From Three-Dimensional Space Vision to Prehensile Hand Movements: The Lateral Intraparietal Area Links the Area V3A and the Anterior Intraparietal Area in Macaques

Myeloarchitecture and Cat-301 staining of the intraparietal areas

Figure 2 shows the myelin-stained and Cat-301-immunostained coronal sections to reveal areal borders of the intraparietal cortex. Where the sections were stained clearly, it was easy to recognize the borders of different areas. The tracks of the electrode penetrations (Fig. 2A,arrowheads) and the recording sites (Fig.2A, arrow) were also apparent in the myelin sections as in the Nissl sections.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 2.

Photographs showing the areal differences of myeloarchitecture (A–C) and Cat-301 immunostaining (D–F) in the intraparietal areas. The sections of the top andbottom rows (A and D,B and E, and C andF) are at similar rostrocaudal levels.A–C are from one of the monkeys in which we made electrophysiological recordings from the lateral intraparietal area (LIP), and sectionsD–F are from one of the monkeys in which we injected tracers into the anterior intraparietal area (AIP) and visual area V3A.Arrowheads show the tracks of recording electrodes, and an arrow shows a recording site. Note that the layers ofAIP are thicker than those of LIP andV3A in the coronal sections. 5, Area 5;5ip, intraparietal part of area 5 [part of PEa excluding the medial intraparietal area (MIP) andAIP]; 7b, area 7b; IP,intraparietal sulcus; LIPd, dorsal part of the lateral intraparietal area; LIPv, ventral part of the lateral intraparietal area; PIP, posterior intraparietal area;VIP, ventral intraparietal area. Scale bars, 1 mm.

In the myelin-stained sections, densely stained bilaminar layers of the inner and outer bands of Baillarger were apparent in LIP (Fig.2B). The bands of Baillarger were stained more densely in the ventral (LIPv) than in the dorsal (LIPd) part of the LIP. The densely stained bilaminar layers were also apparent in the posterior, curved portion of LIP (Fig. 2A). Densely myelinated, thick fibers ran perpendicular to the layers of LIP. In V3A, the outer band of Baillarger was wider but less densely stained than in the LIP, and the inner band of Baillarger was a thin layer that was densely stained and appeared as a clear band. In the posterior intraparietal area (PIP), the area located posterior to VIP, medial to V3 and V3A, and lateral to MIP and PO (Colby et al., 1988), densely stained bilaminar bands were observed, and the inner band of Baillarger was more densely stained than the outer band. In layer 3, many medium-sized myelinated fibers ran parallel to the layer, and in layer 1, thicker myelinated fibers were seen running parallel to the layer. In MIP, the inner band of Baillarger was thin and densely stained, whereas the outer band was not clear but seen as a densely stained wide layer. In the deep part of layer 3, layer 4, and layer 5, medium-sized, densely stained fibers ran parallel to the layers. In the middle to superficial layer 3, fine fibers were seen running in both parallel and perpendicular directions to the layer. In the ventral intraparietal area (VIP), the inner band of Baillarger was very thin but densely stained, whereas the outer band of Baillarger was not clearly seen (Fig. 2B). In layer 3, thick myelinated fibers ran parallel to the layer. In area 5ip, the bilaminar bands of Baillarger were densely stained but were not so apparent as in LIP. Area 5 on the crown of the postcentral gyrus was very densely stained, and thick myelinated fibers were seen to run from layer 6 to the upper part of layer 3. In AIP, the bilaminar bands of Baillarger were wider but not as densely stained as in LIP. The bilaminar pattern was thus not as apparent in AIP as in LIP. In most cases, the myelinated fibers in AIP were not stained as densely as those in LIP, but in some cases, they were stained as densely as those in LIPv. The staining difference in the dorsal and ventral parts, which was apparent in LIP, was not observed in AIP. The myelinated fibers were finer in AIP than in LIP, and a dense mesh of fine myelinated fibers was seen throughout the layers of AIP.

The Cat-301-immunostained sections were also useful in determining the borders between the parietal areas. In the LIP, immunoreactive neurons were distributed in three bands located in layers 3, 5 and 6 (Fig.2E). Large pyramidal neurons and medium to large-sized nonpyramidal neurons were stained in the lower part of layer 3 and in layer 5. Several small to medium-sized pyramidal and nonpyramidal neurons were also Cat-301 positive in the upper part of layer 3 and in layer 6. The staining pattern was similar in both LIPd and LIPv, although fewer Cat-301-positive neurons were observed in LIPd than in LIPv. Large cells were more abundant in LIPv than in LIPd, and the laminar structure was more apparent in LIPv than in LIPd. The same histological appearance was maintained in the posterior curved part of the LIP (Fig. 2D). In V3A, the three-layered pattern was also apparent, but the size of the Cat-301-positive neurons was small. In V3, the Cat-301-positive neurons were scattered throughout the width of layer 3. The Cat-301-positive neurons were also distributed widely in layers 5 and 6. As a result, the three-layered pattern in V3 was not as apparent as in LIP, and it appeared as if the Cat-301-positive neurons were distributed over layers 3–6 with a thin lamina of cell-free zone in layer 4. In PIP, the laminar structure was not so apparent, but large pyramidal neurons in layers 3 and 5 were strongly Cat-301 positive. In MIP, Cat-301-positive neurons were more compactly arranged in the deep layer 3 than in PIP, and some large pyramidal and nonpyramidal Cat-301-positive neurons were scattered in layer 5. Accordingly, they showed as distinct a laminar structure as that seen in LIPv. Compared with LIPv, however, fewer Cat-301-positive neurons were localized in the upper part of layer 3 and in layer 6 of MIP. In VIP, the number of Cat-301-positive neurons was smaller than in LIP or MIP, but these neurons were compactly arranged and the lamination was distinct. In area 5ip, a smaller number of Cat-301-positive neurons was observed, and the neurons showed a less distinct laminar pattern compared with MIP. In area 5, no lamination was found, and only a few Cat-301-positive neurons were scattered in layer 3. In AIP, Cat-301-positive neurons were found only in the deeper half of layer 3, and no large neurons were Cat-301 positive in layers 5 and 6.

Electrophysiological recordings from LIP

Because our previous study showed that neuronal responses to 3-D visual objects were recorded in LIP but not in V3A (Shikata et al., 1996), we mapped the recording sites for 3-D visual objects (Figs.3A, 4A). To clarify whether the neurons responded to a 3-D visual object but not to the orientation of an object in the two-dimensional monitor screen, we examined the orientation tuning of the neuronal response to the object in the 3-D space. The orientation tuning was examined by varying the surface or axis orientation of stimulus around the sagittal, horizontal, or vertical axis (sagittal axis was parallel to the direction of gaze; horizontal and vertical axes were parallel to the frontal plane). When the neurons were more responsive to the stimuli of one orientation than of the other orientations in 3-D space, we considered the neurons to be 3-D orientation selective. The responses to the stimulus presented to either the left or right eye (Fig. 1,Left, Right) or to the 3-D stimulus viewed without polarizing glasses (Fig. 1, Merged) were much less than those to the same stimulus viewed binocularly with polarizing glasses. Thus, all these neurons showed a disparity effect, i.e., the response activity was stronger with polarizing glasses compared with the response to the same stimuli without the glasses. The response activities linearly increased as the size of the object increased to reach some limit, and then, when the size got bigger than the size limit, the response decreased, thus suggesting the existence of surround inhibitory “off” receptive field.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 3.

A, Nissl staining showing the gliosis along the recording needle tracks and the spot of gliosis at the recording sites (arrows). An arrow(bottom right) is in the lateral intraparietal area (LIP), and two arrows(middle) are in the medial intraparietal area (MIP). B, Injection site of WGA-HRP at one of the recording sites in LIP. C, Injection site of BDA in area V3A. D, Surface of the cortex showing back-diffusion of the injected Fast blue through the needle tract (yellow spot at the arrow) after the injection of the tracer into the anterior intraparietal area (AIP). E, Injection site of Nuclear Yellow into AIP. The dashed line shows the border between the gray and white matter.C–E are flipped horizontally, so they appear on the right side of the brain. 5ip,Intraparietal part of area 5 [part of PEa excluding the medial intraparietal area (MIP) and AIP];IP, intraparietal sulcus. Scale bars: A, C, E, 1 mm; B, D, 5 mm.

Our observations were biased toward visually responsive units, but we also observed neurons that were active during hand reaching. For example, in one of the monkeys in which 61 penetrations were made and 462 neurons were isolated, visual and visual-related neurons were 374 (81%), including 23 neurons that fired to both visual stimuli and reaching hand movements (5%). Six neurons were active during reaching hand movements but not during visual testing (1.3%). A total of 38 neurons (8.2%) responded to 3-D visual stimuli.

A total of 470 penetrations were made in six hemispheres, and the responses of 73 visual neurons to 3-D stimuli were studied. On the Nissl-stained sections, the recording needle tracks were clearly seen as lines of gliosis (Fig. 3A). Some of the recording sites were apparent as the large spots of gliosis along the needle track (Fig. 3A, arrows), probably because of long recording sessions (usually 2–3 hr for one recording site, sometimes >5 hr). Of 73 neurons examined with 3-D visual stimuli, 56 neurons showed statistically significant orientation preference. To be conservative, we chose 43 neurons with an orientation index of >0.4 to map the location of these neurons (Fig.4A). For the 43 neurons that were activated with 3-D visual stimuli, of which 22 of that number were in LIP, 17 in MIP, and 4 in V3A. Seventeen neurons showed a preferred response to disk or square plate, whereas 26 preferred bar or cylinder. There neurons were intermingled, and we did not recognize any spatial segregation of these units. We recorded only in the posterior part of the intraparietal sulcus, so we did not record any units in the anterior part of LIP.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 4.

Recording sites of the neurons that responded to three-dimensional visual stimuli and the distribution of labeling after injecting WGA-HRP into one of the recording sites in the posterior part of the lateral intraparietal area (LIP).A, The recording sites of neurons with an orientation index of >0.4 (filled circles).B, Drawings showing orthogradely labeled terminals (dots) and retrogradely labeled neurons (filled circles) after an injection of WGA-HRP (filled area on section 19) into the three-dimensional visual responsive region of the posterior part ofLIP. C, Drawing of the hemisphere showing the levels of the frontal sections in A and B. Thenumerals indicate the distance in millimeters from the rostral tip of the intraparietal sulcus. 5ip,Intraparietal part of area 5 [part of PEa excluding the medial intraparietal area (MIP) and AIP];IP, intraparietal sulcus; PO,parieto-occipital area; PIP, posterior intraparietal area; V3, visual area V3; VIP, ventral intraparietal area.

Injection of WGA-HRP into LIP recording site

After the recordings, we injected WGA-HRP into one of the most typical recording sites in the posterior part of LIP under the control of unit recording of neurons responding to the 3-D visual stimuli (Figs. 3B, 4B). The WGA-HRP injection covered most of the posterior part of LIP (Fig. 4B) and extended from layer 1 to layer 6. The injected WGA-HRP also diffused into the white matter below layer 6, but because the injection track was limited to cortex and because WGA-HRP is not taken up by intact axonal shafts (Mesulum, 1982), we considered the resulting labeling to be the result of the cortical part of the injection. Orthogradely labeled terminals and retrogradely labeled neurons were found abundantly in LIP, VIP, and V3A and slightly in MIP, PIP, PO, and AIP. In AIP, WGA-HRP-labeled terminals were found mainly in layer 3, and a small amount was found in layer 4 (Fig.5A). A few retrogradely labeled neurons were also found in layers 3 and 5. In V3A, many WGA-HRP-labeled neurons were found in layer 3, and only a few were found in layer 5 (Fig. 5B). WGA-HRP-labeled terminals were densely distributed in layer 1, and a few were distributed in layers 3, 5, and 6, but no labeling was seen in layer 4 of V3A. In PO, only a small number of WGA-HRP-labeled neurons were observed in layer 3, and a few were observed in layer 5 (Fig. 5C). In addition, a few WGA-HRP-labeled terminals were found in layers 1 and 3. Other than V3A and PO, we could not find any substantial projections from any of the other occipital visual areas after the LIP injection.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 5.

Dark-field photomicrographs showing orthogradely labeled terminals and retrogradely labeled neurons in the anterior intraparietal area (AIP) (A), the visual area V3A (B), and the parieto-occipital area (PO) (C) after the injection of WGA-HRP into a region of LIP in which three-dimensional responsive neurons were recorded.A, Labeled terminals in layers 3 and4 of AIP. A few labeled neurons were also seen in layer3. B, In V3A, many labeled neurons were seen in layer3, and only a few were observed in layer5. Dense terminal-like labeling was seen in layer1, and light terminal-like labeling was in layers3 and 6. C, Labeled neurons in layer3 and labeled terminals mainly in layer 1in the PO. Note that, as seen in A, the cortex in the coronal sections is thicker in AIP than in V3A or PO. Scale bar, 100 μm.

Double tracer injections into V3A and AIP

To confirm the connections from V3A to AIP relayed by LIP, we injected bidirectional tracers (WGA-HRP, BDA, and Rd) into V3A and retrograde tracers (FB and NY) into AIP. The injections into V3A and AIP were made without recording, but instead by reliance on MRI-derived stereotaxic coordinates, and were later confirmed as being in V3A and LIP by architectonics using myelin-stained sections and Cat-301-immunostained sections. Although we tried a total of 14 injections in V3A of 10 hemispheres, 2 were well localized within the V3A without any diffusion into the white matter or the other areas, and 5 were localized in the V3A with some diffusion into the white matter. For the latter cases, the injection needle track also got into the white matter except in one case. Although a total of 12 injections were made into AIP of nine hemispheres, 4 were well localized within AIP without any diffusion into area 7b or the white matter, but 5 covered AIP and area 7b with diffusion into the white matter. For the latter cases, the injection needle track was limited in the cortex in three cases and got into the white matter in two cases. We successfully injected both V3A and AIP in six hemispheres, and in five of them the injections were good enough to study the overlap of connections in LIP. Figure 3C–E shows a few of these injection sites.

Figure 6 shows the distribution of labeling in one of the multiple tracer experiments. In this case, BDA was injected into V3A (Figs. 3C, 6A), and FB (Figs. 3D, 6B) and NY (Fig.6C) were injected into AIP far beyond the border between LIP and AIP.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 6.

Distribution of labeling in the intraparietal areas demonstrating the overlap in the lateral intraparietal area (LIP) of anterograde labeling from the visual areaV3A and retrograde labeling from the anterior intraparietal area (AIP). Biotinylated dextran amine (BDA) was injected into V3A(A); Fast blue (FB) and Nuclear Yellow (NY) were injected into AIP(B, C). A, After the injection ofBDA into V3A, labeled axons with boutons or terminal arbors (lines) and neurons (filled circles) were mainly distributed inLIP. B, After the injection of FB into the posterior part of AIP, retrogradely labeled neurons (filled circles) were distributed inAIP, LIP, the intraparietal part of area 5 (5ip), the ventral intraparietal area (VIP), and the medial intraparietal area (MIP). C, After the injection ofNY into the anterior part of AIP, retrogradely labeled neurons (filled circles) were distributed in AIP, LIP, 5ip, VIP, and MIP. D, Drawing of the hemisphere showing the levels of the frontal sections in A–C. The conventions are the same as those in Figure 4.

The V3A injection of BDA was well confined to V3A in the lateral part of the annectant gyrus. The BDA injection was centered in the deep part of layer 3 and extended to the upper part of layer 3 and to layers 4 and 5. Because layers 3 and 5 are the main sources of corticocortical projection neurons in most cortical areas (Rockland and Pandya, 1979; Maunsell and Van Essen, 1983; Van Essen and Maunsell, 1983; Andersen et al., 1990a), the limited laminar extent of this injection probably did not result in any missed projections. In this case, orthogradely labeled terminals were mainly found in layer 4, and some were found in layer 3 of the LIP (Fig. 6A). Retrogradely labeled neurons were found in layer 5 with a few in layer 3. Although most of the BDA labeling was distributed in the posterior part of LIP, BDA-labeled terminals were scattered throughout the rostrocaudal extent of LIP. Other than the labeling in LIP, only a few BDA-labeled fibers were found in layers 3, 4, and 6 of VIP and MIP.

The AIP injection of FB was placed in the lateral and posterior part of AIP and extended from layers 4 to 6. Because layer 4 is the main target of forward corticocortical projection neurons in most cortical areas (Maunsell and Van Essen, 1983; Van Essen and Maunsell, 1983; Felleman and Van Essen, 1991), the limited laminar extent of this injection probably did not result in any missed projections from LIP to AIP. Although the injection spread into the white matter, the needle track did not, thus suggesting that no artifactual labeling resulted. It also included all the layers of the medial part of area 7b. Thus the results reflected the connections of AIP and area 7b. The connections of the intraparietal areas other than area 5ip observed in this case were similar to those found in the other cases in which the injection site was restricted in AIP. Retrogradely labeled neurons were found in AIP, LIP, 5ip, VIP, and MIP (Fig. 6B). Most of the labeling in area 5ip and part of the labeling in LIP were probably caused by the involvement of the injection site in area 7b. FB-labeled neurons were localized in layers 3 and 5 of AIP, VIP, and the anterior part of LIP. In LIP and MIP, FB-labeled neurons were localized mainly in layer 3. Because the injection of AIP included only layers 4–6, it is possible that this injection did not fully label afferents to AIP. The main difference in the labeling of this case in comparison with other cases with large injections was a paucity of the labeled neurons in the VIP and the posterior part of the LIP.

The AIP injection of NY was placed in the anterior part of AIP ∼2 mm anterior of the FB injection. The NY injection extended from layers 3 to 6 and also covered the medial part of area 7b. Because the injection needle track got into the white matter, the results included the connections of AIP, area 7b, and the white matter below. As in the FB injection, the connections of the intraparietal areas except area 5ip observed in this case were similar to those found in other cases in which the injection site was restricted in AIP. Retrogradely labeled neurons were distributed in AIP, LIP, 5ip, VIP, and MIP (Fig.6C). The NY-labeled neurons were localized mainly in layer 3 of these areas. In LIP, retrogradely labeled neurons were more densely distributed in the anterior part, but some were also found in the posterior part. The orthograde fibers and terminals from V3A and retrogradely labeled neurons projecting to AIP thus overlapped not only in the posterior part of LIP but also throughout the rostrocaudal extent of LIP.

Figure 7 shows the data of another animal in which WGA-HRP was injected into V3A and NY was injected into AIP. The injection of WGA-HRP was localized in the lateral part of V3A covering layers 1–6 and the white matter above. The axons in the white matter were damaged by the injection needle, so uptake of WGA-HRP might have occurred and affected the results. However, because no projections to LIP from areas more posterior than V3A have yet been reported, we considered the resulting labeling in LIP to be the result of the cortical part of the injection. Terminal-like labeling and labeled neurons were distributed in LIP, VIP, and MIP (Fig. 7A). In LIP, WGA-HRP-labeled terminals were found in layers 3 and 4, and labeled neurons were found in layers 3 and 5. WGA-HRP labeling was distributed in a columnar manner in LIP. In VIP, terminal-like labeling was found in layers 1 and 3, and labeled neurons were found in layer 5. In MIP, only minimal terminal-like labeling was found in layer 1, and a few labeled neurons were found in layer 3.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 7.

Distribution of labeling in the intraparietal areas after injection of WGA-HRP (W-H) into the visual area V3A and Nuclear Yellow (NY) into the anterior intraparietal area (AIP). A, After the injection of WGA-HRP into the visual area V3A, labeled terminals (dots) and neurons (filled circles) were distributed in the lateral intraparietal area (LIP) and the ventral intraparietal area (VIP). B, After the injection ofNY into the posterior part of AIP, retrogradely labeled neurons (filled circles) were distributed in AIP, LIP, the intraparietal part of area 5 (5ip), VIP, and the medial intraparietal area (MIP).C, Drawing of the hemisphere showing the levels of the frontal sections in A and B. The conventions are the same as those in Figure 4.

The injection of NY was well confined to AIP and covered layers 1–6 but also diffused across the intraparietal sulcus into layers 1 to upper 3 in area 5ip. Although the injection spread into the white matter, the needle track did not, thus suggesting that no artifactual labeling resulted. The involvement of the anterior part of area 5ip may have labeled many neurons in the posterior part of area 5ip in this case. Retrogradely labeled neurons were distributed in layers 3 and 5 of AIP, LIP, 5ip, VIP, and MIP (Fig. 7B). The terminals projecting from V3A and retrogradely labeled neurons projecting to AIP thus overlapped not only in the posterior part of LIP but also throughout the rostrocaudal extent of LIP in this case, too.

Figure 8 shows an overlap of the Rd-labeled fibers projecting from V3A and the distribution of NY- and FB-labeled neurons projecting to the ipsilateral and contralateral AIPs. Rd was injected into the lateral part of V3A covering layers 4–6 and diffused into the white matter. Because layers 3 and 5 are the main sources of corticocortical projection neurons in most cortical areas (Rockland and Pandya, 1979; Maunsell and Van Essen, 1983; Van Essen and Maunsell, 1983; Andersen et al., 1990a), the limited laminar extent of this injection probably resulted in a labeling of projections from layer 5 but not from layer 3. The injection needle passed through the white matter above layer 6 and thus damaged the axons. As a result, the uptake of Rd by passing fibers probably occurred and affected the results. Rd-labeled fibers were found in layers 3–6 of LIP, and Rd-labeled neurons were found mainly in layer 3 (Fig.8A).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 8.

Distribution of labeling in the intraparietal areas demonstrating the overlap in the lateral intraparietal area (LIP) of anterograde labeling from the visual areaV3A and retrograde labeling ipsilaterally and contralaterally from the anterior intraparietal area (AIP). Tetramethylrhodamine dextran (Rd) was injected into V3A (A), Nuclear Yellow (NY) was injected into the ipsilateralAIP (B), and Fast blue (FB) was injected into the contralateralAIP (E). A, After the injection of Rd into V3A, labeled fibers with terminals (lines) and neurons (filled circles) were distributed in the entire rostrocaudal LIP. B, After the injection ofNY into AIP, retrogradely labeled neurons (filled circles) were distributed inAIP, LIP, the intraparietal part of area 5 (5ip), the ventral intraparietal area (VIP), and the medial intraparietal area (MIP). C, After the injection ofFB into the left AIP contralateral to theRd injection, retrogradely labeled neurons (filled circles) were distributed in the rightAIP, LIP, and MIP. D, Drawing of the hemisphere showing the levels of the frontal sections inA–C. E, Injection of FBinto AIP contralateral to the Rdinjection. Retrogradely labeled neurons (filled circles) were distributed in AIP,LIP, 5ip, VIP, andMIP. F, Drawing of the hemisphere showing the levels of the frontal sections in E. E and F are flipped horizontally. The conventions are the same as those in Figure4.

NY was injected into AIP covering layers 1–6. The injection also covered layers 1–2 of area 5ip across the intraparietal sulcus and diffused into layer 3 of area 5ip. Diffusion was also seen in areas 7b and 5. Although the injection spread into the white matter, the needle track did not, thus suggesting that no artifactual labeling resulted. The connections in the intraparietal areas, except area 5ip, observed in this case were similar to those found in the other cases in which the injection site was confined to AIP. Most of the labeling in area 5ip was probably caused by the involvement of the injection site in areas 5ip and 7b. Retrogradely labeled neurons were distributed in layers 3 and 5 of AIP, LIP, 5ip, VIP, and MIP (Fig.8B). In LIP, NY-labeled neurons were localized in the rostrocaudal extent of the area. In the posterior part of LIP, the neurons and fibers labeled with Rd were found in the vicinity of AIP-projecting neurons labeled with NY (Fig.9A–C). Some of the NY-labeled AIP-projecting neurons were also found to project to V3A (Fig. 9D–F).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 9.

Photomicrographs of the posterior part of LIP after injecting a bidirectional fluorescent tracer Rd into the visual area V3A and retrograde fluorescent tracers NY and FB into AIP ipsilateral and contralateral, respectively, to the Rd injection. A, A fiber and a neuron in layer 3 of LIP labeled with Rd injected into V3A. B, LIP neurons retrogradely labeled with NY injected into AIP in the same field as that of A. C, A merged photograph of Aand B. D, Rd-labeled neurons in layer 3.E, NY-labeled neurons. F, A merged photograph of D and E showing double-labeled neurons (arrows). G, Rd-labeled neurons and fibers with varicosities and terminal-like swellings (arrowheads) in LIP ipsilateral to Rd injection. H, A neuron in LIP labeled with FB injected into the contralateral AIP in the same field as that of G. I, A merged photograph of G and Hshowing a double-labeled LIP neuron (arrow) that projected to ipsilateral V3A and contralateral AIP, in the vicinity of labeled fibers and terminals (arrowheads). Scale bars, 20 μm.

The injection of FB was confined to AIP contralateral to the V3A injection and covered layers 3–6. Because layer 4 is the main target of forward corticocortical projection neurons in most cortical areas (Maunsell and Van Essen, 1983; Van Essen and Maunsell, 1983; Felleman and Van Essen, 1991), the limited laminar extent of this injection probably did not result in any missed projections from LIP to contralateral AIP. The tracer labeled the connections similar to those found in other cases in which the injection site involved all layers of the cortex. Although the injection spread to the white matter that was adjacent to layer 6, the needle track did not, thus suggesting that no artifactual labeling resulted. The injection thus probably labeled all cortical connections. FB-labeled commissural neurons were distributed mainly in layers 3 and 5 of LIP and in layer 3 of MIP (Fig.8C). The FB-labeled neurons in the posterior part of LIP that projected to the contralateral AIP were also found in the vicinity of the fibers and terminal arbors labeled with Rd (Fig.9G–I). Some of the FB-labeled neurons projecting to the contralateral AIP were also labeled with Rd and thus projected to V3A (Fig. 9G–I). On the side of the intraparietal areas contralateral to the Rd injection into V3A, FB-labeled association neurons were found in layers 3 and 5 of LIP, VIP, 5ip, and MIP (Fig. 8E). Very few FB-labeled commissural neurons were found in the anterior part of the medial bank of the intraparietal sulcus.

Myeloarchitecture and Cat-301 immunostaining of the intraparietal areas

We confirmed the previous reports on the characteristics of myelin staining for areas LIP, VIP, PIP, V3A, and 5 (Seltzer and Pandya, 1980;Blatt et al., 1990; Preuss and Goldman-Rakic, 1991) and added some observation on the myelin structure of area AIP. We also confirmed that Cat-301 antibody is helpful in delineating the borders of the areas relating to the M pathway, such as V3, MIP, VIP, and LIP (Hockfield et al., 1983; McGuire et al., 1989; DeYoe et al., 1990), and further showed that it is useful in distinguishing AIP from the other parietal areas.

The connections of V3A, LIP, and AIP

The present results confirmed the reciprocal connections between V3A and LIP. V3A occupies the rostral part of the dorsomedial area ofBeck and Kaas (1999) and receives major inputs from V2 (Gattas et al., 1997; Beck and Kaas, 1999) and V3 (Felleman et al., 1997; Beck and Kaas, 1999) as well as sparse inputs from V1 (Zeki, 1978, 1980; Van Essen et al., 1986; Beck and Kaas, 1999). V3A also receives inputs from the middle temporal area (Ungerleider and Desimone, 1986; Beck and Kaas, 1999), the middle superior temporal area (Boussaoud et al., 1990;Beck and Kaas, 1999), the fundus of the superior temporal visual area (Boussaoud et al., 1990; Beck and Kaas, 1999), and PO (Colby et al., 1988; Beck and Kaas, 1999). V3A has substantial reciprocal connections with LIP (Cavada and Goldman-Rakic, 1989; Andersen et al., 1990a; Blatt et al., 1990; Morel and Bullier, 1990; Baizer et al., 1991); V3A thus provides main visual inputs to LIP. Accordingly, the neurons in V3A show visual activity that is modulated by gaze (Galletti and Battaglini, 1989; Nakamura and Colby, 2000), and a similar modulation is also observed in the neurons of LIP (Andersen and Mountcastle, 1983;Andersen et al., 1985b, 1990b).

LIP was originally defined on the basis of its connections from the frontal eye field (Andersen et al., 1985a). LIP has been reported to be reciprocally connected with PO, V3, V3A, V4, middle superior temporal area, dorsal prelunate area, area 7a, medial dorsal parietal area, VIP, area 5, area 8a, and area 46 (Andersen et al., 1990a; Blatt et al., 1990). In addition to the reciprocal connections, LIP has been reported to receive projections from areas 7b, 7m, and 24a, and also sends axons to the middle temporal area, the posterior inferior temporal area TEO, and the inferior temporal area TE. LIP thus receives and integrates visual information of both space and objects from areas in the dorsal and ventral streams and somatosensory information from the somatosensory areas and sends the integrated information to the visuomotor and motor-related areas.

This is the first study to inject tracers into AIP. Previous knowledge about connections of AIP were limited to the report that neurons in AIP do not project to the frontal eye field but project to ventral premotor area 6 (Petrides and Pandya, 1984; Matelli et al., 1986; Johnson et al., 1996; Gallese et al., 1997).

LIP relay of connections from V3A to AIP

In contrast to our expectations, the results showed that the projections from V3A terminated throughout the rostrocaudal extent of LIP, and the neurons projecting to AIP were not confined to the posterior part of LIP but were distributed throughout LIP. As a result, visual information from V3A is sent to the entire LIP, and then from the entire LIP to AIP. This finding correlates with the report by Tian and Lynch (1996) in which a single small injection of a retrograde tracer into the frontal eye field labeled neurons throughout the rostrocaudal extent of LIP. In accordance with this finding, the present mapping of the recording sites also revealed that the 3-D-responsive neurons were not restricted to the posterior part of the LIP. Moreover, because we only recorded the neuronal responses to the stimuli of 20–60° in size, we did not record any neurons responding to 3-D visual stimuli of objects smaller than 20° or larger than 60°. The recordings may thus cover only a limited area of the 3-D visually responsive region of the LIP. If so, the modules of the 3-D visual information processing for the hand manipulation may be distributed in a wider region of the LIP and thus may overlap the distribution of other modules such as those related to the saccadic eye movements. Interestingly, the distribution of neurons responding to the 3-D visual stimuli was similar to the location of neurons related to the saccadic eye movements (Gnadt and Andersen, 1988; Barash et al., 1991a) and attention (Robinson et al., 1995), as well as to the distribution of neurons projecting to the intermediate layers of the superior colliculus (Lynch et al., 1985) and to the frontal eye fields (Tian and Lynch, 1996).

Functional consideration of LIP

Neurons in LIP are involved in visuomotor integration functions (Andersen, 1987, 1995; Andersen and Zipser, 1988; Andersen et al., 1992), and are related not only to saccade and saccade preparation (Mountcastle et al., 1975; Lynch et al., 1977; Robinson et al., 1978;Gnadt and Andersen, 1988; Andersen 1989; Barash et al., 1991a,b;Andersen et al., 1992; Duhamel et al., 1992; Goldberg and Colby, 1992;Lynch, 1992) but also to visual response (Yin and Mountcastle, 1977;Robinson et al., 1978; Bushnell et al., 1981; Andersen et al., 1987;Colby et al., 1996). Some neurons in LIP respond to the saccades in 3-D space (Gnadt and Mays, 1995) and thus are related to 3-D space vision. LIP is also related to several aspects of spatial information processing such as attention (Lynch et al., 1977; Robinson et al., 1978, 1995; Bushnell et al., 1981; Mountcastle et al., 1981, 1987;Lynch and McLaren, 1989; Goldberg et al., 1990; Colby et al., 1996;Gottlieb et al., 1998; Corbetta et al., 2000), intention (Andersen, 1995; Bracewell et al., 1996; Mazzoni et al., 1996; Snyder et al., 1997), and anticipation (Colby et al., 1993b; Eskandar and Assad, 1999) and thus is suggested to be related in a more general role in visuospatial behavior (Colby et al., 1996).

In the present study, we focused on 3-D visual responses and their relation to hand manipulation; however, it should also be pointed out that neurons in LIP are also active during reaching hand movements.Mountcastle and colleagues (1975) reported that 24.7% of 474 neurons in area 7 (mostly in LIP) were reach and hand manipulation neurons. They classified 51 of 474 (10.8%) neurons as hand manipulation and 61 of 474 (12.9%) neurons as reach. They also classified, in area 5, 53 of 977 (3.4%) neurons as hand manipulation and 52 of 977 (5.3%) neurons as reach (Mountcastle et al., 1975). This is in accordance with our results that neurons in the MIP were also responsive to 3-D visual stimuli. As was the case in our study, in area 7, much overlap has been reported in the sites at which visual, visuomotor, fixation, and reach/hand manipulation neurons were isolated (Hyvarinen and Poranen, 1974; Mountcastle et al., 1975; Hyvarinen and Shelepin, 1979; Leinonen et al., 1979).

The LIP neurons are related not only to visual information processing but also to motion signal generation (Hyvarinen and Poranen, 1974; Mountcasle et al., 1975; Hyvarinen and Shelepin, 1979). Approximately one-fourth of all the neurons recorded in area 7, mostly in LIP, reported by Mountcastle et al. (1975) and one-third of neurons in area 7 reported by Hyvarinen and Poranen (1974) were manipulation neurons associated with reaching and hand movement. Because the hand motor neurons of the parietal cortex were neither sensory nor motor in nature, Mountcastle et al. (1975) suggested the “command hypothesis,” i.e., the parietal neurons have command functions for manual exploration of surrounding space. The posterior parietal cortex not only reacts to sensory stimuli but also participates in the guidance of voluntary motor acts and integrates sensory and intentional factors (Hyvarinen, 1982). As in LIP, the responses of neurons in AIP are visual (Sakata et al., 1995), visual memory related (Murata et al., 1996), and motor (Gallese et al., 1994) in the grasp-related behavior. LIP and AIP therefore may relate visual and hand motor integration and motor command functions through the connections between these areas.