The modified-oddball paradigm has been used to measure ERP components associated with attention and recognition memory in infancy. Infants are familiarized with 2 stimuli and then exposed to brief presentations of three types of memory stimuli: frequent familiar, infrequent familiar, and infrequent novel. Recognition memory is inferred based on differential cortical responding to each of the memory stimulus types. A middle-latency negative ERP component over central leads labeled Negative Central (Nc) is assumed to reflect a general orienting response associated with attention. The Nc has been found to be greater in amplitude following novel stimulus presentations. Late slow waves proposed to reflect recognition memory include the negative slow wave (associated with novelty detection), and the positive slow wave (associated with an updating of recognition memory). A commonly used behavioral measure of recognition memory is the visual paired-comparison choice trial. Paired-comparison trials involve simultaneous presentation of a familiar and a novel stimulus. Recognition memory for the familiar stimulus is inferred when infants spend a greater proportion of time looking at the novel stimulus (i.e., demonstrate a novelty preference). No study to date has measured ERPs during paired-comparison trials because of the eye-movement artifacts produced during shifts between stimuli. One goal of the present study was to examine infant ERPs during paired-comparison trials by utilizing independent components analysis to identify and remove eye-movement components from the EEG data. A second goal was to examine the consistency between ERP components and behavioral correlates of attention and recognition memory by embedding paired-comparison trials within the modified-oddball paradigm.
Infants 20, 26, and 32 weeks of age served as participants. Infants were familiarized with two stimuli prior to testing. Participants were then exposed to alternating blocks of paired-comparison trials and brief stimulus presentations. The paired-comparison trials and blocks of brief stimulus presentations were alternated in order to measure the infants’ visual preferences as the study progressed. Look durations during the paired-comparison trials were scored off-line to obtain novelty preference scores. Electroencephalographic recordings were made with a 126-channel system and ERP averages were made from -50 ms to 2000 ms around stimulus onset for brief stimulus exposures, and for the duration of the paired-comparison trials. Our ERP analysis focused on the Nc component.
There was a significant effect of age on visual preference. The 26- and 32-week-olds preferred novel stimuli, whereas the 20-week-olds preferred familiar stimuli. In the ERP analysis of paired-comparison trials, greater amplitude Nc was found to the non-preferred stimulus. When infants demonstrated a novelty preference, Nc was greater in amplitude to the familiar stimulus (and vice versa). Results of the ERP analysis of brief stimulus presentations replicated past studies with greater amplitude Nc following novel stimulus presentations. These findings indicate that behavioral measures can be successfully integrated into ERP studies of infant cognitive development, although ERP and behavioral findings may not be entirely consistent. While infants demonstrated greater amplitude Nc to novel stimuli following brief stimulus presentations, greater amplitude Nc was found to familiar stimuli when look duration was indicative of a novelty preference.
Our current studies in this area use this mixed presentation procedure. Infants are presented with paired-comparison trials interspersed with brief presentations. The presentation sequence is designed to elicit preferential looking to a novel or familiar stimulus. In one study stimulus familiarity is elicited by repeated presentations of one stimulus, and the second stimulus changes on each trial. In a second study we are using familiar or novel toys, and mother or stranger faces. With this procedure we can examine the brain correlates of familiarity and novelty preference.