Development of the representation of auditory space in the superior colliculus of the rat

The aims of the present study were, first, to determine the time of the emergence of the topographic map in the superior colliculus (SC) of the rat and, second, to examine the spatial changes in auditory neurons' receptive fields (RFs) throughout the developmental period. For these purposes, recording sessions were conducted on rats of different age groups (P15-18; P21-24; P27-29 and P60-80). Results show that SC auditory neurons' RFs go through multiple changes during early development before the establishment of the adult-like auditory topographic map (P27). These modifications include, first, the refining of directional RFs tuning between P18 and P27 and, second, a shift of sensitivity towards 90° in the contralateral hemispace after P15-18. In addition, data indicate that the neuronal response latencies are shorter in the adult group than in the P15-18 or P21-24 groups. Finally, a diminution in spontaneous firing rate was observed between the P15-18 group and the P60-80 group. The results of the present study revealed that the neural organization representing auditory space in the deep layers of the SC of the rat is not innate, but rather emerges at about P27-29. Moreover, they bring new evidence supporting a two-step process involved in the development of the representation of auditory space. Hence, the development of the auditory space representation follows a logical temporal maturation sequence whereby the RF organization must be completed before a stable topographic map can be formed.