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. 2010 Nov 17;4:172. doi: 10.3389/fnbeh.2010.00172

Figure 7.

Figure 7

S–R and S–S theories to account of classical conditioning. Two theories, i.e., the stimulus–response (S–R) association theory and the stimulus–stimulus (S–S) association theory, have been proposed to account for classical conditioning in higher vertebrates including humans (Rescorla, 1988). In the S–R theory (A), classical conditioning is viewed as the strengthening of a new reflex pathway for the CS to evoke a conditioned response (CR) (i.e., a pathway from neurons that code for the CS to neurons whose activities lead to behavioral response), as a result of pairing of the CS with a US (Rescorla, 1988; Pickens and Holland, 2004; Holland, 2008). According to this view, an initially insignificant event, CS, is incorporated into the reflex system under the control of a more biologically significant stimulus, US, whenever those two events occur in close temporal contiguity. This view accounts for some forms of classical conditioning in higher vertebrates (Rescorla, 1988; Pickens and Holland, 2004). Many other forms of classical conditioning in higher vertebrates, however, have been suggested to involve the strengthening of S–S connection, i.e., connection from neurons representing CS to those representing US (B). According to this view (S–S theory), an association is formed between internal representation of the CS and that of the US are strengthened (i.e., a connection from neurons that code for the CS to neurons that code for the US is strengthened), and the growth of this association permits the CS to activate a representation of the US in the absence of the US itself. This anticipatory activation of the US representation produces the CR. This view is referred to as cognitive account of classical conditioning, since it assumes the formation of internal representation of the relationship between external sensory events (i.e., contingent occurrence of the CS and US) (Rescorla, 1988; Pickens and Holland, 2004; Holland, 2008). Notice that these theories address the question of what kinds of connections are strengthened by learning, not the question of how or by which mechanism such connections are formed.