In previous studies we have described vibrissae-related segmentation in the brainstem, thalamus, and cortex of the neonatal rat. Using succinic dehydrogenase (SDH) histochemistry, we delineated the time course in development of the normal segmentation at each level of the trigeminal system and the aberrant segmentation resulting from follicle damage at birth (Killackey and Belford, '79; Belford and Killackey, '79a, b). The present study examines the aberrant patterns that result from damage to the vibrissae follicles at different ages, comparing the patterns at the different levels of the trigeminal system.

The present study indicates a number of similarities between the central representations of the vibrissae. First, the patterns are similar within each of the three representations in the trigeminal nuclei for removal at a given age in a particular animal. These changes include a decrease in SDH density but a maintenance of normal row widths. Second, the patterns are similar within both the ventrobasal complex and layer IV of somatosensory cortex for removal at a given age in a particular animal. These changes include a fusion of individual clusters into bands, and a decrease in band width, but maintenance of normal SDH density. Third, the effects of damage to a row of vibrissae follicles at different ages are graded. Earlier damage produces more marked aberrations. Fourth, for all of the structures, the transition between bands and clusters occurs with damage at the same age. Further, the last age at which damage produces aberrant patterns is Day 3 for all of the structures. Thus, the data suggest that there is one sensitive period for pattern alteration in the entire central trigeminal system.