Ethics Statement

All the experiments described in the manuscript have been approved by the Pitié-Salpetriere’s ethical committee (Comité Consultatif de Protection des Personnes participant à une Recherche Biomédicale-Promotion: CPP 13–41 for the fMRI and behavioral experiments & P091111 for the SEEG experiment). All subjects gave their written informed consents and all the different investigations were conducted according to the principles expressed in the Declaration of Helsinki.

Participants

Experiment 1: Behavior and fMRI in controls

Twenty six participants were recruited through posted advertisements for the fMRI experiment. Six participants were eliminated due to a technical problem with the fMRI machine. The reported analyses were therefore based on 20 subjects (12 males; age range: 20–30; mean ± sd = 25.3 ± 3.7). Participants completed a screening form for significant medical conditions and were paid 80 euros to participate in the experiment.

Experiment 2: Behavior and intracranial electrophysiological recordings in epileptic patients

Six epileptic patients (3 males, age range: 25–46, mean ± sd = 35.8 ± 7.2) participated in this study. Neuropsychological assessment revealed normal or mildly impaired general cognitive functioning. These patients suffered from drug-refractory focal epilepsy and were implanted stereotactically with depth electrodes as part of a presurgical evaluation. Implantation sites were selected on purely clinical criteria, with no reference to the present protocol and included the left hippocampus region. One of these patients (patient S2135) was also implanted with microelectrodes recording multi-unit activities at the most internal extremity of the left hippocampus electrode.

Experiment 3: Behavior in amnesic Mild Cognitive Impairment (aMCI) patients and matched controls

Twelve amnesic patients (5 males, age range: 50–84, mean ± sd = 69.3 ± 10) and 12 matched control participants (5 males, age range: 54–87, mean ± sd = 73.25 ± 10.4) participated in this part of the experiment. Amnesic patients were recruited and tested at the Memory and Alzheimer’s disease institute at the Pitié-Salpêtriere hospital in Paris (IM2A). Controls were recruited through posted advertisements.

Stimuli and procedure

Experiment 1: Behavior and fMRI in controls

Overview

The experiment was composed of 5 consecutive blocks, during which fMRI data were acquired: two rating blocks (Rating 1 and 2) and two Choice blocks (Choice 1 and 2) and one repetition detection block. Finally, a memory test and a familiarity assessment were performed after the scanning sessions.

Stimuli

Stimuli consisted of 120 colored images of potential vacation destinations. In Rating blocks and in the repetition detection block, one image was presented at the center of the screen with the destination name printed below the image (font size = 30) in each trial. In Choice blocks, two images and their respective names were presented to the left and to the right of the screen, in each trial. The order in which stimuli were presented within each block was random.

Rating 1

Subjects viewed 120 destinations and were asked on each trial to report how much they would like to spend their vacation in that given destination on an eight point scale (1 = “I do not want to go there at all”, 8 = “I would love to go there”). Each trial began with a fixation point that lasted 2 to 3 seconds. Then the destination appeared for 3 second. Subsequently, subjects were presented with an image of two schematic hands with a number from 1 to 8 above each finger (except the thumbs) that randomly varied positions from trial to trial. Subjects were instructed to respond with the finger corresponding to the rating they would like to give. This technique allowed us to avoid different handedness related biases. This screen was presented until response. If subjects answered in less than 3 seconds, a fixation dot was presented, so the interval between the end of the destination presentation and the next trial lasted at least 3 seconds.

Choice 1

Following the Rating1 block, subjects were presented with pairs of destinations they had rated equally (difference of R1 scores ≤ 1) and had to indicate with a button press at which one they would rather take their vacation. Note that all individual medians of R1 scores differences were null. The mean of averaged R1 differences was equal to 0.12 ± 0.04. Importantly, RCR and RRC pairs did not differ in terms of R1 scores difference in each of the groups we tested (fMRI controls; SEEG patients; MCI controls; MCI patients; all paired t-tests p-values were ≥ 0.1). Similarly no significant difference could be observed between remembered and forgotten pairs, both for RRC and RCR conditions (all p-values ≥ 0.2). In each of the Choice tasks, subjects viewed 25 pairs of destinations based on ratings given in Rating 1. This means a total of 100 destinations out of the 120 were used to form the Choice blocks. The 20 remaining destinations (the same ones for all subjects) were used as repetition targets for the following repetition detection task described below.

Out of the 100 destinations 50 were used to form 25 pairs in Choice 1 and the 50 other formed the 25 pairs of Choice 2. Each trial began with a fixation point that lasted 2 to 3 seconds. Then the paired destinations appeared for 3 seconds, followed by a fixation point, during which subjects had to give their choice. This fixation screen lasted at least 3 seconds and at most until subject’s response.

Incidental task: Repetition detection task

In this task subjects were presented with all 120 destinations as each appeared at the screen with the same presentation time as rating 1 and rating 2. 1/6th of the time either the destination name or the image, or both were repeated contiguously (only for the 20 images mentioned in Choice 1 description). Subjects had to press a button to signal that they detected this repetition. This repetition detection task was implemented in order to make sure subjects were attentively attending all destinations and their respective names. The main aim of this block was to probe the timing of preference change during a passive condition. Analyses of this block did not reveal any significant result. Given the focus of the present study on memory effects, we do not report here these analyses for sake of concision.

Rating 2

This block was strictly identical to Rating 1.

Choice 2

This block was strictly identical to Choice 1, but only included the other half of the stimuli as described earlier.

Post-scanning questions

After the scanning sessions, participants were asked to perform two additional tests outside the fMRI. First they performed a memory test concerning the choices they had made in Choice 1 and Choice 2 sessions. In order to avoid any explicit memorization of the items, subjects were not informed about this memory test at the beginning of the experiment. Every trial began with the appearance of a destination picture and its name at the center of the screen. Below the item, were stated the options “chosen” and “rejected” at the left and right of the screen. Using the keyboard arrows, subjects had to indicate whether they remember choosing or rejecting that given item during the choice tasks. This question tested objective memory of the choices. After giving their answer, appeared a sentence at the center of the screen “Are you sure of your response?” underneath which were presented the two options “ I am sure” and “I guessed” at left and right of the screen. Again subjects had to use the keyboard arrow to specify their answer. This second question tested the more subjective aspect of subjects’ memory. There was no time limitation and the trials concerned all 100 items that were seen during Choice 1 and Choice 2.

Finally, subjects rated the familiarity of each of the 120 destinations using an eight-point scale on the pc’s numeric keypad (1 = “I do not know this country at all”, 8 = “I am very familiar with this country”).

Experiment 2: Behavior and intracranial electrophysiological recordings in epileptic patients

The procedure of this experiment was strictly identical to the fMRI experiment in all regards expect in the three following aspects:

1 Given the gain in temporal resolution of SEEG, as compared to fMRI, pictures of the destinations were presented for a duration of 2 seconds rather than 3 seconds. This shortened the overall length of the experiment making it more adapted for implanted patients. 2 As opposed to the fMRI experiment, in which handedness related activations were important to control, in rating 1 and rating 2 of this experiment subjects did not respond using a schematic hand but rather the PC’s numeric keypad. 3 For sake of concision with these patients, the experiment did not include the familiarity test.

Experiment 3: Behavior in amnesic Mild Cognitive Impairment (aMCI) patients and matched controls

The procedure of this experiment was strictly identical to the fMRI experiment in all regards expect in the three following aspects: