One way to better understand clinical disorders is to combine research from cognitive experiments to dissociate information processing impairments for various conditions, such as major depressive disorder. Ruminations, which are associated with depression, place an increased load on basic cognitive processes, such as working memory and attention, which are necessary for more advanced reasoning and problem solving. Research has revealed a connection between performance decrements in an array of activities requiring effortful processing, or processes that rely on working memory and controlled attention . Depression impairs such cognitive functions as verbal fluency [2,3], verbal memory , mental flexibility , effortful processes and executive functions .
While working memory and attention are important contributing processes to fluid intelligence , mental processing speed also contributes to variance in higher level reasoning and problem solving [8,9], yet little research has been devoted to understanding mental speed in the depressed individual. Measures of mental processing speed can include reaction time for decision making. For example, in a study by Kalb, Dorner, and Kalb (2006), depressed patients were administered both simple and choice reaction time measures . Depressed patients were slower than control participants on measures using briefly presented stimuli. After administration of antidepressants, the depressed patients showed decrease in reaction time, linking the medication to better speed of processing, however, performance errors increased. The results of this research indicate differences in information processing occurring very early at the stage of pre-attentive processing.
Another way to examine mental speed and depression is to use inspection time measures . Inspection time assessments entail manipulating the exposure duration of stimuli. Most inspection time measures require a decision, such as auditory or visual discrimination of masked stimuli. Inspection time is considered to be an indirect measure of neural firing speed in the brain, specifically for the processes necessary for an accurate decision. Individuals with fast neural speed can make an accurate representation of the stimulus in less time, which could facilitate subsequent decision making and problem solving. If depression is characterized by slowed mental speed on inspection time measures, then decision making is also impaired. Depressive symptoms, as characterized by the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-V), include feeling slowed down, fatigued, and inhibited concentration and thinking . According to
Evidence for cognitive slowing has been found in a study by Tsourtos, Thompson, and Stough (2002), in which inpatients with depressive disorder were compared with controls on visual inspection time tasks using various sizes of lines . Here, the depressed patients showed decrements on mental speed tasks with exposure durations as short as 60 ms. The visual inspection time measures required discrimination judgments for briefly presented lines.
The focus of the current research endeavor was to examine the potential relationship between depression and mental speed using inspection time measures, and expanding the variation to speeded detection, identification, and discrimination of letters presented at durations of 80 to 16 ms. Since inspection time for line discrimination was found to be impaired in depressed patients , letter discrimination is also expected to be impaired in the current study. Since the other measures, detection and identification, are unexplored, it is unknown whether they will be impacted by depression. Unlike the existing research examining inspection time, this study used a college student population, and self-report on the Beck Depression Inventory . It is important to examine the potential impairments in a non-clinical sample, especially students, who must rely on peak cognition to maintain academics in a college setting.
The sample consisted of 217 college student participants from a highly selective liberal arts college in the Midwest. Age ranged 18-23, with a gender distribution of 144 females, 73 males. Participants signed up online or called for individual appointments, and were given the option of receiving monetary compensation or research credit in a psychology course, if enrolled.
For each trial a “Ready” screen appeared with a prompt for participants to self-initiate the trial sequence. Once a trial was initiated there was a refractory period of 500ms. Next a forward visual mask, a“#” sign, was displayed for 30 ms, followed by a potential letter presentation or a blank screen for either 80, 50, 30, 20, or 16 ms, depending on the block. A backward visual mask followed, which was another “#” sign for 300ms. The general perception in this task is you must decide if a letter is present between the “#”. Presence of blank trials was random for each specific inspection time block. Responses for letter detection were indicated by a key press on a computer key pad, with designated keys marked (“1” for yes or “3” for no). If participants indicated a letter was detected, the next instruction was to attempt to identify the letter by choosing it on the keyboard. Feedback was provided on accuracy at the end of each trial. Measures attained by this task are accuracy for detection and identification, for each presentation duration block (80-16 ms). Refer to Figure 1 for a representation of stimulus events.
Participants were tested in individual appointments at the Kenyon College Cognition Lab, in sound attenuated rooms, and were instructed to work at a comfortable pace. After completing surveys, participants were screened on the Snellen Eye Chart. The participants then completed the first computerized inspection time task for detection and identification. For the performance measures, each participant was seated in a stationary chair 17 inches from the monitor to control for visual angle. The participants then completed the first computerized inspection time task for detection and identification, which lasted approximately 10 to 15 minutes. Then after a break, a second computerized task was administered for discrimination. At debriefing, participants were provided with contact information for counseling services in the local area and encouraged to seek support if concerned about survey responses.
3.1. Beck depression inventory
Scores on the BDI had a mean of 9.67, with a standard deviation of 7.87, a range of 0-39, and high internal consistency with Chronbach’s Alpha of 0.895. BDI scores were positively skewed, with 92 participants qualifying for minimal depression, 97 for mild depression, 27 for moderate, and 5 for severe (see Figure 2 for participant frequencies on BDI categories). With so few participants meeting criterion for severe depression, all subsequent analyses for hypothesis testing compared those in both the moderate and severe group, giving a total of 32 in the depressed group to compare with those not qualifying at that intense of a criterion level on the BDI. For this sample, 14.75% fall into the category of moderate to severe depression.
3.2. Inspection time measures
3.3. Group performance data
Results of this study confirm that individuals reporting symptoms of moderate to severe depression, based on the Beck Depression Inventory criterion, do have impaired mental speed, as measured by the discrimination version of the inspection time measure. By including the additional speed measures for detection and identification, the goal was to see if depression would also be associated with changes in the more basic speed measures.
This finding is consistent with previous research using patient samples . Speeded discrimination is a standard paradigm for inspection time measurement, is widely used, and has been found to be sensitive to participant variables, such as age and other diagnostic criteria, and has now been found to be sensitive to depression symptoms.
With multiple studies now concluding that depressive disorder is associated with impairments in mental processing speed, it is important to focus on why the relationship exists. Based on both the description in the DSM-V, and reported symptoms of patients and volunteer participants, the experience of depression is that of feeling slowed, lethargic, and fatigued. In this sense, depression may be categorized by a general nervous system slowing in the brain, which impacts neural firing necessary for processing briefly presented stimuli, (as measured by the inspection time tasks). A depressed individual needs more time with the stimuli in order to make an accurate decision about features. It may possibly take more time to form an accurate, detailed representation of stimuli when depression is more severe. The findings of this study are consistent with the
What some may identify as a central limitation of this study, the student sample, may also be considered a strength. The participants had to meet strict academic criterion to enroll in a selective private liberal arts college, and should score high in intelligence and reasoning. Even though the sample is not comprised of patients in clinical treatment institutions, differences in performance are consistent with previous findings , and it is likely that depression symptoms and cognitive deficits should be exacerbated in a more representative community or patient sample. Even with a seemingly healthy, capable subject pool, moderate to severe symptoms are associated with reduced mental speed performance. We assume that the differences found in discrimination speed would have a larger effect size with a sample more representative of the general public. Since there were only 5 individuals meeting criterion for severe depression, the fact that moderate depression symptoms yield impaired discrimination speed performance is an indication that one does not need to be an extreme case or patient in a clinic in order for cognition to be impaired. It is also important to note that with a patient sample, clinicians may be more able to assess the length of the depressive episodes and possibly gather more information as to whether depressed symptoms are a first time experience or related to isolated events, and thus attributed to state depression as opposed to trait depression. Knowing if the depression is a first time episode may also be critical to determine because research has shown that the first incidence may not impair cognition on a visual search paradigm . The value of examining a student population is that the sample requires good cognition for everyday problem solving related to the goal of achieving a diploma. Therefore, any problems that are observed with basic cognitive processes are notable, as they may be associated with difficulty in higher-order decision making and reasoning necessary to complete assignments for courses.
Although participants provided only self-reported symptoms for depression, the findings are consistent with other research using undergraduate samples and self-reported data on the BDI finding cognitive impairments , as well as research using patient populations that have been assessed for depressive disorder via a structure diagnostic interview from a clinician . Results of a study comparing medicated and non-medicated patients with depression indicate that both show impairments in relation to non-patient controls, however, those who were medicated performed better than those not . Future research should examine potential changes in performance before and after medication as a treatment to better understand the potential for intra-individual improvements in mental speed. Findings of the current research with student samples indicate that this population is also vulnerable to depression and cognitive impairment, and thus administrators and educators should be aware that young adult college students also suffer, and that the impact may be seen in academics.
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