What Method Did Researchers Use So That an Observer Would

Multiple Choice

What method did researchers use so that an observer would be conscious of a visual stimulus on some trials and not conscious of the same stimulus on other trials?

A) They presented the stimulus sometimes during wakefulness and sometimes during sleep.
B) They gave the observer drugs that block activity in the visual cortex.
C) They presented masking stimuli before and after the stimulus.
D) They asked the observer sometimes to pretend not to see the stimulus.