Components of Cognitive Functioning and Their Measurement​

George I. Papakostas, MD

From the Clinical Trials Network and Institute, Massachusetts General Hospital, Boston

Cognition is central to the classification of MDD as a medical disorder; it involves both execution of mental tasks including the purposeful perception and processing of affect. Cognitive functioning is a conscious or largely conscious activity involving the integration of memory and sensory input. Its result is the completion of a task, so it is considered to be a purposeful and goal-oriented activity. Cognitive problems such as difficulty concentrating and making decisions are symptoms of MDD and are recognized by the DSM-5. In addition to being a symptom of the illness, cognitive functioning is also a functional outcome measure, as impaired cognition hinders the restoration of a patient’s psychosocial functioning to its premorbid level,1 allowing for continued difficulty at work, home, and in social settings, even after formal syndromal remission of MDD. Therefore, clinicians must routinely assess and measure cognitive functioning in patients with MDD.

General Components of Cognitive Functioning

The basic components of cognitive functioning include attention, immediate and delayed memory, cognitive speed, and executive functioning.1 These components are broadly defined as follows:

  • Attention: the ability to focus on a stimulus of interest against a background of stimuli considered irrelevant and potentially distracting
  • Immediate memory: the ability to remember something for a short time after it was presented
  • Delayed memory: the ability to recall something presented in the distant past
  • Cognitive speed: the rate at which different mental processes and tasks happen
  • Executive functioning: the ability to integrate sensory input and memory in order to complete a task

Thus, executive functioning is a multi-dimensional and complex process that has several requirements. First, executive functioning requires focusing motivational input or positive affect—the desire to participate in an activity or the anticipation of enjoyment or reward—to complete a task or achieve a goal.2 Second, executive functioning requires being able to ignore negative affective stimuli. For example, when a person is frightened, anxious, or nervous, these emotions can overpower problem-solving skills and organized action, thereby increasing the chance of errors or even inactivity. Given that the two core symptoms of a major depressive episode involve the presence of excess negative affect and/or a deficit in positive affect (anhedonia), it is therefore no surprise that cognitive and executive dysfunction is a common complaint of patients. The final requirements involve the ability to ignore irrelevant stimuli, allowing the person to focus on the task at hand and to create a plan for accomplishing it, as well as the ability to quickly and accurately access their memory.

Lack of Formal Cognitive Assessment in Clinical Practice

Despite the common occurrence of cognitive deficits in MDD and other psychiatric disorders, a lack of consensus exists among clinicians regarding their proper assessment. A recent international survey3 focusing on 61 psychiatrists exemplifies this point. In that study, psychiatrists were asked whether they regularly assessed psychiatric symptoms and, if so, with what tool. The study found that 93% of respondents regularly assessed cognition in patients with depression, while 7% did not. In addition, those who did not assess cognition regularly reported that they did not view cognitive functioning to be a relevant aspect of MDD (AV 1).

AV 1. Testing Of Patients With MDD For Cognitive Problems in Practice (00:42)

Data from El Hammi et al3

In addition, the majority (61%) of respondents indicated that they relied on patient history alone to assess cognitive functioning rather than more structured and objective formal tests of cognition.3 Among those clinicians who did use cognitive instruments for patients with MDD, little consensus emerged on which was the optimal one. While the MMSE proved slightly more common, this tool often is not sensitive enough to detect subtle deficits in cognition in patients without dementia or a frank psychosis such as patients with MDD. Many other tools mentioned in the survey were measures of symptom severity for major depression, schizophrenia, and Alzheimer’s disease rather than specific measures of cognitive functioning.

Clinician-Administered Scales for Identifying Cognitive Symptoms

The following tools are useful for accurately measuring various components of cognitive functioning, although one should also keep in mind that there is often overlap in component measurement with each of these. While these tools are sensitive and reliable, they are not always easily administered in clinical practice since they require time to be administered as well as trained personnel to administer them. However, practicing clinicians should be somewhat familiar with the nature of these tests to better understand the results of published studies that report on their use.

Mini-Mental State Examination (MMSE). The MMSE is widely used to test overall cognitive function. The test includes 19 items to assess orientation in place and time, learning and memory, construction ability, attention, and calculation skills, but excludes executive functioning. Possible scores range from 0 to 30; a score of 24 or higher indicates no impairment.4 The total score of the MMSE has shown an association with education level, which is problematic for those who would like to use the tool across a population with diverse educational statuses.

Digit Symbol Substitution Test (DSST). In the DSST, participants match symbols with the corresponding number based on a 9-digit coding table. A higher score, based on the number of symbols coded in 90 seconds, indicates better performance.5 The DSST’s requirement of response inhibition for successful completion heavily weights its measure of global cognitive function towards executive function.

Stroop test. The Stroop Test presents a series of words, printed in color, that may name another color (eg, the word “blue” in red letters). Participants must name the color of the text instead of reading the word. Several versions of the test exist, but scores are typically based on the number of items completed correctly in the time limit. The Stroop Test is a measure of attention and executive functioning because of its requirement for response inhibition and set-shifting (AV 2).6

AV 2. Using the Stroop Test to Assess Cognitive Impairment in MDD (01:54)

Trail Making Test (TMT). The TMT consists of 2 timed parts. In Part A, participants draw lines connecting numbered circles in ascending order from 1 to 25. In Part B, participants draw lines connecting circles in an ascending pattern, alternating between numbers and letters. The number of seconds required to complete the task determines the score; a higher score indicates greater impairment. Both parts measure motor speed and visual attention, with Part B measuring executive functioning.7,8

Rey Auditory Verbal Learning Test (RAVLT). In the RAVLT, participants are read a list of 15 unrelated words, asked to repeat as many as possible immediately after hearing them, and then asked to repeat the words again after a period of time. The test measures immediate memory, verbal learning, and working memory. Score is determined simply by the number of words recalled.9,10

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Simple Reaction Time (SRT). The SRT test involves delivery of a simple stimulus and a response. Participants may be asked to press a button when they hear a sound or see a light; timing of the stimulus is the only uncertainty. The SRT test measures alertness and motor speed, with scoring based on the participant’s average reaction time.11

Choice Reaction Time (CRT). The CRT test is, as the name indicates, similar to the SRT test; however, participants are instead given 2 possible stimuli with 2 possible responses. If the participant is asked to press button A at one sound, he or she will be asked to press button B at the second. Scoring is based on mean reaction time. In addition to the alertness and motor speed measured by the SRT test, the CRT test measures the ability to filter out irrelevant information.11

Letter-Number Sequencing Test (LNST). The LNST, like the DSST, is a component of the WAIS III.5 The test administrator reads numbers and letters out loud, then the patient sequences them by value of numbers and alphabetical order of the letters. The sequence length is increased from 2 to 8, until 3 trials have failed. The LNST tests working memory and executive functioning.

Two-Digit Cancellation Test (TDCT). In the TDCT, the patient has 45 seconds to search for 2 specific digits among rows of numbers on a page. The number of target digits found, minus the number of errors and the number of times a subject needs reminders, is the score. This test measures attention and executive function.12

Patient-Rated Scales for Tracking Cognitive Symptoms in MDD

While the above tests reliably and consistently identify cognitive impairment, their use may be less feasible in clinical practice than in research trials. In routine practice, clinicians need easy-to-use, validated scales that are sensitive to change. Presently, 2 scales are available for clinicians to track cognitive symptoms in patients with MDD, and both are patient-rated. The advantage of such scales is that they are easy to administer and score. The disadvantage is that they rely on patient report rather than actual performance. This can be particularly pronounced when trying to assess executive functioning.

Massachusetts General Hospital Cognitive and Physical Functioning Questionnaire (MGH CPFQ). The first is the MGH CPFQ,13 a 7-item questionnaire measuring motivation, wakefulness, energy, focus, recall, word-finding ability, and mental acuity. The MGH CPFQ asks patients to compare their functioning during the previous month with their best level of functioning. Administering this scale in patients with MDD after antidepressant therapy has shown that cognitive impairment remains in many patients despite successful treatment.13

Perceived Deficits Questionnaire (PDQ). The other instrument, the PDQ, is a 20-item scale validated for use in MDD.14 The 20 questions assess 4 components: attention and concentration; prospective memory; retrospective memory; and planning and organization.

Conclusion

Cognition comprises 5 broad components: attention, immediate memory, delayed memory, cognitive speed, and executive functioning. Each can be measured with a number of reliable and consistent clinician-administered tests in randomized clinical trials. Clinicians should be familiar with these tests, to better understand findings of studies on cognition in MDD. However, in their practices, clinicians should integrate easy-to-use, validated, change-sensitive scales, such as the MGH CPFQ and the PDQ.

Clinical Points

  • Clinicians should consider cognitive symptoms when developing a treatment plan for a patient with MDD, because impaired cognition hampers the restoration of psychosocial functioning
  • Recognizing the components of cognition and some common tests for these components is helpful for evaluating results of research trials
  • Using scales such as the MGH CPFQ and the PDQ can be helpful to measure and track change in cognitive symptoms during depression treatment

Abbreviations

CRT = Choice Reaction Time; DSM-5 = Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition; DSST = Digit Symbol Substitution Test; LNST = Letter-Number Sequencing Test; MDD = major depressive disorder; MGH CPFQ = Massachusetts General Hospital Cognitive and Physical Functioning Questionnaire; MMSE = Mini-Mental State Examination; PDQ = Perceived Deficits Questionnaire; RAVLT = Rey Auditory Verbal Learning Test; SRT = Simple Reaction Time; TDCT = Two-Digit Cancellation Test; TMT = Trail Making Test; WAIS III = Wechsler Adult Intelligence Scale, 3rd Edition

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References

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