Cognitive functions in depressive disorders: evidence from a population-based study

Background. Most of the available evidence on the effects of depression is based on in- and out-patient samples focusing on individuals suffering from major depression. The aims of this study were to examine cognitive functioning in population-based samples and to determine whether cognitive performance varies as a function of depression subgroup. Method. Population-based samples (aged 20-64 years) with major depression (N = 68), dysthymia (N = 28), mixed anxiety-depressive disorder (N = 25) and minor depression (N= 66) were examined on a variety of cognitive tasks (i.e. episodic memory, verbal fluency, perceptual-motor speed and mental flexibility). One hundred and seventy-five non-depressed individuals served as controls. Results. The total group of depressed individuals showed impairments in tasks tapping episodic memory and mental flexibility. Of more interest, however, was the observation that the pattern of impairments varied as a function of depression subgroup: the major depression and mixed anxiety-depressive disorder groups exhibited significant memory dysfunction, whereas individuals with dysthymia showed pronounced difficulties in mental flexibility. Minor depression did not affect cognitive performance. Verbal fluency and perceptual-motor speed were not affected by depression. Conclusions. These results indicate that persons with depressive disorders in the population exhibit cognitive impairments in tasks tapping episodic memory and mental flexibility and that cognitive impairment varies as a function of depressive disorder.