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  • Eight males and 18 females (20–40 years of age, mean age = 28.3±6.3 years) participated in the experiment. All participants reported normal or corrected-to-normal vision and none of them had any motor or sensory abnormalities. They gave written informed consent to participate in the study, which was approved by the NTT Communication Science Laboratories Research Ethics Committee. All participants were right-handed. This experiment consisted of two phases: a color-action association learning phase (practice phase) and a test phase. Considering the aim of this study, we excluded the following participants from the analyses: 1) Those who (two participants) noticed the existence of the prime stimulus during the experiment due to violation of the instructions. For example, they were fixating not the fixation point but prime-mask stimuli themselves. 2) Those who (three participants) showed no (or little) effect of implicitly driven motor behavior (by invisible prime) on action evaluation in the practice phase. As for the latter exclusion case, since we wanted to examine what information (online sensorimotor information, endpoint error, or an external cue) is the more critical clue for the action evaluation in the test phase, we needed an association between color and action in the practice phase, which means a higher score in the congruent-green condition and a lower one in the incongruent-blue condition (see Procedure and Result sections for details). In the test phase, we divided the participants into two groups: an incongruent-green group (three males and eight females, mean age = 28.3±6.7 years) and congruent-blue group (three males and seven females, mean age = 27.6±6.9 years) as described below in detail. Participants viewed the CRT display (60 Hz) from a distance of approximately 42 cm, with their head movement restricted by a chin-rest (see Fig. 1A). Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0034985.g001 Experimental setup and protocol. (A) Configuration of the experimental apparatus. Participants rested their heads on the chin rest and were required to make reaching movements from the button switch to the target displayed on the monitor while they watched the fixation point. The motion of the reflective marker was recorded by the motion capture system as the kinematics of the reaching movement. (B) Visual stimulus layout. The diameter of the fixation point participants were asked to watch during the trials was 0.85 deg (shown as a green filled circle). Right- or left-pointing prime and mask stimuli were regular triangles whose sides were 1.5 and 6.0 deg, respectively. The centroid of the prime triangle was located at 10 deg above the fixation point in the vertical plane. Possible targets (white filled circle, diameter of 0.6 deg) were located at 6.8 and 9.3 deg horizontally from the centroid on each side, respectively. (C) Time sequence of the visual stimuli (prime-mask combination) in each condition. While the fixation point remained green in the congruent(incongruent)-green and neutral conditions after the button release, the color of the fixation point was switched from green to blue in the incongruent(congruent)-blue conditions at the same timing of prime onset. The outer contour of the prime stimuli fit exactly within the inner contour of the cutout of the mask triangles. Except for trials in the neutral condition, either the right- or left- target point (white filled circle, diameter: 0.6 deg) reappeared according to the direction of the mask stimulus, and feedback about the reaching endpoint (green filled circle, diameter: 0.6 deg) was also provided after the reaching movement (i.e., contact with touch panel). Right- or left-pointing prime and mask stimuli were regular triangles whose sides were 1.5 and 6.0 degrees, respectively. The centroid of the prime triangle was located at 10 deg above the fixation point in the vertical plane. The outer contour of the prime stimuli fit exactly within the inner contour of the cutout of the mask triangles. A neutral mask was formed from the superimposition of the two triangles (see Figs. 1B and 1C). The prime triangle was presented 100 ms after a button had been released at the start position. It was presented for 17 ms. The prime-mask stimulus-onset asynchrony (SOA) was set at 50 ms and mask presentation was 17 ms (see Fig. 1C). Right and left target points were simultaneously presented with the mask stimulus in each near or far condition (near: 6.8 deg, far: 9.3 deg from the center of the cutout, see Fig. 1B). The mask stimulus was also a cue stimulus for reaching direction. The reaching end position was recorded with a touch panel (Keytec. Inc). The right-hand position (monitored by a reflective marker placed on the back of the hand around the bottom of the ring finger) was recorded with a three-dimensional motion capture system (ProReflex, Qualisys, Sweden) at a frequency of 250 Hz. Before a reaching task, we needed to obtain the perceptual threshold of prime stimulus intensity so that participants would not be able to recognize the prime triangle in the main experiment. For this purpose, we had participants perform a prime identification task using a Bayesian adaptive psychometric procedure, QUEST [34]. Participants were requested to report the direction of the prime triangle in the two-alternative choice manner for prime (right, left) and mask (right, left) combinations randomly selected (80 trials). QUEST is a procedure for running each trial at whatever signal strength would contribute most to minimizing the variance of the final threshold estimate (see also [35]). Such a procedure combines the experimenter's prior knowledge (tGuess = −1, tGuessSd = 5.0, beta = 3.5, delta = 0.01, gamma = 0.5 in QUEST software of Psychophysics Toolbox [36]) and the observer's responses (i.e., right or left) in past trials in choosing the signal strength for the next trial, and, in the end, estimating threshold. After each response, a Gaussian probability density function is updated by Bayes’ rule. Each trial is placed at the current maximum-likelihood estimate of the threshold. The threshold of each participant was set at 51%, which is rather conservative compared to the threshold generally assigned (i.e., about 70%). We confirmed that the final threshold estimates appropriately converged. With this setting, we assumed that the performance level of each participant would be almost chance level. After this task, we also confirmed, from participants’ reports (cf. [37]), that participants could not identify the direction of the prime stimulus at all. Next, participants performed the reaching task using the backward masking paradigm in a darkened room. As shown in Figs. 1A and 1B, the participants were asked to place their heads on a chin-rest and to fixate a green fixation point (diameter of 0.85 deg). Each trial started by participants’ pressing a button placed at the start position with the index finger. The initial reaching target (white dot, diameter of 0.6 deg) was presented 10 deg above the fixation point for 500 ms from the start time of the button press, and participants were instructed to intend to reach for this initial target. After the target point disappeared, two beeps at intervals of 200 ms were provided. Participants were required to start moving (i.e., release the button) at the second “go” beep and reach for the target point. When a neutral mask stimulus was presented, the participants were required to reach for the initial target corresponding to the center of the cutout. The participants had to correct their reaching movements according to the direction of the mask triangle and reach for the point consistent with the mask direction out of the two presented target points when a right- or left- pointed triangle mask was presented (the mask stimulus was thus also a cue stimulus for reaching direction as mentioned above). After the reaching movements, the target point and reaching endpoint were shown in white and green, respectively, except in the neutral condition. There was no feedback error information when the neutral condition was applied. The participants were instructed not to shift their gaze to the triangle (mask) stimulus or the target point but to keep watching the fixation point during the trial. After each trial, except those in the neutral condition, the participants were requested to answer the following two questions: Did the fixation point color remain green or did it switch from green to blue (the color identification task)? To what extent did you appropriately perform the movement according to your intention of reaching for the target based on the stimulus (mask) direction? The second question was reported on a 5-point scale (i.e., 5 - very smooth; 1 - very clumsy). The reaching task consisted of two phases: a color-action association learning phase (practice phase) and a test phase. In the practice and test phases, 384 and 192 trials were completed, respectively. A short break was inserted every 48 trials. In the practice phase, there were three conditions as shown in Fig. 1C: 1) congruent-green (50%), in which the prime direction was congruent with the mask one [prime-mask combinations of right-right (RR) and left-left (LL)] and the color of fixation point remained green; 2) incongruent-blue (33%), in which the prime direction was incongruent with the mask one [prime-mask combinations: right-left (RL) condition and left-right (LR)] and the color of fixation point was switched to blue; 3) neutral (17%), in which neutral mask triangle without the prime was presented and the color of the fixation point remained green. In the test phase, an additional color-stimuli combination condition, either incongruent-green or congruent-blue, was inserted in addition to the congruent-green, incongruent-blue, and neutral conditions (see Fig. 1C). The rate of each condition was as follows. Congruent-green condition 50%, incongruent-blue condition 25%, incongruent-green or congruent-blue condition 8%, and neutral condition 17%. Trial order was pseudo randomized in both phases. The participant group in the practice phase was divided into two groups in the test phase: an incongruent-green group, in which the incongruent-green condition was inserted, and a congruent-blue group, in which the congruent-blue condition was inserted. The hand position data were temporally aligned with respect to the button release and were filtered offline using a fourth-order Butterworth filter (double-sided) with a cutoff frequency of 10 Hz. The velocity was calculated from three-point numerical time differentiations of the filtered position data. Our main interest is the effect of the prime direction on motor behavior, so we analyzed the following data pooled from both target positions (i.e., near and far conditions). To quantify the effect of the prime stimulus on motor behavior, we calculated the mean velocity of a 100-ms-time window from 200 to 300 ms after prime onset in each condition. We adopted the mean X-velocity as the index of the prime effect because velocity is more suitable for detecting the transient behavioral changes than position (trajectory). The response onset to the prime stimulus was defined as the time from which the velocity differences between RR (LL) and LR (RL) continuously exceeded a threshold value of 30 mm/s for at least 40 ms in a window between 150 and 300 ms after the prime onset. The threshold value approximately corresponded to 2 SDs of the velocity during a period of 0–50 ms after the button release (cf. [38]). Endpoint error was calculated as the Euclidean (linear) distance in two-dimensional space (on the vertical plane) between the endpoint of the reaching movement recorded with the touch panel and the target position (see Figs. 1A and 1C). Mean endpoint error in each condition was averaged by the sum of each trial’s endpoint error in each condition. Repeated measures ANOVAs were applied to these mean values. Specifically, the prime direction (right, left) and mask one (right, left) were within-participant factors for velocity, and mask direction (right, left) and conditions [congruent-green, incongruent-blue] were within-participant factors for the endpoint error and action evaluation score in the practice phase. In the test phase, the prime direction (right, left) and condition [congruent-green, incongruent-blue, incongruent-green (or congruent-blue)] were within-participant factors for the velocity, and the mask direction (right, left) and conditions [congruent-green, incongruent-blue, incongruent-green (or congruent-blue)] were within-participant factors for the endpoint error. Tukey's HSD procedure was used for post-hoc comparison of means (alpha level  = .05). As for the action evaluation score, we are interested in whether the action evaluation is modulated by the color cue itself, so we performed planned t tests on the mean evaluation scores for incongruent-blue and incongruent-green conditions in the incongruent-green group and on those for congruent-green and congruent-blue conditions in the congruent-blue group. Finally, to identify the information used in the action evaluation, we introduced path analyses. We have the assumption that online corrections induced by the invisible prime stimulus and/or consequent endpoint error are potential candidates that could affect the action evaluation in the practice phase. In the test phase, the color cue would inferentially contribute to the evaluation of one’s own action. The color cue variable introduced in the test phase was nominal scale, so we used a dummy variable for the analysis; that is, blue was transformed to 0 and green was transformed to 1. For each variable, we calculated the variance inflation factor (VIF), which, as suggested by Myers [39], should be less than 10 to avoid multicollinearity problems. We focused on the standardized path coefficients between variables. Standardized path coefficients indicate the relative effect of variables within the model.
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