DOI: 10.4226/95/590bbd3954504
title: Eye movements and implicit source memory

description: During source memory studies, knowledge of some detail about the context of a previously experienced item or event is tested. Here, participants attended to different objects presented at different quadrants on a screen. In a later test phase, a single object was presented in all four quadrants, and participants verbally reported whether the object was new or previously seen (item recognition), and if it was previously seen, they indicated the original screen location (source memory recollection). We combined this test with eye-tracking to determine whether attention to an object during encoding would correlate with later recognition of the object and recall of its source location, and whether eye movements at test can reveal attention to the correct source location in the absence of correct explicit verbal responses. Number of fixations on an object during encoding was not related to later object recognition or source recollection. However, when participants correctly recognized an object but incorrectly indicated the source information, there were significantly more fixations on the correct source location than on incorrect, non-selected locations. Also, when participants correctly recognized an object but said they could not recall the source information, there were significantly more fixations on the correct source location. These findings provide evidence that accurate source information can be expressed through gaze direction even when explicit recollection or verbal expression of that information is incorrect or has failed.
description: Materials A total of 96 colored images of common objects were employed. The images were developed and described by Cansino [12]. From the pool of images, a set of 64 images were selected to be displayed during the encoding phase. During the retrieval phase, 32 images from the encoding phase were used again, along with 32 new images. The stimuli were presented on the screen of a laptop computer (1366 × 768 pixels; 60 Hz) using e-Prime software version 2.0. A Tobii Technology X2-30 eye tracker was positioned below the screen to track eye movements. The subjects were seated so that their eyes were 50-80 cm from the screen. Procedure The experiment was conducted in a single session that consisted of an encoding phase, a retention interval, and a retrieval test phase. Eye movements were tracked during the encoding phase and the test phase. Before beginning the study, each subject read and signed an informed consent form and filled out a computerized demographic questionnaire. Participants started the experiment by calibrating their eyes to the eye-tracking software. Once the calibrations were satisfactory, the encoding phase began, which lasted for approximately 6 minutes. The participants were presented with an illustrative encoding slide and instructed to study the objects that appeared on screen for a later test. After the illustration slide was shown, a series of 32 additional slides was presented. Each encoding slide was divided into four quadrants by a red cross. In two of the quadrants randomly selected pairs of objects were presented. Participants were not instructed to remember image locations or image pairings. Unknown to them, one of the objects from each slide would be part of the retrieval test (the target) and the other would not be seen again (the distractor). Quadrants that contained stimuli were always diagonally opposed: either top left and bottom right, or top right and bottom left (Fig 1, top panel). These screen positions, as well as the position of the object that would be presented again later, were counterbalanced across the session and presented in individually randomized orders. Before each new slide was displayed, the participants were presented with a fixation cross for 1000 ms to center their focus. Each slide was presented for 3000 ms, after which the images and the quadrants disappeared and were replaced with the message “press space to continue”. By pressing spacebar, the participant could then choose when they would see the next slide. On average, participants took 1439 ms to continue onto the next slide. Examples of stimuli layout. In the encoding phase (top panel), a red cross divided the screen into quadrants and the stimuli were presented randomly in opposite quadrants. In the test phase (lower panel), a red cross divided the screen into quadrants and the stimuli were presented in all four quadrants. Immediately after the encoding phase there was a ten minute retention interval. During this interval participants completed a distractor task that involved solving simple arithmetic problems. The participants were asked to select the correct count-down series by threes out of 3 options. They had to do this for 10 different numbers ranging from 17-32. For the test phase, participants were presented with test screens divided into four quadrants by a red cross. The same object was displayed in each quadrant (Fig 1, bottom panel). The subjects were first presented with a sequence of three illustrative test trials and advised that after viewing the objects in each trial they would be asked to either point with their hand to the quadrant that they had previously seen the image in, say “new” if they had not seen the image previously, or say “don’t know where” if they had seen the image before but could not recall where its previous location was. For each test trial, a fixation cross in the center of the screen appeared for 1000 ms before each slide was presented to enable the participant to center their focus. The objects appeared for 3000 ms, then they were removed while the red dividing cross remained. At that point, the verbal responses from the subject were collected. These verbal reports were recorded by the experimenter. The subjects were presented with 64 test slides, 32 of which contained objects that were previously shown in the encoding phase and the other 32 contained new objects. The sequence of new images and previously presented images in the test screens was individually randomized. Coding of responses The third author noted participants’ response as they were being given and later coded these responses according to the following guidelines: • Correct item recognition – previously seen objects which were not labelled as new by the participant. • Correct source location – previously seen objects for which the participant had pointed to the correct quadrant. • Incorrect source location – previously seen objects for which the participant had pointed to an incorrect quadrant. • Don’t know where – previously seen objects which were recognized but of which the participant said not to know the location. • Incorrect item recognition – previously seen objects which were labelled as new by the participant. New objects could also be correctly identified as new or falsely considered to have been seen before. These were not analyzed further.
description: Data files are accessible with Excel and EPrime proprietary software.