Elsevier

Cognition

Volume 109, Issue 3, December 2008, Pages 363-371
Cognition

Action goal selection and motor planning can be dissociated by tool use

https://doi.org/10.1016/j.cognition.2008.10.001Get rights and content

Abstract

The preparation of eye or hand movements enhances visual perception at the upcoming movement end position. The spatial location of this influence of action on perception could be determined either by goal selection or by motor planning. We employed a tool use task to dissociate these two alternatives. The instructed goal location was a visual target to which participants pointed with the tip of a triangular hand-held tool. The motor endpoint was defined by the final fingertip position necessary to bring the tool tip onto the goal. We tested perceptual performance at both locations (tool tip endpoint, motor endpoint) with a visual discrimination task. Discrimination performance was enhanced in parallel at both spatial locations, but not at nearby and intermediate locations, suggesting that both action goal selection and motor planning contribute to visual perception. In addition, our results challenge the widely held view that tools extend the body schema and suggest instead that tool use enhances perception at those precise locations which are most relevant during tool action: the body part used to manipulate the tool, and the active tool tip.

Introduction

Perception is enhanced at the goal of body movements during their preparation, relative to other locations. Such movement-related perceptual enhancement has been shown for saccadic eye movements (e.g., Deubel and Schneider, 1996, Doré-Mazars et al., 2004), manual reaching (Deubel, Schneider, & Paprotta, 1998) and grasping movements (Deubel and Schneider, 2004, Schiegg et al., 2003). Despite the fact that such an effect of action on perception has been observed in many studies, it remains unclear what cognitive processes underlie this effect. The intentional selection of a visual object as the goal of the next movement could lead to privileged perceptual processing at the spatial location of the goal; alternatively, planning a movement with specific motor coordinates could lead to enhanced perception at the end position of the planned movement. Usually, our actions are appropriate to the objects they aim for, and thus the position of the movement goal and that reached by the body movement coincide spatially. However, to examine which action process – goal selection or motor planning – underlies perceptual enhancement, these two positions must be dissociated experimentally.

The aim of the current study was to systematically dissociate the goal from the motor endpoint of a manual pointing movement. We did this by asking participants to point to visual targets with the tip of different hand-held tools (see illustrations in Fig. 1a). The position that must be reached by the finger while moving the tool (i.e., the motor endpoint) was different from the movement goal (the visual target the participants were to point to with the tool tip), the difference between the two depending on the shape of the tool. Participants prepared to place the tip of the tool on the visual target cued by a central arrow. During movement preparation, visual discrimination stimuli were presented at different positions, corresponding to the movement goal (tool endpoint), the motor endpoint (fingertip endpoint) or one of several other locations.

If perceptual enhancement at aimed-for locations depends on goal selection processes, then perception should be enhanced only when the discrimination target appears at the tool endpoint location (i.e., at the cued movement target location). If, on the contrary, motor planning processes lead to perceptual enhancement, then performance should be better when the discrimination target appeared at the location of the finger motor endpoint. A third possibility is that both goal selection and motor planning influence perception. In that case, we would expect perceptual performance to be better at both the cued location and at the motor endpoint relative to other locations.

We ran five experiments. Experiments 1–4 tested discrimination performance at different locations on and around the finger and tool with tools of various shapes. Our aim was to determine at which locations perceptual enhancement occurred. Experiment 5 examined performance in a task which required two targets to be compared. The goal was to determine whether perceptual enhancement occurred in parallel or serially between cued and motor endpoint locations.

Section snippets

Participants

Nine right-handed participants (aged 20–42 years, mean 30 years; five women) with normal visual acuity and no known neurological disorders took part in exchange for payment or course credit. One of the authors (T.C.) took part. All others were naïve regarding the object of the experiment. All gave their informed consent prior to starting the experiment, which was carried out according to the ethical standards of the Declaration of Helsinki (2008).

Stimuli and instruments

Visual stimuli were 1° by 1° black asterisks

Methods

Six participants (aged 20–40, mean 27; two women; one author) who also participated in Experiment 1 took part in exchange for payment. All gave their informed consent prior to starting the experiment, which was carried out according to the Declaration of Helsinki (2008).

Stimuli and procedures were identical to Experiment 1, except that eight placeholders indicated the locations at which the DT might occur. The four new placeholders were located midway between the original four locations.

Methods

Six volunteers (aged 21–28, mean 24; two women) received payment in exchange for their participation in Experiment 3. All had normal visual acuity and no known neurological disorders. None had participated in the previous experiments and all were naïve regarding the object of the experiment, except one author (T.C.). All gave their informed consent prior to starting the experiment, which was carried out according to the Declaration of Helsinki (2008).

Experiment 3 differed from Experiment 1 in

Methods

Eight volunteers (aged 21–28, mean 24; two women) took part in Experiment 4 in exchange for payment. All had normal visual acuity and no known neurological disorders. Four had participated in Experiment 3, including one author (T.C.). All gave their informed consent prior to starting the experiment, which was carried out according to the Declaration of Helsinki (2008).

The tool was mirror-reversed with respect to Experiment 1: the tip extended to the right of the little finger (Fig. 1a, bottom),

Methods

Eight volunteers (aged 21–28, mean 24; three women) received payment in exchange for their participation. All had normal visual acuity and no known neurological disorders. Five had participated in Experiment 4, including one author (T.C.). All gave their informed consent prior to starting the experiment, which was carried out according to the Declaration of Helsinki (2008).

The stimuli and tool were identical to Experiment 1. However, participants performed a matching task based on that

General discussion

Our results suggest that both goal selection and motor planning processes contribute to orienting visuo-spatial attention throughout the visual field during the preparation of a manual pointing movement. Perception was enhanced for the movement endpoints of those two objects that are most relevant during tool use – the fingertip and the active part of the tool. In contrast, perception was not enhanced at other spatial locations. Perception was high at the spatial location which was to be

Acknowledgments

T.C. is supported by an Alexander von Humboldt Foundation Research Fellowship. The authors thank Sybille Röper and Jonathan Schubert for help with data acquisition and Rainer Schäfer for technical assistance.

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