A response to one stimulus and not another is called discrimination. Subjects can be trained to discriminate by reinforcing one stimulus and not another.
Discrimination training figures in both classical and operant conditioning, but is perhaps most associated with research into operant conditioning conducted by B.F. Skinner.

See also generalisation, a response to two similar stimuli.

Activities :

1. Read and reflect : Generalisation and discrimination
2. Read : For examples of discrimination training in second language learning, see Woodsworth, 1965 (PDF p. 49)