Study looking into biomechanical constraints of approach-avoidance movements in MR-scanner


Additional information to Volman et al., 2016, ENeuro

Designed and performed by Alessandra Galli 

Donders Institute for Brain, Cognition and Behaviour, The Netherlands




Previous studies using the approach-avoidance task (AA-task, Volman et al., 2013; Radke et al., 2015) contained a strong movement effect due to biomechanical constraints of the joystick. Here, we performed a separate study to qualify these constrains.



The movement AA-task consisted of 4 blocks (with 12 trials per block) during which participants had to respond to a visually presented bow or ellipse by either pulling a joystick towards themselves (approach) or by pushing it away from themselves. The participants had to categorize the picture as bow or ellipse. This resulted in 4 different trials representing the picture (bow, ellipse) x movement (approach-avoid) combinations. At the start of each block participants received written instructions regarding the required response mapping. Each picture was presented for 100 ms, preceded by a 300 ms blank screen and followed by the participant’s response, a blank screen and by a pseudorandom inter-trial-interval (1-3 s). A baseline period of 21-24 s preceded each block. Joystick displacements > 80% along the sagittal plane within 2 s from stimulus presentation were marked as valid responses. Invalid responses were signalled for 1 s with written feedback stating ‘‘you did not move your joystick far enough.’’ After moving the joystick, participants had to return to the starting position (defined as the central area extending 20% along the sagittal plane) before the end of the inter-trial-interval (ITI). Otherwise, visual feedback indicated ‘‘return the joystick to the starting position’’ and the ITI was repeated after participants returned the joystick. An MR-compatible joystick (Fiber Optic Joystick, Current Designs, sampling rate = 550 Hz) was placed on participants’ abdomen to ensure comfortable push and pull movements. Data from 21 participants (age range: 20-25 years) was collected, all female and right-handed, in a mock scanner with imaging background noise.


Bow and ellipse stimuli:    


Behavioral Analysis:

Behavioral data was analyzed using matlab 7.9 (Mathworks, Natick, MA) and PASW statistics 18 (SPSS Inc., Chicago, IL). First, to obtain a precise measure of movement onset (reaction time: RT), the joystick movement for each trial was reconstructed using the joystick displacement measurements. Excluded trials showed either a joystick movement in the wrong direction; an extreme RT (<150 or >1500 ms), peak velocity (<0.1 cm/s) or movement time (>400 ms); or an error rate of above chance level in a block (in that case the whole block was excluded. Second, a 2-way repeated measures analyses of variance (ANCOVArm) on mean RT was performed, with factors Movement (approach, avoid), and Object (Ellipse, Bow). The a-level was set at P < 0.05.



A significant movement effect was observed (F(1,20) = 8.6, P = 0.008, see Figure and table). No additional significant effects were present.


Figure. Mean RTs (± SEM) for the approach and avoidance movements of the AA-task, showing increased reaction times for avoidance (away) compared to approach (towards) movements.


Table. Reaction times (RT in ms)







427.0 (14.3)

477.9 (20.4)


440.1 (12.4)

466.7 (21.2)

Note: Values are presented as mean (SE).



This study shows that the responses provided with this joystick in the AA task are biomechanically faster when the participant moves the joystick towards themselves than away from themselves, while laying in a supine position.