Early studies of the development of recognition memory in infants demonstrated a negative component over central leads (Nc) with greater amplitude in event-related potentials following presentations of a novel stimulus than presentations of a familiar stimulus. In contrast, subsequent investigations utilizing a familiarization phase prior to stimulus exposure found that Nc amplitudes do not vary significantly following novel versus familiar stimulus presentations. The present study investigated the effects of familiarization and attention on electrophysiological correlates of recognition memory in infants. In addition, equivalent current dipole analysis was used to localize cortical sources associated with recognition memory.

Background

Results

Summary

The EEG was segmented beginning 48 ms prior to the onset of the brief visual stimulus and through 2 s following. The EEG was stacked and independent components analysis (ICA) was used to do a "spatial components analysis“. The coefficients for each ICA component were a scalp topographical map of temporal-spatial organized activity. The top 20 components from all infants were examined further by clustering individual components together according to their topographical similarity. Component activations were examined with experimental factors (age, attention, stimulus type). Equivalent current dipole analysis was conducted on the individual components for cortical source localization. Cortical source locations were standardized across individuals by transforming the locations to Talairach spatial locations.

Analyses

ParticipantsSeventy infants in the following three age groups were used as participants: 20, 26, and 32 weeks of age. Only infants born full-term, weighing greater than 2500 g at birth, with no pre- or perinatal medical complications were used.

Methods

Cortical Source Localization of Infant Visual Attention and Recognition Memory

ProcedureInfants were initially familiarized with two visual stimuli. In the experimental phase, one of the familiar stimuli was presented 60% of the time (frequent familiar), the other was presented 20% of the time (infrequent familiar), and novel stimuli were used for the remaining 20% of stimulus presentations (infrequent novel). A Sesame Street video was utilized as a background stimulus throughout the experimental phase. The stimuli used in the familiarization phase for the familiarization control group were never presented during the experimental phase.

Heart Rate and EEG RecordingHeart rate was recorded during testing as an index of attention. Three HR-defined attention phases were identified throughout testing: stimulus orienting, sustained attention, and attention termination. Stimulus orienting was defined as the period beginning 1 s following the onset of the primary stimulus continuing for a period of 4 s. Sustained attention was defined as the period following a HR deceleration, when the HR remained below its prestimulus level. Attention termination was defined as the period of time following sustained attention when HR returned to its prestimulus level. A high-density 128 channel EEG recording system was used. The vertex was used as the reference for the recordings. The sampling rate of the EEG was 250 Hz, and band-pass filters were set from 0.1 to 100 Hz, with 20K amplification.

Greg D. Reynolds & John E. Richardshttp://jerlab.psych.sc.edu/pdf/expt58.pdf

Effects on the Negative Central ComponentThe familiarization group demonstrated greater amplirude Nc following novel stimulus presentations, this effect was not found for familiarization conrtols.

The impact of familiarization and attention on ERP and ICA components associated with recognition memory in human infants was investigated using HD-EEG and ECD analyses. An ICA cluster with dipoles located in frontal regions (inferior and middle frontal gyri, anterior cingulate cortex) is a likely source for the NC component commonly found in ERP studies of recognition memory. The Nc was greater in amplitude following novel stimulus presentations for the familiarization group but not for familiarization controls. Differences in late slow waves based on stimulus type were demonstrated at frontal electrodes sites and for frontal ICA clusters during attention.

ICA Component ClustersThe analysis revealed several ICA components. These components were located in frontal, occipital, parietal, and temporal regions.

Effects on the Late Slow Wave ComponentsDifferences in the Late Slow Waves for stimulus type were found during sustained attention. During periods of inattentiveness (i.e., attention termination), no differences were found for stimulus type.

Frontal Pole

Parietal

Left Temporal

Occipital

ICA Cluster Projections Equivalent Current Dipole Locations

-20V

20V

Prefrontal ICA Cluster: Medial Frontal Gyrus (25) , Inferior Frontal Gyrus (47), Anterior Cingulate Cortex (8)

(Talairach coordinates: 9.4, 42.9, 16.4)

ICA ProjectionGrand Average ERP

20V -20V

Nc for Familiarization Group

Frequent Familiar Infrequent Familiar Infrequent Novel

Infrequent Familiar Infrequent Novel

Nc for Familiarization Control Group

-20 V +20 V

Frequent Familiar

ICA Activations for Frontal Clusters following Familiar Stimulus Presentations

ICA Activations for Frontal Clusters following Novel Stimulus Presentations

Late Slow Wave During AttentionFrequent Familiar

1.0 – 1.25 s 1.25 – 1.5 s 1.5 – 1.75 s 1.75 – 2.0 s

1.0 – 1.25 s 1.25 – 1.5 s 1.5 – 1.75 s 1.75 – 2.0 s

1.0 – 1.25 s 1.25 – 1.5 s 1.5 – 1.75 s 1.75 – 2.0 s

Infrequent Familiar

Infrequent Novel

Response to Infrequent Novel at Frontal Electrodes for Attention

vs. Inattention

Late Slow Wave at Frontal Electrodes during Attention

Late Slow Wave ICA Activations for Prefrontal Cluster following Familiar Stimulus

Presentations

Late Slow Wave ICA Activations for Prefrontal Cluster following Novel Stimulus

Presentations

Late Slow Wave ICA Activations for Occipital Cluster

ERP Late Slow Wave Effects

ICA Late Slow Wave Effects

Response to Infrequent Familiar at Right Temporal Electrodes for

Attention vs. Inattention

Late Slow Wave at Right Temporal Electrodes during

Attention