Item memory, source memory, and the medial temporal lobe: concordant findings from fMRI and memory-impaired patients

Abstract

We studied item and source memory with fMRI in healthy volunteers and carried out a parallel study in memory-impaired patients. In experiment 1, volunteers studied a list of words in the scanner and later took an item memory test and a source memory test. Brain activity in the hippocampal region, perirhinal cortex, and parahippocampal cortex was associated with words that would later be remembered (item memory). The activity in these regions that predicted subsequent success at item memory predicted subsequent source memory to a similar degree. In experiment 2, memory-impaired patients with damage thought to be limited to the hippocampal region were given an item memory test and a source memory test, as in experiment 1. The patients were similarly impaired on the item memory test and the source memory test. Together, the findings suggest that medial temporal lobe structures broadly support recognition memory function and that item memory and source memory similarly depend on these structures.

Fig. 1.

Fifteen participants rated their confidence in both their item- and source-memory decisions. (A) Increasing confidence in the item decision (1, 2, or 3) correlated with increasing item memory success. (B) Increasing confidence in the source decision (1, 2, or 3) correlated with increasing source memory success. (C) Increasing confidence in the item decision (1, 2, or 3) also correlated with increasing source memory success. Brackets show SEM.

Fig. 2.

The remembered vs. forgotten contrast found two regions in the medial temporal lobe that predicted subsequent item memory success, irrespective of source memory success (Top and Bottom). Within both regions, activity for words that would later be remembered along with correct source memory judgments (item and source) was similar to activity for words that would later be remembered but with incorrect source memory judgments (item only) (Middle). Asterisks indicate a difference relative to the forgotten condition (P < 0.05). Brackets show SEM.

Fig. 3.

The item and source vs. forgotten contrast found five regions in the medial temporal lobe that predicted subsequent item memory and source memory success (Middle and Bottom). Within all of these regions, activity predicted subsequent item memory success (Top). Also, activity for words that would later be remembered along with correct source memory judgments (item and source) was similar to activity for words that would later be remembered but with incorrect source memory judgments (item only) (Middle). Asterisks indicate a difference relative to the forgotten condition (P < 0.05). Brackets show SEM.

Fig. 4.

The item-only vs. forgotten contrast found two regions in the medial temporal lobe that predicted subsequent item memory success along with incorrect source memory judgments (Middle and Bottom). Within both regions, activity predicted subsequent item memory success (Top). Also, activity for words that would later be remembered along with correct source judgments (item and source) was similar to activity for words that would later be remembered but with incorrect source judgments (item only). Asterisks indicate a difference relative to the forgotten condition (P < 0.05). Brackets show SEM.

Fig. 5.

The item and source vs. item-only contrast found one region in the medial temporal lobe that predicted item memory success along with correct source memory judgments relative to item memory success but with incorrect source memory judgments (Middle and Bottom). Within this region, activity for words that would later be remembered along with correct source judgments (item and source) was greater than activity for words that would later be remembered but with incorrect source judgments (item only). Activity did not predict subsequent item memory success (Top). Asterisks indicate a difference between the activity for item and source words relative to item-only words (P < 0.05). Brackets show SEM.

Fig. 6.

Five memory-impaired patients with damage limited to the hippocampus (H) and six controls (CON-1) learned 25 words by imagining an indoor or outdoor scene associated with each word. Each participant saw the 25 words three times each (25 × 3). Patients were impaired relative to controls on both item judgments and source judgments. Six additional controls (CON-2) saw 100 words once each (100 × 1) and performed similarly to the patients on both item judgments and source judgments. The item score is the hit rate minus the false alarm rate (chance = 0%). The source score is the proportion of hits that were followed by a correct indoor/outdoor judgment (chance = 50%).

Fig. 7.

Inside an fMRI scanner, participants learned 175 words by imagining an indoor or outdoor scene associated with that word. At study, participants were shown a cue (indoor/outdoor) for 1 s, the study word for 1 s, and then had 4 s to form an indoor or outdoor image as they viewed a fixation cross. Trials were separated by zero, two, four, or six 2-s trials of the baseline task (odd/even number judgments). At test (5–10 min later), participants made old/new decisions for the 175 studied words and 175 novel foils followed by confidence ratings (–3). For words endorsed as “old,” participants also made source judgments (indoor or outdoor) and confidence ratings (–3).