Eye movements in patients in early psychosis with and without a history of cannabis use

Abstract

It is unclear whether early psychosis in the context of cannabis use is different from psychosis without cannabis. We investigated this issue by examining whether abnormalities in oculomotor control differ between patients with psychosis with and without a history of cannabis use. We studied four groups: patients in the early phase of psychosis with a history of cannabis use (EPC; n = 28); patients in the early phase of psychosis without (EPNC; n = 25); controls with a history of cannabis use (HCC; n = 16); and controls without (HCNC; n = 22). We studied smooth pursuit eye movements using a stimulus with sinusoidal waveform at three target frequencies (0.2, 0.4 and 0.6 Hz). Participants also performed 40 antisaccade trials. There were no differences between the EPC and EPNC groups in diagnosis, symptom severity or level of functioning. We found evidence for a cannabis effect (χ² = 23.14, p < 0.001), patient effect (χ² = 4.84, p = 0.028) and patient × cannabis effect (χ² = 4.20, p = 0.04) for smooth pursuit velocity gain. There was a large difference between EPC and EPNC (g = 0.76-0.86) with impairment in the non cannabis using group. We found no significant effect for antisaccade error whereas patients had fewer valid trials compared to controls. These data indicate that impairment of smooth pursuit in psychosis is more severe in patients without a history of cannabis use. This is consistent with the notion that the severity of neurobiological alterations in psychosis is lower in patients whose illness developed in the context of cannabis use.

Fig. 1. Conceptual models for understanding cannabis-psychosis vulnerability.

a In the decreased vulnerability model, cannabis represents a distinct risk factor and less heritable predisposition to schizophrenia is required to reach the psychotic threshold. If oculomotor deficits represent a heritable endophenotype for schizophrenia, we would expect impairment in EPNC vs EPC. b In the shared vulnerability model there is no difference in heritable component of EPC vs EPNC and we would not expect to see any oculomotor difference in EPC vs EPNC. EPC early psychosis with cannabis use, EPNC early psychosis without cannabis use, HCC healthy controls with cannabis use, HCNC healthy controls without cannabis use, SPEM smooth pursuit eye movements, AS antisaccades.

Fig. 2. SPEM mean gain × group.

EPC early psychosis with cannabis use, EPNC early psychosis without cannabis use, HCC healthy controls with cannabis use, HCNC healthy controls without cannabis use, SPEM smooth pursuit eye movements. Boxplot plotted using ggplot geom_boxplot defaults in R: centre lines represent median, box hinges indicate Quartile 1 & 3, whisker 1.5x Interquartile Range from hinge or furthest data point, whichever is closer.

Fig. 3. AS error × group.

EPC early psychosis with cannabis use, EPNC early psychosis without cannabis use, HCC healthy controls with cannabis use, HCNC healthy controls without cannabis use, SPEM smooth pursuit eye movements. Boxplot plotted using ggplot geom_boxplot defaults in R: centre lines represent median, box hinges indicate Quartile 1 & 3, whisker 1.5x Interquartile Range from hinge or furthest data point, whichever is closer.