The role of vision, speed, and attention in overcoming directional biases during arm movements

Abstract

Previous research has revealed directional biases (preferences to select movements in specific directions) during horizontal arm movements with the use of a free-stroke drawing task. The biases were interpreted as a result of a tendency to generate motion at either the shoulder or elbow (leading joint) and move the other (subordinate) joint predominantly passively to avoid neural effort for control of interaction torque. Here, we examined influence of vision, movement speed, and attention on the directional biases. Participants performed the free-stroke drawing task, producing center-out strokes in randomly selected directions. Movements were performed with and without vision and at comfortable and fast pace. A secondary, cognitive task was used to distract attention. Preferred directions remained the same in all conditions. Bias strength mildly increased without vision, especially during fast movements. Striking increases in bias strength were caused by the secondary task, pointing to additional cognitive load associated with selection of movements in the non-preferred directions. Further analyses demonstrated that the tendency to minimize active interference with interaction torque at the subordinate joint matched directional biases in all conditions. This match supports the explanation of directional biases as a result of a tendency to minimize neural effort for interaction torque control. The cognitive load may enhance this tendency in two ways, directly, by reducing neural capacity for interaction torque control, and indirectly, by decreasing capacity of working memory that stores visited directions. The obtained results suggest strong directional biases during daily activities because natural arm movements usually subserve cognitive tasks.

Interlimb differences of directional biases for stroke production

“…Directional biases were revealed by peaks of the histograms. The dominant preference to produce strokes in the diagonal directions documented in our previous studies for the dominant arm (Dounskaia and Goble 2011; Goble et al 2007) is apparent for both arms in both secondary task conditions. Furthermore, Fig.…”

“…The calculated torques were used to compute the two cost functions, optimization of which accounted for directional biases in the D arm in our previous studies (Dounskaia and Goble 2011; Dounskaia et al 2011; Goble et al 2007): $$I_{\text{INTE}}=\frac{1}{T_1-T_0}\sum _{t=T_0}^{T_1}\frac{\mid {\text{INTE}}_t\mid }{\mid {\text{INTE}}_t\mid +\mid {\text{MUSE}}_t\mid },$$ $$I_{\text{INTS}}=\frac{1}{T_1-T_0}\sum _{i=T_0}^{T_1}\frac{\mid {\text{INTS}}_t\mid }{\mid {\text{INTS}}_t\mid +\mid {\text{MUSS}}_t\mid },$$ Here, T 0 and T 1 are the time moments of the beginning and the end of the stroke, INTE and INTS are interaction torques, and MUSE and MUSS are muscle torques at the elbow and shoulder at each moment of time T 0 ≤ t ≤ T 1 . These cost functions represented a tendency to produce predominantly passive motion either at the elbow ( I INTE ) or at the shoulder ( I INTS ) by generating low MUS compared with INT.…”

“…Dounskaia and Goble (2011) used a free-stroke drawing task during which movements were not limited to a circular space, and horizontal strokes were initiated anywhere in front of the subject. Directional biases were again evident, although in intrinsic and not extrinsic space.…”

“…Attention manipulation with the use of a cognitive secondary task was also included, taking into account the finding of Dounskaia and Goble (2011) that this manipulation markedly enhanced directional biases in the D arm. It was expected that differences in directional biases between the two arms would be more pronounced during simultaneous performance of the secondary task than without it.…”

Reciprocal aiming precision and central adaptations as a function of mechanical constraints

“…To this aim, another organisational principle that can play a role on the control of movement accuracy is the relationship between the load and central demands incurred by the CNS to perform rapid aiming movements (e.g., Ketelaars et al, 1999;Khan et al, 2006). In other words, from our results, we investigated whether neural effort for movement control is an important factor that influences how CNS arrives at kinematics and associated muscle activation patterns when mechanical constraints are altered (Dounskaia and Goble, 2011;Kistemaker et al, 2010). However, the alterations of accuracy movement caused by the load were not paralleled by a change in mental effort as assessed through simple reaction times.…”