Howard S. Hoffman
Dept. of Psych., Dalton Hall, Bryn Mawr College, Bryn Mawr, PA 19010
More than a century ago, Helmholtz noted the ``curious fact'' that by mere conscious effort one can focus attention on any portion of the visual field and that the process ``is entirely independent of the position and accomodation of the eyes.'' A sequence of experiments in the Bryn Mawr laboratory, provided strong support for Helmholtz's assertion. When eyeblinks were elicited by a tap between the eyebrows, the response was inhibited by a light flash (i.e., a prepulse) presented in various parts of the visual field. Subjects were able to modify the amount of inhibition engendered by the flash by shifting their attention toward or away from its impending location and they were able to do so without moving their eyes. A second experiment examined the effects engendered by foreknowledge as to the modality (auditory versus tactile) of an impending startle eliciting stimulus. Foreknowledge was found to reduce response amplitude. In other experiments foreknowledge as to when a startle eliciting signal would occur also reduced response amplitude. Overall the results imply that foreknowledge acts selectively with respect to the modality of a given sensory input and it also acts selectively as to where in the nervous system that input is directed.