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 dominat ed by inputs from the distance senses (vision, audition). In contrast, the rodent SC relies more heavily on non-visual inputs, such as touch and nocic eption, possibly as an adaptive response to the proximity of dangers encoun tered during their somatosensory-dominant search behaviors. The ferret (a c arnivore) 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 adap tations 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). Furthe rmore, tactile responses were driven primarily from hair-receptors (like ca ts), 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) noxio us stimulation, but further tests could not establish these responses as no ciceptive. Collectively, these data indicate that the ferret SC is best ada pted for the animal's visual/acoustically guided activities and most closel y resembles the SC of its phylogenetic relative, the cat.