5. Conclusions and future directions
As outlined in the previous sections, deficits in inhibitory processes may play a central role in the occurrence of ruminative responses. According to Nolen-Hoeksema and her collaborators, what characterizes rumination and differentiates it from negative automatic thoughts is that it is a style of thought rather than just negative content (Nolen-Hoeksema, 1991). Thus, rumination is defined by the process of recurring thoughts and ideas often described as a “recycling” of thoughts and not necessarily by the content of these recurring thoughts. While depressive rumination is characterized by negative self-focused thoughts, rumination in an angry or happy mood state features other contents although the process may be similar (Feldman, Joormann, & Johnson, in press; Rusting & Nolen-Hoeksema, 1998). More research that focuses on the underlying cognitive processes and the relation of rumination to biases in memory and attention processes is clearly needed. The majority of studies on rumination to date have been concerned with consequences of ruminative responses. While this line of research informs us about the devastating effects of rumination, it is not particularly helpful in determining why it is so difficult for some people to redirect their thoughts and control their attention before it becomes dysfunctional. Finding an answer to this question might increase our understanding of cognitive processes in depression and inform our interventions. Although a comprehensive discussion of the following issue is beyond the scope of this article, we should nevertheless point out that the concept of inhibition has been criticized in research on attention and memory (e.g., Friedman & Miyake, 2004; MacLeod, Dodd, Sheard, Wilson, & Bibi, 2003). Thus, the construct validity of several measures that have been proposed to assess inhibition has been questioned, and some investigators have pointed out that researchers often do not justify their assumptions that specific measures actually involve inhibitory processes (Friedman & Miyake, 2004). Specifically, research on the negative priming task has led investigators to propose a number of alternative mechanisms that could underlie the observed effects. Indeed, MacLeod et al. (2003) argued that many results that are interpreted in terms of inhibitory processes can be explained without reference to this concept. This is an ongoing debate, however, and Tipper (2001) recently concluded that although different explanations have been discussed, there is no firm evidence to discount inhibition models; indeed, Anderson (2003) has been more forceful in discussing repeatedly why forgetting requires the recruitment of inhibitory control mechanisms to override prepotent responses (see also Anderson & Spellman, 1995). One possible alternative explanation for the presented results is that there is differential initial activation of positive and negative material in the depressed group in the absence of group differences in the strength of inhibition. Clearly, future studies are needed to investigate whether inhibition, or any of these proposed alternative mechanisms, provides the best explanation of the observed effects. We believe, however, that the findings of difficulties in redirecting attention, disengaging from negative material and increased elaboration of negative content that we have summarized in this article represent important result that can inform our models of cognition in depression even if the precise underlying mechanisms remain open to debate.
