The primary aim of this study is to investigate whether there will be a difference between behavioral and brain hemodynamic responses during the visual-spatial WM task in groups with low and high depression scores, although there is no clinical diagnosis. Considering the average age of the participants and the difficulty of the visuospatial 2-back task, it is predicted that the groups will not show a behavioral difference, but there will be a difference in the hemodynamic brain response, and this difference will be observed as an increase in the right PFC. In this section, the findings of the study are discussed in the light of previous studies in the literature.
When sociodemographic characteristics were examined, studies were generally conducted with groups in which individuals between the ages of 35-45 participated, some of the patient participants were treated with drugs in the past/currently using drugs, and the duration of education was equalized in years (7, 17, 18, 20–22, 35–39).
This research was conducted with equal groups according education norms, and the number of participants in the groups with low and high BDI scores was equal. Since the relationship between depression symptoms and WM in the young population is curious, the mean age is smaller than other studies in the literature (high BDI score mean±sd=22.2±2.28, BDI score low mean±sd=23.9±3.04). While some studies were conducted with only the patient group without a control group, some of them included both the patient and the control group. Our study also differs from some studies in the literature in that it includes a control group. Our study differs from most studies in the literature in that it was conducted with participants who were not diagnosed with depression and determined according to their BDI scores.
Tasks used to measure WM are generally n-back (7, 19, 22, 35, 38, 40) and verbal fluency tests (17, 18, 21, 37), but different experiments (20, 36, 39) are available.
In this study, a structured n-back task was designed on the visuospatial component of WM. It was expected that the participants would keep in mind the place of the stimulus that came out 2 before the stimulus they saw and decide whether it should be in the same place with the stimulus they saw by making a comparison. While designing this task, the stimuli used in the experiment were divided into emotional and neutral to see whether the behavioral performance and the brain hemodynamic response of the participants with a high BDI score differed when an emotional stimulus was present. The fact that the locations of the stimuli are kept in the mind and manipulated during the experiment is a cognitive task that serves the visual spatial sketchpad component of Baddeley and Hitch's memory model. In addition, the visuospatial n-back task is a unique paradigm in that it consists of both verbal and visual stimuli and allows measuring emotional and neutral stimuli together.
While some of the studies reported that there were behavioral differences between MDD and healthy control groups during the WM task, some found no difference. The inconsistent results of different studies with similar years of education show that a consensus cannot be reached in the literature. In this study, no significant difference was observed between the two groups in behavioral data. The reason for this is thought to be that the sample is from the young population. Even if the BDI score is high, the participants performed the same as the healthy group. Emotional stimuli also did not have a negative effect on the performance of the group with a high BDI score. The reason for this brings to mind the idea that emotional stimuli are not stimulating enough. When we ignore group and condition differences, right VLPFC activity appears to be increased in the visuospatial WM task. On the other hand, it was observed that the group with high depression scores showed increased activation during the WM task and in the right DLPFC.
This research was carried out using the NIRS method, which has been used frequently in recent years and has come to the fore with its easy and fast applicability. Although it is similar to magnetic resonance imaging (MRI) in the principle of measuring hemodynamic activity, but it cannot reach the activation of subcortical brain regions. Due to the similarity with MRI, MRI and NIRS studies were emphasized while examining the literature for this research.
In line with the studies examined in the literature, it is seen that the findings of our study are compatible with some information, but also contradictory with some results. The reason for this may be the average age of the participants, as well as the fact that the groups with MDD received medical treatment, the variety of WM tasks used, the difference in the difficulty of these tasks, the difference in the number of participants in the groups and the higher average age compared to our study. In our study, even though they were not diagnosed with MDD, people with high scores on BDI showed the same performance as people with low scores, and increased oxyHb change was observed in the right PFC, especially in DLPFC. In line with this finding, it is observed that the group with depressive features exhibited more PFC activation to achieve the same success.
The findings in the literature state that there is a hemispheric asymmetry in depression, and this asymmetry is observed as either hypoactivation in the left PFC or hyperactivation in the right PFC. Our study also supports the theory of hyperactivation in the right PFC (41).