| The aetiology of schizophrenia seems to be complex and the Vulnerability-Stress model (Nuechterlein & Dawson, 1984) proposes that genetic vulnerability factors, in conjunction with a particular history of environmental stress, would act together to drive the individual into a clinical state of psychosis.;The first study of this thesis (Brenner et al., 2007) examines the reliability, validity and factor structure of the Community Assessment of Psychic Experiences (CAPE) (Stefanis et al., 2002), with a French and English speaking sample form the north American population, a 42-item self-report questionnaire assessing psychotic-like symptoms in the general community: positive (or psychotic), negative (or vegetative) and depressive symptoms. This questionnaire was completed by 2 275 individuals from the Montreal area and the results support the internal consistencies of the original three subscales. Further, the exploratory factor analysis suggests 3--5 factor solutions, where the 4- and 5-factor solutions propose separating the positive symptoms into more specific subcategories. Finally, this study suggests a shorter version of the CAPE with 23 items, representing the same three original scales. These results support the existence of the continuum of psychosis, in which variation of psychotic symptoms can be found in the clinical population as well as in the general community.;The second study of this thesis (Brenner et al., 2009) examines the extent to which cortisol reaction to a psychosocial stress test, the Trier Social Stress Test (TSST) (Kirschbaum et al., 1993), can discriminate between controls and schizophrenia patients while controlling for important variables. A group of 30 schizophrenia patients and a group of 30 individuals from the general community, recruited from the first study of this thesis, participated in this study designed as a case-control experiment. The control group includes people with subclinical symptoms, and an overlap in the severity of psychotic symptoms exists between the two groups. Following the stressor, both groups have significant increases in heart rate and mean arterial pressure. However, their cortisol responses diverge, with schizophrenia patients exhibiting a smaller response than the controls, but with statistically significant group differences at only one time point, immediately after the stressor. These significant results are obtained after controlling for the severity of positive symptoms, which is a discriminating factor between the two groups. The level of cortisol measured immediately after the stressor is actually a marker of the critical threshold capable of discriminating between the groups. Cortisol levels in patients have a tendency to peak later than that of the controls.;As a general conclusion, the two groups' reactions to stress are another way to observe the continuity of a certain behaviour, up until it reaches a critical threshold, which allows the differentiation between clinical psychosis and absence of illness.;The principal objective of this thesis is to better understand the physiological reaction to a psychological stressor in people with schizophrenia, while conceptualizing psychotic symptoms along a continuum, rather than as a restrictive dichotomy. In order to distinguish between schizophrenia patients and controls from the general community, beyond the severity of their psychotic symptoms, they are compared on their stress reaction, and the phenomenon of critical threshold is explored within the cortisol reaction for its ability to distinguish between the two groups.;Keywords: schizophrenia, non clinical population, CAPE, validation, stress, TSST, cortisol. |
