Results Tectopulvinar clusters and convergence As previously reported [9], injections of biotinylated dextran amine (BDA) in the SC label tectopulvinar axons that terminate in nontopographically organized widespread boutons (‘‘diffuse’’ projections; Figure 1A) as well as topographically organized clusters of boutons (‘‘specific’’ projections; Figure 1B). By comparing BDA-labeled tectopulvinar synapses to synapses labeled by the vGLUT2 antibody (a marker for tectothalamic terminals; [9,10]), we previously concluded that both the diffuse and specific projections are convergent [9]. We confirmed this by staining tissue that contained BDA-labeled tectopulvinar axons with the vGLUT2 antibody. As illustrated in Figure 1C–G, for both types of tectopulvinar axons, the BDA-labeled terminals made up a small proportion of the total terminals within a vGLUT2-stained cluster. However, vGLUT2-stained clusters contained at most 1 bouton contributed by ‘‘diffuse’’ axons (Figure 1F), whereas ‘‘specific’’ axons contributed several boutons to each cluster (Figure 1C–E, G). This suggests the ‘‘diffuse’’ pathway exhibits a greater degree of convergence onto individual dendrites than does the ‘‘specific’’ pathway. Tectopulvinar Synapses and Synaptic Potentials Specific synapses are longer than diffuse synapses We re-analyzed electron micrographs collected in our previous ultrastructural study of tectopulvinar terminals labeled by the anterograde transport of BDA injections into the SC [9] to compare the synapse length of ‘‘specific’’ terminals (Figure 2A, n = 47) imaged within Pc tissue and ‘‘diffuse’’ terminals (Figure 2B, n = 49) imaged in Pd tissue. As illustrated in Figure 2C, the distribution of the synapse lengths overlapped, but as a population, specific synapses (0.2560.07 mm) were found to be significantly longer than diffuse synapses (0.2260.07 mm, P,0.05). We found no significant difference in the size of the specific and diffuse terminals measured at the site of synaptic contacts (specific 0.4660.21 mm2; diffuse 0.4760.21 mm2). In this single section analysis most terminals made single synaptic contacts (46 in Pc, 47 in Pd), but a few made more than one contact (1 in Pc, 2 in Pd). Tectopulvinar terminals contain both synapsin I and synapsin II Because the expression of synapsins has been related to short- term plasticity [36], we examined the synapsin content of RS, RM and RL profiles in the dorsal lateral geniculate nucleus (dLGN, Figure 2. ‘‘Specific’’ tectopulvinar synapses are longer than ‘‘diffuse’’ tectopulvinar synapses. Electron micrographs illustrate examples of specific tectopulvinar terminals in the Pc (A) and diffuse tectopulvinar terminals in the Pd (B) labeled by the anterograde transport of biotinylated dextran amine (BDA) from the superior colliculus. The synapse length (arrows) of each terminal type was measured. The distribution of synapse lengths is plotted (C). As a population, the length of specific synapses was found to be significantly longer than the length of diffuse synapses (P,0.05). Scale bar = 0.5 mm. doi:10.1371/journal.pone.0023781.g002 PLoS ONE | www.plosone.org 4 August 2011 | Volume 6 | Issue 8 | e23781