Cognitive Reserve in Major Depression-Associations with Cognitive Status, Age, Education, Personality, and Depression Severity

Research Article

Austin J Psychiatry Behav Sci. 2014;1(3): 1015.

Cognitive Reserve in Major Depression–Associations with Cognitive Status, Age, Education, Personality, and Depression Severity

Coloma Andrews LC1* and Zihl J2,3

1Institute for Stroke and Dementia Research, Ludwig Maximilian University, Germany

2Max Planck Institute of Psychiatry, Munich University, Germany

3Department of Psychology, Ludwig Maximilian University, Germany

*Corresponding author: Coloma Andrews LC. Institute for Stroke and Dementia Research, Medical Center, Ludwig Maximilian University, Marchioninistr, 15, 81377 Muenchen, Germany

Received: March 31, 2014; Accepted: April 01, 2014; Published: April 03, 2014


Cognitive reserve (CR) is understood as a latent potential underlying the flexible adaptation to mental challenges. By optimizing cognitive performance, it can be used to cope with high task demands. This study examines CR in 40 inpatients with unipolar depression compared to 24 healthy control subjects. The size of CR was assessed by calculating the maximal performance improvement over retesting with a digit symbol substitution task. Furthermore, the relation between CR, cognitive status, age, education, and the personality traits Openness for experience and Neuroticism was explored. CR did not differ significantly between controls and the whole group of depressed patients. However, patients who displayed cognitive deficits in one–time neuropsychological testing (50%) showed a lower CR than controls while patients without deficits showed a marginal higher CR. In patients, CR was positively associated with attention, short term memory, and openness for experience. In controls, CR was relatively independent from cognitive status but showed a negative association with age. Our results support the idea that subgroups of patients can be differentiated through cognitive status as well as CR. Furthermore, the marginal higher CR in patients without cognitive impairment suggests that CR acts as a buffer against the development of cognitive deficits in depression.

Keywords: Cognitive Reserve; Cognitive Plasticity; Depression; Cognition; Personality; Testing–the–limits.


CR: Cognitive Reserve; Ttl: Testing–The–Limits; DSST: Digit Symbol Substitution Test; STM: Short Term Memory; WM: Working Memory; MADR–S: Montgomery Asberg Depression Rating Scale; MMSE: Mini–Mental State Examination; SSRI: Selective Serotonin Reuptake Inhibitors; NASSA: Noradrenergic and Specific Serotonergic Antidepressant; NARI: Selective Noradrenaline Reuptake Inhibitors; D: Depressed Patients without Cognitive Deficits; Ddef: Depressed Patients with Cognitive Deficits.



Cognitive deficits in depression are well documented. Impairments have been found in many cognitive domains, including attention [1–3], memory [4,5], and executive function [6,7]. However, it has been doubted whether the standard one–time testing reflects the “latent competence” of a subject. This competence is understood as cognitive reserve (CR), which is activated when required to cope for functional consequences of brain pathology [8]. Therefore, people with high CR have a better ability to compensate for pathologies like Alzheimer’s or Parkinson’s disease than people with low CR [9–12]. In healthy individuals CR has been suggested as a protective factor against cognitive decline in normal ageing [13]. CR is also sought to enable individuals at any age to cope with increased task demands by optimizing cognitive performance, possibly by using neural networks high in efficiency or capacity [8]. To measure CR, proxy variables are often used which are hypothesized to reflect cognitive functioning, like education or intelligence (for a discussion, see [14]). Other authors have favored a more dynamic measure of CR by assessing the potential to adapt performance to a challenging cognitive task (e.g. [15]). In the testing–the–limits (TtL) procedure one’s maximal performance improvement due to training or practice is assessed [16–18]. Within this procedure, CR differs from cognitive status as it reflects the latent competence one has available when high performance is needed.

The first objective: The first objective of the present study is to examine CR in depressed patients with a TtL procedure using a simple retest design. As a CR measure, we used the individual performance improvement in the Digit Symbol Substitution Test (DSST, revised Wechsler Intelligence Scale, german version; Aster [19]), which is a simple but multi faced measure incorporating multiple cognitive abilities [20,21]. Since depression is associated with a wide range of structural and functional neuronal abnormalities (e.g. Hickie and Rogers [22,6], not only detriments in cognitive status but also in CR may be found in depression [23,24]. By using performance gain after memory training as a CR measure, Calero and Galiano [25] found no difference in CR between older subjects with and without high scores in a self–rating depression scale. However, generalization of those results is restricted, since subjects with an elevated score might not have been clinical depressed according to DSM–IV criteria [26]. The present study therefore compares clinically diagnosed depressed patients with healthy controls. As depression is not necessarily associated with cognitive impairments in one–time–testing [27,3], we additionally compared CR in patients with and without clinically relevant cognitive deficits.

The second objective: The second objective of the present study is to explore a number of variables associated with CR in patients and healthy subjects: age, education, cognitive status, and the personality factors Openness and Neuroticism. As healthy aging is associated with a mild decline in cognitive functions [28], higher age may also be negatively associated with CR [16,18,29]. Since past research has repeatedly shown that the DSST is age sensitive (for a meta–analysis, s. Hoyer [30]), this may be especially prone in a DSST–based CR measure. A positive association with education and cognition has been suggested by Richards and Deary [31] and Scarmeas and Stern [32] as it may help to establish efficient cognitive skills. However, positive associations between general cognitive abilities and CR have only been reported in some (e.g. Singer [29]) but not all [15,21] studies using performance improvement as a CR measure. Past research has shown that the “Big Five” [33] personality factor Openness to experience has a direct positive effect on intelligence through environmental enrichment [34] and is positively related to cognitive abilities like memory and executive functions [35– 37]. Neuroticism has frequently reported to be negatively related to cognition (e.g. Chapman [38]). Therefore, Openness (+) and Neuroticism (−) may also be important for the development of CR. However, the relationship between performance improvement and personality has only been studied by few studies, yielding different results [39–41]. Nevertheless, one can assume that a low Neuroticism on the one hand and good cognitive abilities, a high education and a high Openness on the other hand not only favors the development of CR by the accumulation of efficient strategies but are also beneficial in mobilizing CR to improve performance in a current test situation.

In depression, the mobilization of CR may also be affected by the severity of depression as the severity of symptoms can influence the extent of cognitive impairment [42].

The following hypotheses: The following hypotheses have been pre specified in the present study:


Study design

Depressed patients and healthy control subjects were tested to assess the individual size of CR, cognitive status, and personality characteristics. Patients were tested on two separate days to avoid fatigue effects. The first day included a routinely administered test battery to comprehensively examine performance in attention, memory, and executive function. The second day implied further study specific measures. Testing in control subjects was executed on a single day including only measures used for the quantitative analysis.


Depressed in–patients admitted to the Max Planck Institute of Psychiatry in Munich were originally included when meeting the following inclusion criteria: first episode of unipolar major depression or recurrent depression. Diagnosis of depression was made by the treating psychiatrist according to DSM–IV criteria [26]. Exclusion criteria were other primary psychiatric diagnoses than depression, depression with psychotic symptoms, electroconvulsive therapy within the last three month, present or past neurological illness, present or past substance abuse, un medicated hypo– or hypertonia, diabetes and thyroid dysfunction.

Healthy control subjects without a history of psychiatric or neurological illness were recruited through notices at the Ludwig Maximilian University in Munich and through a control–sample which previously participated in a non–cognitive study at the Max Planck Institute of Psychiatry.

All patients and controls gave written informed consent according to the latest version of the Declaration of Helsinki.

Procedure and Material

Cognitive reserve

CR was measured by a TtL procedure using a retest paradigm with ten consecutive trials of the Digit Symbol Substitution Test (DSST, revised Wechsler Intelligence Scale, german version; Aster [19]) without providing feedback or strategies to improve performance. This simple paradigm was chosen to maximize compliance in spite of the expected reduction of motivation and drive in patients. In each trial, a coding key is presented on the top of the sheet assigning the numbers 1–9 to corresponding symbols. Subjects are asked to use the coding key to note the corresponding symbols under blank fields below a series of digits. The same test was administered ten times in a row with a one minute break in between. To avoid ceiling effects, processing time was limited to 90 sec. The number of correctly written symbols per test served as the outcome measure.

Cognitive status, personality and severity of depression

Table (1) shows the cognitive tests used in the test battery routinely administered at the Max Planck Institute of Psychiatry in Munich. To compare patients with and without cognitive deficits, performance in every test was rated. A clinical relevant deficit was defined as having a test score lower than 1.5 SD below the test specific norm in one or more measures. To examine variables associated with CR, we limited the analysis to three cognitive tests as valid measures of short term⁄ working memory, selective attention, and problem solving.

CRi = 1+(1+x1 ⁄ xmaxgroup )*[(xi - x1 ) ⁄ xmaxgroup ]

Selective visual attention: Selective visual attention was measured by the d2 test of attention [43]: Participants were required to cross out the letter d with two dashes out of 14 lines of letters p and d with one to four dashes arranged above and below each letter. The number of correctly crossed out letters minus the number of errors served as the attention score.

Verbal short term and working memory: Verbal short term and working memory was measured by the subtest digit span forward and backward of the revised Wechsler Memory Scale (german version; Härting [44]). Subjects were asked to repeat strings of digits in increasing length either in the same (short term memory; STM) or the reversed order (Working Memory; WM).

Problem solving : Problem solving was measured by the subtest matrix reasoning (matrices) of the Wechsler Adult Intelligence Scale– III (german version; Aster [19]): Subjects were instructed to complete 26 geometric patterns of increasing difficulty by choosing the correctout of five inserts.

Personality: Personality was assessed by a computerized version of the NEO–Fife–Factor–Inventory (NEO–FFI; german version; Borkenau and Ostendorf [46]). Subjects were instructed to indicate on a five–point Likert scale how each of 60 statements suits their personality. The test–score for Neuroticism reflecting nervousness and anxiety and Openness to experience reflecting curiosity and creativity were used for analyses.

Severity of depression: Severity of depression in patients was measured by the Montgomery Asberg Depression Rating Scale (MADR–S; Montgomery and Asberg [47]). The MADR–S is an external rating instrument consisting of ten items representing symptoms of major depressive disorder. The rating was based on a structured interview [48] (Table 1).