Experiments

Experiment 1

Twelve subjects participated in the first experiment. We examined the representation of the object’s dynamics during reciprocal object rotations. Specifically, we asked whether the dynamics of CW and CCW rotations were represented by separate adaptive mechanisms, and whether these mechanisms interacted. To this end, we conducted a similar paradigm as in the previous chapter (Group 1; Fig. 5.1G1). The experiment included pairs of CW and CCW rotations back and forth between two angular targets (Fig. 6.1A). The first trial of each pair was considered as the "Out" rotation, followed by the second trial as the "Return" rotation (these were discrete trials). We counterbalanced the choice of Out and Return rotations as half of the subjects performed the paired trials of CW-CCW as Out-Return rotations, while the other half performed the CCW-CW pair as Out-Return rotations. The data from these subgroups was properly combined for the analysis and presentation. Each subject had a familiarisation session in which s/he performed 20 pairs of null trials. These trials were not analysed.

The experiment started with a short block of 18 trials (9 pairs of Out-Return trials), that included four pairs of error-clamp trials as well as 5 pairs of null trials (Fig. 6.2A). This short block provided the baseline level for PD and adaptation ratio. The experiment continued with three main phases: the first probe phase (P1), the Training phase, and the second probe phase (P2). During P1 (35 pairs of trials), subjects experienced the full dynamics of the object in the Out rotations, while the Return rotations were all under error-clamp trials. At the end of P1 there was two more pairs of error-clamp trials (in both rotations) to measure the final adaptive state of Out and Return movements. Subjects then deadapted to the object dynamics in a following washout period with 22 pairs of null trials in both rotations.

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The Training phase included 50 pairs of exposure trials, in which subjects adapted to the object dynamics concurrently in both Out and Return rotations. This phase was completed with two pairs of error-clamp trials in the end of the phase to measure the final adaptive state in each rotation. The Training phase was also followed by 22 pairs of null trials to deadapt subjects to the object dynamics. Finally, subjects performed the P2 phase which was identical to P1 (35 pairs of trials), with exposure trials in the Out and error-clamp trials in the Return rotation. The P2 phase was completed with two more pairs of error-clamp trials (in both rotations) in the end of the phase, followed by final washout period of 17 pairs of null trials.

Experiment 2

A new group of 12 subjects participated in the experiment. In this paradigm, we conducted a more extensive experiment in which we tested all possible combinations of task dynamics (trial types) between Out and Return rotations and examined how the interaction between the adaptive processes associated with each rotation changed throughout the experiment.

Each subject had a familiarisation session in which they performed 20 pairs of null trials to get used to the task and the apparatus. During the main experiment, subjects performed 13 consecutive blocks of trials, each consisting of 13 to 18 pairs of Out-Return rotations (Fig. 6.2B). Within each block, the dynamics of the task between Out and Return rotations was of a different combination (Fig. 6.2C). In some blocks we applied the same dynamics for both rotations (i.e., the diagonal elements in Fig. 6.2C). While in the other blocks, we had different object dynamics associated with each rotation (blocks 5, 7, 9, and 12; the off-diagonal elements of Fig. 6.2C). Each block was followed by two pairs of error-clamp trials in which we measured the adaptation level at the end of that block for each rotational direction (as shown by grey patch in Fig. 6.2B).

In the first block, subjects performed 13 pairs of null trials as the baseline. After that, in blocks 2 to 10, we alternated the blocks of simultaneous exposure in both rotations (i.e., 2,4,6,8 and 10) with probe blocks in which we varied the dynamics between the two rotations (blocks 3,5,7,9). We examined all possible combinations of dynamics in probe blocks for when the Return movement was a clamp trial, while the Out movement was either a clamp

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Fig. 6.2 Experimental paradigm. A. Experiment 1: the experiment consisted of three main phases: P1, the Training phase and P2, each followed by a period of washout (null trials).

In P1 and P2, subjects experience exposure trials (yellow patch) in Out movements, and error-clamp trials (grey patch) in Return trials. These phases are followed by washout trials (null; white patch) w.o.1 and w.o.2. During the Training phase, subjects perform under exposure trials in both Out and Return rotations. B. Experiment 2: subjects perform 13 blocks of trials with different dynamic manipulations between Out and Return movements.

Multiple rest breaks were provided for the subjects (60-90 seconds) during each experiment as shown with the dashed lines. C. different combinations of blocks between the Out and Return movements used in experiment 2. D. Two groups of subjects participated in this experiment. In group 1, the visual information about the orientation of object was provided, whereas in group 2, the object was ambiguous as to orientation. The dynamics of the task for both groups was the same.

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(block 3), exposure (block 5), or null (block 7). We also examined cases where the Out movement was a null trial and the Return movement was either a clamp (block 7) or exposure (block 9) trial. In block 11, subjects had a washout period in both rotations to return their performance back to the baseline. We then examined learning after a washout in block 12 in which the Out movement was an exposure and Return movement was a clamp trial. Finally, subjects finished the experiment with a null block for both movement rotations in block 13.

All subjects underwent the same order of probe blocks so that the behaviour in each block was comparable across subjects (i.e., for each block, the preceding trials and the following trials were the same for all subjects; this is important as the behaviour in each block is highly influenced by the history of adaptation preceding that block). This way we could appropriately average the time course of adaptation and PD across all subjects, and perform the model fitting analysis on the averaged data over the whole trial sequence. Accordingly, we chose the order of probe blocks as follows. For the first probe block (block 3; Fig. 6.2B) we used error-clamp trials in both rotations so that we could measure the baseline rate of memory decay in each rotation, after subjects first adapted to simultaneous exposure in block 2 (note that all probe blocks were preceded by simultaneous exposure blocks). In blocks 5 and 7, we examined the decay rate of adaptation in the Return rotation (error clamp trials), while having exposure and null trials in the Out rotation, respectively. In this case, we chose to introduce the exposure-clamp trials in block 5, before the null-clamp trials in block 7.

This was encouraged by some predictions of the model which we will discuss in detail in the discussion section. We then introduced block 9 with null trials in Out and exposure in Return.

Finally, blocks 11, 12 and 13 were introduced last to resemble the P2 phase of experiment 1, where we examined the induced adaptation behaviour (i.e., exposure-clamp trials, preceded and followed by washout trials; Fig. 6.2A and B).

We further aimed to examine how the visual information about the object’s shape and geometry would affect the representation of the object dynamics in Out-Return rotations. In this experiment, subjects were provided with the information about the shape of the object (i.e., the square mass attached to a rod). We recruited another group of subjects (n=12), who performed the same paradigm as the first group in this experiment, except that visual cue

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about the object’s actual shape was masked by an ambiguous object (Fig. 6.2D). Therefore, knowledge of the orientation of the object was only possible by the haptic perception of the object dynamics. We asked whether the interaction of Out and Return adaptive mechanisms was modulated by the visual information of the object.