AN EXPERIMENTAL INVESTIGATION ON THE NATURE OF EXTINCTION

Under conditions of bilateral simultaneous presentation a right brain-damaged patient consistently extinguished visual stimuli presented for 200 msec to the left visual field. He was submitted to a series of experiments aimed at assessing the variables that influence this phenomenon. Extinction persisted unmodified when stimulus presentation was lengthened to 500 msec, when the size of the left stimulus was four-fold that of the right stimulus and for whatever position the stimuli occupied on the two halves of the display. Single stimuli were always perceived, but their localization was transposed rightward. The only condition in which extinction was partially reduced was when the instructions called for total disregard of the right stimulus. The patient also extinguished the leftmost of two stimuli presented in the left space, while he always perceived both stimuli when they were presented in the right space. However, if attention was covertly moved to a point in the right space and two stimuli were displayed to either side of it,left stimuli were neglected 70% of the time. Though a left stimulus was never perceived, it slowed the RT to the identification of a right stimulus. These data are interpreted in the frame of the directional bias theory, which attributes extinction to a shift of attention towards the side ipsilateral to the lesion, caused by the imbalance between the opponent turning processors, controlled by the right and left hemisphere. However, the striking difference in the rate of extinction when both stimuli were presented to the left or to the right field implies a stepwise decrement in the deployment of attention by the damaged processor, when it enters in the opposite space.