Object
Finding one’s way and remembering the location of objects in the world is essential for everyday living. Deficits in these capacities are of critical relevance in several psychopathological and neurological syndromes (e.g., Denis et al., in press). This research addresses the issue of the impact of schizophrenia on visuo-spatial memory, focussing especially on the sequential spatial memory in schizophrenic patients.
Functional MRI studies have shown consistently that a hippocampal volume reduction is one of the structural abnormalities found in schizophrenia. The hippocampus is a region of the brain that is crucial to memory function, and its role has been established as essential in spatial cognition (e.g., Maguire et al., 2003). Spatial memory of traveled paths requires memorizing the spatial layout of the environment and keeping track of the sequence of the landmarks encountered.
In investigating memory for scenes of viewed along this route in schizophrenic patients, we expect to be able to distinguish between sequential retrieval from mere scene recognition.
Methodology
In a research collaboration with P. Boyer and C. Morès (UMR 7593), we conducted an experiment involving a set of four related tests. The experiment took place on the premises of a Parisian hospital, the Hôpital de la Salpêtrière. This ancient and historical hospital looks like a small city, with a number of main streets, alleys, cross junctions, gardens and old as well as new buildings.
Two groups of participants were set up : a schizophrenic patients group (n=20) and a healthy participants one (n=28). The diagnosis of schizophrenia was achieved primarily by psychiatric evaluation, according to the DSM-IV.
The learning phase consisted for the participants to navigate along on a pre-determined route, through the hospital environment (ten minutes walk). The participants were accompanied by the experimenter and were invited to pay attention to this route, since they would be required to describe it later. The navigation was repeated twice.
In the first two tests, the participants were requested to verbally describe the route they previously went all over, then to draw it in the form of a sketch map. They were invited to be as accurate as possible in order to allow a supposed addressee to find easily the route. The verbal descriptions were recorded on a tape, before transcription and analysis. The sketches of the route were drawn on a blank map of the hospital environment (the map did not contain any building or street names on it).
In the last two tests, the participants were shown a set of 32 photographs on the screen of a computer. For each of the photographs, they had to identify whether the view belonged to the route or not. Thereafter, they were shown 28 pairs of photographs and asked to decide for each pair which view had been encountered before the other one on the route (the two photographs of each pair were simultaneously displayed on the screen). For these two experiments, response times were recorded.
Results
Route descriptions
Not surprisingly, due to their communication difficulties, schizophrenic patients produced shorter descriptions than the control participants: an average of 33.7 propositions versus 60.7 (p<.005).
Their descriptions were less prescriptive (they mentioned less actions to be done, 14.8 versus 27.1 p<.0005). They were also less descriptive : the patients’ descriptions referred to a lesser number of landmarks (they produced an average of 12.1 propositions mentioning a landmark, versus 28.5 for the control participants’, p<.01). Conversely, they produced more comments than the control participants did (20% of their descriptions versus 8%, p<.01).
To summarize, the patients’ route descriptions are obviously quite unlikely to help an addressee to find a route. This deficit, however, may result from two factors: the patients’ inherent language weaknesses and the specific cognitive difficulties in the processing of spatial information.
Examining the sketch maps was quite informative in this respect.
Map drawings of the route
- Compared to the controls, the sketch maps drawn by the schizophrenic patients reflected a real difficulty in recalling the landmarks and the overall shape of the route.
- In a same way as for the verbal descriptions, schizophrenic patients produced quite poor sketches, especially regarding the following two criteria :
- Number of relevant landmarks (located at appropriate reorientation places). While control participants mentioned 7.3 crucial landmarks located at intersections or reorientation points, the schizophrenic patients ignored a large number of them and drew only 2.9 landmarks on the map (p<.00002)
- Number of correctly reported reorientations. The schizophrenic patients reported 4.1 turns or changes of orientation versus 6.2 out of the 7 turns on the route (p<.001).
Recognition of views
The participants were shown a series of 32 views of the hospital environment, half of them had been actually encountered during the navigation phase. The other half belonged also to the Salpêtrière hospital and presented the same architectural characteristics of the hospital, but had not been seen along the route.
When presented with each of the photographs in random order, schizophrenic patients achieved very good recognition performance, and did as well as controls: on average, they identified correctly 24.1 scenes out of 32 (versus 25.9 for the control participants). They did not need significantly more time than controls to respond (5,4 s versus 5,3 s)
These results show that in spite of their cognitive deficit, the schizophrenic patients are able to correctly memorize the views they have encountered along a route. They have fully preserved ability to distinguish a scene from a similar other one and to identify that it belongs to the route.
Identification of the sequential order of views
The participants were asked to identify the temporal sequence between pairs of views. They were presented pairs of photographs and had to remember which view of the two was encountered first along the route.
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Example of a pair of photographs from the Salpêtrière Hospital |
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Results
Schizophrenic patients had poorer performance on the order recognition task than the control participants. Out of the 28 presented pairs, the patients identified correctly the sequential order for only 18 pairs versus 24 pairs for the control participants (p<.001).
Conclusion
Route descriptions are essentially sequentially organized materials. Schizophrenic patients show severe deficits regarding the chronology of the events encountered along the routes. They are fully able to recognize objects, but they cannot correctly localize these objects in space. They have difficulties in remembering the spatial relations among landmarks in the environment. When recalling routes previously learned, they are impaired in the recall of the sequence of turns and the sequence of visual landmarks. To summarize, to refer to the classic distinction (cf. Landau & Jackendoff), they can quite easily answer the "what" question, while experiencing much difficulty with the "where" one.