Disorganized Memory after Right Dorsolateral Prefrontal Damage

Ptak, R., Shnider A. (2004).Disorganized Memory after Right Dorsolateral Prefrontal Damage. Neurocase, 10(1), 52-59.

Dorsolateral prefrontal cortex (DLPFC) damage may lead to deficits in memory tasks such as self-ordered pointing, working memory, or source memory

Patients with dorsolateral damage may exhibit deficient strategy implementation, control of interference and cognitive estimation

She presented severely disordered memory, planning and decision making. These failures were due to deficient selection and scheduling of actions, and the inability to maintain goals for ongoing behavior

CASE DESCRIPTION

A 52 yr. old retired secretary suffered severe head trauma in a car accident 13 yrs. ago. After experiencing disorganized behavior for deficient memory she was referred to a rehab clinic. Since her brain injury she has not been able to work. Although she has applied for jobs, she has failed to be punctual for interviews and exhibited confused behavior. She was aware of her problems and suffered from social isolation.

The patient suffered from DLPFC damage to areas 9/46.

Behavioral Observations

• She would forget meetings because she would engage in another activities

• She showed difficulty in initiating actions

• Lack of reliability made her lose friends resulting in social isolation

• A marked abnormality was communication; she was very talkative but failed to express a clear thought. She was aware of her agrammatism, her tendency to produce, “too many words which I cannot put in order”

• She had compulsive writing, using post-it notes and several organizers (Fig. 2)

•The patient described herself as being unable to respect a time plan. She would arrive 20-30 minutes late for all appointments.

Neuropsychological Examination

Her neuropsychological examination was quite normal. She was fully oriented and followed instructions.

She had normal intelligence, visual, visuo-spatial, arithmetic abilities, concentration and information processing speed.

Unlike many patients suffering from frontal dysfunction she displayed normal idea production in fluency tasks, control of interference, cognitive flexibility, and concept formation

She had excellent ability to find similarities between two concepts

Her performance in standard memory tests were also normal.

Neuropsychological ExaminationThe patient showed difficulty in tasks which required active organization of her responses

She showed severe deficits in unstructured verbal tasks producing highly abnormal responses

Her performance in the comprehension subtests of the WAIS-R was severely deficient (percentile < 1) which contrasts with her performance in the similarities subtest (percentile 99).Both tasks measure reasoning.

The patient failed in tasks with a high degree of freedom requiring verbal reasoning and tasks which require the resolution of several problems in an unstructured setting.

Four hypotheses were tested to explain the patients deficits in organization and inadaptable behavior to suit the needs of particular situations and to environmental situations

EXPERIMENT 1

Does the patient fail to anticipate behavioral consequences of her behavior ?

To test this they used the Gambling Task; participants choose a card out of four decks , depending on their choice they win or lose money. In order to solve this task they must learn to anticipate a negative outcome of their choice.

The patients performance was comparable to that of healthy controls. She shows a preserved ability to sense which choices were advantageous and avoided choices that were disadvantageous in the long run

EXPERIMENT 2

Does the patient have difficulty in suppressing memories lacking current relevance?

A possible explanation for the patients deficient action scheduling is that she was unable to differentiate between currently relevant and previously relevant but currently irrelevant action plans

This task consists of two runs of a continuous recognition test composed of the same set of line drawings

She scored within the range of healthy controls

EXPERIMENT 3

Does the patient fail to organize her behavior in time because of impaired time estimation?Structured actions may require the presence of an internal clock indicating the time elapsed since the start of an activity

The six-elements test requires self-paced switching between different tasks and prevents the patient from counting. The researchers tested estimation of three different time intervals, and asked the patient how much time she thought had elapsed.

The patient scored better than healthy controls

EXPERIMENT 4

Does the patient have deficient working memory?

The n-back task was used.

The patient was shown numbers on a screen, she was told to press the space bar when a number was the same as the second back (2-back series), 3-back, and 4-back.

This tasks increases the demands on working memory

Her performance decreased with an increasing working memory load

She evaluated her performance optimistically and was disappointed with her results

An MRI scan showed an isolated right dorsolateral cortical contusion, extending to the prefrontal white matter.

MRI Scan

Her lesion was localized in the middle and superior frontal gyrus (BA9/46) and extended subcortically to the anterior horn of the lateral ventricle

Superior Frontal gyrus BA46

Middle frontal gyrus BA9

• This study provides evidence that damage to the DLPFC area 9/46 may lead to severe behavioral disorder which manifests itself in daily activities requiring active planning and action scheduling and unstructured experimental situations

• This patient is unique from others;

• She was aware of her problems with respect to time and this behavior does not conform to social rules resulting in social isolation

• Literature states that damage to the dorsolateral area tends to produce passivity, this patient was motivated and showed increased activity

• Research has shown damage to the DLPFC results in failure to anticipate risks, she did not show any decision-making deficits

DISCUSSION

• The patient performed well on all tests of memory and attention• She showed no difficulties in distinguishing between memories

pertaining to ongoing reality and currently irrelevant memories• She showed no problems with temporal processing• The only test she did poorly on was the n-back task which requires

maintenance and online manipulation of several pieces of information that are used for the guidance of behavior

• Previous functional imaging studies have shown working memory tasks activate the DLPFC

• Reports have also shown that the n-back task activates area 9/46• This is the first known reported case of a patient with area 9/46

damage showing a selective failure in the n-back task, showing support that this area is important for working memory

DISCUSSION

• Current modals of working memory suggests that there at least two components; a capacity-limited component involved in maintenance of information and the executive system which permits manipulation, monitoring and rehearsal of this information

• This patient may have damaged her short term store consistent with her limited digit span, reports have shown limited digit span is a consequence of DLPFC damage

• A second possibility is impaired mental rehearsal and maintenance of rapidly changing information in WM. A WM model by Petrides explains the role of the mid-DLPFC (area 9/46) in the active manipulation and monitoring of this information

• Several authors suggest active monitoring in WM is necessary to protect relevant information against distracters

DISCUSSION

• This case report displays the importance of dorsolateral area 9/46 for action scheduling based on online manipulation and maintenance of ongoing information

• Absence of these cognitive tasks may induce disorganization of behavior and an inability to use knowledge in a coordinated fashion to guide goal-directed behavior.

DISCUSSION