Attention to eyes is present but in decline in 2-6-month-old infants later diagnosed with autism

Abstract

Deficits in eye contact have been a hallmark of autism since the condition's initial description. They are cited widely as a diagnostic feature and figure prominently in clinical instruments; however, the early onset of these deficits has not been known. Here we show in a prospective longitudinal study that infants later diagnosed with autism spectrum disorders (ASDs) exhibit mean decline in eye fixation from 2 to 6 months of age, a pattern not observed in infants who do not develop ASD. These observations mark the earliest known indicators of social disability in infancy, but also falsify a prior hypothesis: in the first months of life, this basic mechanism of social adaptive action--eye looking--is not immediately diminished in infants later diagnosed with ASD; instead, eye looking appears to begin at normative levels prior to decline. The timing of decline highlights a narrow developmental window and reveals the early derailment of processes that would otherwise have a key role in canalizing typical social development. Finally, the observation of this decline in eye fixation--rather than outright absence--offers a promising opportunity for early intervention that could build on the apparent preservation of mechanisms subserving reflexive initial orientation towards the eyes.

Figure 1. Example stimuli, visual scanpaths, regions-of-interest, and longitudinal eye-tracking data from 2 until 24 months of age

a, Data from 6-month-old later diagnosed with ASD, red. b, Data from typically-developing (TD) 6-month-old, blue. Two seconds of eye-tracking data are overlaid on each still image, onscreen at the midpoint of the data sample. Saccades plotted as thin white lines with white dots; fixation data plotted as larger colored dots. c, Corresponding regions of interest for each image in a and b, shaded to indicate eye, mouth, body, and object regions. Trial data with FDA curve fits plotting percentage of total fixation time on eyes, from 2 until 24 months of age, for d, 2 children with ASD and e, 2 TD children.

Figure 2. Growth charts of social visual engagement for typically-developing children and children diagnosed with ASD

Fixation to eyes, mouth, body, and object from 2 until 24 months in a, TD and, b, ASD. c, Contrary to a congenital reduction in preferential attention to eyes in ASD, d, children with ASD exhibit mean decline in eye fixation. Longitudinal change in fixation to e, eyes; f, mouth; g, body; and h, object regions; between-group comparisons by functional ANOVA. Dark lines indicate mean growth curves, light lines indicate 95% CI. Top panels in e-h plot percent fixation; middle panels plot change in fixation (the first derivative, in units of % change per month); and bottom panels plot F value functions for between-group pointwise comparisons. Significant differences are shaded in medium gray for comparison of fixation data and light gray for comparison of change-in-fixation data.

Figure 3. Visual fixation between 2 and 6 months relative to diagnosis at year 3

Individual curve fits for, a, eye fixation data, and, b, change-in-fixation data for typically-developing infants (blue) and infants later diagnosed with ASD (red). c, The extent of between-group overlap in distributions of change-in-fixation data. For internal validation, each infant was tested as a validation case in relation to the remainder of the data (leave-one-out cross-validation, LOOCV). Area plots in d and e show LOOCV mean and 95% prediction intervals for individual trajectories of d, eye fixation, and e, change-in-fixation data; f, shows extent of between-group overlap in change-in-fixation data (mean and 95% CI). g–i and j–l repeat the same analyses for body fixation. m, plots the joint distribution of change in eye and body fixation. n, 6 male infants, not part of the original sample, were tested as an external validation.

Figure 4. Visual fixation between 2 and 6 months relative to outcome levels of affectedness

At 36 months, infant siblings at high-risk for ASD were confirmed either as having ASD; as having subthreshold signs of ASD (HR-ASD_BAP, Broader Autism Phenotype); or as unaffected (HR-ASD_No-Dx). ROC curves in a, b, and c, quantify overlap in measures of change in eye fixation relative to outcome (95% CI by LOOCV). The behaviour of unaffected siblings is overlapping with that of TD children, c, while the behaviour of infants later diagnosed with ASD, a, and that of infants with subthreshold signs, b, differs significantly from typical children. d, Eye fixation varies systematically across all outcome groups, with significant interaction of Outcome by Age (by HLM). e, Individual change in eye and body fixation for N=70 infant males (N=29 TD, 25 original sample, 4 external validation; N=13 ASD, 11 original sample, 2 external validation; N=18 HR-ASD_No-Dx; and N=10 HR-ASD_BAP). Probability density functions on ordinate and abscissa indicate distribution of measures for each outcome group.

Extended Data Figure 1. In infants later diagnosed with ASD, decline in eye fixation during the first 2 years is significantly associated with outcome levels of symptom severity

Functional Principal Component Analysis (FPCA) was used to extract growth curve components explaining variance in trajectory shape about the population mean. (a) Population mean for fixation to eyes in children with ASD (red line) plotted with lines indicating direction of individual trajectories having positive principal compenent one (PC1) scores (line marked by plus signs) or negative PC1 scores (line marked by minus signs). (b) Outcome levels of social disability (as measured by ADOS Social-Affect) as a function of decline in eye fixation (measured as eyes PC1 score). (c) Outcome levels of social disability as a function of decline in eye fixation using subsets of the longitudinal data (i.e., measuring decline in eye fixation using only data collected between months 2–6, excluding data thereafter; then between months 2–9, etc.). Decline in eye fixation predicts future outcome at trend levels by 2–9 months (P = 0.100), and is statistically significant thereafter.

Extended Data Figure 2. Developmental differences in visual fixation between 2 and 6 months of age

Raw data for eyes fixation (a–c), mouth fixation (d–f), body fixation (g–i), and object fixation (j–l) between 2 and 6 months for typically-developing infants (in blue) and infants later diagnosed with autism spectrum disorders (in red). Darkly shaded data markers indicate the interquartile range (spanning 25th to 75th percentiles). Data show significant associations with chronological age, but the slopes of the associations differ for ASD and TD outcome groups, with significant interactions of Diagnosis by Age for eyes, F(1,787.928) = 9.27, P = 0.002; for body, F(1,25.557) = 5.88, P = 0.023; and for object, F(1,21.947) = 5.24, P = 0.032; but not for mouth, F(1,47.298) = 0.019, P = 0.89. Analyses by HLM. Plots in c, f, i, and l show mean trend lines and 95% CI.

Extended Data Figure 3. Percentage of total time spent fixating and saccading between 2 and 6 months of age

Raw data for percentage of total time spent fixating (a–c) and time spent saccading (d–f) between 2 and 6 months for typically-developing infants (in blue) and infants with autism spectrum disorders (in red). Darkly shaded data markers indicate the interquartile range (spanning 25th to 75th percentiles). Data show significant associations with chronological age, but the slopes of the associations do not differ for ASD and TD outcome groups, F(1,20.026) = 0.88, P = 0.359 for time spent fixating; and F(1,26.430) = 0.56, P = 0.460 for time spent saccading. Analyses by HLM. Plots in c and f show mean trend lines and 95% CI.

Extended Data Figure 4. Developmental change in visual fixation between 2 and 24 months of age in typically-developing children

Raw data for eyes fixation (a), mouth fixation (c), body fixation (e), and object fixation (g) between 2 and 24 months for typically-developing children. Darkly shaded data markers indicate the interquartile range (spanning 25th to 75th percentiles). Black lines indicates mean growth curves via hierarchical linear modelling (HLM). Mean fixation curves with 95% confidence intervals for eyes fixation (b), mouth fixation (d), body fixation (f), and object fixation (h) between 2 and 24 months for typically-developing children.

Extended Data Figure 5. Developmental change in visual fixation between 2 and 24 months of age in children with ASD

Raw data for eyes fixation (a), mouth fixation (c), body fixation (e), and object fixation (g) between 2 and 24 months for children with ASD. Darkly shaded data markers indicate the interquartile range (spanning 25th to 75th percentiles). Black lines indicates mean growth curves via hierarchical linear modelling (HLM). Mean fixation curves with 95% confidence intervals for eyes fixation (b), mouth fixation (d), body fixation (f), and object fixation (h) between 2 and 24 months for children with ASD.

Extended Data Figure 6. Developmental change in visual fixation on the eyes relative to outcome levels of affectedness

Percent fixation on eyes for (a) typically-developing infants; (c) infants at high-risk for ASD who showed no evidence of ASD at 36 months (HR-ASD_No-Dx); (e) infants at high-risk for ASD who showed some sub-thresholds signs of the Broader Autism Phenotype at 36 months but did not meet clinical best estimate diagnosis of ASD (HR-ASD_BAP); and (g) infants diagnosed with ASD at 36 months. External validation participants not included (in contrast to Figure 4 in main text). Darkly shaded data markers indicate the interquartile range (spanning 25th to 75th percentiles). Black lines indicates mean growth curves via hierarchical linear modeling (HLM). Plots in b, d, f, and h show mean fixation curves with 95% CI. Plots i–l highlight the first 6 months of life in each group, and m plots the relationship across groups.

Extended Data Figure 7. Mean fixation curves by PACE/FDA with the effects of adding (+) or subtracting (−) principal component (PC) functions

(following the convention of Ramsay and Silverman, ref. 21). a, eyes fixation, b, mouth fixation, c, body fixation, and d, object fixation for both typically-developing infants (in blue) and infants with autism (in red). For each region and each group, number of plots is dictated by number of PC functions. Number of PC functions was determined by the Akaike Information Criterion. The fraction of variance explained (FVE) is given in parentheses in the upper left corner of each plot. The mean functions in each case match those plotted in Main Text Figure 2.

Extended Data Figure 8. Calibration accuracy from 2 until 24 months of life in typically-developing children (TD) and in children diagnosed with an autism spectrum disorder (ASD)

In plots in a, the cross marks the location of mean calibration accuracy, while the annulus marks the 95% confidence interval (CI). In b, kernel density estimates plot the distribution of fixation locations relative to fixation targets for Typically-Developing (TD) children. In c, kernel density estimates plot the distribution of fixation locations relative to fixation targets for children diagnosed with an Autism Spectrum Disorder (ASD). Smoothing bandwidth for kernel density estimates was equal to 1°. Targets for testing calibration accuracy consisted of spinning and/or flashing points of light and cartoon animations, ranging in size from 1° to 1.5° of visual angle, presented on an otherwise blank screen, all with accompanying sounds

Extended Data Figure 9. Growth charts of social visual engagement and their relationship to dimensional and categorical outcome, with data from month 2 included versus excluded

Comparison of growth curves with month 2 data included or excluded for a, Typically-Developing males (TD, in blue) and, b, for males with an autism spectrum disorder (ASD, in red). Exclusion of the month-2 data does not significantly alter the trajectories themselves, nor does it alter the between-group comparisons. c, Outcome levels of social disability (as measured by ADOS social-affect score) as a function of decline in eyes fixation (measured as eyes PC1 score, as in Extended Data Figure 1) using subsets of the longitudinal data (i.e., decline in eye fixation using only data collected between months 2–6 or 3–6, excluding data thereafter; then between months 2–9 or 3–9, etc.). In top row, month 2 data are included; in bottom row, month 2 data are excluded. When month 2 data are included or excluded, decline in eye fixation still significantly predicts future outcome; this relationship reaches trend level significance by 3–9 months (P = 0.097), and is statistically significant thereafter (with r = −0.714, P = 0.014 for 3–12 months). d, ROC curves for classification of infants with confirmed ASD outcomes relative to typically-developing infants. Using leave-one-out cross-validation, plots show mean and 95% confidence intervals for classification based on change in eye fixation (first two plots from left), change in body fixation (middle two plots), and change in both eye and body fixation (last two plots at right) between 2 and 6 months of age. Plots show ROC classification using data from months 2–6 and for the comparison of months 2–6 relative to months 3–6. With month-2 data excluded, confidence intervals for the cross-validated ROC curves increase in size (as expected, in proportion to the reduction in data by excluding month 2), but the curves remain significantly different from chance, and the ROC curves with month 2 data included or excluded are not significantly different from one another.