Many measures of visual function reach adult levels by about age 5, but some visual abilities continue to develop throughout adolescence. Little is known about the underlying functional anatomy of visual cortex in human infants or children. We used fMRI to measure the retinotopic organization of visual cortex in 15 children aged 7–12 years. Overall, we obtained adult-like patterns for most children tested. We found that significant head motion accounted for poor quality maps in a few tested children who were excluded from further analysis. When the maps from 10 children were compared with those obtained from 10 adults, the magnitude of retinotopic signals in visual areas V1, V2, V3, V3A, VP, and V4v was essentially the same between children and adults. Furthermore, one measure of intra-area organization, the cortical magnification function, did not significantly differ between adults and children for V1 or V2. However, quantitative analysis of visual area size revealed some significant differences beyond V1. Adults had larger extrastriate areas (V2, V3, VP, and V4v), when measured absolutely or as a proportion of the entire cortical sheet. We found that the extent and laterality of retinotopic signals beyond these classically defined areas, in parietal and lateral occipital cortex, showed some differences between adults and children. These data serve as a useful reference for studies of higher cognitive function in pediatric populations and for studies of children with vision disorders, such as amblyopia.

*intra*-areal function.

^{2}. The area of an ROI is the sum of the area of

*n*triangles. Due to the inherent difficulty of segmentation of white from gray matter, segmentation is a potential source of error in the reconstructions. However, the accuracy of the programs has been validated (e.g., by test-retest comparisons) (Dale et al., 1999) and the accurate estimates of cortical thickness (Fischl & Dale, 2000; Rosas et al., 2002). Furthermore, each reconstruction is manually inspected for errors.

*F*) values were computed for each voxel by using a comparison between the Fourier domain amplitude at the stimulation frequency and the average amplitude at other (nonharmonic) frequencies. For these types of calculations, the phase/magnitude data are converted from polar coordinates to the equivalent data in rectilinear coordinates (i.e., complex numbers with real and imaginary components). The

*F*statistic accommodates this multivariate analysis. The

*F*statistic is computed as the sum of the squared real and imaginary signal components divided by the noise variance. Finally, a registration procedure within Freesurfer was used to align (in all 3 planes) the T1 contrast anatomical images collected in each functional session with the cortical surface model. The same registration matrix was then applied to the functional images to view the results on the cortical surface.

*x*is the cortical distance and

*y*is the eccentricity. The process for V2 was equivalent, except that the 30-deg filter was centered on oblique polar angles (135 deg for V2v and 45 deg for V2d), so as to fall along the center of those areas. One adult subject and one child subject, with poor quality polar angle maps, did not satisfy our criteria of least squares

*R*

^{2}fit greater than 0.5, and were excluded from this analysis.

*F*-value data sets were then re-sampled into spherical space and subsequently averaged across subjects using a fixed-effects model (i.e.,

*F*ratio numerator is summed real plus imaginary components squared, and denominator is the summed noise variance divided by number of subjects). Extension of these methods to random effects is desirable, but will require accounting for multiple comparisons on the surface, and clustering approaches may not be appropriate for retinotopic data. Finally, the average

*F*-value map was painted (via the spherical transformation) onto the inflated surface of one adult. Separate maps were created for the eccentricity and polar angle stimulus in each hemisphere. The entire analysis was subsequently repeated for the group of children.

*t*test approached significance (

*p*= .06 and

*p*= .05, respectively).

*R*

^{2}= 0.07) or the adult group (

*R*

^{2}= 0.34).

*qualitative*inspection of the retinotopic maps from representative children and adults did not suggest consistent differences between groups. However, it is important to be able to

*quantify*measures from individual visual areas that can be averaged across subjects to address any systematic group differences. For the 10 adults and 10 children with interpretable field sign maps, regions of interest were created for the six visual areas (V1, V2, V3, V3A, VP, and V4v). A seventh ROI entitled “All” was defined to include all six visual areas. These ROIs were used to separately extract from our data the average fMRI signal Fourier magnitude of all voxels located in each of the six visual areas. We also directly calculated the surface area of visual area ROIs as described in Section 2.2.

Visual Area | Child LH polar | Adult LH polar | Sig. {itp} LH polar | Child LH eccen | Adult LH eccen | Sig. {itp} LH eccen |
---|---|---|---|---|---|---|

V1 | 3.1 | 2.6 | — | 7.6 | 5.4 | — |

V2 | 3.7 | 3.3 | — | 9.5 | 6.7 | — |

V3 | 5.7 | 4.7 | — | 12.4 | 9.3 | — |

V3A | 5.3 | 4.1 | — | 9.3 | 7.2 | — |

VP | 4.0 | 3.9 | — | 9.8 | 6.4 | — |

V4v | 3.9 | 4.1 | — | 5.7 | 5.6 | — |

All | 25.7 | 22.7 | — | 54.3 | 40.6 | — |

Visual Area | Child RH polar | Adult RH polar | Sig. {itp} RH polar | Child RH eccen | Adult RH eccen | Sig. {itp} RH eccen |
---|---|---|---|---|---|---|

V1 | 2.0 | 3.0 | — | 5.8 | 4.9 | — |

V2 | 3.1 | 4.0 | — | 8.4 | 7.1 | — |

V3 | 6.3 | 5.1 | — | 13.0 | 7.6 | — |

V3A | 4.5 | 3.1 | — | 7.8 | 6.6 | — |

VP | 3.2 | 5.2 | 0.007 | 8.6 | 8.3 | — |

V4v | 4.9 | 4.1 | — | 8.9 | 5.8 | — |

All | 24.0 | 24.5 | — | 52.4 | 40.3 | — |

*t*tests. For the polar angle stimulus, only the right VP was significantly different, having a larger value in adults. Next section we consider this difference in relation to the size of the visual areas.

Visual Area | Child LH mm^{2} | Adult LH mm^{2} | Sig. {itp} LH mm^{2} | Child LH % | Adult LH % | Sig. {itp} LH % |
---|---|---|---|---|---|---|

V1 | 1102 | 1182 | — | 1.3 | 1.4 | — |

V2 | 851 | 1194 | 0.02 | 1.0 | 1.4 | 0.02 |

V3 | 417 | 580 | 0.04 | 0.5 | 0.7 | 0.05 |

V3A | 513 | 678 | — | 0.6 | 0.8 | — |

VP | 496 | 678 | — | 0.6 | 0.8 | — |

V4v | 484 | 738 | 0.01 | 0.6 | 0.9 | 0.02 |

All | 3865 | 5049 | 0.02 | 4.7 | 5.5 | 0.03 |

Visual Area | Child RH mm^{2} | Adult RH mm^{2} | Sig. {itp} RH mm^{2} | Child RH % | Adult RH % | Sig. {itp} RH % |
---|---|---|---|---|---|---|

V1 | 1219 | 1155 | — | 1.5 | 1.4 | — |

V2 | 866 | 1161 | 0.02 | 1.1 | 1.4 | 0.04 |

V3 | 381 | 502 | 0.04 | 0.5 | 0.6 | 0.07 |

V3A | 549 | 540 | — | 0.7 | 0.6 | — |

VP | 462 | 667 | 0.003 | 0.6 | 0.8 | 0.02 |

V4v | 597 | 648 | — | 0.7 | 0.8 | — |

All | 4073 | 4673 | 0.07 | 5.1 | 5.6 | — |

^{2}, except for V1 and V2, which ranged from 800–1200 mm

^{2}. The results indicated some small but significant differences between the adults and children. Adults showed a slightly larger extent of visual areas V2, V3, and V4v in the left hemisphere. In the right hemisphere, visual areas V2, V3, and VP were significantly larger in adults. In contrast, there was no difference in the extent of V1 between groups.

^{2}for the children, and 87,009 and 86,011 mm

^{2}for adults. The adult’s brains were larger than the children’s brains, but the effect was not significant (

*p*= 0.12 for left hemisphere;

*p*= 0.13 for right hemisphere). Nevertheless, given the obvious developmental trend, a measure of ROI size relative to the total neocortical sheet may be a valuable measure (Tables 3 and Table4|4). Overall, individual visual areas range from 0.5–1.5% of the cortical sheet, and all six visual areas combined occupy about 5% of the neocortex in one hemisphere. When the adults and children were compared, the results were highly consistent with the comparisons of absolute size in that similar extrastriate areas proved to be slightly larger in adults. The total proportional size of the visual areas of both hemispheres in children and adults is shown graphically (Figure 4).

*SD*divided by the mean. Fourier magnitude measures for children’s visual areas had a CV in the range of 50–55%; adults produced values ranging from 35–45%. Measures of areal size (absolute and normalized) had a CV in the range of 15–30% for both children and adults.

*R*

^{2}> 0.5). Moreover, the average adult and children curves were fit with

*R*

^{2}values of 0.94 and 0.96, respectively (Figure 5). The mean fitted exponent was 0.74 for the adults and 0.71 for the children. The two subject groups did not have a different distribution of exponential values. This was true when the fit was done after placing data in 1-deg bins to make Figure 5 (

*p*= .66) and in the case of the non-binned data (

*p*= .96). Thus, our data indicate no difference in the precise retinotopic mapping function, although it can be seen that the variance of the child group was somewhat greater.

*R*

^{2}values of 0.98 and 0.97 for adults and children. The mean fitted exponential value is 0.073 for the adults and 0.057 for the children. For V2d,

*R*

^{2}values are 0.96 and 0.92 for adults and children. The mean fitted exponential value is 0.081 for the adults and 0.080 for the children. The two subject groups do not have a different distribution of exponential values for V2v (

*p*= .66) or V2d (

*p*= .99). With regard to any differences between the exponential function for V2v and V2d, there is a significant difference for the children (

*p*= .01) with V2d showing a steeper slope than V2v. The same trend is observed in adults.

*F*statistic. Average

*F*-statistic maps were produced for the adult group and the children’s group. The results for the eccentricity stimulus indicate a larger extent of signal in the adult subjects in parietal cortex (Figure 7). Inspection of the individual data revealed that this was a consistent trend in the groups, for both hemispheres. Seven out of 10 adults showed some activation in the middle and/or anterior extent of the intraparietal sulcus, whereas only three children passed these criteria.

*F*statistic in adults and children appeared qualitatively similar in the ventral temporal cortices of both hemispheres (Figure 9).

*negative*BOLD response in the visual cortex of sedated infants, raising the possibility of drastic developmental changes in the BOLD mechanism (Born, Rostrup, Leth, Peitersen, & Lou, 1996, 1998; Yamada, Sadato, & Konishi, 1997). Possible reasons for differences in the BOLD signal in children include higher metabolic rates at rest than in adults, perhaps supporting higher synaptic density (e.g., Chugani et al., 1988). However, sedation or sleep may instead be the important variable here. Recently, a negative BOLD response was obtained in children and in adults during slow wave sleep, as well as in some cases of sedated adults (Born et al., 2002). Regardless of the precise role of these factors in infants and young children, our own results and those of others suggest that a positive BOLD response dominates in awake children by age 7-8 (Martin et al., 1999).