Is territorial expansion a mechanism for crossmodal plasticity?

Crossmodal plasticity is the phenomenon whereby, following sensory damage or deprivation, the lost sensory function of a brain region is replaced by one of the remaining senses. One of several proposed mechanisms for this phenomenon involves the expansion of a more active brain region at the expense of another whose sensory inputs have been damaged or lost. This territorial expansion hypothesis was examined in the present study. The cat ectosylvian visual area (AEV) borders the auditory field of the anterior ectosylvian sulcus (FAES), which becomes visually reorganized in the early deaf. If this crossmodal effect in the FAES is due to the expansion of the adjoining AEV into the territory of the FAES after hearing loss, then the reorganized FAES should exhibit connectional features characteristic of the AEV. However, tracer injections revealed significantly different patterns of cortical connectivity between the AEV and the early deaf FAES, and substantial cytoarchitectonic and behavioral distinctions occur as well. Therefore, the crossmodal reorganization of the FAES cannot be mechanistically attributed to the expansion of the adjoining cortical territory of the AEV and an overwhelming number of recent studies now support unmasking of existing connections as the operative mechanism underlying crossmodal plasticity.

Responses to innocuous, but not noxious, somatosensory stimulation by neurons in the ferret superior colliculus.

The intermediate and deep layers of the superior colliculus (SC) are known for their role in initiating orienting behaviors. To direct these orienting functions, the SC of some animals (e.g., primates, carnivores) is dominated by inputs from the distance senses (vision, audition). In contrast, the rodent SC relies more heavily on non-visual inputs, such as touch and nociception, possibly as an adaptive response to the proximity of dangers encountered during their somatosensory-dominant search behaviors. The ferret (a carnivore) seems to employ strategies of both groups: above ground they use visual/auditory cues, but during subterranean hunting ferrets must rely on non-visual signals to direct orienting. Therefore, the present experiments sought to determine whether the sensory inputs to the ferret SC reveal adaptations common to functioning in both environments. The results showed that the ferret SC is dominated (63%; 181/286) by visual/auditory inputs (like the cat), rather than by somatosensory inputs (as found in rodents). Furthermore, tactile responses were driven primarily from hair-receptors (like cats), not from the vibrissae (as in rodents). Additionally, while a majority of collicular neurons in rodents respond to brief noxious stimulation, no such neurons were encountered in the ferret SC. A small proportion (4%; 13/286) of the ferret SC neurons were responsive to long-duration (>5 s) noxious stimulation, but further tests could not establish these responses as nociceptive. Collectively, these data indicate that the ferret SC is best adapted for the animal's visuallacoustically guided activities and most closely resembles the SC of its phylogenetic relative, the cat.

Corticothalamic and corticotectal somatosensory projections from the anterior ectosylvian sulcus (SIV cortex) in neonatal cats: an anatomical demonstration with HRP and 3H-leucine.

Corticothalamic and corticotectal projections from the anterior ectosylvian sulcus (AES) in neonatal cats were studied with anterograde and retrograde neuroanatomical techniques. When the injection site was relatively restricted to the sulcal walls and fundus of the rostral AES (i.e., the SIV cortex), heavy ipsilateral thalamic label was observed in the medial subdivision of the posterior group, in the suprageniculate nucleus, and in the external medullary lamina. No terminal label was seen in the contralateral thalamus although the contralateral homotopic cortex was heavily labeled. Within the ventrobasal complex (VB), dense axonal label was observed in fascicles that traversed VB, but only light terminal label was observed within VB itself. However, in cases where the tracer spread into adjacent SII, terminal label in VB was pronounced. Similarly, when the injection site extended into auditory cortex, terminal label was observed in the lateral and intermediate subdivisions of the posterior group. Rostral AES injections produced distinct, predominantly ipsilateral, terminal label in the superior colliculus that was distributed in two tiers: a discontinuous band in the stratum griseum intermedium and a more diffuse band in stratum griseum profundum. Caudally, dense terminal label was seen in the intercollicular zone and dorsolateral periaqueductal gray. When the injection site did not include rostral AES, no label was observed in the superior colliculus. Horseradish peroxidase injections into the superior colliculus of neonates produced retrogradely labeled neurons throughout the AES, but none was found on the crown of the gyrus where SII is located. Thus, the neonatal corticotectal somatosensory projection arises exclusively from AES and parallels that found in adults. These data indicate that the elaboration of a major descending somatosensory pathway from AES to the thalamus and midbrain is largely a prenatal event. The in utero anatomical maturation of the corticofugal projections from SIV cortex to the superior colliculus contrasts with the protracted postnatal development of the corticotrigeminal projections from SI cortex but is consistent with the mature anatomical state of ascending trigeminotectal projections.