MATERIALS AND METHODS
Subjects
Ten normal (five female) right-handed subjects (ages 22 - 29 years) gave informed
consent and were paid for their participation in the study. The experimental protocol
was approved by the Ethics committee of the Laboratory for fMRI, Robarts Research
Institute, London, Canada. Each subject contributed to two measurements by
repeating the experiments on a different day. Subjects learned different sequences in
the two repetitions of the experiments. Due to technical problems in data recording
and large head movement, final data analysis was carried out on two repetitions by six
subjects and one repetition by two other subjects.
Experimental task
We used a modified 2x6 sequence task (Hikosaka et al., 1995; Bapi et al., 2000). Two
square cells (called a set) were illuminated simultaneously on the 3x3 grid. Subjects
learned, by trial and error, the correct order of pressing the corresponding keys. A
sequence of six such sets constitutes a hyperset. (Fig. 1 a). Subjects were asked to
execute the sequence as fast as they could at all times to facilitate smooth
performance of finger movements. We fixed the inter-set gap within a hyperset to 3
seconds to enable presentation of the sets at an even pace. Trial was terminated upon
an error and learning started again from the beginning of the hyperset.
Normal and Rotated settings
Subjects practiced the same hyperset alternately in normal and rotated (visual or
motor) settings (Fig. 1 b). While in the normal setting the display and keypad were
arranged in the usual upright position, the display-to-keypad relationship was altered
in the rotated settings. In the visual setting, the keypad was rotated by 180°, while the
display remained unaltered. In the motor setting both the keypad and the display on
the screen were rotated by 180°. Consequently, in the visual setting the sequence of
visuo-spatial cues (visuo-spatial sequence) remained identical, while in the motor
setting the sequence of finger movements (somato-motor sequence) remained the
same as that of the normal setting. Further, it is to be noted that the display-to-keypad
mapping was identical between the motor and visual settings.
Experiments
We utilized an on-off (box-car) design for the experiments where subjects alternated
between control and test conditions (Fig. 1 c). In the control condition we used the
1x12 follow task wherein subjects pressed one key at a time following random visual
cues. In the test condition we used the 2x6 sequence task and subjects practiced the
same hyperset alternately in normal and rotated (visual or motor) settings. Before
every scanning experiment, subjects performed the experimental tasks in a practice
session for half-an-hour using a hyperset different from the one used during scanning.
Subjects were informed of the display and keypad rotations and hence our task is an
explicit sequence learning task.
Scan parameters