Solution: Thanks to @wakecarter and also refer to the following post: Here is the core of solving this problem.
The basic idea is to use repetition to mimic the back and forth. Set 1000 reps and each time show a given instruction based on the pressed key.
Attached is a revised version that works to walk the subject thru three instruction back and forth and, when finished, jump to some test.
Instruction walk thru.psyexp (29.6 KB)
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