As expected, there was a significant attenuation of the RIF effect in MDD. Moreover, there was a negative correlation with depressive severity and rumination but positive correlation with verbal memory learning and delayed recall in MDD. The associations suggest that there may be a spiral of events whereby increasing depressive severity associated with rumination as well as difficulties in disengaging attention from negative material leads to increased elaboration of negative material in MDD (van Minnen, et al., 2005).
None of the verbal learning variables showed a significant correlation in healthy control participants, but there was a positive correlation between immediate and delayed recall correlated with RIF in MDD participants. This suggests that verbal learning is not related to the RIF effect generally, but that there is a specific impairment in MDD. Interestingly, IQ measures were not correlated with RIF, thus highlighting how the RIF task is not a reflection of general intelligence. Although wide ranging deficits across a number of neuropsychological tests have been found in MDD (Snyder, 2013), RIF involves memory retrieval (Soares, Polack and Miller, 2016) which are mediated through impairments in inhibition processes (Murayama et al., 2015). The inhibitory mechanism that is impaired in MDD is not well captured by measures of general cognitive functioning.
In support of inhibitory processes, participants who endorsed smoking demonstrated a greater RIF effect in comparison to those who were not smokers. In the total sample, the number of cigarettes smoked per day was positively correlated with RIF, while within the MDD group, smoking status showed a negative relationship with RIF. Edginton and Rusted (2003) reported that nicotine, a cholinergic agonist, enhanced inhibition in RIF, which is consistent with the known effects of nicotine on inhibitory processes and specific aspects of cognition that require effortful processes (Rusted et al., 2009). The present findings suggest that if inhibition can be potentially enhanced with nicotine, it may also be possible to enhance suppression of intrusive negative thoughts and reduce rumination in MDD (Nolen-Hoeksema, 2000).
In contrast, there was a positive correlation in the capacity for mindfulness with RIF in MDD. Mindfulness is a training practise which involves being present in the moment. Mindfulness-based cognitive therapy is associated with improved meta-awareness in a decentred manner in MDD (Hargus et al., 2010). Mindfulness-based interventions are associated with neurophysiological measures that reflect a state of relaxed alertness (Lomas, Ivtzan and Fu, 2015), and in a meta-analysis, Im et al. (2021) found significant improvements in executive function although limited effects in attention, working memory and long term memory.
MDD is associated with a negative bias in memory recall (Clark and Teasdale, 1982), future thinking (Roiser, Elliot and Sahakian, 2012), emotion recognition for words (Bradley, Mogg and Williams, 1995), images (Donaldson, Lam and Mathews, 2007) and in facial expressions (Ridout et al., 2003; Fu et al., 2004, 2008). Joormann et al. (2004) demonstrated that dysphoric participants and participants with a history of depressive episodes exhibited reduced inhibition of negative material that they were instructed to ignore, indicating that RIF may be an important underlying inhibitory mechanism in MDD. Moreover, the observed attenuation in neutral words in the present study suggests that there is a more wide-ranging impact of the executive function impairment that is not limited to negative emotions.
In translating the findings to clinical practice, it has been proposed that rehearsing positive forms of stimuli or memories as in an attention bias modification strategy might improve dysphoric moods, and cognitive behavioural therapy has been proposed to intervention which facilitates positive information relative to negative information (Joorman et al., 2005, Brewin, 2006). However, the clinical efficacy of attention bias modification interventions has been limited (Fodor et al., 2020), and cognitive behavioural therapy shows clinical effectiveness in MDD with limited benefit over other forms of psychotherapy (Cuijpers et al., 2013, 2023). There is significant heterogeneity in MDD in which the mechanisms underlying impairments might not be simply attributed to observable biases (Fu, Fan and Davatzikos, 2019).
In conclusion, the current findings demonstrate a link between ruminations and depressive severity to the RIF effect in MDD. In contrast, the capacity for mindfulness was associated with improved RIF in MDD, and the relationship between smoking and RIF could reflect enhanced inhibition effects. Clinical implications support interventions that lead to a state of relaxed self-awareness in MDD.