Medium Inter-Stimulus Intervals Are Optimal for Infant Engagement and Sustained Attention


This study by Xie and Richards found that medium inter-stimulus intervals of 600–1,000 milliseconds are optimal for infant engagement and sustained attention during stimulus presentation.

The researchers found that using the shorter inter-stimulus intervals (the amount of time between the presentation of stimuli) resulted in more visually fixated trials and reduced the frequency of disengagement per experimental block. They also found larger heart rate changes during sustained attention for the two shortest inter-stimulus intervals, and larger Nc (sensitive to attending to visual stimuli) and P400 (sensitive to infant faces) event-related potential (ERP) component amplitude (or signal size) for the medium inter-stimulus interval. The two face-sensitive ERP components were differentially affected by the inter-stimulus interval manipulations. The earlier N290 was relatively unaffected, but the P400 was largest for the medium inter-stimulus interval. This difference suggests that the neural mechanisms underlying the N290 and P400 components play different roles in infant face processing and attention allocation. In conclusion, this study showed that increasing the presentation rate by shortening the inter-stimulus interval increases the amount, complexity, and novelty of visual information presented in a fixed period of time, which in turn enhances infant engagement and attention in an ERP study.


Article Title:

Effects of interstimulus intervals on behavioral, heart rate, and event-related potential indices of infant engagement and sustained attention (Paywall)




Wanze Xie and John E. Richards




University of South Carolina, United States




The researchers wanted to primarily examine the effects of inter-stimulus interval duration on behavioural and physiological indicators of infant sustained attention in an event-related potential (ERP) experiment. Specifically, their goal was to examine the effects of shortening the inter-stimulus interval for presentation on infants’ fixation to presentation, heart rate-defined attention, and ERP components, with the goal of producing better fixation and attention patterns than with the typical presentation rate.

They hypothesized that the shorter inter-stimulus intervals would increase the frequency of visual fixation, decrease the frequency of disengagement, increase the amount of heart beat (or interbeat interval) change during sustained attention, and extend the duration for sustained attention.

Given an overall increase in attentiveness and the relation between attention and the amplitude of the negative central (Nc) ERP component, they also expected that using the short and medium inter-stimulus intervals would elicit greater Nc amplitude than using the long interval. A second goal of this study was to examine whether the facilitation effect of shortening the inter-stimulus interval duration on infant sustained attention would benefit infant face-sensitive ERPs.

The researchers hypothesised that using the short and medium inter-stimulus intervals that are expected to enhance sustained attention would lead to greater amplitude of the N290 and P400 responses.




The researchers noted that studies with infant participants often use EEG and ERPs as measures of infant attention and cognitive processes. One issue in this research is the relatively high attrition (or drop out) rate of participants compared to that in adult research. They stated that infants’ fussiness and poor data quality are two frequent contributors to the high attrition rate in infant EEG/ERP research. They suggested maximizing infant attention and engagement to stimulus presentation should enhance the processing quality and minimize the probability of a study being terminated early because of infants’ fussiness, which should in turn reduce the attrition rate.

They hypothesized that accelerating the presentation rate of visual stimuli that enhances the complexity of information load would facilitate infant sustained attention. The researchers noted that a typical inter-stimulus interval of 1,000 – 2,500 milliseconds (ms) allows only one or two presentations during the period of infant stimulus orienting, and may not be optimal for further stimulus processing and initiating the sustained attention phase. At the end of a period of sustained attention, the infant disengages from the stimulus presentation, and the probability increases of being distracted from the fixated location by a stimulus in another location. However, stimulus and presentation characteristics may extend the period of sustained attention. They stated that shortening the inter-stimulus interval during stimulus presentations would increase the amount of information that infants are being exposed to, eliciting sustained attention, increasing stimulus complexity, which then should facilitate infant sustained attention and engagement in the experiment, and extending the period of sustained attention. They also suggested that increasing the presentation rate by shortening the inter-stimulus interval would also increase the signal-to-noise ratio in the ERP data by obtaining more artifact (error)-free trials.




The researchers analysed the data from a final sample of 36 infants who were tested at 3, 4.5, or 6 months of age, which means that 12 infants each were tested at these three ages. Infants were presented with a block of stimulus presentations that began with a Sesame Street attractor at the center of the screen overlaid on a background image. The three types of stimuli were female faces, infant faces, and toys, presented randomly with equal probability in an experimental block. An experimenter judged if the infant was looking toward the attractor, and the computer program initiated a sequence of brief image presentations. If the infant looked away from the visual presentation, a Sesame Street attractor was presented and lasted until the infant looked back to the center of the screen. The brief stimulus presentations then resumed. The brief stimulus presentations consisted of a stimulus presentation for 500 ms followed by an inter-stimulus interval, whose duration for any stimulus presentation was randomly selected within the minimum and maximum range for an inter-stimulus interval type: short (400–600 ms), medium (600–1,000 ms), and long (1,500–2,000 ms). One of the three types was chosen at the beginning of an experimental block and used for the brief stimulus presentations during the entire block. Heart rate and ERPs were measured during the task.


ScientiFix tip: This study is promising in showing that a medium inter-stimulus interval may be optimal for maintaining infant attention and engagement. However, important evidence to support this claim would have to come from researchers using this interval length in future studies. This would allow researchers to then compare their results to past studies where they used the same methodology but with a different interval length. They could then compare attrition rates, when infants have to drop out or end the study session early, as well as their measures of engagement and attention relative to studies where they used alternative interval lengths. While this is a promising study, future studies that could make these kinds of comparisons would be very informative.


Leave a comment