Impact of Monocular Retinal Lesions on Blob Size in Adult Human V1.

We studied the attributes of cytochrome c oxidase (CytOx)-rich blobs and ocular dominance columns (OD) in human V1 associated with monocular retinal lesions. Interblob distance, blob cross-sectional area, OD width, and OD arrangement pattern were analyzed in CytOx-reacted tangential sections of flat-mounted V1 preparations. Monocular deprivation induces differential expression of CytOx in the corresponding ODs in V1. We were thereby able to identify the V1 regions associated with the lesioned area in the retina, assign which OD was associated with each eye, and assign the corresponding blob in Layer III as deprived or nondeprived of visual input. We found that nondeprived blobs are more conspicuously stained than blobs outside the lesioned area. Notably, we found a selective expansion of blobs associated with the nonlesioned eye, whereas blobs associated with the deprived eye showed no significant change in size. Blob size in the latter condition was similar to the one observed in normal participants. These effects were present throughout the representation of the lesion in V1, suggesting that the underlying plasticity mechanisms do not depend on eccentricity. Retinal lesion caused no change in interblob distance, which was comparable to the normal brain (i.e., participants with no retinal lesion). This indicates that blob center is a stable hallmark of cortical organization. Finally, the width of ODs associated with the nonlesioned eye tended to be larger compared with ODs of the lesioned eye. However, this effect did not reach statistical significance. The stability of ODs thereby contrasts with blob plasticity, suggesting that the retinal lesion-triggered imbalance in the thalamocortical projection to Layer IVc has a limited impact on OD CytOx reactivity. On the other hand, we argue that ocular imbalance supports intracortical lateral competition that increases CytOx reactivity in the periblob region associated with the nonlesioned eye, accounting for the blob expansion we observe.

Distribution of cytochrome oxidase-rich patches in human primary visual cortex.

We studied the tangential distribution of cytochrome oxidase (CytOx)-rich patches (blobs) in the striate cortex (V1) of normally sighted Homo sapiens. We analyzed the spatial density and cross-sectional area of patches in CytOx-reacted tangential sections of flat-mounted preparations of V1 and surrounding areas. CytOx-rich patches were most clearly defined in the supragranular cortical layers of V1, particularly at middle levels of layer III. Variations in patch spatial density were subtle across different visual eccentricity representations. Within the binocular representation of V1, the average patch spatial density decreased slightly with increasing cortical eccentricity, from around 1.0 patch/mm2 in the foveal representation to 0.6 patch/mm2 at the representation of ∼60° eccentricity, but seemed to increase again at the representation of the monocular crescent. Across the entire sample, the cross-sectional area of patches (i.e., patch size) varied from approximately 0.2-0.8 mm2 , with a mean value of 0.32 mm2 . Notably, there was no significant variation in the mean patch size across eccentricity representations. Human patches are on average larger than those reported for nonhuman primate brains, and analysis of species with different brain sizes suggests an approximately linear relationship between V1 area and patch size. The relative constancy of patch metrics across eccentricities is in stark contrast with the exponential variation in V1 cortical magnification, suggesting a nearly invariant modular organization throughout human V1.