The effect of an external load on the force and timing components of mentally represented actions

Abstract

The chronometry of real and imagined movements was investigated in a group of eight subjects under varying conditions. The visually-guided pointing task was used to investigate the speed for accuracy trade-offs that occur as target size is varied for both real and imagined performance. The task was performed both with and without an external load of 2 kg. For the no-load condition and load conditions, the speed for accuracy trade-off for both real and imagined performance conformed to Fitts’ law. Movement durations of real movements remained largely unaffected by the addition of the load, however, movement durations of imagined movements increased significantly with the addition of the load. These patterns of results suggest that the weight disrupted the force calculation component of imagined movements but not the relative timing.

Introduction

Motor imagery is a dynamic state in which an individual mentally simulates the performance of a specific motor action or motor task. [2], [4], [9]. Studies using positron emission tomography and functional magnetic resonance imaging find that similar brain areas are activated when subjects either perform a motor task or imagine performing the same task. This suggests that common neurocognitive networks subserve motor imagery and the intentional generation of overt motor movements [2], [6], [11], [12]. Behavioural studies also demonstrate that imagined motor performance is subject to the same environmental and physiological constraints as real motor performance [2], [5], [6], [16]. For example, the time to complete motor movements in imagination is highly correlated with the time necessary to actually perform the same movements in both normal subjects and in subjects with movement disorders arising from focal brain lesions or neurodegenerative disorders [2], [5], [6], [7], [11], [16], [19], [20].

A recent dissociation between the timing and duration of imagined movements [6] suggests that imagined movements may be assembled using different force, timing and sequencing components in the same way as real movements are assembled [10], [13], [14], [15]. In the Decety et al. study [6], a high correlation was found between the duration required to actually walk different pre-specified distances and the duration required to imagine walking the same distances. When the same task was repeated with subjects shouldering a 25-kg rucksack, the duration of real movements was unchanged while a systematic slowing of imagined movements was observed. It was hypothesised that this slowing occurred because subjects programmed a greater muscle force in order to overcome the additional weight when moving. When subjects actually walked, this additional force allowed subjects to maintain the same speed carrying the weight as when they carried no weight. However, as increases in force are generally associated with increased movement duration, the additional force programmed when subjects imagined walking with the weight was interpreted as representing that the duration of their movements should be increased [1], [6]. Importantly, despite this increase in duration, the relationship between real and imagined movements remained proportional. Thus, the weight disrupted the calculation of the force component of imagined movements but not the relative timing. These data suggest that motor imagery cannot be considered a fully formed and irreducible motor representation akin to a motor memory.

Before this hypothesis can be explored further, a number of criticisms that have been made of the Decety et al. [6] study must be addressed. First, as the subjects were blindfolded during the experiment their performance may reflect changes in memory function, or in their visual-spatial representation of the space rather than, or in addition to, any internal representations of the motor task [8]. Second, if the weight was attached to the limbs of subjects rather than placed on their back, then real movements may have been slowed in the same manner as imagined movements [1]. Finally, as subjects were blindfolded, they may not have increased imagined effort in the blindfolded condition because, in the absence of movement, visual feedback may be necessary for adjusting effort [3]. If true, these criticisms cast doubt on the hypothesis that internal representations of imagined motor performance and actual motor performance are assembled from the same component processes. However, all of these criticisms can be rejected if it can be demonstrated that motor imagery is still slowed with additional weight when subjects are not blindfolded in the imagery task and the external load is attached to a limb. Therefore, the aim of the current study was to investigate real and imagined performance on the visually guided pointing task under conditions where an external load was and was not applied to the limb performing the task.

It was hypothesised that the external weight would slow the duration of imagined but not real movements although the external load would not effect the logarithmic relationship between target width and the duration of either real or imagined movements.