Dysfunctions of Spatial Cognition in Schizophrenic Patients

ABSTRACT

Twenty outpatients who fulfilled the criteria for a diagnosis of schizophrenia and 28 control participants were invited to learn a route through a complex outdoor environment. They were then tested in tasks intended to explore various aspects of their memorized representation of the navigational episode. Compared to controls, the patients showed significant impairment in both the verbal production of route directions and the drawing of sketch maps. They referred to fewer landmarks and provided fewer directional instructions than the controls, while making a greater number of irrelevant comments. When invited to distinguish between photographs showing views of landmarks encountered along the route and distractors, they performed as well as the controls, and they had similar response times. However, when they were presented with pairs of actual photographs taken along the route, they displayed special difficulty in deciding which of the two landmarks was encountered first along the route. This difficulty in retrieving the sequential structure of the navigational episode suggests that the patients' memories were not accurately linked to one another in their mental representation of the route. These findings are interpreted in the context of current hypotheses about the hippocampal impairment that affects schizophrenic patients.

Keywords:

• hippocampal deficit
• navigation
• route directions
• schizophrenia
• sketch maps
• temporal information

Notes

1. The symbolic distance literature commonly refers to the “metric” properties of visuospatial representations on which distance comparisons are executed. Strictly speaking, the term “metric” implies some numerical value combined with a unit of measurement. However, given that comparisons are executed accurately regardless of the scale of a participant's representation, the assumption of an ordinal representation of the distances (rather than the representation of precise distances) is acceptable. It would, therefore, be more appropriate to refer to the concept of “relative metrics.”

2. The contrast between route and survey representations is common in the spatial cognition literature (e.g., Denis & Loomis, 2007). We are aware that drawing a sketch map generates a survey-like graphic representation, but that an accurate map can also be drawn from a route representation in the memory. In other words, people who are drawing a map do not necessarily have to have a survey representation in their mind before they draw the map. It is therefore conceivable that both a verbal description and a sketch map could be based on the same internal route representation.