We examined the projections from low-level visual cortex to caudal auditory cortex. Injections of retrograde tracers were placed into caudal areas of the monkey lateral sulcus via direct pressure injections or through the cannulae of a linear multielectrode recording array (see Materials and Methods).
Individual injections were mainly limited to one cortical area but in most cases also involved adjacent areas in the lateral sulcus and occasionally in the superior temporal sulcus (). Therefore, those injection sites were designated according to the primary/secondary areas involved, for example, CM/primary auditory cortex (A1). In each case, we describe the injections according to the cortical area mainly involved by the injection site and then discuss the potential consequences of involvement of adjacent areas. The injection sites 1 to 13 were previously described in detail in a study of the local cortical and thalamic connections of caudal belt areas (Hackett et al. 2007
; Smiley et al. 2007
). summarizes the characteristics of individual injections. In both and the following descriptions, the injections are organized into groups according to the areas injected. These groups include the following: 1) injections into A1 and middle medial area (MM), 2) injections mainly into CM, 3) injections into caudal lateral auditory belt area (CL) and caudal parabelt auditory area (CPB), 4) injections into temporoparietal area (Tpt), and 5) injections mainly into area SII on the dorsal bank of the lateral sulcus.
Labeling in visual areas was plotted on 0.96-mm–spaced histological sections. Although both hemispheres were searched, labeling in visual areas V2 and prostriata was found only in the ipsilateral hemisphere.
Group 1: A1 and Medial Belt
Injection 12 (MAG Fruby) was made through a cannula and was a comparatively small injection clearly confined to area A1 near its caudal medial border. The injection was mainly confined to the middle cortical layers and did not reveal projections from visual cortex.
Injection 4 (ME DY), in medial belt area MM, was performed using a cannula in a location centered between A1 and retroinsular area (Ri). A small amount of tracer leaked into area Ri medially. Electrophysiological recordings performed in the site prior to injection showed response to somatosensory and auditory stimuli (Hackett et al. 2007
; Smiley et al. 2007
). This injection revealed no projecting cells from visual cortex.
Group 2: Caudal Belt Area CM
We placed 7 injections in area CM. Some of these also involved the adjacent medial areas A1, Ri, and SII, and one also involved area Ipa in the fundus of the superior temporal sulcus. Injection 1 (MAB FR) was made using a multiarray injectrode and was confined to area CM at the caudal and medial border of area A1. This injection labeled 5 cells in area prostriata and only one in area V2, distributed over 2 consecutive histological sections. Of the remaining 6 injections, 2 (nos 2 and 8) produced no labeling in visual areas and 4 (nos 3, 5, 6, and 7) produced weak labeling (). It is unlikely that the adjacent somatosensory area Ri receives robust visual projections as 2 of these injections (nos 6 and 7) had significant involvement of that area.
Overall, the injections into CM and the adjacent medial areas produced weak and variable projections from areas V2 and prostriata. The labeled cells were located in the dorsal bank of the anterior calcarine sulcus, within ~3 mm from its rostral end. At this location, the rostral extent of V1 is still present in the fundus of the sulcus and areas prostriata and V2 extend medially across the dorsal bank of the sulcus. Area V2 on the dorsal bank represents the lower visual quadrant (Van Essen et al. 1984
); area V2 at the very anterior part of calcarine sulcus represents the peripheral visual field, beyond 30° of eccentricity, possibly restricted to the monocular field (Van Essen et al. 1984
; Gattass et al. 1997
). The organization of area prostriata is less documented, but it is known to be highly connected with the peripheral visual representation of areas V1, V2, and middle temporal visual area (MT) and thus is probably involved in dorsal visual stream processing (Gattass et al. 1997
; Palmer and Rosa 2006
). Area prostriata was identified by histological criteria as well as its previously described topography (see Materials and Methods section). shows, on a medial view of the brain, the rostral-to-caudal extent of the location of labeled cells in the visual cortex. This same location was labeled by injections of CM as well as by injections in the other auditory areas described below.
Figure 2. Histological sections are shown to illustrate the topography of visual cortical areas and labeling in calcarine sulcus (CS), white matter (WM), and layer IV (IV). This example is taken from injection 10, which was located in gyral Tpt (MAB CTb). (A1) (more ...)
Group 3: Caudal Lateral Belt and CPB
Three injections were placed in the CL and CPB areas. These produced labeled cells in the same region of V2 and area prostriata seen with injections in area CM, but there was clearly a larger number of labeled cells. Injection 13 (MS Fruby) was the only injection confined to area CL. This injection was small, and caused comparatively sparse labeling in auditory cortex, but nevertheless labeled a slightly larger number of cells in visual cortex than any of the CM injections (). In the absence of additional injections confined to CL, it is uncertain if this finding is representative of CL's connections. Injection 9 (MAG FB) was a large injection (0.4 μL) that targeted area CL and encroached on part of CPB (). This injection labeled 76 cells in peripheral area V2, over a region spanning a rostrocaudal distance of approximately 3 consecutive 0.96-mm–spaced sections. In addition, this injection produced relatively dense labeling in area prostriata, with 23 projecting cells distributed over 5 consecutive sections. Injection 19 (MR FB) was placed in the caudal CPB and revealed moderate projections from both areas V2 (13 cells) and prostriata (7 cells) distributed across 3 consecutive sections.
Group 4: Area Tpt
Five injections involved the gyral and medial (planum) parts of area Tpt. In most of these, the number of labeled cells in areas V2 and prostriata was usually at least as great as that produced by CPB injections. The exception was injection 14 (MY FR), located on the medial planum surface, which produced only a few cells in visual areas. Injection 10 (MAB CTb), which was lateral to injection 14 on the planum surface and encroached on caudal belt area CL, produced moderate labeling in both V2 (17 cells) and prostriata (34 cells). Injections 11, 16, and 17 were placed in the gyral portion of Tpt. All 3 showed moderate labeling of area V2 (). Two injections (11 and 16) had strong labeling of prostriata (49 and 90 cells, respectively), but injection 17 did not label the prostriata. In all Tpt injections, the location of labeled cells in the rostral calcarine sulcus was the same as that found with belt and parabelt injections.
Figure 3. The distribution of individual labeled cells (black squares) produced by 2 different injections is shown, plotted directly onto photomicrographs of histological sections. In each example, consecutive 0.96-mm–spaced sections are arranged in caudal-to-rostral (more ...)
Group 5: Area SII
Injections 15 and 18 (MY FB and MZ FB) were centered on somatosensory area SII on the dorsal bank of the lateral sulcus and did not cause labeling of early visual cortex. In both cases, there was some leakage of the tracer into auditory areas A1 or CL on the ventral bank of the sulcus. However, only the very superficial layers of auditory areas were involved, and lack of label in visual cortex may not be representative of their connections. Labeled cells were found in the parietal areas of the postcentral gyrus, intraparietal sulcus, and inferior parietal gyrus, consistent with the known connections of SII (Disbrow et al. 2003
Quantitative Comparison of Visual Inputs to Different Auditory Areas
As described above, the number of labeled cells in visual areas was typically low after injections in CM and higher after injections of CPB and Tpt (). This difference is illustrated by statistical analysis of the number of labeled visual cells produced by these injections. For this purpose, we compared the relative number of cells in visual V2/prostriata, expressed as a percent of the total cells plotted on the lateral surface of the temporal and parietal lobes (Smiley et al. 2007
). An alternative approach that simply analyzed the total number of plotted cells in these visual areas produced nearly identical statistical differences (not shown). In our previous study (Smiley et al. 2007
), an error was made in computing the total number of cells in injections 1–10, and the correct values are presented in . The extent of underreporting was similar across layers and areas and had little effect on previously reported within-injection comparisons.
Due to the inhomogeneous variance of these samples, a nonparametric test (Kruskal–Wallis, SPSS 12.0) was used to compare differences between injection groups 2 (mainly CM), 3 (CPB/CL), and 4 (Tpt) (). This demonstrated a significant group difference (chi square
8.78, degrees of freedom
0.01). Post hoc Mann–Whitney tests showed significant differences only between group 2 and each of the other groups, after Bonferroni correction for 3 comparisons (P
0.05/3). The findings indicate that areas CPB and Tpt have a higher density of visual inputs than area CM. This difference is not easily explained by the types of tracers used as CM injections labeled few visual cells even using large injections of the more sensitive tracers CTb and FB (injections 5, 6, and 8). Only one injection (no. 13) was confined to area CL, and more data are needed to evaluate whether the relatively high number of labeled visual cells is representative of the connections of CL.
After most injections, there were roughly 3 times as many cells in prostriata as in V2 (). However, 2 injections in CPB (nos 9 and 19) had proportionally more cells in V2 than prostriata (). Although statistical comparison of the proportion of cells in V2 compared with prostriata in groups 3 and 4 was not significant (Mann–Whitney tests, P
0.1), it is possible that larger sample sizes would show preferential connections of V2 and prostriata with different auditory areas.
Laminar Distribution of Projecting Cells in Areas Prostriata and V2
The distribution of labeled cells in areas V2 and prostriata after injections of auditory areas was mainly infragranular. In V2, 86% of the labeled cells were infragranular; only infragranular cells were found in 8 of the 11 injections that labeled V2. In prostriata, 87% of the labeled cells were infragranular; only infragranular cells were found in 7 of the 12 injections that labeled area prostriata ().
These values are approximate because we did not use a high sampling frequency (0.96 mm spacing), and it is possible that we overlooked small foci of supragranular cells (Barone and Kennedy 2000
; Vezoli et al. 2004