Cognitive deficits present in almost all patients with schizophrenia, and account for considerable functional disability, but as typically measured in schizophrenia are unrelated to hallucinations and delusions. The absence of this relationship may be caused in part by the lack of an organizing principle of psychosis-related cognitive impairment. Most studies of cognition in schizophrenia involve standard neuropsychological tests that were devised for measuring brain injury in patients with no previous relevant illness. Human perception, thought and action—the basic elements of maintaining reality—are based upon a hierarchical process that conjoins memory and external stimuli, which we refer to as learning-dependent predictive perception. We propose that impairments in this elemental process lead to psychosis in patients with schizophrenia.

The hierarchical model posits that the nature of the output from a given area of cortex depends on temporal coincidence with the patterns of the bottom-up input it receives. If an individual experiences stimuli that do not clearly fit any top-down hypotheses derived from previous experience, a given area of cortex relays the details of the patterns it receives to higher cortical areas; the signals are passed on to the next highest layer and this pattern extends until a match is achieved. As a situation becomes more familiar, the representations of a given level of analysis are shifted to lower cortical areas, freeing higher areas for the detection of high-level patterns. The correct identification of objects, sensations, and processes in the environment is thus based upon probabilistic prediction determined by the accumulation of memories of how the perceptual world is organized and how it operates (Hawkins and Blakeslee, 2004; Purves et al, 2001).

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