Trajectories of crystallized cognition over 28 years: Testing the neurodegenerative hypothesis of schizophrenia

Cognitive impairment is common among individuals with schizophrenia. However, it is unclear whether these impairments reflect perturbed neurodevelopmental or neurodegenerative processes. While crystallized cognition is expected to increase over the lifespan, neurodegenerative processes are associated with stalled or declining trajectories of crystallized cognition. Trajectories of crystallized cognition may therefore provide a window into the nature of cognitive impairment in schizophrenia. Previous studies have been limited by small sample sizes, brief follow-up periods, and a lack of data extending into midlife. The present study charted trajectories of crystallized cognition in schizophrenia and other psychotic disorders.

Participants are from the Suffolk County Mental Health Project, an epidemiological first-admission psychosis cohort. Vocabulary scores were used as a measure of crystallized cognition. Scores were collected from 2-, 20-, 25-, and 28-year follow-ups, and data from 297 people who had vocabulary scores at 2 or more follow-ups were analyzed. Vocabulary score trajectories were estimated using hierarchical linear models.

Individuals with schizophrenia did not accrue vocabulary (B = 0.01, 95% C.I. = (-0.02, 0.03), p = 0.631), while those with other psychotic disorders gained one point every 25 years (B = 0.04, 95% C.I. = (0.02, 0.06), p < 0.001). The interaction between time and diagnosis was significant (p = 0.037). Among 144 individuals with schizophrenia, vocabulary scores increased over time in 56, and decreased or remained the same in 88.

The trajectory of the schizophrenia group's vocabulary scores, coupled with a higher proportion of individuals showing stable or declining crystallized cognition, is consistent with a neurodegenerative process.