The nature of recollection across months and years and after medial temporal lobe damage

Abstract

We studied the narrative recollections of memory-impaired patients with medial temporal lobe (MTL) damage who took a 25-min guided walk during which 11 planned events occurred. The recollections of the patients, recorded directly after the walk, were compared with the recollections of controls tested directly after the walk (C1), after one month (C2), or after 2.6 years (C3). With respect to memory for the walk, the narrative recollections of the patients were impoverished compared with C1 but resembled the recollections of volunteers tested after long delays (C2 and C3). In addition, how language was used by the patients in their recollections resembled how language was used by groups C2 and C3 (higher-frequency words, less concrete words, fewer nouns, more adverbs, more pronouns, and more indefinite articles). These findings appear to reflect how individuals, either memory-impaired patients or controls, typically speak about the past when memory is weak and lacks detail and need not have special implications about language use and MTL function beyond the domain of memory. A notable exception to the similarity between patient narratives and the narratives of C2 and C3 was that the control groups reported the events of the walk in correct chronological order, whereas the order in which patients reported events bore no relationship to the order in which events occurred. We suggest that the MTL is especially important for accessing global information about events and the relationships among their elements.

Keywords: autobiographical memory; forgetting; hippocampus; language.

Fig. 1.

Map indicating where 11 planned events occurred during a 25-min guided walk: 1: Discard cup. 2: Find change in vending machine. 3: View portraits. 4: Point out coffee cart. 5: Find book in library. 6: Receive bike lock. 7: Lock bike. 8: View statue. 9: Buy banana. 10: Tie shoes. 11: Drink from fountain. Sidewalks are light gray. Buildings are dark gray. Arrows indicate the path taken during the walk. Also see Materials and Methods. Reprinted from ref. .

Fig. 2.

Mean number of events referred to by each group during 6-min narrative recollections about the walk (A). Performance on 40 two-alternative, forced-choice questions about the events of the walk (B). C1 (n = 8), controls tested directly after the walk. C2 (n = 7), controls tested 1 mo after the walk. C3 (n = 7), controls from the C1 group tested 2.6 y after the walk. MTL (n = 5), patients with medial temporal lobe lesions.

Fig. 3.

Mean number of words in the 6-min narrative recollections used to report accurate episodic details about the walk (A). Mean number of accurate episodic details about the walk reported by each group in the 6-min narrative recollections (B). Groups are as in Fig. 2.

Fig. 4.

Mean richness of the episodic details describing each event in 6-min narrative recollections about the walk. Richness was rated on a 0–6 scale (see Materials and Methods). Groups are as in Fig. 2.

Fig. 5.

Characteristics of the content words (nouns, adjectives, verbs, adverbs) used to report accurate episodic details in the 6-min narrative recollections about the walk. (A) Word frequency of the content words. (B) Concreteness of the words. (C) Imageability of the words. Word frequency is based on a corpus of 190,000 words. Concreteness and imageability are based on ratings (100–700); see Materials and Methods. Groups are as in Fig. 2.

Fig. 6.

Percentage of nouns among the words used to report accurate episodic details in the 6-min narrative recollections about the walk (A). Percentage of adverbs used to report accurate episodic details in the 6-min narrative recollections about the walk (B). Groups are as in Fig. 2.

Fig. 7.

Percentage of pronouns among the words used to report accurate episodic details in the 6-min narrative recollections (A). Percentage of definite articles used to report episodic details (B). Percentage of indefinite articles used to report episodic details (C). Groups are as in Fig. 2.

Fig. 8.

The data points show when, on average, the events from the walk were referred to during the 6-min narratives. The control groups tended to describe events in the order that they occurred (A–C). The order in which the MTL patients mentioned events was unrelated to the order in which the events occurred (D). Lines represent significant fits to the data. No C2 described event 1. No C3 described event 4. No patient described events 10 and 11.

Fig. 9.

Coherence of the 6-min narrative recollections based on ratings (0–3) for each of three characteristics of the narratives: context, chronology, and theme (see Materials and Methods). Groups are as in Fig. 2.