The preference-for-signalled-shock phenomenon: Signalling shock is reinforcing only if shock is modifiable

Inescapable, unavoidable, 5-sec. shocks were preceded by 5-sec. white-noise signals contingent on previously food-trained (FR 40) bar pressing of 20 rats. Twenty yoked-control subjects receiving the same, but non-contingent, series of signals indicated that the signal was reliably reinforcing only if the shocks were delivered through an alternate, rather than scrambled, floor-grid system. Implications are discussed for previous reports of the preference for signalled shock phenomenon.