Impaired filter function in patients with somatoform disorders and major depression, as reflected by auditory evoked potentials in an oddball and an active distraction paradigm

Introduction: In the in influential perception-filter model of somatoform disorders
(SD), three stages of symptom perception are postulated: (1) bodily
signals, which may be amplified by stress, (2) a filter system, which distinguishes
between relevant and irrelevant stimuli, and (3) cortical perception
of physical symptoms. As there is dearth of evidence so far supporting the
relationship between (1) bodily signals and (2) filter processes in SD, this
study investigated if filter processes are altered in SD and if stress may affect
filter mechanisms.
Methods: Twenty-four patients with SD, 24 with depression and 24 healthy
control individuals were assessed. Event-related potentials (ERPs) with two
different auditory distraction procedures were recorded over four blocks, one
before and three after either a socially evaluated cold pressor test (SECPT)
or a control procedure. We manipulated both the frequency of and the attentional
focus on stimuli to reflect filter processes.
Results: We found smaller P3b amplitudes (reflecting memory storage) in patients
with depression and SD, as compared to healthy controls. Furthermore,
both patient groups showed a smaller P3a amplitude (reflecting attention),
when counting the infrequent tone ("Oddball task") and a smaller N1 amplitude
when counting the frequent tone ("active distraction"). In patients
with SD, the SECPT had a decreasing effect on P3a amplitudes.
Conclusions: Both late filter processes, reflecting attention (P3a) and memory
storage (P3b), may be impaired in SD and depression. As acute stress
affected attention in SD patients only, the impact of bodily signals (1) on
filter processes (2) may be specific for SD.