Slowness and the preceding preparatory interval effect in schizophrenia

We measured the reaction times (RTs) of schizophrenic (n = 25), matched normal (n = 69), and bipolar (n = 14) subjects to a tone preceded by a preparatory interval (PI; the time between the warning signal and imperative stimulus) of varying length. RTs increase when the PI for the immediately preceding trial (the PPI) is longer than the PI for the current trial. Several studies have shown that this PPI effect (difference score) is heightened in schizophrenia. We replicated this finding. However, we found that the size of the PPI effect within groups increased with overall slowness and that the least squares regression line relating the PPI effect difference score to overall slowness did not differ between groups, nor did schizophrenic subjects' regression line differ from that of normal subjects. Group differences on the PPI effect were also analyzed by taking residuals for members of all groups from the normal subjects' regression line of the PPI effect difference score on overall slowness. Groups did not differ on these residuals, nor did schizophrenic subjects differ from normal subjects. We conclude that the heightening of the PPI effect in schizophrenia is like that observed in equally slow normal subjects. This finding suggests that the PPI effect does not appear to be a promising marker of a distinctive schizophrenic pathology.