Tongue protrusion mediated by spared anterior ventrolateral neocortex in neonatally decorticate rats: behavioral support for the neurogenetic hypothesis.

Many changes in anatomical organization and behavior follow circumscribed, neonatal cortical ablations. These include functional sparing and compensatory anatomical changes. An objective of this study was to examine the generality of such changes by reversing the usual experimental procedure, removing all cortex but a circumscribed area and examining whether the remnant made new anatomical connections, adopted new functions and whether it continued to subserve the typical functions of that area. All neocortex and cingulate cortex, except a small portion of anterior-lateral neocortex, which is normally involved in tongue and mouth use, was removed from one-day-old or adult rats. Fluorescent labelling and behavioral tests were used to evaluate its function. The results showed: (1) The remnant cortical tissue maintained similar connections in neonatal and adult groups and similar connections to those found in rats that had received no lesions. (2) The rats still displayed behaviors normally supported by this cortex, including tongue protrusion to obtain food and picking up and eating hard food efficiently. (3) Impairments were obtained on tests normally mediated by the ablated cortex, including skilled reaching and grooming. (4) When the cortical remnant was removed, tongue protrusion and efficiency of food consumption were similarly impaired in both neonate and adult groups. An additional serendipitous finding was a dissociation between two types of tongue movement: licking from a ventrally-located surface survived cortical removal but tongue protrusion did not. The results show that bilaterally spared small remnants of neocortex maintain normal functions and do not assume new functions or make new connections despite neonatal decortication. The results provide behavioral support for the neurogenetic hypothesis, which postulates that cortical circuitry is specified during early embryonic development. This suggests that there are constraints on neural remodelling and behavioral recovery following neonatal lesions. The implications of these results are discussed with respect to the documented remodelling and sparing that occur following partial or unilateral lesions within functional systems.