We compared thresholds for discriminating spatial frequency for children aged 5, 7, and 9 years, and adults at two baseline spatial frequencies (1 and 3 cpd). In Experiment 1, the minimum change from baseline necessary to detect a change in spatial frequency from either baseline decreased with age from 34% in 5-year-olds to 11% in 7-year-olds, 8% in 9-year-olds, and 6% in adults. The data were best fit by an exponential function reflecting the rapid improvement in thresholds between 5 and 7 years of age and more gradual improvement thereafter (*r* ^{2} = 0.50, *p* < 0.0001). In Experiment 2, 5-year-olds' thresholds were higher than those of adults, even when memory demands were eliminated by presenting the two spatial frequencies side by side for an unlimited time. The pattern of development for sensitivity to spatial frequency (this study) resembles those for the development of sensitivity to orientation (T. L. Lewis, S. E. Chong, & D. Maurer, 2009) and contrast (D. Ellemberg, T. L. Lewis, C. H. Lui, & D. Maurer, 1999). The similar patterns are consistent with theories of common underlying mechanisms in primary visual cortex (A. Vincent & D. Regan, 1995; W. Zhu, M. Shelley, & R. Shapley, 2008) and suggest that those mechanisms continue to develop throughout childhood.

*L*

_{max}and

*L*

_{min}are the maximum and minimum mean local luminance values. The space-average luminance of the background was maintained at 13 cd/m

^{2}and the contrast of the stimuli was maintained at 89 ± 2%. Maximum luminance within each grating was 43.8 cd/m

^{2}and the minimum was 2.5 cd/m

^{2}.

*Demonstration and criterion trials*. The demonstration consisted of four trials with the maximum spatial frequency comparisons (1 versus 2 cpd or 3 versus 6 cpd). Two trials had the lower spatial frequency (thicker stripes) in the first interval, and the other two trials had the higher spatial frequency (thinner stripes) in the first interval. For each demonstration trial, the experimenter was aware of the stimuli and pointed out the fatter stripes. After the demonstration trials, the procedure continued with criterion blocks of 4 trials with the maximum comparison but the participant was expected to indicate whether the fatter stripes were in interval 1 or 2. No feedback was provided unless an error was made, after which the remaining trials in that block were used as demonstration trials and the criterion test resumed in the following block. Participants met criterion by getting all four trials in a block correct and had three chances to do so; four 5-year-olds failed to meet this criterion and were replaced in the final sample; the remaining participants usually passed within the first block. The experimenter was unable to see the stimuli during the criterion trials and all subsequent phases of the experimenter.

*Practice run*. Thresholds were calculated using a maximum-likelihood threshold estimation procedure (ML-PEST) in which the spatial frequency difference on the first trial was set at a one octave difference from the reference spatial frequency (where an octave is a halving or doubling of a value) and the value on each subsequent trial was the best estimate of the subject's threshold based on the history of the run (Harvey, 1997). Threshold was defined as the minimum change of spatial frequency required to identify accurately the interval with the thicker stripes. Specifically, threshold measurement stopped at the value corresponding to 82% correct responses with a confidence interval of 95% that the estimate of threshold was accurate within ±0.1 log units. There was no pre-specified maximum number of trials.

*Test of thresholds*. The test of threshold was identical to the practice run. Demonstration and test phases were then repeated for the second reference spatial frequency. The mean number of trials per staircase was 56 (range = 37–121) for 5-year-olds, 56 (range = 38–91) for 7-year-olds, 61 (range = 38–123) for 9-year-olds, and 69 (range = 41–156) for adults. Participants were given as many breaks as necessary, and all participants completed the testing protocol in a single session that lasted no longer than 1 h.

*f*is the minimum difference in spatial frequency required to discriminate stripe width accurately, and

*f*is the reference spatial frequency. The Weber fractions were subsequently subjected to an outlier removal procedure outlined by Kirk (1990). Specifically, each Weber fraction was converted to a

*Z*score using the mean and standard deviation for that age and reference spatial frequency.

*Z*scores greater than +2.5 or less than −2.5 were treated as outliers and were replaced with the original group mean (i.e., the mean threshold for the condition before removal of the outliers). Three data points were replaced: one from an adult tested with a reference spatial frequency of 1 cpd, one from a 9-year-old tested with a reference spatial frequency of 1 cpd, and one from a 7-year-old tested with a reference spatial frequency of 3 cpd. (Note that analyses completed on the original data set, without the removal of outliers, did not differ from the pattern of results when the analyses were completed with outliers removed.) The data were log transformed before analyses because a Levene's test indicated non-homogeneity of variance in the original data set (

*p*s < 0.00001). After the log transformation, homogeneity of variance was improved but remained imperfect (

*p*s < 0.0001). Partial eta squared (

*η*

_{ ρ }

^{2}) values were used for estimates of effect size when examining more than two groups. The figures show the original untransformed data.

*F*(3,76) = 57.79,

*p*< 0.0001, partial eta squared

*η*

_{ ρ }

^{2}= 0.70, power = 1.0. There was no main effect of reference spatial frequency,

*F*(1,76) = 1.66,

*p*> 0.20, and no significant interaction between age and reference spatial frequency,

*F*(3,76) = 0.72,

*p*> 0.55. Tukey post-hoc tests revealed a significant difference in sensitivity between 5-year-olds and the older observers (

*p*s < 0.0001) and between 7-year-olds and adults (

*p*< 0.0001), but not between 7-year-olds and 9-year-olds (

*p*> 0.11). Nine-year-olds did not differ significantly from adults (

*p*> 0.21). Compared to adults, the minimum change necessary to discriminate spatial frequency, when averaged across the two reference spatial frequencies, was 5.7 times (0.29 log units) higher in 5-year-olds, 1.9 times higher in 7-year-olds, and 1.4 times higher in 9-year-olds. Curve fitting indicated that thresholds decreased exponentially with age,

*R*

^{2}= 0.50,

*y*= 1996 * exp(−0.856

*x*) + 6.505. The best-fitting exponential function collapsed across spatial frequency is shown as the smooth black curve in Figure 1.

*σ*= 2) static horizontal sinusoidal gratings. The luminance profile of the Gabor stimulus is described by

*A*is the signal contrast (or amplitude modulation) set at 89 ± 2%, and

*f*is the spatial frequency ranging from 1 to 2 cpd. The Gabor stimuli were rendered in a 15° circular aperture on a gray background and phase was jittered randomly. The resulting blurred edges of varying phase prevented participants from making judgments based on a direct comparison of the adjacent edges of the stimuli in the spatial condition. The space-average luminance of the background was maintained at 45.4 cd/m

^{2}and the contrast of the stimuli was maintained at 89 ± 2%. Maximum (

*L*

_{max}) luminance within each grating was 97.6 cd/m

^{2}and the minimum (

*L*

_{min}) was 5.6 cd/m

^{2}. The reference spatial frequency for the gratings was 1 cpd, and the corresponding comparison spatial frequency ranged from 1.01 to 2 cpd.

*n*= 30 trials, because pilot data from 5-year-olds indicated that 5-year-olds had difficulty completing an entire practice run combined with two test conditions. Demonstration and test phases were then repeated for the second test paradigm.

*p*> 0.08). The figures show the original non-transformed data.

*F*(1,36) = 113.89,

*p*< 0.0001, partial eta squared

*η*

_{ ρ }

^{2}= 0.76, power = 1.0, and an interaction of condition and order,

*F*(1,36) = 10.46,

*p*< 0.003, partial eta squared

*η*

_{ ρ }

^{2}= 0.23, power = 0.88. There was no main effect of order,

*F*(1,36) = 3.46,

*p*> 0.07, or condition,

*F*(1,36) = 0.32,

*p*> 0.58, no significant interactions of age with either order,

*F*(1,36) = 1.40,

*p*> 0.25, or condition,

*F*(1,36) = 3.47,

*p*> 0.07, and no significant 3-way interaction among order, condition, and age,

*F*(1,36) = 0.58,

*p*> 0.45. Regardless of order or condition, 5-year-olds' thresholds were significantly worse than those of adults.

*t*

_{19}= 2.1,

*p*< 0.04; order 2:

*t*

_{19}= −2.3,

*p*< 0.03).