These impairments tend to be present even before the onset of the positive symptoms such as delusions (e.g., paranoid thinking) or hallucinations (e.g., hearing voices). They can often present as problems with concentration, memory, and speed of processing. However, not all patients with schizophrenia experience these deficits in a prodromal phase or pre-illness phase. For some patients, the onset of the positive symptoms can occur suddenly and without any prior difficulties such as problems with cognition.