Ординатура / Офтальмология / Английские материалы / Eye, Retina, and Visual System of the Mouse_Chalupa, Williams_2008

the amplitude of SLREMs induced by the electrical stimulation was well fitted by the linear regression line (for temporal SLREMs: slope = 43.1, r = 0.86; for nasal SLREMs: slope = 30.3, r = 0.83), which was similar to that of the spontaneous SLREMs (see figure 19.2B), although values of the electrically induced SLREMs are lower than those of the spontaneously evoked SLREMs.

All these results indicate that SLREMs in mice share common properties with those previously described in cats and monkeys, a finding suggesting the existence of similar saccade generator circuits downstream of the mouse SC.

Local circuits of the rodent superior colliculus

Cellular Properties A 1974 study by Langer and Lund used Golgi staining to reveal the cytoarchitecture of local circuits in the SC superficial layers. This study showed that the local circuit in the superficial layers involves five neuron types: narrow-field vertical cells, wide-field vertical cells, piriform or stellate cells, horizontal cells, and marginal cells (figure 19.4). Among these, wide-field vertical cells are the major projection neurons connected to the deeper layers of the SC (Mooney et al., 1988b) and to the lateral posterior nucleus of the pulvinar (Lane et al., 1993). Their dendritic arbors extend widely into a horizontally wide area of the superficial layer. In addition, narrow-field vertical and marginal cells are also projection neurons targeted to the SC deeper layers and the parabigeminal nucleus. Subsequent immunohistochemical studies suggested that horizontal, stellate, and piriform cells are GABA-ergic, based on the morphology of soma and proximal dendrites (Mize, 1992). Compared with neurons in the superficial layers, the morphological properties of neurons in the intermediate layer are more heterogeneous and less

distinct. They are classified into multipolar, pyramidal, fusiform, horizontal, and wide-field vertical cells (Norita, 1980; Ma et al., 1990).

The electrophysiological properties of SC neurons are very heterogeneous. Based on the firing pattern in response to the depolarizing current step, they are classified into regular spiking, late spiking, burst spiking, fast spiking, and rapid adaptation types (Saito and Isa, 1999). The regular spiking type constitutes the majority of the neurons both in the superficial (50%; Isa and Saito, 2001) and intermediate layer (50%; Saito and Isa, 1999). Surprisingly, no clear correlation has so far been observed between the electrophysiological properties and somatodendritic morphology of neurons, except the expression of large and fast hyper- polarization-activated current (Ih) in wide-field vertical cells. These neurons exclusively express Ih of large amplitude and fast activation time course (Lo et al., 1998; Saito and Isa, 1999).

Among a variety of SC neurons, the morphological and electrophysiological properties of deeper layer neurons projecting to the contralateral paramedian pontine reticular formation (PPRF) were studied by whole-cell patch-clamp recordings from cells that had been retrogradely labeled with dextran-conjugated Texas red injected into the PPRF a few days before the experiments (Sooksawate, Saito, et al., 2005). Among the 112 identified projection neurons in the SGI, regular, burst, late, and fast spiking and rapid inactivation types accounted for 73%, 12%, 11%, 0%, and 4%, respectively. Among the 76 projection neurons that were successfully stained with biocytin, multipolar, fusiform, pyramidal, horizontal, and round-shaped cells accounted for 66%, 13%, 8%, 11%, and 3%, respectively. Thus, multipolar and regular spiking neurons made up the largest population of SGI neurons projecting to the contralateral PPRF.

Figure 19.4 Morphological properties of neurons in the mammalian SC. Five major subclasses of neurons in the superficial layer (marginal cell, narrow-field vertical cells, piriform/stellate cells, horizontal cells, and wide-field vertical cells) and five major subclasses of neurons in the intermediate layer (multipolar cells, hori-

zontal cells, pyramidal cells, fusiform cells, and wide-field vertical cells) are illustrated. Somata and dendrites are shown in black and axons in gray. SGI, stratum griseum intermediale; SGS, stratum griseum superficiale; SO, stratum opticum. (Modified from Isa and Saito, 2001.)

GABAergic Circuits Recently, we studied the electrophysiological and morphological properties of GABAergic neurons in GAD67 (Gad1)-GFP knock-in mice (Endo et al., 2003, 2005; Sooksawate, Isa, et al., 2005), in which GABAergic neurons are labeled with fluorescence of green fluorescent protein (GFP) (Tamamaki et al., 2003).

The superficial layers contain a very high density of GABAergic neurons. Mize (1992) noted that about 45% of neurons in the SGS are GABAergic. In the GAD67 (Gad1)- GFP knock-in mice, all the recorded GABAergic neurons revealed the morphology of horizontal cells, which issued relatively long dendritic trees (Endo et al., 2003). They exhibited the firing pattern of regular spiking (18 of 65, 28%), burst spiking (14 of 65, 22%), or fast spiking (18 of 65, 28%) patterns. They are likely to be involved in lateral inhibition of visual responses in the SGS. So far we have not been able to stain the cells with piriform or stellate cell-type morphology, which were proposed to be GABAergic in the immunohistochemical study by Mize (1992), among the neurons with GFP fluorescence in the GAD67 (Gad1)-GFP knock-in mice. The reason may be that neurons of this subclass exhibit weak GFP expression in this mouse genotype.

We analyzed the electrophysiological and/or morphological properties of 231 GABAergic neurons in the SGI of the GAD67 (Gad1)-GFP mice (Sooksawate, Isa, et al., 2005). In the electrophysiological analysis, a majority of these cells exhibited either fast spiking (135 of 231, 58%) or burst spiking (67 of 231, 29%) properties. Based on the axonal trajectories, the GABAergic cells that were successfully stained by intracellular labeling (n = 115) were classified as

(1) intralaminar local interneurons (19 cells), (2) intralaminar horizontal neurons (19 cells), (3) interlaminar interneurons (38 cells), (4) commissural neurons (5 cells), or (5) extrinsic projection neurons (34 cells). The axodendritic morphology of individual subclasses of neurons suggests they may provide recurrent inhibition or lateral inhibition. Detailed information on the input-output relationship of each cell type is required to determine the function of individual subtypes of GABAergic neurons.

Interlaminar Connections As described earlier in the chapter, the visual topography of the superficial layer and the distribution of SLREM vectors in the deeper layers showed good coincidence. This is a similar to what is seen in cats and monkeys, but in these animal species, the connection between the superficial and deeper layers had long been a subject of debate (Edwards, 1980; Mays and Sparks, 1980; Mooney et al., 1988a; Moschovakis et al., 1988; Behan and Appell, 1992).

In the late 1990s, the existence of the interlaminar connection was investigated in detail by using electrophysiological techniques in combination with intracellular staining

techniques in slice preparations of the rodent SC (Lee et al., 1997; Isa et al., 1998). In this section, we summarize the observations obtained in slice studies.

Figure 19.5A summarizes the design of our experiments in slices obtained from rats ages 17–22 days, roughly 1 week after eye opening. Stimulating electrodes were placed in the SGS and in the optic tract (OT) near the lateral border of the optic layer (SO), where the OT comprises a bundle of fibers. Electrical pulses delivered through the OT electrode induced EPSPs with long and fluctuating onset latencies, presumably of dior oligosynaptic origin, in SGI neurons following stimulation of the OT (figure 19.5B and C). These responses were markedly enhanced by application of GABAA receptor antagonists such as bicuculline (see figure 19.5C) or SR95531; the bursting spike discharges evoked in the SGI cells were superimposed on large clusters of EPSPs, which could last longer than 1 s, even when single brief electrical pulses were delivered through the OT electrode. The longlasting depolarization and bursting spike discharges were evoked in an all-or-none fashion at threshold stimulus intensities (see figure 19.5C). Stimulation of the SGS induced monosynaptic EPSPs in SGI neurons, which were again amplified into bursting spike discharges superimposed on the longlasting depolarization following application of bicuculline (figure 19.5D). These results confirm the existence of an excitatory pathway from the OT to SGI neurons, presumably mediated by SGS or SO neurons, as previously demonstrated by Lee et al. (1997) in SC slices from the tree shrew. These observations in slice preparations were confirmed in in vivo experiments. The bursting responses evoked in SGI neurons by OT stimulation were observed following blockade of GABAA receptor-mediated synaptic transmission in anesthetized rats (Katsuta and Isa, 2003) and more recently in anesthetized monkeys (N. Nikitin, R. Kato, and T. Isa, unpublished observations).

Thus, the existence of the interlaminar connection has been demonstrated, and signal transmission through the pathway is enhanced by disinhibition from GABAA receptormediated inhibition. It has been argued that the interlaminar connection is likely used for “express saccades,” which are executed with extremely short reaction times (see Isa, 2002; Isa et al., 2003; Helms et al., 2004; Isa and Sparks, 2006) in monkeys. The function of this interlaminar connection remains elusive in rodents.

The results of the present experiments were confirmed in mice.

Mechanism of Burst Generation Saccade-related activities in the deeper layers of the SC have been intensively investigated in nonhuman primates. Under conditions in which the subject can anticipate the forthcoming target location, a gradual increase in activity is observed in a subpopulation of deeper layer neurons (“prelude” or

Figure 19.5 Interlaminar connection in the rodent SC. A, Experimental setup of in vitro slice experiments. Stimulating electrodes are placed in the bundle of the optic tract, at the most lateral portion of the optic layer (SO), or in the superficial gray layer (SGS). Whole-cell recordings were made from neurons in the intermediate gray layer (SGI). B, The morphology of the recorded neuron, stained with biocytin. Dendrites are drawn as thick lines. The axon and its collaterals are drawn as thin lines. C1 shows the synaptic responses of this neuron to the OT stimulation (50 μA) in

“buildup” activity), and the sudden change from low-level activity to high-frequency bursts is thought to trigger the initiation of saccades. The timing of the high-frequency burst of collicular neurons is tightly coupled to saccade onset, leading the movement by 18–20 ms (Sparks et al., 2000). Accordingly, clarifying the cellular or circuit mechanism of burst generation in the SC deeper layer neurons should facilitate understanding of the neural mechanism underlying the saccade initiation. In vitro slice preparations of the SC that have been separated from other brain structures can be a powerful tool in this regard. The bursting responses of neurons may arise via several alternative mechanisms related to either the intrinsic membrane properties of individual neurons or the structure of the circuit.

As a first step in exploring this issue, it was essential to know the intrinsic membrane properties of individual identified tectofugal cells. We found that most of the crossed tectoreticular SGI cells identified by retrograde labeling with a fluorescent tracer exhibited regular spiking properties and a quasi-linear f-I relationship (Sooksawate, Saito, et al., 2005). In contrast, as shown in figure 19.5C, the bursts emitted by SGI neurons in response to stimula-

the control solution. C2 and C3 show the effects of application of 10 μM Bic (C2, control; C3, after application of Bic). C4 shows the synaptic responses to the critical stimulus strength (17 μA) for induction of longlasting responses with the application of 10 μM Bic. D, Effect of SGS stimulation on another SGI neuron. Shown is the effect of 10 μM Bic on the EPSP (compare D1 and D2) and of the additional application of 50 μM APV (D3) or 50 μM APV plus 10 μM CNQX (D4). (Modified from Isa et al., 1998.)

tion of the SGS in the presence of bicuculline were suppressed by application of 50 μM APV, an NMDA-type glutamate receptor antagonist, and thus the bursts depended on activation of NMDA-type glutamate receptors. It is well known that NMDA receptors have a J-shaped current-voltage relationship. Due to Mg2+ block, the NMDA-type glutamate receptors admit inward currents only when the cell is sufficiently depolarized. Once the membrane potential exceeds the value necessary for activation of the NMDA receptor, a regenerative process ensues and further enhances their depolarization. This nonlinear activation of NMDA receptors can account for the all-or-none character of the bursts emitted by SGI neurons.

Presaccadic neurons of the primate SC are known to have recurrent collaterals that ramify in the neighborhood of the parent somata (Moschovakis et al., 1988). To further investigate whether a local circuit including these neurons may support their longlasting depolarization and bursting activity, we obtained simultaneous dual whole-cell patch-clamp recordings from pairs of adjacent SGI neurons in rats (Saito and Isa, 2003, 2004, 2005). Figure 19.6A illustrates an

Figure 19.6 Simultaneous recordings from a pair of SGI neurons in an SC slice. A, Photomicrograph of a pair of recorded SGI neurons (injected with biocytin intracellularly). B, Spontaneous membrane potentials in control solution (1) and in the presence of 10 μM bicuculline (Bic) and low (0.1 mM) Mg2+ (2). C, Sponta-

example obtained from a pair of neurons horizontally separated by less than 100 μm. When we applied 10 μM bicuculline (or SR95531) to block GABAA receptors and reduced the extracellular concentration of Mg2+ from 1.0 mM to 0.1 mM to enhance the NMDA receptor-mediated responses, the SGI neurons exhibited bursting spike activity superimposed on repetitive, spontaneous, depolarizing potentials (figure 19.6B). Interestingly, the spontaneous depolarization and the bursting spike activity occurred almost simultaneously in both neurons. Since the spiking discharges of two adjacent presaccadic burst neurons are synchronous, synchronization of SC neuron discharges could underlie the generation of their presaccadic bursts in vivo. Such synchronous depolarization could be observed when spikes were blocked by intracellular application of QX-314 (figure 19.6C), which suggested common input to the neuron pair. We further found that activation of NMDA-type glutamate receptors is essential for such synchronous depolarization to occur, since it was completely abolished by APV (figure 19.6D). These results suggest that a recurrent excitatory network would generate synchronous bursting responses in the SC deeper layer neurons via NMDA receptordependent synaptic transmission. The experiments were conducted in rats; however, later studies confirmed the results in mice.

Action of Neuromodulators It has been reported that the SC receives innervation from several neuromodulator

neous membrane potentials recorded simultaneously from a pair of SGI neurons (cell-1 and cell-2) in the presence of 10 μM Bic and low (0.1 mM) Mg2+. The intracellular solution contained 5 mM QX-314. D, Effect of 50 μM APV on the synchronous depolarization.

systems, such as cholinergic, serotonergic, and purinergic systems (Binns, 1999). These neuromodulator systems may modulate the signal processing in the SC local circuits in a context-dependent manner. Among these, the actions of cholinergic inputs are becoming clear.

The superficial layer of the SC (sSC) receives cholinergic innervation from the parabigeminal nucleus (Graybiel, 1978a; Sherk, 1979; Hall et al., 1989). The cholinergic action on the sSC was examined by testing the current responses induced by acetylcholine (ACh) in the GAD67 (Gad1)-GFP knock-in mouse (Endo et al., 2005). Brief air pressure application of 1 mM ACh elicited nicotinic inward current responses in both GABAergic and non-GABAergic neurons. The inward current responses in the GABAergic neurons were highly sensitive to a selective antagonist for α3β2 (Gabra3/Gabrb2 subunits)- and α6β2 (Gabra6/ Gabrb2 subunits)-containing receptors, α-conotoxin MII (αCtxMII). A subset of these neurons exhibited a faster α- bungarotoxin-sensitive inward current component, indicating the expression of α7 (Chrna7)-containing nicotinic ACh receptors (nAChRs). We also found that activation of presynaptic nAChRs induced release of GABA, which elicited a burst of miniature inhibitory postsynaptic currents mediated by GABAA receptors in non-GABAergic neurons. This ACh-induced GABA release was mediated mainly by αCtxMII-sensitive nAChRs and resulted from the activation of voltage-dependent calcium channels. Morphological analysis of the recorded neurons revealed that

recorded GFP-positive neurons were interneurons and GFPnegative neurons include projection neurons. These findings suggest that nAChRs are involved in the regulation of GABAergic inhibition, especially enhancement of contrast of visual responses by facilitation of lateral inhibition in the sSC.

On the other hand, the SGI receives cholinergic innervation mainly from the pedunculopontine and laterodorsal tegmental nuclei of the midbrain (PPTN and LDTN; Graybiel, 1978b; Illing and Graybiel, 1985; Beninato and Spencer, 1986; Hall et al., 1989). The effect of bath application of carbachol, an agonist of both nicotinic and muscarinic ACh receptors, has been analyzed in 246 randomly sampled SGI neurons (Sooksawate and Isa, 2006). It has been clarified that carbachol application induces postsynaptic responses with various combination of nicotinic inward, muscarinic inward, and muscarinic outward currents. Pharmacological analysis clarified that nicotinic inward currents are mainly mediated by the α4β2 (Gabra4/Gabrb2 subunits) subtype of nicotinic receptors, and muscarinic inward currents are mainly mediated by M3 (Chrm3)-type and muscarinic outward currents by M2 (Chrm2)-type muscarinic receptors. Among these, projection neurons of the SGI, which were identified by their axonal trajectories, mainly exhibit combination of nicotinic inward + muscarinic inward + muscarinic outward currents (15 of 28), or nicotinic inward + muscarinic inward currents (9 of 28). Thus, the major action of the cholinergic inputs on SGI output neurons would be excitatory.

On the other hand, cholinergic inputs suppress GABAergic synaptic transmission in the SGI with presynaptic mechanisms, mediated by M1 (Chrm1)- and M3 (Chrm3)-type muscarinic receptors (Li et al., 2004). Both of these results suggest that cholinergic inputs to the SGI mainly facilitate the generation of output command from the SGI. Thus, cholinergic inputs to the sSC and SGI originate from different sources and exhibit different roles in modulation of sensorimotor processing in the SC.

Conclusion

High-speed video-oculography system has made it possible to analyze SLREMs in head-fixed mice. We found that mice exhibit spontaneous SLREMs with kinematic properties similar to those seen in cats, monkeys, and humans. In addition, electrical stimulation of the deeper layer of the SC induces SLREMs. Investigation of SLREMs in genetically engineered mice should allow us to study the molecular mechanisms underlying the operation of saccade generator circuits.

For this purpose, an in-depth understanding of the structure of the saccade control systems is required. Considerable knowledge is emerging on the structure and the way signals

are processed in local circuits of the SC, including their modulation by the neuromodulator system. Further understanding of the saccade control system as a whole, including the downstream circuits of the SC located in the brainstem reticular formation and upstream structures in the cerebral cortex and basal ganglia, will facilitate a multiscale analysis of saccade control, from molecular control to neural circuits to the final behavioral motor output.

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Visual cortical functions are impaired in many disorders that affect children. For example, in autism spectrum disorders, individuals have a predisposition to see local stimulus features but have difficulty grouping these features to recognize global shapes (Dakin and Frith, 2005). In Williams syndrome, individuals suffer from a visuospatial construction deficit that makes it difficult for them to put together even the simplest of puzzles (Meyer-Lindenberg et al., 2004). Both of these disorders are linked to a variety of gene mutations (Polleux and Lauder, 2004; Eckert et al., 2006; Grice and Buxbaum, 2006) and are associated with structural abnormalities that reduce the connectivity of the parietal cortex, which belongs to the dorsal cortical processing stream (Van Essen et al., 2006; Just et al., 2007). To develop an understanding of the mechanisms that underlie these structural malformations, there is growing interest in using a mouse model in which the expression of specific genes can be manipulated and the effects on the visual cortical network can be studied (Eckert et al., 2006). To interpret these effects, it is important to understand the connectivity in wild-type mice and to ask whether the mouse is a good model of the primate visual cortex.

Investigation of the connectivity of mouse visual cortex was pioneered in the 1970s by Vernon Caviness, Ursula Dräger, and Alan Pearlman, and to this day, our understanding of mouse visual cortical connectivity is dominated by their contributions. After the publication of their classic papers, however, research focused more on the cortical anatomy of rats and squirrels than on mice, with a corresponding lag in understanding the connectivity of mouse visual cortex. This chapter considers primarily studies in mice, to collect the research and establish a clear picture of what is known today.

Area map of mouse visual cortex

Cytoarchitectonic Fields It is widely accepted that the brain operations responsible for perception and cognitive processing of visual information occur in an interconnected network of functionally specialized cortical areas. Knowing where these areas are, how they are connected and what they do is therefore key to understanding how the network

processes visual information. In the tradition of Brodmann’s work on the structural diversity of cortical areas (Garey, 2006), early studies of mouse cerebral cortex identified areas based on variations in cell size, cell density, and lamination (Isenschmid, 1911; De Vries, 1912; Rose, 1912; Fortuyn, 1914). Using such cytoarchitectonic features, Rose (1929) distinguished an astonishingly large number of 55 fields, and speculated that they may be linked by 1,458 possible connections. In Rose’s 1929 map, occipital cortex was dominated by a large elliptical region, which, owing to its thick granular layer, became known as striate area. Many years later this area was shown to undergo structural changes after enucleation (Valverde, 1968). This discovery was followed by labeling the structure by transneuronal transport of 3H-proline from the eye (Dräger, 1974), which identified the area as primary visual cortex (i.e., striate cortex, V1).

The first hint that visual cortex extends beyond the striate area came from a lesion study that revealed degenerating connections to the surrounding medial and lateral extrastriate cortex (Valverde and Estéban, 1968). A short time later, studies showed that these cortical regions contain visuotopic maps and demonstrated that receptive fields (RFs) in mouse striate and extrastriate cortex are tuned to moving oriented light bars (Dräger, 1975; Métin et al., 1988). The topographic organization of Dräger’s (1975) area 18a adjacent to lateral V1 resembled area 18 in cat and area V2 in hedgehog and gray squirrel (Hubel and Wiesel, 1965; Kaas et al., 1970; Hall et al., 1971). Area 18 on the medial side of mouse V1 (Dräger, 1975) resembled the prostriate area (Sanides and Hoffman, 1969) or the visually responsive splenial cortex in cats (Kalia and Whitteridge, 1973). Based on these similarities, Caviness (1975) reinterpreted Rose’s (1929) map and proposed that, similar to what is seen in rat (Krieg, 1946), mouse V1 is adjoined laterally by area 18a and medially by area 18b (figure 20.1A). In more recent maps, however, considerable uncertainty persists about the shape, size, and boundaries of areas 18a and 18b (Caviness, 1975; Caviness and Frost, 1980; Frost and Caviness, 1980) and how these areas relate to the cytoarchitectonically defined visual areas V2L, V2ML, V2MM, and TeA depicted in the most widely used atlas, by Paxinos and Franklin (2001) (figure 20.1B).

Figure 20.1 Different maps of mouse visual cortex. A, Flat map of cytoarchitectonic areas (black outlines) in the left hemisphere of mouse cerebral cortex published by Caviness and Frost (1980). Red shaded regions represent schematic outlines of visuotopically organized areas, identified by Wang and Burkhalter (2006). Note that areas 18a and 18b contain multiple visuotopic areas. B, Flat map of cytoarchitectonic areas of the left mouse cerebral cortex constructed by David C. Van Essen by unfolding coronal sections taken from the atlas of Paxinos and Franklin (2001). Red outlines represent schematic borders of visuotopically defined areas identified by Wang and Burkhalter (2007). Note that areas V2L and

Topographically Defined Areas Studies in monkey have shown that only a minority of areas can be identified based on cytoarchitectonic features. In fact, even within the original borders of Brodmann’s area 18, visuotopic mapping has revealed as many as four complete topographic representations of the visual field that are considered discrete visual areas (Van Essen, 1979; Van Essen and Maunsell, 1981). To study whether cytoarchitectonic areas correspond to visuotopically defined areas, Wagor et al. (1980) recorded RFs in mouse visual cortex and showed that area 18a contains two complete visuotopic maps, V2 and V3, that

V2ML contain multiple visuotopically organized areas. C, Visuotopic organization of the left mouse visual cortex derived by mapping of receptive fields, published by Wagor et al. (1980). In this map, cytoarchitectonic area 18a contains areas V2 and V3 and cytoarchitectonic area 18b contains the rostral and caudal medial visual areas, Vm-r and Vm-c. D, Area map of left mouse visual cortex derived by topographic mapping of V1 connections and receptive field mapping (Wang and Burkhalter, 2007). Blue shading represents the distribution of callosal connections in superficial layers. See color plate 7.

flank V1 on the lateral side (figure 20.1C). On the medial side of V1, Wagor et al. (1980) found a rostral and a caudal visual area, Vm-r and Vm-c, both of which were contained within area 18b (see figure 20.1C). Using intrinsic optical signal imaging, Kalatsky and Stryker (2003) confirmed that mouse V1 is adjoined laterally by a single area, V2. In addition, they showed that V2 is adjoined laterally by area V3, and that V3 shares its lateral border with area V4. On the medial side of V1, they found only a single area, V5. Although Kalatsky and Stryker (2003) suggest that their optical map resembles the map of Wagor et al. (1980), it is

important to note this has not been demonstrated directly. Using a similar optical imaging approach, Schuett et al. (2002) found a completely different map, in which mouse V1 is adjoined on the lateral side not by a single area V2 but by the lateromedial area, LM, and the anterolateral area, AL. These conflicting maps prompted Wang and Burkhalter (2007) to revisit the issue by using a combination of axonal tracing of V1 connections, RF mapping, and referencing projection and recording sites to fixed callosal landmarks and myeloarchitectonic boundaries. The tracing experiments revealed a total of 15 V1 projection fields, nine of which contained a complete representation of the visual field and can therefore be considered distinct areas (Felleman and Van Essen, 1991). Based on similar studies in rat visual cortex (Olavarria and Montero, 1984; Montero, 1993) we have designated these fields posterior (P), lateromedial (LM), anterolateral (AL), rostrolateral (RL), anterior (A), posteromedial (PM), anteromedial (AM), laterointermediate (LI), and postrhinal (POR) areas (figure 20.2A, B, and D). Each of these areas has a characteristic location relative to the fixed pattern of callosal connections (figure 20.2C), but the callosally connected regions do not necessarily correspond to areal boundaries (see figure 20.2B and C). Some of these locations (e.g., LM, AL, LI, P, AM) correspond to projection sites that were previously identified by tracing connections of mouse V1 with 3H-proline (Olavarria and Montero, 1989). On the medial side of V1, Wang and Burkhalter’s (2007) connection map strongly resembles the RF map of Wagor et al. (1980) (see figure 20.1C and D). However, unlike Wagor et al. (1980), who found a single area V2 on the lateral side of V1, Wang and Burkhalter (2007) found that lateral V1 is adjoined by five distinct areas, P, LM, AL, RL, and A. One explanation for these differences is that Wang and Burkhalter’s (2007) map was constructed by highresolution mapping in single animals and the referencing of projection and recording sites against fixed callosal and myeloarchitectonic landmarks. In contrast, Wagor et al. (1980) faced the challenging task of constructing maps by pooling recording sites across animals with a dearth of fixed anatomical references. As a result, Wagor et al. (1980) overlooked the reversal of the elevation map at the LM/AL border and the polarity change of the azimuthal map at the vertical meridian representation that coincides with the AL/ RL border (see figure10A and B of Wang and Burkhalter, 2007). In addition, Wagor et al. (1980) mistook the horizontal meridians of LM and RL as a single split horizontal meridian representation at the V2/V3 border. Thus, the string of areas on the lateral side of V1 resembles more closely the complex organization found in rat (Montero, 1993) than the simple organization found in many rodent and nonrodent species, in which lateral V1 is adjoined by a single area V2 (Rosa and Krubitzer, 1999). However, unlike in rat (Thomas and Espinoza, 1987; Montero, 1993), Wang and Burkhalter

Figure 20.2 Topographic maps of V1 connections in mouse visual cortex. Representation of azimuth in extrastriate visual cortex is shown in horizontal sections of left occipital cortex. The maps were generated by making three simultaneous injections of fluororuby (FR, red), fluoroemerald (FE, green), and biotinylated dextran amine (BDA, yellow) into V1, followed by tripleanterograde tracing of intracortical connections. A, Darkfield image showing heavy myelination in primary visual cortex (V1) and the barrel field of primary somatosensory cortex (S1). Arrowheads indicate myeloarchitectonic borders. Arrows indicate injection sites in V1. B, Fluorescently labeled axonal connections after injections of FR, FE, and BDA at different nasotemporal locations (azimuth) of the upper visual field representation in V1. Dashed lines indicate areal borders, which were determined by mapping inputs from the perimeter of V1. Solid lines indicate myeloarchitectonic borders. C, Overlay of BDA-labeled axonal projections shown in B and bisbenzimide-labeled callosally projecting neurons (blue). D, Higher magnification image of axonal labeling shown in area A (inset in B). A, anterior; L, lateral; M, medial; P, posterior. Scale bar = 1 mm (A–C), 0.1 mm (B, inset), and 0.3 mm (D). See color plate 8.

(2007) found that in mouse, the vertical meridian is represented only at the V1/LM border, suggesting that LM is homologous to V2 (Van Essen, 1979). Thus, in respect to V1 and LM, mouse visual cortex is simple. But, unlike the simple extrastriate cortex of primates, in which V1 is