Fig. 3. SC targeting synapses in different phases of development.

In figures only FAES (left panels) and non-FAES synapses (right panels) are shown. AEV and non-AEV synapses exhibit similar development. In the initial condition (first row), non-AEV synapses are similar but narrower than non-FAES, while AEV synapses are inactive as FAES. X-axis and y-axis represent the position of the pre-synaptic and post-synaptic neuron respectively, while the grayscale denotes the strength of each synapse. Thus, each single row in a panel represents the synapses that target one specific SC neuron. In the immature stage (top row in the figure) the SC receives effective (but weak) synapses only from non-AES areas (specifically, non-FAES for auditory and non-AEV for visual inputs). These inputs provide the sole sensory drive to SC neurons. Connections from AES (i.e., AEV, visual area and FAES auditory area) are not effective, and the very large RFs of SC neurons, in the neonatal condition, reflect the diffuse nature of this projection. In an early stage of the development (after 40.000 steps, second row of panels in the figure), the model presented the first SC multisensory neurons with mature AES-SC synapses (light-grey stripes in the left figure), and pruned non-AES connections (right figure); these synaptic patterns led to a contraction in their RFs. These neurons resulted capable of integrating cross-modal stimuli. In an intermediate stage of the development (after 60.000 steps, third row in the figure), the simulated SC presented an increased number of integrative multisensory neurons, with similar synaptic patterns described above. In a late phase of development, there are just a few non-integrative multisensory neuron in the modeled SC, characterized by widespread projections from non-AES areas (light-grey stripes in the bottom right figure), and non-effective synapses from AES.