Sources of individual differences in IT

Interest in inspection time (IT), uniformly thought to index ‘speed-of-processing’, has been maintained because of its established empirical correlation with general mental ability and performance IQ measures. The IT procedure generally consists of a simple visual discrimination task displayed at various critical exposure durations and immediately followed by a suitable mask. Processing is assumed to terminate at mask onset, which seems inappropriate in light of the target/mask composite necessarily available with integration theories of backward masking. The present paper reports five experiments involving attended and unattended secondary stimuli and additional factor analyses of previous studies involving IT. From these it is proposed that IT is better thought of as indexing the power an individual can bring to bear within a specific cognitive domain rather than ‘speed-of-processing’ per se. The consequence is that the observed IT-IQ correlation is merely the inevitable outcome of measuring the same domain with two different tasks; rather than due to some elemental factor such as mental speed underlying both tasks. Ways in which this model could be tested are discussed.