To quantify the influence of sensory predictions, we calculated the
survival probability of percepts across temporary stimulus removals in
each participant. A lower survival probability indicates a weaker tendency
towards percept stabilization and thus aweaker influence of sensory
predictions in perceptual inference. Due to the relatively fast but
long sequence of stimulus presentations, in some trials participants
missed the required response, andwe found the number ofmissed trials
to be higher in schizophrenia patients than in healthy controls (4.2% vs
1.1% of trials [medians], z=2.7, p b 0.01,Wilcoxon rank-sumtest). This
raises the possibility that between-group differences in the number of
missed responses would systematically bias the results in favor of our
hypothesis of lower survival probabilities in schizophrenia patients
compared to healthy controls. To preclude such a type-I-error, we considered
trials in which participants did not make a response as trials in
which the percept had survived the preceding stimulus removal.
This procedure not only seems reasonable given that in the vast
majority of trials the stimulus was indeed perceived in the same
configuration as in the preceding trial (see Results), but also implies
that in participants with high numbers of missed responses (i.e.
schizophrenia patients) survival probabilities would be rather
over- than underestimated, thereby rather decreasing than inflating
the hypothesized group difference between schizophrenia patients
and healthy controls.
2.3. Measurement of delusional ideation