Visuotopic organisation and neuronal response selectivity for direction of motion in visual areas of the caudal temporal lobe of the marmoset monkey (Callithrix jacchus): Middle temporal area,middle temporal crescent,and surrounding cortex

On the basis of extracellular recordings in marmoset monkeys, we report on the organisation of the middle temporal area (MT) and the surrounding middle temporal crescent (MTc). Area MT is approximately 5-mm long and 2-mm wide, whereas the MTc forms a crescent-shaped band of cortex 1-mm wide. Neurones in area MT form a first-order representation of the contralateral hemifield, whereas those in the MTc form a second-order representation with a field discontinuity near the horizontal meridian. The representation of the vertical meridian forms the border between area MT and the MTc. In both areas, the fovea is represented ventrocaudally, and the visual field periphery is represented dorsorostrally. Analysis of single units revealed that 86% of cells in area MT show a strong selectivity for the direction of motion of visual stimuli. The proportion of direction-selective cells in the MTc (53%), whereas lower than that in area MT, is much higher than that observed in most other visual areas. Neurones in the cortex immediately rostral to area MT and the MTc are direction selective, with receptive fields predominantly located in the visual field periphery. In contrast, only a minority of the cells in the cortex ventral to the MTc are direction selective, and their receptive fields emphasise central vision. The results suggest that the MTc is functionally closely related to area MT, and distinct from the areas forming the dorsolateral complex. The MTc may have a role in combining information about motion in the visual field, processed by area MT, with information about stimulus shape, processed by the dorsolateral complex. J. Comp. Neurol. 393:505–527, 1998. © 1998 Wiley-Liss, Inc.     

Chemoarchitecture of the middle temporal visual area in the marmoset monkey (Callithrix jacchus): laminar distribution of calcium-binding proteins (calbindin, parvalbumin) and nonphosphorylated neurofilament

We studied the distributions of interneurons containing the calcium-binding proteins parvalbumin and calbindin D-28k, as well as that of pyramidal neurons containing nonphosphorylated neurofilament (NNF), in the middle temporal visual area (MT) of marmoset monkeys. The distributions of these classes of cells in MT are distinct from those found in adjacent areas. Similar to the primary visual area (V1), in MT, calbindin-immunopositive neurons can be objectively classified into "dark" and "light" subtypes based on optical density of stained cell bodies. Calbindin-positive dark neurons are particularly concentrated in layers 2 and 3, whereas light neurons have a more widespread distribution. In addition, a subcategory of calbindin-positive dark neuron, characterized by a "halo" of stained processes surrounding the cell body, is found within and around layer 4 of MT and V1. These cells are rare in most other visual areas. In comparison, parvalbumin-immunopositive cells in area MT have a relatively homogeneous distribution, although with a trend toward higher spatial density in lower layer 3, and are relatively uniform in terms of density of staining. Finally, MT shows a characteristic trilaminar distribution of NNF-immunopositive pyramidal cells, with stained cell bodies evident in layers 3, 5, and 6. Although the laminar distribution of cells stained for the three markers overlap to some extent, these subcategories can be readily distinguished in terms of morphology, including cell body size. Chemoarchitectural parallels observed between MT and V1 suggest comparable physiological requirements and neuronal circuitry.     

The second visual area in the marmoset monkey: Visuotopic organisation,magnification factors,architectonical boundaries,and modularity

The organisation of the second visual area (V2) in marmoset monkeys was studied by means of extracellular recordings of responses to visual stimulation and examination of myelin- and cytochrome oxidase-stained sections. Area V2 forms a continuous cortical belt of variable width (1–2 mm adjacent to the foveal representation of V1, and 3–3.5 mm near the midline and on the tentorial surface) bordering V1 on the lateral, dorsal, medial, and tentorial surfaces of the occipital lobe. The total surface area of V2 is approximately 100 mm², or about 50% of the surface area of V1 in the same individuals. In each hemisphere, the receptive fields of V2 neurones cover the entire contralateral visual hemifield, forming an ordered visuotopic representation. As in other simians, the dorsal and ventral halves of V2 represent the lower and upper contralateral quadrants, respectively, with little invasion of the ipsilateral hemifield. The representation of the vertical meridian forms the caudal border of V2, with V1, whereas a field discontinuity approximately coincident with the horizontal meridian forms the rostral border of V2, with other visually responsive areas. The bridge of cortex connecting dorsal and ventral V2 contains neurones with receptive fields centred within 1° of the centre of the fovea. The visuotopy, size, shape and location of V2 show little variation among individuals. Analysis of cortical magnification factor (CMF) revealed that the V2 map of the visual field is highly anisotropic: for any given eccentricity, the CMF is approximately twice as large in the dimension parallel to the V1/V2 border as it is perpendicular to this border. Moreover, comparison of V2 and V1 in the same individuals demonstrated that the representation of the central visual field is emphasised in V2, relative to V1. Approximately half of the surface area of V2 is dedicated to the representation of the central 5° of the visual field. Calculations based on the CMF, receptive field scatter, and receptive field size revealed that the point-image size measured parallel to the V1/V2 border (2–3 mm) equals the width of a full cycle of cytochrome oxidase stripes in V2, suggesting a close correspondence between physiological and anatomical estimates of the dimensions of modular components in this area. J. Comp. Neurol. 387:547–567, 1997. © 1997 Wiley-Liss, Inc.     

Resolving the organization of the New World monkey third visual complex: the dorsal extrastriate cortex of the marmoset (Callithrix jacchus)

We tested current hypotheses on the functional organization of the third visual complex, a particularly controversial region of the primate extrastriate cortex. In anatomical experiments, injections of retrograde tracers were placed in the dorsal cortex immediately rostral to the second visual area (V2) of New World monkeys (Callithrix jacchus), revealing the topography of interconnections between the "third tier" cortex and the primary visual area (V1). The data indicate the presence of a dorsomedial area (DM), which represents the entire upper and lower quadrants of the visual field, and which receives strong, topographically organized projections from the superficial layers of V1. The visuotopic organization and boundaries of DM were confirmed by electrophysiological recordings in the same animals and by architectural characteristics which were distinct from those found in ventral extrastriate cortex rostral to V2. There was no electrophysiological or histological evidence for a transitional area between V2 and DM. In particular, the central representation of the upper quadrant in DM was directly adjacent to the representation of the horizontal meridian that marks the rostral border of V2. The present results argue in favor of the hypothesis that the third visual complex in New World monkeys contains different areas in its dorsal and ventral components: area DM, near the dorsal midline, and a homolog of area 19 of other mammals, located more lateral and ventrally. The characteristics of DM suggest that it may correspond to visual area 6 (V6) of Old World monkeys.     

Organization of visual cortex in the northern quoll, Dasyurus hallucatus: evidence for a homologue of the second visual area in marsupials

Two visual areas, V1 and V2 (first and second visual areas), appear to be present in the posterior neocortex of all eutherian mammals investigated so far. However, previous studies have not established whether an area homologous to V2 also exists in metatherian mammals (marsupials). Using electrophysiological techniques, we mapped the visual receptive fields of neurons in the striate and peristriate cortices of the northern quoll, an Australian marsupial. We found that neurons in a 2-mm-wide strip of cortex rostrolateral to V1 form a single, relatively simple representation of the complete contralateral hemifield. This area resembles V2 of eutherians in several respects: (i) neurons in the medial half of the peristriate area represent the lower visual quadrant, whereas those in the lateral half represent the upper visual quadrant; (ii) the vertical meridian of the visual field is represented adjacent to V1, while the visual field periphery is represented along the lateral and rostrolateral borders of the peristriate area; (iii) there is a marked anisotropy in the representation, with a larger magnification factor parallel to the V1 border than perpendicular to this border; and (iv) receptive fields of multiunit clusters in the peristriate cortex are much larger than those of cells in V1 at comparable eccentricities. The cortex immediately rostral and lateral to V2 did not respond to visual stimulation under our recording conditions. These results suggest that V1 and V2 together form a 'core' of homologous visual areas, likely to exist in all therian mammals.     

Cortical connections of the dorsomedial visual area in new world owl monkeys (Aotus trivirgatus) and squirrel monkeys (Saimiri sciureus)

The dorsomedial visual area (DM) is an extrastriate area that was originally described in owl monkeys as a complete representation of the visual hemifield in a heavily myelinated wedge of cortex just rostral to dorsomedial visual area V2. More recently, connections of DM in owl monkeys have been described (Krubitzer and Kaas [1993] J. Comp. Neurol 334:497–528). As part of an effort to determine whether DM exists in other primates, we compared the architecture, connections, and visual topography of DM in owl monkeys and the presumptive DM in squirrel monkeys. In both species of New World monkeys, the DM region was more heavily myelinated than adjacent cortex, and this region was connected with the first and second visual areas, the middle temporal area (MT), the medial area, the ventral posterior parietal area, the dorsointermediate area, the dorsolateral area, the ventral posterior and ventral anterior areas, the medial superior temporal area, the fundal area of the superior temporal sulcus, the inferior temporal cortex, and frontal cortex in or near the frontal eye field. In squirrel monkeys, both blob and interblob regions of V1 contributed equally to DM, whereas the blob regions provided most of the projections to V1 in owl monkeys. In squirrel monkeys, connections were also found with cortex on the ventral surface in the ventral occipital temporal sulcus. In owl monkeys and squirrel monkeys, connections were with both the upper and lower visual field representations in V1, V2, and MT, demonstrating that DM contains a complete representation of the visual field. These similarities in architecture, connections, and retinotopy argue that DM is a visual area of both owl and squirrel monkeys. J. Comp. Neurol. 400:18–34, 1998. © 1998 Wiley-Liss, Inc.     

Ventral posterior visual area of the macaque: visual topography and areal boundaries

Using both physiological and anatomical techniques, we have studied the topographic organization of extrastriate visual cortex on the ventral surface of the occipital lobe in macaque monkeys. Our results show that a topographically organized representation of the superior contralateral quadrant of the visual field lies immediately anterior to the ventral half of V2. This area is organized in a mirror symmetric fashion to ventral V2: it shares a horizontal meridian representation with V2 and a representation of the superior vertical meridian forms its anterior border. A well-defined strip of callosal inputs runs along the vertical meridian representation, thereby providing a reliable anatomical marker for areal boundaries in ventral extrastriate cortex. We refer to this area as the ventral posterior area (VP) because it is, in all these respects, notably similar to VP in the owl monkey. Ventral V2 has strong reciprocal connections with VP, and the topography of the V2 projection agrees closely with the topography revealed in our physiological mapping experiments. The visual field representation in VP is strikingly anisotropic, with linear magnification factor being much larger along contours of constant polar angle than along contours of constant eccentricity.     

Cortical projections of the dorsolateral visual area in owl monkeys: the prestriate relay to inferior temporal cortex

The dorsolateral visual area (DL) is one of a number of visual areas that have been defined by electrophysiological mapping procedures and cortical architecture in the extrastriate cortex of owl monkeys. The projections of DL were determined by the intra-axonal transport of 3H-proline, 3H-acetyl-wheat germ agglutinin, and horseradish peroxidase after cortical injections. The major ipsilateral projection of DL defined a new subdivision of the visual cortex in owl monkeys, the caudal inferior temporal cortex. Single injections in DL sometimes produced label in two separate regions in the caudal inferior temporal cortex, suggesting that functional subdivisions exist in this projection zone. Other targets of DL included the region of the frontal eye fields, the dorsomedial visual area, the dorsointermediate visual area (DI), a region of the cortex rostral to DI which we call the temporoparietal cortex, and possibly the ventral (V) and posterior parietal areas. A major feedback projection of DL was to V-II. Projections from DL to V-II and the dorsomedial visual area were roughly retinotopic. Projections from DL to the contralateral cerebral hemisphere were to DL and the inferior temporal cortex. Overall, the results support the concept that a major relay of visual information proceeds from V-I to V-II to DL and then to the inferior temporal cortex. In addition, similarities in connection patterns of DL in owl monkeys and V4 in macaque monkeys suggest that DL and much or all of V4 are homologous.     

Microstimulation and architectonics of frontoparietal cortex in common marmosets (Callithrix jacchus)

We investigated the organization of frontoparietal cortex in the common marmoset (Callithrix jacchus) by using intracortical microstimulation and an architectonic analysis. Primary motor cortex (M1) was identified as an area that evoked visible movements at low levels of electric current and had a full body representation of the contralateral musculature. Primary motor cortex represented the contralateral body from hindlimb to face in a mediolateral sequence, with individual movements such as jaw and wrist represented in multiple nearby locations. Primary motor cortex was coextensive with an agranular area of cortex marked by a distinct layer V of large pyramidal cells that gradually decreased in size toward the rostral portion of the area and was more homogenous in appearance than other New World primates. In addition to M1, stimulation also evoked movements from several other areas of frontoparietal cortex. Caudal to primary motor cortex, area 3a was identified as a thin strip of cortex where movements could be evoked at thresholds similar to those in M1. Rostral to primary motor cortex, supplementary motor cortex and premotor areas responded to higher stimulation currents and had smaller layer V pyramidal cells. Other areas evoking movements included primary somatosensory cortex (area 3b), two lateral somatosensory areas (areas PV and S2), and a caudal somatosensory area. Our results suggest that frontoparietal cortex in marmosets is organized in a similar fashion to that of other New World primates.     

Architectural subdivisions of medial and orbital frontal cortices in the marmoset monkey (Callithrix jacchus)

Although the common marmoset has become a model for the study of several neurological conditions that affect the frontal lobe, knowledge of the boundaries of the areas located in the orbital and medial frontal regions has remained incomplete. Here we examined histological sections stained for myelin, Nissl substance, and cytochrome oxidase, allowing identification of likely homologues of most of the architectural fields defined in Old World monkeys. Ventrally, we identified three granular fields at or near the frontal pole (area 10, and the medial and lateral subregions of area 11), and two granular fields along the lateral margin of the orbitofrontal cortex (medial and orbital subdivisions of area 12). More caudal and medially, dysgranular and agranular cortices included four subdivisions of area 13 as well as rostral and caudal subdivisions of area 14 (at the ventromedial convexity). The ventral frontotemporal transition encompassed at least two subdivisions of agranular insular cortex, as well as the likely homologues of the gustatory cortices. Most of the medial surface was encompassed by area 10 (which projected a caudomedial finger‐like extension toward the subgenual cortex), together with a relatively large dysgranular area 32 and an agranular area 25 (in subgenual cortex). Finally, the caudal limit of the medial frontal cortex included two fields of agranular cingulate cortex (areas 24a and 24b). These findings enhance our understanding of the architectural organization of the marmoset frontal cortex and highlight a highly conserved basic organization across simian primates, allowing the informed interpretation of experimental neurological studies. J. Comp. Neurol. 514:11–29, 2009. © 2009 Wiley‐Liss, Inc.     

Cortical connections of inferior temporal area TEO in macaque monkeys

In macaque monkeys, lesions involving the posterior portion of the inferior temporal cortex, cytoarchitectonic area TEO, produce a severe impairment in visual pattern discrimination. Recently, this area has been shown to contain a complete, though coarse, representation of the contralateral visual field (Boussaoud, Desimone, and Ungerleider: J. Comp. Neurol. 306:554–575, '91). Because the inputs and outputs of area TEO have not yet been fully described, we injected a variety of retrograde and anterograde tracers into 11 physiologically identified sites within TEO of seven rhesus monkeys and analyzed the areal and laminar distribution of its cortical connections. Our results show that TEO receives feedforward, topographically organized inputs from prestriate areas V2, V3, and V4. Additional sparser feedforward inputs arise from areas V3A, V4t, and MT. Each of these inputs is reciprocated by a feedback projection from TEO. TEO was also found to have reciprocal intermediate-type connections with the fundus of the superior temporal area (area FST), cortex in the most posteromedial portion of the superior temporal sulcus (the posterior parietal sulcal zone [area PP]), cortex in the intraparietal sulcus (including the lateral intraparietal area [area LIP]), the frontal eye field, and area TF on the parahippocampal gyrus. The connections with V3A, V4t, and PP were found only after injections in the peripheral field representations of TEO. Finally, TEO was found to project in a feedforward pattern to area TE and to areas anterior to FST on the lateral bank and floor of the superior temporal sulcus (areas TEm, TEa, and IPa, Seltzer and Pandya: Brain Res. 149:1–24, '78), all of which send feedback projections to TEO. Feedback projections also arise from parahippocampal area TH, and areas TG, 36, and possibly 35. These are complemented by only sparse feedforward projections to TG from central field representations in TEO and to TH from peripheral field representations. The results thus indicate that TEO forms an important link in the occipitotemporal pathway for object recognition, sending visual information forward from V1 and prestriate relays in V2–V4 to anterior inferior temporal area TE. © 1993 Wiley-Liss, Inc.

1 This article is a US Goveriiment work and, as such, is in the public domain in the United States of America.

     

Receptive fields of neurons in areas 3b and 1 of somatosensory cortex in monkeys

Receptive fields of neurons within the separate representations of the glabrous hand in areas 3b and 1 of somatosensory cortex were studied in cynomolgus monkeys. Many neurons in area 1 have center-surround receptive fields with separate 'on' and 'off' zones, while neurons in area 3b exhibit largely uniform or homogeneous receptive fields.     

Topographic organization of the middle temporal visual area in the macaque monkey: representational biases and the relationship to callosal connections and myeloarchitectonic boundaries

We have used physiological and anatomical techniques to address three general issues concerning the topographic organization of the middle temporal visual area (MT) of the macaque monkey. First, we carried out a quantitative analysis of irregularities and asymmetries in the visual representation in MT. This analysis revealed a striking overemphasis on a restricted portion of the visual field that runs obliquely through the inferior contralateral quadrant and largely avoids both the horizontal meridian and the inferior vertical meridian. This corresponds to the portion of the visual field that would be maximally stimulated during visually guided hand movements. Second, the physiologically determined topographic organization of MT was compared to the pattern of callosal inputs in the same hemisphere, which are known to be distributed irregularly within MT. Callosal inputs tended to be densest near the representation of the vertical meridian, but there were numerous exceptions to this trend. Thus, topographic irregularities account for only part of the irregularities in callosal inputs to MT. Finally, comparison of these data with previous reports shows a strong correlation between body weight and the average size of MT. The representation in myeloarchitectonically defined MT was found to include much of the visual periphery, although it is unclear from our data whether this representation is invariably complete.     

Patterned distribution of immunoreactive astroglial processes in the striate (V1) cortex of new world monkeys

The possibility of a modular organization of non-neuronal elements was analyzed in the opercular region of the striate neocortex in adult New World monkeys. For this purpose, and in order to follow possible correlations in the general organization of neuronal and astroglial elements, immunocytochemical procedures for Glial Fibrillary Acid Protein (GFAP) and Microtubule Associated Protein 2 (MAP-2), in addition to cytochrome oxidase (COX) histochemistry, were applied to tangential and coronal sections and analyzed by using computer-assisted procedures. Astroglial interlaminar processes stemming from superficial laminae did not traverse lamina IVA, and thus did not appear in deeper layers. Clearly definable interlaminar processes were predominantly concentrated in laminae II–III. A honeycomb- like lattice was observed in tangential sections, with a “cell” size distribution similar to the MAP-2-IR lattice, suggesting an intimate association with the pyramidal columns. Additionally, analysis of similar sections disclosed the periodic appearance of large patches with high density of interlaminar processes, indicating a nonhomogeneous distribution of GFAP-IR processes in the striate cortex. COX “blobs” appeared frequently to coincide with areas expressing high density of GFAP-IR elements. These findings add a new perspective to current concepts of astroglial organization in the striate cortex of primates and reveal the existence of a non-neuronal modular organization in the primate striate cerebral cortex, and suggest that possible correlations between relative distributions of neuronal and astroglial elements should be further analyzed in cortical areas with a clear modular organization such as the striate cortex. GLIA 25:85–92, 1999. © 1999 Wiley-Liss, Inc.     

Representation of the visual field in the primary visual area of the marmoset monkey: Magnification factors,point‐image size,and proportionality to retinal ganglion cell density

The primary visual area (V1) forms a systematic map of the visual field, in which adjacent cell clusters represent adjacent points of visual space. A precise quantification of this map is key to understanding the anatomical relationships between neurons located in different stations of the visual pathway, as well as the neural bases of visual performance in different regions of the visual field. We used computational methods to quantify the visual topography of V1 in the marmoset (Callithrix jacchus), a small diurnal monkey. The receptive fields of neurons throughout V1 were mapped in two anesthetized animals using electrophysiological recordings. Following histological reconstruction, precise 3D reconstructions of the V1 surface and recording sites were generated. We found that the areal magnification factor (M_A) decreases with eccentricity following a function that has the same slope as that observed in larger diurnal primates, including macaque, squirrel, and capuchin monkeys, and humans. However, there was no systematic relationship between M_A and polar angle. Despite individual variation in the shape of V1, the relationship between M_A and eccentricity was preserved across cases. Comparison between V1 and the retinal ganglion cell density demonstrated preferential magnification of central space in the cortex. The size of the cortical compartment activated by a punctiform stimulus decreased from the foveal representation towards the peripheral representation. Nonetheless, the relationship between the receptive field sizes of V1 cells and the density of ganglion cells suggested that each V1 cell receives information from a similar number of retinal neurons, throughout the visual field. J. Comp. Neurol. 521:1001–1019, 2013. © 2012 Wiley Periodicals, Inc.     

Gamma-frequency fluctuations of the membrane potential and response selectivity in visual cortical neurons

Fluctuations at frequencies of 25-70 Hz is an inherent property of cortical activity. These rapid, gamma-range fluctuations are apparent in the local field potentials, in spiking of cells and cell groups, and in the membrane potential of neurons. To investigate stimulus dependence of the gamma-frequency fluctuations of the membrane potential, we have recorded intracellularly responses of cells in cat visual cortex to presentation of moving gratings. We found gamma-range fluctuations of the membrane potential in both simple and complex cells. The strength of the gamma-frequency fluctuations correlated with the stimulus optimality. Furthermore, the amplitude of the gamma-frequency fluctuations correlated with the phase of stimulus-imposed slow changes of the membrane potential. The combination of these features makes cortical neurons capable of encoding the slow changes in the visual world in a kind of amplitude modulation of the high frequency fluctuations. This assures reliable transformation of the membrane potential changes into spike responses without compromising the temporal resolution of visual information encoding in the low frequency range.     

Cortical and thalamic connections of the parietal ventral somatosensory area in marmoset monkeys (Callithrix jacchus).

Microelectrode mapping methods were used to define the parietal ventral somatosensory area (PV) on the upper bank of the lateral sulcus in five marmosets (Callithrix jacchus). In the same animals, neuroanatomical tracers were placed into electrophysiologically identified sites in PV and/or the second somatosensory area (S2). Foci of anterograde and retrograde label were related to electrophysiological maps of cortical areas and cortical and thalamic architecture. The results lead to the following conclusions: (1) Multiunit recordings from cortex on the upper bank of the lateral sulcus demonstrate that PV is somatotopically organized, with the face representation adjoining area 3b and the hindlimb and tail representations away from this border in cortex deep on the upper bank of the lateral sulcus. The forelimb representation is caudal in PV adjacent to the S2 forelimb representation. The body surface representation in PV approximates a mirror image of that in S2; (2) Areas PV and S2 are less myelinated and have less cytochrome oxidase enzyme activity than area 3b; (3) The ventroposterior inferior nucleus (VPI) of the thalamus provides the major somatosensory projections to PV. PV is reciprocally connected with VPI and anterior pulvinar; (4) PV has ipsilateral cortical connections with areas 3a, 3b, 1, and M1 and higher order somatosensory fields, and at least most of these connections are somatotopically matched; and (5) Callosal connections of PV are with S2 and PV of the other cerebral hemisphere. These results further establish PV as one of at least four somatosensory areas of the lateral sulcus of primates.     

Frequency representation in auditory cortex of the common marmoset (Callithrix jacchus jacchus)

The location and characteristics of the primary auditory cortex of the common marmoset, Callithrix jacchus jacchus, were determined in five anesthetized male adult animals by mapping the responses of cortical units and unit clusters to pure tone stimuli presented to the contralateral ear. The primary auditory cortex lies largely ventral to the lateral sulcus, the only major fissure on the lateral cortex of this smooth-brained primate, but in some animals it may extend significantly down the ventral bank of this sulcus. Responses are distributed such that low best frequencies are found rostroventrally whereas high best frequencies occur caudally. The disposition of frequency-band contours is fan-shaped, with contours separating low-frequency octaves nearly parallel to the lateral sulcus and high-frequency (greater than 8 kHz) contours perpendicular to that sulcus. Best frequencies range from 0.6 to 30 kHz across the primary field, but there is a disproportionate representation of the three octaves between 2 and 16 kHz. The most sensitive thresholds (as low as -2 dB SPL) are found between 7 and 9 kHz. The primary auditory cortex is similar in cytoarchitecture to that reported for the cat, showing a blurring of lamination in the middle layers (II-IV) and a preponderance of small cells in these merged layers, giving a highly granular appearance. The accessibility of the cochlear representation on the gyral surface makes the marmoset an attractive animal for studies of primate auditory cortex.     

The projections from striate cortex (V1) to areas V2 and V3 in the macaque monkey: asymmetries, areal boundaries, and patchy connections

The organization of projections from V1 to areas V2 and V3 in the macaque monkey was studied with a combination of anatomical techniques, including lesions and tracer injections made in different portions of V1 and V2 in 20 experimental hemispheres. Our results indicate that dorsal V1 (representing the inferior contralateral visual quadrant) consistently projects in topographically organized fashion to V3 in the lunate and parietooccipital sulci as well as to the middle temporal area (MT) and dorsal V2. In contrast, ventral V1 (representing the superior contralateral quadrant) projects only to MT and ventral V2. A corresponding dorsoventral asymmetry in myeloarchitecture supports the idea that V3 is an area that is restricted to dorsal extrastriate cortex and lacks a complete representation of the visual field. The average surface area of myeloarchitectonically identified V3 was 89 mm2. Additional information was obtained concerning the laminar distribution of connections from V1 to V2 and V3, the patchiness of these projections, and the consistency of projections to other extrastriate areas, including V4 and V3A.     

Cytoarchitectonic subdivisions of the dorsolateral frontal cortex of the marmoset monkey (Callithrix jacchus), and their projections to dorsal visual areas

We describe the organization of the dorsolateral frontal areas in marmoset monkeys using a combination of architectural methods (Nissl, cytochrome oxidase, and myelin stains) and injections of fluorescent tracers in extrastriate areas (the second visual area [V2], the dorsomedial and dorsoanterior areas [DM, DA], the middle temporal area and middle temporal crescent [MT, MTc], and the posterior parietal cortex [area 7]). Cytoarchitectural field 8 comprises three subdivisions: 8Av, 8Ad, and 8B. The ventrolateral subdivision, 8Av, forms the principal source of frontal projections to the "dorsal stream," having connections with each of the injected visual areas. The cytoarchitectural characteristics of 8Av suggest that this subdivision corresponds to the marmoset's frontal eye field. The intermediate subdivision of area 8 (8Ad) has efferent projections to area 7, while the dorsomedial subdivision (8B) has few or no connections with extrastriate cortex. Area 46, located rostrolateral to area 8Av, has substantial connections with the medial extrastriate areas (DM, DA, and area 7) and with MT, while the cortex lateral to 8Av (area 12/45) projects primarily to MT and to the MTc. The rostromedial prefrontal (area 9) and frontopolar (area 10) regions have very few extrastriate projections. Finally, cells in dorsal area 6 (6d) have sparse projections to DM, MT, and the MTc, as well as strong projections to DA and to area 7. These results illuminate aspects of the evolutionary development of the primate frontal cortex, and serve as a basis for further research into cognitive functions using a marmoset model.