RESUMEN
In a behavioral experiment on 60 healthy volunteers, the Tower of London was employed as a complex visuo-spatial planning task. After each trial, participants were asked how difficult they found the task and whether they thought their solution was optimal. Results showed that objective problem difficulty affected behavioral performance as well as subjectively experienced difficulty and performance certainty. This pattern was influenced by experimental manipulation of participants' knowledge of objective problem difficulty: For optimally solved problems, performance certainty remained at high levels if such knowledge was provided, and strongly declined in more difficult problems if it was withheld. For nonoptimally solved problems, subjects' ratings indicated awareness of errors when they were informed about objective problem difficulty; otherwise, performance certainty declined from intermediate to low levels in more difficult problems. No such interaction was observed with regard to ratings of subjective problem difficulty. Additional structural equation modeling revealed that subjective awareness of errors and processing conflicts can be considered as independent only for optimally solved trials in which the optimal solution was known to the participants. We conclude that participants' ratings of problem difficulty and performance certainty can be regarded as indicators of at least partly distinct processes of performance monitoring, and that studies of complex problem solving incorporating such subjective measures may enhance the empirical basis of current theories of executive functioning.