Descripción de los ambientes de aprendizaje

At the beginning of a trial, the subject aligned the eye and right index finger with the glued fixation spot in the centre of the screen. The target was a small white spot 0.5 deg in diameter, which appeared at –8° visual angle (8° to the left from the fixation spot) and started to move towards the right. Its vertical coordinate was always 0°. The spot initially moved at a fixed velocity of 6°/sec before changing either to 2°/sec or 10°/sec (slow or fast final velocity). This final velocity was visible for either short (approx. 50 ms), intermediate (approx. 100 ms), or long duration (approx. 150 ms) (final velocity duration) before the spot disappeared. More specifically, the frame rate of 72 Hz (i.e., the duration of one frame was 13.88 ms) allowed presentation times of only 42 or 56 ms (3 or 4 frames) for the short final velocity duration, 97 or 111 ms (7 or 8 frames) for the intermediate final velocity duration, and 139 or 153 ms (10 or 11 frames) for the long final velocity duration. The target always started and disappeared at the same position.

Trial types resulting from the different combinations are schematically illustrated in Figure 2.

fast final velocity (10˚/sec) 150 ms 100 ms 50 ms -8˚ 0˚ +4˚ fixation target offset target onset 3.70˚ 3.12˚ 2.66˚

slow final velocity (2˚/sec)

final velocity duration 150 ms 100 ms 50 ms target distance position of velocity change 3.93˚ 3.73˚ 3.84˚

Figure 2 Schematic representation of target position in the six different trial types. The change in target velocity occurred at the same moment, but at different positions depending on the final velocity and the final velocity duration of the target.

At the moment the target arrived at +4°, the target and the background illumination of the monitor were simultaneously turned off and the subject was in complete darkness. A blank screen was the cue for the subject to begin moving.

The subjects had been told previously that the spot continued to move in the darkness. They were asked to “intercept” this occluded target with their eyes only, hand only (single- task conditions), or eyes and hand concurrently (dual-task condition). Figure 3 shows a sample trial of the dual-task condition. The subjects were also told that target motion would change to a higher or lower velocity in each trial. They were, however, unaware that the target always disappeared at the same position.

0 500 1000 1500 2000 2500 3000 -8 -6 -4 -2 0 2 4 6 8 10

time (ms)

target position (deg)

eye hand target position of velocity change latency error

Figure 3 Sample trial of eye and hand movements to a fast target with long final velocity duration. The dotted line represents the actual position of the target if it had continued to move visibly. The difference between this position and the hand or eye end position shows the end position error (identical with the amplitude error when the movement starts from the center). Latency is the difference between target offset and movement onset.

In tasks involving the hand, the subjects were informed about the plexiglass reinforcement of the screen and asked to place their fingertip directly on the screen. No feedback on performance was given. Because all the subjects had gained experience with this task in several preceding pilot studies, no practice trials were performed. As the subjects had received no feedback on performance in these pilot studies, participation probably did not lead to a training of the response, but only to an increased familiarity with the rather complex task.

Each condition consisted of 120 trials with the two final velocities and the three final velocity durations in a pseudorandomised order. Each subject participated in all three experimental conditions, the order of which was counter-balanced.

5.3.4 Measurement of hand and eye movements

An ultrasonic speaker 1 cm in diameter was attached to the tip of the subject’s right index finger. The spatial 3-D location of this speaker was measured at a sampling rate of 200 Hz by means of an ultrasonic device (Zebris, Isny, Germany). A calibration was performed at the beginning of each session based on a set of four markers with known 3-D coordinates. There was a mean accuracy of 3.4 mm over all sessions. Another calibration procedure involved having the subject point to targets at known eccentricities at the beginning, the middle, and the end of each session. Pointing position was defined as the horizontal coordinate of the index finger marker.

Eye movements were monitored with an infrared corneal reflection device (IRIS, Skalar, Delft, Netherlands), the output of which was digitized at a rate of 1 kHz. Several calibration trials were performed at the beginning, in the middle, and at the end of each session by having the subject fixate targets at known eccentricities.

All data were stored and analyzed off-line. Calibration of eye and hand movements was performed by means of a third-order polynomial calibration based on fixation data or pointing data, respectively. The beginning of a hand or eye movement was defined as the moment at which the velocity of the hand or eye exceeded 10% of peak velocity. Movements that did not reach 20 deg/sec hand velocity or 50 deg/sec eye velocity were eliminated from further analysis. The end of the hand or eye movement was defined as the moment at which the velocity of the hand or eye fell below 10% of peak velocity. Maximal latency for a hand or eye movement was set at 1000 ms, minimal latency at 80 ms. Thus, trials with movement onset before target offset were also excluded from the analysis. Only data for the left eye are presented.

During the hand-alone condition, trials in which a saccade occurred were discarded. Eye and hand amplitudes were calculated for each factor level (see data analysis). Those values deviating more than two standard deviations from the respective cell mean were considered outliers and were omitted from further analyses.

Overall, each subject performed an average of 347 trials, 40% of which were discarded mainly because of the large number of staircase saccades occuring in the dark and because of blink artefacts.

5.3.5 Data analysis

Four dependent variables were analyzed for eye and hand movements: amplitude, amplitude error, latencies, and movement time. The amplitude of eye and hand movements was defined as the distance between the respective start and end position. The amplitude error was calculated as the primary amplitude of eye or hand minus target amplitude. Target amplitude was defined as the movement amplitude required to hit the virtual target that had continued to move in the dark at one of the two final velocities. It was computed as the time difference between the moment the eye or hand landed on the screen and the moment the target disappeared multiplied by the final velocity of the target + 4°. Negative error values indicate an undershoot, positive values an overshoot. A smaller error therefore indicates smaller movement amplitude. Latency was defined as the time interval between target offset and response initiation. Movement time was defined as the time interval between response initiation and the end of the movement.

Amplitude error, latency, and movement time were analyzed by means of a 2x2x2x3 repeated measures analysis of variance with the factors movement type (eye, hand), task condition (single, dual), final velocity (slow, fast), and final velocity duration (short, intermediate, long). Post hoc analyses were performed using the Scheffé test.

Generally, movements with a longer latency require larger movement amplitude. An incorrect amplitude, i.e., an amplitude error, can arise either because the velocity of the target or the individual latency is not taken into account. In other words, either the amplitude does not vary with the latency or it does not vary with the final target velocity. For this reason, a further analysis of the amplitude was performed to determine whether movement amplitudes differed across latencies and/or final velocities (as described in the Results).

5.4 Results