Fig. 12. SC targeting synapses in different phases of development with disparate cross-modal inputs.

The figure displays the maturation of synaptic connections between the four unisensory input areas and the neuron in position 80 in the SC area, as the result of a repeated exposure to two cross-modal inputs coincident in time, but not in space. The x-axis represents the position of the pre-synaptic unisensory neurons, while the y-axis reports the synaptic strength of the connections. In particular in this case the visual input is placed in the center of the corresponding RF, whereas the auditory inputs are placed in the RF of unit at position 40 (i.e., about 70° far from the corresponding visual inputs). Panel A) shows the synaptic strength in an early stage of this developmental process. The connections from AES regions are weak and the ascending projections still present an immature arrangement. In this phase the observed SC neuron is maximally responsive for visual and auditory inputs placed in the same position (position 80). Panel B) shows synapses in a late training phase. Finally, panel C) reports the final synaptic configuration. It’s worth noting that the center of the SC auditory RF is spatially shifted (in fact, the RF of the auditory neuron was placed at position 40 in the immature phase, but it is now centered at position 80), reflecting the stimulus position during the training period; as a consequence, the unisensory RFs (i.e. visual and auditory) are no longer overlapped.