Janis volunteered to participate in a psychology experiment. when she arrives at the lab a lab assistant standing on the other side of a counter greets her. the lab assistant explains the informed consent procedure and asks her to sign the form. as the lab assistant reaches for the form he drops it behind the counter. he drops down behind the counter to pick up the form, but, unbeknownst to janis, another person stands up holding the form. after janis signs the form, she is asked if she noticed the change. she replies that she did not. this phenomenon is known as: