Cognitive dysfunction in late-onset depression

Marko Pišljar
Psychiatric Hospital Idrija, Idrija, Slovenia

coauthors: Zvezdan Pirtošek, Grega Repovš

ZP: Department of Neurology, University Medical Centre Ljubljana, Ljubljana, Slovenia;
GR: Department of Psychology, University of Ljubljana, Ljubljana, Slovenia

Introduction: The evidence suggests that 10-20% of depressed patients show impairment on variety of cognitive tasks. Most functional neuroimaging studies of major depression observed hypoactivity in frontal regions, particularly in medial prefrontal cortex and in dorsal anterior cingulate cortex. Given that the frontostriatal circuitry may be disturbed in depression, it might be expected that depressive patients would show deficits in tasks that are dependent on the integrity of frontal cortex.
Subjects and methods: In our study we investigated cognitive functioning in a group of patients (aged 65 and older) with late onset depression, after they had achieved remission from major depression. They were treated with selective serotonin reuptake inhibitors. At study entry patients had met ICD-10 criteria for unipolar depression. The severity of depression was measured using the Geriatric Depression Scale and Hamilton Depression Rating Scale. The subjects were considered in remission if they no longer met criteria for depression and had an HDRS score of 10 or lower for 3 consecutive weeks. A group of patients, compared with age and education matched healthy volunteers, were assessed for deficits in executive functioning. We performed the Stroop Color – Word test, simultaneously with event related potentials recording. We had two neutral stimuli (color patches and verbal control), congruent and incongruent stimuli. Computer version of tasks enabled a more detailed reaction time (RT) analysis and the use of modern electrophysiological techniques. In our experimental protocol, verbal response (RT) and P300 event related potentials were recorded. EEG activity was recorded with Ag / AgCl electrodes attached at Fz, Cz and Pz sites (EEG 10 – 20).
Results and conclusion: Patients with late onset depression showed increased absolute reaction times in Stroop Color-Word test. They had longer RTs for verbal control stimulus, the differences between the second control and incongruent RTs were smaller. Significant differences in the magnitude of individual interference effects point toward a characteristic change in attention processes in depressed patients. In contrast to RTs, which are different in Stroop task, P300 latency demonstrated no change across color / word congruence conditions. The ERP findings indicate that the Stroop effects occur after stimulus evaluation had been completed. We found lower P300 amplitude in depressed patients, meaning that the stimulus evaluation may be less efficient. This finding is pointing toward central cognitive impairment, not only to impairment of efferent, motor processing, which is expressed by reaction time.

Keywords: late onset depression, cognitive dysfunction, Stroop color-word test, event related potentials