Aaron Beck, the famous psychotherapist and one of the founding fathers of cognitive behaviour therapy, thought that people with depression had automatic negative thoughts about situations, created by pessimistic assumptions and underlying negative thought processes. Research has confirmed these theories but there has been less study of inhibition in depression. It's impossible to pay attention to everything going on in the world around us as this would be overwhelming so inhibition helps us to choose which stimuli to pay attention to and which to ignore ; it's what helps us to listen to a particular conversation in a crowded room. Researchers in Canada thought that people with depression would find it harder to screen out negative stimuli. They studied 111 people, with and without depression, attempting to distract them from completing a task using positive, neutral and negative words. Overall depressed people were more easily distracted and they were particularly distracted by negative words. The more prone people were to negative thoughts and rumination (literally chewing negative thoughts over and over) the more easily distracted they were by negative stimuli.
Lau, Mark A. ... [et al] - Inhibitory deficits for negative information in persons with major depressive disorder Psychological Medicine 2007, 37, 1249-1259