Histograms showing the differences in retrograde labeling distributions in the inferotemporal cortex (area TE) and the MTL.

<p>A, Case of WGA-HRP injections into V4. B, Case of rabies injections into V4 at the 3-day postinjection period. C, Case of rabies injections into V4 at the 4-day postinjection period. Cell counts were performed in every 12th section (60 µm thick; 720 µm apart). Data from the WGA-HRP injection case were obtained in monkey V4, while those taken at the 3- or 4-day postinjection period represent the average of the labeled neuron numbers in monkeys V4-3a and V4-3b or monkeys V4-4a and V4-4b, respectively. Asterisks indicate the areas in which cell counts could not be done, as a tremendous number of rabies-labeled neurons appeared diffusely. The subicular complex (SC) consists of the subiculum, presubiculum, and parasubiculum. Each of these regions contained are 34, 15, and 12, respectively). HCF, hippocampal formation; IT, inferotemporal cortex; PHC, parahippocampal cortex; PRC, perirhinal cortex; TG, area TG. Other abbreviations are as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0052115#pone-0052115-g001" target="_blank">Figures 1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0052115#pone-0052115-g002" target="_blank">2</a>.</p

Distribution of retrograde labeling in the MTL 3 days after rabies injections into V4.

<p>Six representative coronal sections through the MTL in monkey V4-3a are arranged anteroposteriorly (A–F). The approximate anteroposterior levels of the sections are indicated in the lateral view of the brain (inset). The gray region represents the approximate whole extent of multiple injection sites in monkey V4-3a. Each green dot in the sections (A–F) corresponds to one labeled neuron. Dotted lines denote the borders of layer 4 in each area. amt, anterior middle temporal sulcus; Ent, entorhinal cortex; FST, fundus of the superior temporal sulcus area; ot, occipitotemporal sulcus; pmt, posterior middle temporal sulcus; rh, rhinal sulcus; st, superior temporal sulcus; TEa, anterior portion of area TE; TEm, medial portion of area TE; TEO, area TEO; TEp, posterior portion of area TE; TF, area TF; TH, area TH; 35, area 35; 36, area36.</p

Summary diagram showing possible multisynaptic pathways from the MTL to V4.

<p>Our results indicate that the entorhinal cortex (Ent) provides trisynaptic input to V4 via either the parahippocampal cortex (PHC) or the inferotemporal cortex (IT), while the hippocampus (HC) does through “short-cut” pathways that bypass the Ent, as denoted by orange arrows. The gray area represents the approximate location whole extent of the rabies injection sites in 4 monkeys used for the present study. For clarity, only presumably major pathways are shown here (see Discussion for details). HC, hippocampus; 1st, first-order neurons; 2nd, second-order neurons; 3rd, third-order neurons.</p