Our goal was to challenge both normal controls and patients with seasonal affective disorders (SAD) to various light histories and then measure their retinal response modulation using the electroretinogram (ERG) in both winter and summer. In winter and summer, 11 normal controls and 12 SAD patients were exposed to three different light conditions for 1 h (10,000, 100 and 5 lux) followed by an ERG. Groups showed similar ERG amplitudes in the 100 lux condition. Compared with the 100-lux condition, in controls, the ERG response was significantly increased in the 5-lux condition; in SAD, it was significantly decreased in the 10,000-lux condition. This pattern was present in both seasons. This is the first time a retinal response modulation anomaly has been observed in SAD patients in both the depressed and euthymic states. Retinal response modulation may represent an interesting biomarker of the disease for future research.
In patients with seasonal affective disorder (SAD), a mood lowering is observed during fall and winter, with remission usually occurring in spring and summer, when the photoperiod increases (Rosenthal et al., 1984). Although SAD patients are less exposed to natural light in winter, their pattern of light exposure is not different from the one observed in the healthy population (Guillemette et al., 1998 and Hébert et al., 1998). However, a defect in retinal light sensitivity has been hypothesized to explain the vulnerability of SAD patients to seasonal light changes (Beersma, 1990 and Remé et al., 1990). We and others have demonstrated in fact a rod sensitivity anomaly in SAD patients as measured with the electroretinogram technique (ERG) (Lam et al., 1992, Hébert et al., 2002 and Hébert et al., 2004). The ERG is a bio-potential recorded at the surface of the eye, which allows the functional assessment of various retinal cells such as rods (night; scotopic vision) and cones (day; photopic vision), but also ON-bipolar cells (Heynen and van Norren, 1985, Stockton and Slaughter, 1989 and Lei and Perlman, 1999). Many aspects could influence the ERG response, one of which appears to be the light history (Gagné et al., 2007). In the latter study, we exposed participants for 60 min (on different days) to three light conditions prior to cone and rod ERG assessment. It was found that rods were particularly sensitive to prior light exposure, with increased rod ERG maximal amplitude observed after being exposed to 5 lux for 1 h, when compared with 100 lux or 10,000 lux exposures, the two latter conditions yielding similar rod maximal response. No difference was found on cone response.
Knowing that SAD patients demonstrate a rod retinal sensitivity anomaly (observed only during the depressive episode), we aimed to challenge these patients with the above light history protocol in order to detect if the rod adaptation mechanism would be altered. Moreover, due to the recent discovery of a cone ERG anomaly in SAD patients (Lavoie et al., 2009), cone function was also assessed during the light history protocol with the ERG. Since in Lavoie et al. (2009) the rod and cone anomalies in SAD patients were observed to normalize in summer or after successful treatment with light therapy, our protocol was therefore applied in the same patients during both the depressed and remitted states, which corresponded to the winter and summer seasons, respectively. Our goal was to see how patients with SAD would adapt to this light history challenge and, if anomalies were present, at when they would be detected, that is, only during winter (state marker) or during both seasons (trait marker).
