Visual scan paths in first-episode schizophrenia and cannabis-induced psychosis

Abstract

Objective: Patterns of successive saccades and fixations (scan paths) that are made while viewing images are often spatially restricted in schizophrenia, but the relation with cannabis-induced psychosis has not been examined. We used higher-order statistical methods to examine spatiotemporal characteristics of scan paths to determine whether viewing behaviour was distinguishable on a continuum.

Methods: Patients with early acute first-episode paranoid schizophrenia (SCH; n = 11), cannabis-induced psychosis (CIP; n = 6) and unaffected control subjects (n = 22) undertook a task requiring free viewing of facial, fractal and landscape images for 5 seconds while their eye movements were recorded. Frequencies and distributions of saccades and fixations were calculated in relation to image regions examined during each trial.

Results: Findings were independent of image category, indicating generalized scanning deficits. Compared with control subjects, patients with SCH and CIP made fewer saccades and fewer fixations of longer duration. In turn, the spatial distribution of fixations in CIP patients was more clustered than in SCH and control subjects. The diversity of features fixated in subjects with CIP was also lower than in SCH patients and control subjects.

Conclusion: A continuous approach to characterizing scan path changes in different phenotypes suggests that CIP shares some of the abnormalities of SCH but can be distinguished with measures that are sensitive to cognitive strategies active or inhibited during visual exploration.

Fig. 1: Fixated feature subspaces. Dimensions of the subspace closest to all 3 participant groups' principal component spaces are shown for each stimulus category (minimum 86% of total variance). C = control subject, CIP = cannabis-induced psychosis, SCH = first-episode schizophrenic disorder of paranoid type, Rand = random fixation simulation for landscape (L'scape) scenes. For simplicity, only the first 10 of 30 dimensions are shown, reconstructed as eigenimages. The minimum angular deviation (in degrees) between each direction and group is given for each stimulus category and increases with higher dimensions, indicating greater divergence associated with diversity of features fixated. Graph inserts titrate subspace dimension (abscissas) against accumulated angular divergence (ordinates). A clear pattern emerged irrespective of the stimulus category: CIP > SCH > control. Landscapes were chosen as a control because they are commonly experienced, contain complex natural features and lack overt social relevance. Increasing separation (greater angular deviation) of random viewing from the landscape subspace with respect to control subjects indicates staring and unsystematic sampling of image content.