Electrophysiological correlates of infant recognition memory.

Event-Related Potentials were recorded from 6-month-old infants in order to examine the electrophysiological correlates of recognition memory. In 1 study, infants were shown 1 face briefly, followed by the same face presented with high probability, and a novel face presented with low probability. 2 components were observed that distinguished between the novel and familiar events. Differences between these components in latency, polarity, and scalp topography led to the suggestion that 2 processes were involved in recognizing the stimuli. In a second study infants were again presented with 1 face briefly, followed by the same face and a novel face presented with equal probability. 1 component at central scalp distinguished between the novel and familiar events. In a third study, infants were simply presented with 2 previously unseen faces equally often. Infants responded as if the faces were identical, suggesting that previous experience with 1 stimulus directs infants' attention to the novel stimulus during the test portion of the task. The extent to which the observed responses reflect the updating of working memory is discussed, as is the extent to which infants' electrophysiological response to stimulus novelty and uncertainty resembles the adult's.

The coordination of eye and head movements during early infancy: a selective review.

A review of work dealing with the human infant's ability to use coordinated eye and head movements is presented. Primary emphasis is given to performance under two conditions: (1) the shifting of fixation to a visual stimulus in the periphery; and (2) the use of eye movements to maintain fixation by compensating for the effects of externally imposed head movements.

The effect of change in a stimulus sequence on P300.

In past studies, the P300 amplitude of the ERP has been found to be enhanced for infrequent, unpredictable and/or task-relevant eliciting events. A fourth factor, namely the physical change between two consecutive stimuli in event sequences used, has often been confounded with frequency, predictability and relevance. The current study examined whether change also increases the P300 amplitude. Fourteen adults viewed sequences of slides in which predictability, task (counting) and change varied within subject. ERP was recorded at Cz, Pz, Oz and Fz. P300 amplitudes were significantly larger: (1) for unpredictable than for predictable events, (2) for counted than for non-counted stimuli, and (3) for stimulus change rather than no stimulus change. The change effect interacting with counting suggested that P300 amplitude may be particularly increased by stimulus changes imbedded within only certain cognitive tasks.

Young infants' event-related potentials (ERPs) to familiar and unfamiliar visual and auditory events in a recognition memory task.

Three studies were conducted to test whether 3-month-old infants demonstrate differential brain activity to familiar and unfamiliar characteristics in compound visual-auditory stimuli. Infrequent changes in the stimuli occurred in: (1) visual characteristics alone (VC study); (2) auditory characteristics alone (AC study); or (3) both visual and auditory characteristics (VAC study). Single trial and average event-related potentials (ERPs) were computed at Oz, Pz, and Cz. In the VC and VAC studies a strong LPC effect was found mainly at Oz and Pz for the average ERPs to the unfamiliar stimuli during the interval 150-600 msec after stimulus onset. In the AC study no evidence for an LPC effect was found in the average ERPs at any location. Linear discriminant functions were used to assess LPC effects for single trial ERPs. A local probability index (LPI) was calculated to examine the effects of prior presentations of familiar events on the infants' responses to specific occurrences of unfamiliar events. In the VC study the LPC was present more often at Pz for infrequently occurring trials preceded by a sequence of three or more familiar stimuli than by only one or two. The same effect was present at Cz in the AC study. Concordance among the discriminants' assessment at each scalp site for individual trials was calculated. The discriminant concordances among the combinations of scalp sites were different for each study resulting in a unique spatial representation of the infants' brain responses in different experimental conditions. These studies provide a paradigm to assess LPC effects during recognition memory tasks using both averaged and single trial evoked potential data in young infants.

Infant saccadic eye movements to visible and previously visible targets.

Recent evidence has shown that 1- and 2-month-old infants localize a peripheral target by means of a series of saccades. Step size in a series was directly related to target distance, but within a localizing series did not vary appreciably as the line of sight approached the target. The current study examined whether, during localization of a peripheral target, step size and number of steps were determined prior to the first saccade in a localizing series. 2-month-olds viewed peripheral targets which either remained on (continuous condition) or were extinguished (interrupted condition) prior to target localization. It was found that on the majority of trials a series of saccades was made toward the target hemifield. In both conditions the size of each saccade in a multiple saccadic series was approximately equal within a given trial, but varied with initial target distance. Multiple saccades on both continuous and interrupted trials were similar in form, differing only in the total extent of eye rotation. This suggests that a similar oculomotor program was operating under both conditions.

Acuity and contrast sensitivity in 1-, 2-, and 3-month-old human infants.

The importance of assessing infant visual function is indicated by recent demonstrations that early visual experience in part determines the eventual state of adult visual function. It is argued that the contrast sensitivity function (CSF) could be a valuable index in the assessment of infant vision because it provides information concerning several aspects of vision. CSF's were measured in 1-, 2-, and 3-month-old infants. The "cut-off" spatial frequencies, which are estimates of visual acuity, were 2.4 cy/deg for 1-month-olds, 2.8 cy/deg for 2-month-olds, and 4.0 cy/deg for 3-month-olds. Sensitivity to contrast was shown to increase between 1 and 3 months of age. The CSF's measured also provide evidence for the presence of a low-frequency fall-off at 2 and 3 months.

Infant visual acuity as a function of viewing distance.

Dynamic retinoscopy has suggested that near vision may be more acute than far vision during early infancy. To test this, acuity thresholds were determined by presenting square wave gratings in a preference paradigm to 1- and 2-month-old human infants at 4 viewing distances. Gratings were paired with unpatterned fields; direction of first fixation was the dependent measure. Infants exhibited the same acuity at each of the distances at which gratings were presented. The results were interpreted as compatible with the fact that considerable optical defocusing does not seriously affect a visual system, such as the infant's, that is sensitive only to low spatial frequencies.

Developmental changes in the scanning of faces by young infants.

6 1-month-old infants and 6 2-month-old infants each viewed 3 faces (his mother's, a strange woman's, and a strange man's) while his eye movements were recorded by corneal photography. The 1-month-olds fixated away from the faces most of the time, and they looked at their mothers even less often than at the strangers. When they did fixate a face, they usually chose a limited portion of the perimeter. By constrast, 2-month-olds fixated the faces most of the time, looked at more features, and were more likely to look at internal features, especially the eyes. This scanning resembles that reported previously for 2-dimensional shapes, although in some respects it appears unique to faces.