|Mean reaction time for same side trials and size of the inhibition effect (same side RTs-opposite|
side RTs) as a function of the number of same side probes that occur in a row. Data are from
Experiment II and include only valid and neutral conditions.
size of the inhibition tends to decrease as the response to stimulus interval increases2.
Table 3.3 indicates the RTs when either 0, 1, 2, or 3 targets occur on the same side. The 0 trials means that the other side had been stimulated on the previous trial. This forms the baseline in both the neutral and valid conditions to evaluate the effects of repetition. It is clear that the first repetition shows the largest inhibition effect. This is about 45 millisec in both the neutral and valid conditions. If a second successive target occurs on the same side, the inhibition is reduced to 26 millisec and by the third successive same side target it is reduced still further to only 10 millisec.
It seems fair to conclude from these two experiments that selectivity based on a central cue is not established or maintained very well under any of the conditions. The first few trials following a cue show the greatest selectivity but even this is only 10 to 15 millisec. Cost plus benefit is less than a fifth of what occurs in a design in which a central arrow is used as the cue before each successive trial (e.g., Posner, Snyder, & Davidson, 1980). Moreover, there is no tendency for the effect to become larger or more automatic as the block continues.
A major reason for the failure to show strong selectivity is due to the negative sequential dependency (inhibition) that occurs when the same stimulus is presented twice in a row on the same side. Repeated target events occur most often on the valid side since the probability of a stimulus occurring there is. .8, whereas____________________