The dependent variable was saccade latency to the probe target, defined as the duration between probe target onset and initiation of an eye movement that ended in a fixation, on. the target stimulus. Fixations were defined as portions of eye movement records in which the POG remained within a 1° radius for at least 100-msec. We reasoned that distractor suppression during the prime would be revealed by a difference in saccade latency between RD and control conditions. No sex differences in performance were revealed, nor were there order effects that informed the principal hypotheses; therefore analyses in the three experiments were collapsed across these two variables.
There were differences in overall saccade latency among the three age groups, F(2, 38) = 32.55, p < .001, η2 = .63. Linear contrasts showed that 3-month-old infants were slower (M= 391.30-msec, SD = 63.36) than 6-month-olds (M = 300.70-msec, SD = 59.17), and both were slower than 9-month-old infants (M = 240.89-msec, SD = 25.31), all ps < .01, Bonferroni adjusted alpha level of .017 (.05/3). Hence, we treated average saccade latency as a covariate in the subsequent general linear model (GLM) repeated measures analysis of variance (ANOVA) for two reasons: (a) our principal hypotheses were concerned with indexing target excitation or distractor suppression during the prime display, before oculomotor output saccades were executed to the probe target, and (b) we aimed to reduce effects of within-group variability in saccade latencies, especially in the youngest group, on the RD versus control manipulation of interest.
Average saccade latency values per participant were treated as a covariate in a 3 (ISI: 550, 200, or 67-msec) × 2 (condition: RD vs. control) × 3 (age group: 3-, 6-, 9-month-old infants) GLM repeated measures ANOVA. The analysis yielded a significant main effect of ISI, F(2, 74) = 3.31, p < .05, η2p = .08. Pairwise comparisons showed that latencies were longer overall (all ps < .001, Bonferroni adjusted alpha level of .017) in the 550-msec ISI condition (M = 336.82-msec, SD = 94.82) relative to both the 200-msec and 67-msec ISI conditions (M = 283.57-msec, SD = 93.65, η2 = .37 and M = 286.85-msec, SD = 80.77, η2 = .32, respectively).
illustrates average RD and control saccade latency values by age group. The analysis indicated no main effect of condition but yielded a significant Condition × ISI interaction, F(2, 74) = 8.35, p = .001, η2p = .18. Planned comparisons (simple effects tests) revealed no reliable differences between RD and control trials in either the 550-msec or 200-msec ISI conditions, but saccade latencies were faster to RD relative to control trials at the shortest ISI, F(1, 40) = 9.56, p < .005, η2 = .19. That is, there was facilitation at 67-msec. We also found a significant Condition × ISI × Age interaction, F(4, 74) = 2.69, p < .05, η2p = .13. Planned comparisons examining performance as a function of age group showed that 9-month-olds were slower on RD relative to control trials at the longest ISI, F(1, 16) = 2.84, p < .05, η2 = .34, and marginally significant at the 200-msec ISI, F(1, 16) = 1.66, p = .117, η2 = .15, and nothing reliable at the 67-msec ISI condition, F(1, 16) = −1.62, ns. Given sufficient time, therefore, 9-month-old infants appear to have suppressed the distractor stimulus during prime target selection. In contrast, 6-month-old infants provided no evidence of inhibition but did show facilitation in both the 200-msec, F(1, 11) = −2.37, p < .05, η2 = .34, and 67-msec ISI conditions, F(1, 11) = −3.81, p < .005, η2 = .57. Finally, 3-month-olds showed some evidence of facilitation only in the 67-msec ISI condition, F(1, 11) = −2.11 p < .06, η2 = .29. These data suggest that the time course of inhibition in 3- and 6-month-old infants is slower than in 9-month-olds.
Repeated distractor and control saccade latency data for infants in each of the three ISI conditions. Only 9-month-olds provided reliable evidence of spatial negative priming. Three- and 6-month-old infants showed only facilitation.
Inefficiency in distractor suppression during the prime might lead to interference from the distractor. One possible consequence is an increased tendency to look at the distractor, a tendency that might be greater in infants who are the most susceptible to interference. shows looks to the distractor during the prime, calculated as a proportion of total number of trials provided per participant. An ANOVA examining differences in distractibility as a function of age revealed significant differences in the proportion of looks to the distractor during the prime display, F(2, 38) = 33.88, p < .001, η2 = .64. Three-month-olds were reliably more distractible than both 6- and 9-month-old infants (all ps < .001, Bonferroni adjusted alpha level of .017).
Proportion of looks to the distractor during the prime display. Three-month-olds looked reliably more often at the distractor than did either 6- or 9-month-old infants, suggesting greater susceptibility to interference.