable target was briefly shown. After a variable interval, at the same position, the same (75%) or a different target (25%) was presented. Participants had to press a key in response to target appearance. Superimposed to the video showing cue and target, an agent performing the reaching and grasping of the target was presented. The kinematics of the action was or was not suitable for grasping the cued target, according to the absence or presence of the sharp tips. Results showed that response was modulated by the probability of target identity but only when the observed kinematics was suitable to grasp the attended target. A further experiment clarified that response modulation was never present when the superimposed video always showed the agent at a rest position. These findings are discussed at the light of neurophysiological and psychophysical literature, considering the relationship between the motor system and the perception of objects and of others' actions. We conclude that the prediction of the mechanical events that arise from the interactions between the hand and the attended object is at the basis of the capability to select a graspable object in space.

Keywords: action observation; action prediction; affordances; attention to objects; premotor theory of attention; simple reaction times.

FIGURE 1

Experiment 1. Five frames relative to each of the six videos used as stimuli. The object shown at step (a) for 1000 ms is the Cue and represents the Target appearing in 60% of the trials (Same trials). In 20% of the trials the uncued Target was presented (Different trials). In 20% of the trials the Target was not presented. Participants had to respond by tapping with their right index finger on a conducting pad placed on the table when Target object appeared (flat–flat, flat–sharp, sharp–sharp, sharp–flat), and to refrain to tap when the object did not reappear (flat-catch, sharp-catch). In one session the kinematics of the agent was suitable to grasp the cued object (Cue congruent kinematics). In the other session the kinematics of the agent was not suitable to grasp the cued object (Cue incongruent kinematics). See text for more details.

FIGURE 2

The two objects presented in the videos. On the left, the object grasped by the experimenter in the original video (flat–flat video). On the right, the object artificially introduced via software as Cue (sharp–flat and sharp-catch videos), as Target (flat–sharp video), and as Cue and Target (sharp–sharp video).

FIGURE 3

Mean reaction times (RTs) of responses. Data for valid and invalid trials when the kinematics of the agent was suitable (congruent kinematics) and when it was not suitable (incongruent kinematics) to grasp the cued target are shown. Thin lines above histograms indicate standard error of the mean. Ordinates are in milliseconds.

FIGURE 4

Experiment 2. Three frames relative to each of the six videos used as stimuli. The stimuli and procedure are those described in Figure 1. The only difference consisted in the fact that the agent shown in the background was not moving, as in Experiment 1, but she was always presented at a rest position as in step (a) of Figure 1.