Weakness of the “missing-stimulus effect” in hooded rats: Gross asymmetry in the Sokolovian orienting response

Abstract

The conventional Sokolovian orienting response (OR) model predicts that reflexive orientation will be evoked whenever unexpected stimulus change occurs, irrespective of whether the stimulus increases or decreases in intensity. Orientation toward expected but nonappearing stimuli has been termed the “missing-stimulus effect.” There are few data to support the existence of the phenomenon, however, and in man, it would appear that the omission of an expected stimu- lus does not reliably evoke an OR. The present study investigated the “missing-stimulus effect” in hooded rats. Although rats traversing a runway were successfully distracted by unexpected flashing lights presented in the visual periphery at the midpoint of the run, they showed remarkably little response to the omission of “expected” light flashes (to which they had been previously habituated.) The same result was obtained in two further conditions: when the peripheral lights were simply turned off, or when lights that flashed constantly throughout the test session were suddenly omitted. It was argued that orientation to stimulus appearance, and stimulus disappearance, may be mediated by “short-loop” and “long-loop” neural mechanisms, respectively. The latter may be more easily overridden by collateral behaviors than the former. This distinction is consistent with recent modular and neural OR models.