Retinotopic maps in the pulvinar of bush baby (Otolemur garnettii).

Despite its anatomical prominence, the function of the primate pulvinar is poorly understood. A few electrophysiological studies in simian primates have investigated the functional organization of pulvinar by examining visuotopic maps. Multiple visuotopic maps have been found for all studied simians, with differences in organization reported between New and Old World simians. Given that prosimians are considered closer to the common ancestors of New and Old World primates, we investigated the visuotopic organization of pulvinar in the prosimian bush baby (Otolemur garnettii). Single-electrode extracellular recording was used to find the retinotopic maps in the lateral (PL) and inferior (PI) pulvinar. Based on recordings across cases, a 3D model of the map was constructed. From sections stained for Nissl bodies, myelin, acetylcholinesterase, calbindin, or cytochrome oxidase, we identified three PI chemoarchitectonic subdivisions, lateral central (PIcl), medial central (PIcm), and medial (PIm) inferior pulvinar. Two major retinotopic maps were identified that cover PL and PIcl, the dorsal one in dorsal PL and the ventral one in PIcl and ventral PL. Both maps represent central vision at the posterior end of the border between the maps, the upper visual field in the lateral half and the lower visual field in the medial half. They share many features with the maps reported for the pulvinar of simians, including the location in pulvinar and the representation of the upper-lower and central-peripheral visual field axes. The second-order representation in the lateral map and a laminar organization are likely features specific to Old World simians.

The morphology of the koniocellular axon pathway in the macaque monkey.

The key objective of this study was to determine the distribution and morphology of koniocellular (K) lateral geniculate nucleus (LGN) axons in primary visual cortex (V1) of the macaque monkey. In particular, we were interested in understanding whether subpopulations of K axons exist in this species and, if so, if these subpopulations arise from different K layers of the LGN. Restricted injections of the tracers, biotinilated dextran amine, or Phaseolus vulgaris leucoagglutinin were targeted to specific LGN K layers under electrophysiological guidance and immunocytochemistry was used to visualize labeled axons in cortex that were subsequently reconstructed through serial sections. A total of 36 complete axons and 166 axon segments were reconstructed. Our results identified at least 2 main subpopulations of K axons in macaque V1 based on branching patterns and bouton distribution. Axons that arise primarily from LGN layers K1 and K2 are morphologically simple and tend to branch in cortical layers 1 and 3A. These axons give rise to fewer boutons than seen in axons arising from the dorsal K LGN layers K3-K6. Axons that arise from LGN layers K3-K6 terminate as complex, focused arbors in the cytochrome oxidase (CO) blobs in layer 3Balpha, with only occasional simple projections to the more superficial layers of cortex. Combined with previous observations, our data suggest that there are at least 3 subclasses of K LGN axons in macaque monkey that are similar to K axons identified earlier in both nocturnal simian owl monkeys (Ding and Casagrande 1997) and in prosimian, bush babies (Lachica and Casagrande 1992) suggesting that the LGN K channels that terminate in the CO blobs and in layer 1 are not unique to macaque monkeys but are a common primate feature.

Correlates of motor planning and postsaccadic fixation in the macaque monkey lateral geniculate nucleus.

There is significant controversy regarding the ability of the primate visual system to construct stable percepts from a never-ending stream of brief fixations and rapid saccadic eye movements. In this study, we examined the timing and occurrence of perisaccadic modulation of LGN single-unit activity in awake-behaving macaque monkeys while they made spontaneous saccades in the dark and made visually guided saccades to discrete stimuli located outside the receptive field. Our hypothesis was that the activity of LGN cells is modulated by efference copies of motor plans to produce saccadic eye movements and that this modulation depends neither on the presence of feedforward visual information nor on a corollary discharge of signals directing saccadic eye movements. On average, 25% of LGN cells demonstrated significant perisaccadic modulation. This modulation consisted of a moderate suppression of activity that began more than 100 ms prior to the initiation of a saccadic eye movement and continued beyond the termination of the saccadic eye movement. This suppression was followed by a large enhancement of activity after the eyes arrived at the next fixation. Although members of all three LGN relay cell classes (magnocellular, parvocellular, and koniocellular) demonstrated significant saccade-related suppression and enhancement of activity, more cells demonstrated postsaccadic enhancement (25%) than perisaccadic suppression (17%). In no case did the timing of the modulation coincide directly with saccade duration. The degree of modulation observed did not vary with LGN cell class, LGN receptive field center location, center sign (ON-center or OFF-center), or saccade latency or velocity. The time course of modulation did, however, vary with saccade size such that suppression was longer for longer saccades. The fact that activity from a percentage of LGN cells from all cell classes was modulated in relationship to saccadic eye movements in the absence of direct visual stimulation suggests that this modulation is a general phenomenon not tied to specific types of visual stimuli. Similarly, because the onset of the modulation preceded eye movements by more than 100 ms, it is likely that this modulation reflects higher order motor-planning rather than a corollary of mechanisms in direct control of eye movements themselves. Finally, the fact that the largest modulation is a postsaccadic enhancement of activity may suggest that perisaccadic modulations are designed more for the facilitation of visual information processing once the eyes land at a new location than for filtering unwanted visual stimuli.

The role of L1 in axon pathfinding and fasciculation.

The neural cell adhesion molecule L1 has been found to play important roles in axon growth and fasciculation. Our main objective was to determine the role of L1 during the development of connections between thalamus and cortex. We find that thalamocortical and corticothalamic axons in mice lacking L1 are hyperfasciculated, a subset of thalamocortical axons make pathfinding errors and thalamocortical axon growth cones are abnormally long in the subplate. These defects occur despite formation of six cortical layers and formation of topographically appropriate thalamocortical connections. The loss of L1 is accompanied by loss of expression of ankyrin-B, an intracellular L1 binding partner, suggesting that L1 is involved in the regulation of Ank2 stability. We postulate that the pathfinding errors, growth cone abnormalities and hyperfasciculation of axons following loss of L1 reflect both a shift in binding partners among axons and different substrates and a loss of appropriate interactions with the cytoskeleton.

A comparison of koniocellular, magnocellular and parvocellular receptive field properties in the lateral geniculate nucleus of the owl monkey (Aotus trivirgatus).

1. By analogy to previous work on lateral geniculate nucleus (LGN) magnocellular (M) and parvocellular (P) cells our goal was to construct a physiological profile of koniocellular (K) cells that might be linked to particular visual perceptual attributes. 2. Extracellular recordings were used to study LGN cells, or their axons, in silenced primary visual cortex (V1), in nine anaesthetized owl monkeys injected with a neuromuscular blocker. Receptive field centre-surround organization was examined using flashing spots. Spatial and temporal tuning and contrast responses were examined using drifting sine-wave gratings; counterphase sine-wave gratings were used to examine linearity of spatial summation. 3. Receptive fields of 133 LGN cells and 10 LGN afferent axons were analysed at eccentricities ranging from 2.8 to 31.3 deg. Thirty-four per cent of K cells and only 9 % of P and 6 % of M cells responded poorly to drifting gratings. K, P and M cells showed increases in centre size with eccentricity, but K cells showed more scatter. All cells, except one M cell, showed linearity in spatial summation. 4. At matched eccentricities, K cells exhibited lower spatial and intermediate temporal resolution compared with P and M cells. K contrast thresholds and gains were more similar to those of M than P cells. M cells showed lower spatial and higher temporal resolution and contrast gains than P cells. 5. K cells in different K LGN layers differed in spatial, temporal and contrast characteristics, with K3 cells having higher spatial resolution and lower temporal resolution than K1/K2 cells. 6. Taken together with previous results these findings suggest that the K cells consist of several classes, some of which could contribute to conventional aspects of spatial and temporal resolution.

Optic nerve sheath fenestration with a novel wavelength produced by the free electron laser (FEL).

BACKGROUND AND OBJECTIVE: To determine whether 6.45-microm free electron laser (FEL) energy can successfully perform optic nerve sheath fenestration and to compare the acute and chronic cellular responses with this surgery. STUDY DESIGN/MATERIALS AND METHODS: Optic nerve sheath fenestration was performed in rabbits by using either FEL energy (< 2.5 mJ, 10 Hz, 325-microm spot size) or a knife. The optic nerve integrity and glial response were evaluated histologically acutely or 1 month postoperatively. RESULTS: The FEL at low energy effectively cut the optic nerve sheaths with minimal reaction in the underlying nerve. With FEL or knife surgical techniques, a mild astrocytic hypertrophy only adjacent to the fenestration was observed acutely in the glial fibrillary acidic protein (GFAP) -immunoreacted sections. The chronic healing responses after either technique appeared similar with: (1) a thin fibrous scar at the fenestration site, (2) cells uniformly distributed (hematoxylin and eosin), and (3) up-regulation of GFAP and S100beta in astrocytes adjacent to the fenestration site. CONCLUSION: The FEL at low energy performs an optic nerve sheath fenestration in a small space with ease. Both FEL and knife incisions cause a similar rapid glial response near the fenestration site that remains 1 month later.

The connections of layer 4 subdivisions in the primary visual cortex (V1) of the owl monkey.

The primary visual cortex (V1) of primates receives signals from parallel lateral geniculate nucleus (LGN) channels. These signals are utilized by the laminar and compartmental [i.e. cytochrome oxidase (CO) blob and interblob] circuitry of V1 to synthesize new output pathways appropriate for the next steps of analysis. Within this framework, this study had two objectives: (i) to analyze the con- nections between primary input and output layers and compartments of V1; and (ii) to determine differences in connection patterns that might be related to species differences in physiological properties in an effort to link specific pathways to visual functions. In this study we examined the intrinsic interlaminar connections of V1 in the owl monkey, a nocturnal New World monkey, with a special emphasis on the projections from layer 4 to layer 3. Interlaminar connections were labeled via small iontophoretic or pressure injections of tracers [horseradish peroxidase, biocytin, biotinylated dextrine amine (BDA) or cholera toxin subunit B conjugated to colloidal gold particles]. Our most significant finding was that layer 4 (4C of Brodmann) can be divided into three tiers based upon projections to the superficial layers. Specifically, we find that 4alpha (4Calpha), 4beta (4Cbeta) and 4ctr send primary projections to layers 3C (4B), 3Bbeta (4A) and 3Balpha (3B), respectively. Examination of laminar structure with Nissl staining supports a tripartite organization of layer 4. The cortical output layer above layer 3Balpha (3B) (e.g. layer 3A) does not appear to receive any direct connections from layer 4 but receives heavy input from layers 3Balpha (3B) and 3C (4B). Some connectional differences also were observed between the subdivisions of layer 3 and the infragranular layers. No consistent differences in connections were observed that distinguished CO blobs from interblobs or that could be correlated with differences in visual lifestyle (nocturnal versus diurnal) when compared with connectional data in other primates. Re-examination of data from previous studies in squirrel and macaque monkeys suggests that the tripartite organization of layer 4 and the unique projection pattern of layer 4ctr are not restricted to owl monkeys, but are common to a number of primate species.

The distribution of NADPH diaphorase and nitric oxide synthetase (NOS) in relation to the functional compartments of areas V1 and V2 of primate visual cortex.

The primary visual cortex (V1) of primates receives visual signals from cells in the koniocellular (K), magnocellular (M) and parvocellular (P) layers of the lateral geniculate nucleus (LGN). The functional role of the K pathway is unknown, but one proposal is that it modulates visual activity locally via release of nitric oxide (NO). One goal of this study was to examine the distribution of nitric oxide synthetase (NOS), the enzyme that produces NO, using immunocytochemistry for brain NOS (bNOS) or histochemistry for nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase activity in the V1 target cells of the K pathway and within the LGN itself. A second goal was to examine bNOS and NADPH diaphorase activity within proposed functional compartments in the second visual area (V2). We examined the LGN, V1 and V2 in squirrel monkeys, owl monkeys and bushbabies. In V1 and V2, we found that dense neuropil staining for NADPH diaphorase mirrored the pattern of high metabolic activity shown with cytochrome oxidase (CO) staining but did not necessarily mirror the pattern of immunolabeling seen with antibodies against NOS. The smooth stellate cells stained for NADPH diaphorase or bNOS were sparse and did not colocalize with LGN recipient zones in V1 or with the CO compartments in V2. LGN cells projecting to V1, including K, M and P cells, were negative for bNOS and NADPH diaphorase. Therefore, high levels of NOS are not limited to the K pathway. Instead, dense NOS activity is present in interneurons and within the neuropil of V1 and V2 that exhibit high metabolic demand.

Differential contributions of magnocellular and parvocellular pathways to the contrast response of neurons in bush baby primary visual cortex (V1).

How neurons in the primary visual cortex (V1) of primates process parallel inputs from the magnocellular (M) and parvocellular (P) layers of the lateral geniculate nucleus (LGN) is not completely understood. To investigate whether signals from the two pathways are integrated in the cortex, we recorded contrast-response functions (CRFs) from 20 bush baby V1 neurons before, during, and after pharmacologically inactivating neural activity in either the contralateral LGN M or P layers. Inactivating the M layer reduced the responses of V1 neurons (n = 10) to all stimulus contrasts and significantly elevated (t = 8.15, P < 0.01) their average contrast threshold from 8.04 (+/- 4.1)% contrast to 22.46 (+/- 6.28)% contrast. M layer inactivation also significantly reduced (t = 4.06, P < 0.01) the average peak response amplitude. Inactivating the P layer did not elevate the average contrast threshold of V1 neurons (n = 10), but significantly reduced (t = 4.34, P < 0.01) their average peak response amplitude. These data demonstrate that input from the M pathway can account for the responses of V1 neurons to low stimulus contrasts and also contributes to responses to high stimulus contrasts. The P pathway appears to influence mainly the responses of V1 neurons to high stimulus contrasts. None of the cells in our sample, which included cells in all output layers of V1, appeared to receive input from only one pathway. These findings support the view that many V1 neurons integrate information about stimulus contrast carried by the LGN M and P pathways.

Does the visual system of the flying fox resemble that of primates? The distribution of calcium-binding proteins in the primary visual pathway of Pteropus poliocephalus.

It has been proposed that flying foxes and echolocating bats evolved independently from early mammalian ancestors in such a way that flying foxes form one of the suborders most closely related to primates. A major piece of evidence offered in support of a flying fox-primate link is the highly developed visual system of flying foxes, which is theorized to be primate-like in several different ways. Because the calcium-binding proteins parvalbumin (PV) and calbindin (CB) show distinct and consistent distributions in the primate visual system, the distribution of these same proteins was examined in the flying fox (Pteropus poliocephalus) visual system. Standard immunocytochemical techniques reveal that PV labeling within the lateral geniculate nucleus (LGN) of the flying fox is sparse, with clearly labeled cells located only within layer 1, adjacent to the optic tract. CB labeling in the LGN is profuse, with cells labeled in all layers throughout the nucleus. Double labeling reveals that all PV+ cells also contain CB, and that these cells are among the largest in the LGN. In primary visual cortex (V1) PV and CB label different classes of non-pyramidal neurons. PV+ cells are found in all cortical layers, although labeled cells are found only rarely in layer I. CB+ cells are found primarily in layers II and III. The density of PV+ neuropil correlates with the density of cytochrome oxidase staining; however, no CO+ or PV+ or CB+ patches or blobs are found in V1. These results show that the distribution of calcium-binding proteins in the flying fox LGN is unlike that found in primates, in which antibodies for PV and CB label specific separate populations of relay cells that exist in different layers. Indeed, the pattern of calcium-binding protein distribution in the flying fox LGN is different from that reported in any other terrestrial mammal. Within V1 no PV+ patches, CO blobs, or patchy distribution of CB+ neuropil that might reveal interblobs characteristic of primate V1 are found; however, PV and CB are found in separate populations of non-pyramidal neurons. The types of V1 cells labeled with antibodies to PV and CB in all mammals examined including the flying fox suggest that the similarities in the cellular distribution of these proteins in cortex reflect the fact that this feature is common to all mammals.

Relationships between cytochrome oxidase (CO) blobs in primate primary visual cortex (V1) and the distribution of neurons projecting to the middle temporal area (MT).

The cytochrome oxidase (CO) blobs and interblobs in layer 3B of primate visual cortex have different sets of corticocortical connections. Cortical layers below layer 3B also project corticocortically, but the relationship of efferent projections from the deeper layers to the overlying blob/interblob architecture is less clear. We studied the tangential organization of neurons projecting from primary visual cortex (V1) to the middle temporal visual area (MT) and their relationship to the CO blobs. MT-projecting neurons in two primate species, bush babies and owl monkeys, were retrogradely labeled, then charted in tangential sections, and compared to the positions of the overlying CO blobs. In both primate species, MT-projecting neurons in layer 3C were unevenly distributed in the tangential plane, with dense patches of labeled cells that were aligned with the CO blobs. A novel two-dimensional spatial correlation method was used to show the colocalization of MT-projecting cells with the overlying blobs. Chi-square analyses performed with the cortical surface equally divided into compartments of blob, interblob, and blob/interblob borders showed that blob columns tended to have about 1.5 times more MT-projecting cells (P < 0.0001) than interblob columns. Similar analyses were applied to published data on V1 cells projecting to area MT in macaque monkey (Shipp and Zeki [1989] Euro J Neurosci 1:310-332). Again, the results showed a significant correlation between the cell distribution and CO blobs. Taken together, these results suggest that layer 3C is not uniform but is made up of a mosaic of cells that project to area MT and cells that project to some other location. These findings also indicate that the mosaic organization of layer 3C is related in some unique way to the overlying CO architecture.

Endothelial nitric oxide synthetase (eNOS) in astrocytes: another source of nitric oxide in neocortex.

The distribution of the endothelial form of nitric oxide synthetase (eNOS) was examined in the visual cortex of three species of primate and in the rat using immunocytochemistry. Labeled cells were found in both the gray and white matter. These cells were stellate in appearance and labeled cell processes were seen contacting blood vessels or the pia, suggesting that, by morphological criteria, the cells were astrocytes. All eNOS positive cells were double labeled with an antibody against S100beta. Although all cells were double labeled in the white matter, in the gray matter, some S100beta positive cells did not contain detectable levels of eNOS. eNOS positive astrocytic processes appeared to form prominent and distinctive structures next to neurons, especially in cortical layer IIIC. We postulate that these eNOS-positive structures form astrocytic perisynaptic sheaths on neuronal somas in the cortex. If this is true, then nitric oxide can influence neuronal transmission directly at axosomatic synapses in the cortex. In addition, the presence of eNOS in astrocytes and in their processes that contact blood vessels suggests that the link between local cortical activity and changes in cerebral blood flow could be mediated by astrocytic release of nitric oxide.

Synaptic and neurochemical characterization of parallel pathways to the cytochrome oxidase blobs of primate visual cortex.

The primary visual cortex (V1) of primates is unique in that it is both the recipient of visual signals, arriving via parallel pathways (magnocellular [M], parvocellular [P], and koniocellular [K]) from the thalamus, and the source of several output streams to higher order visual areas. Within this scheme, output compartments of V1, such as the cytochrome oxidase (CO) rich blobs in cortical layer III, synthesize new output pathways appropriate for the next steps in visual analysis. Our chief aim in this study was to examine and compare the synaptic arrangements and neurochemistry of elements involving direct lateral geniculate nucleus (LGN) input from the K pathway with those involving indirect LGN input from the M and P pathways arriving from cortical layer IV. Geniculocortical K axons were labeled via iontophoretic injections of wheat germ agglutinin-horseradish peroxidase into the LGN and intracortical layer IV axons (indirect P and M pathways to the CO-blobs) were labeled by iontophoretic injections of Phaseolus vulgaris leucoagglutinin into layer IV. The neurochemical content of both pre- and postsynaptic profiles was identified by postembedding immunocytochemistry for gamma-amino butyric acid (GABA) and glutamate. Sizes of pre- and postsynaptic elements were quantified by using an image analysis system, BioQuant IV. Our chief finding is that K LGN axons and layer IV axons (indirect input from M and P pathways) exhibit different synaptic relationships to CO blob cells. Specifically, our results show that within the CO blobs: 1) all K cell axons contain glutamate, and the vast majority of layer IV axons contain glutamate with only 5% containing GABA; 2) K axons terminate mainly on dendritic spines of glutamatergic cells, while layer IV axons terminate mainly on dendritic shafts of glutamatergic cells; 3) K axons have larger boutons and contact larger postsynaptic dendrites, which suggests that they synapse closer to the cell body within the CO blobs than do layer IV axons. Taken together, these results suggest that each input pathway to the CO blobs uses a different strategy to contribute to the processing of visual information within these compartments.

Morphology of P and M retinal ganglion cells of the bush baby.

P/midget ganglion cells mediate red-green color opponency in anthropoids. It has been proposed that these cells evolved as a specialization to subserve color vision in primates. If that is correct, they must have evolved about the same time as the long-wavelength ('red') and medium-wavelength ('green') pigment genes diverged, thirty million years ago. Strepsirhines are another group of primates that diverged from the ancestor of the anthropoids at least 55 million years ago. If P/midget ganglion cells evolved to subserve color vision, they should be absent in strepsirhines. We tested this hypothesis in a nocturnal strepsirhine, the greater bush baby Otolemur. The retinal ganglion cells were labeled with the lipophilic tracer Dil and the results show that bush babies have P/midget and M/parasol cells similar to those found in the peripheral retinas of anthropoids. A number of studies have shown that the P and M pathways of bush babies share many similarities with those of anthropoids, and our results show that the same is true for their retinal ganglion cells. These results support the hypothesis that the P system evolved prior to the emergence of red-green color opponency.

The distribution and morphology of LGN K pathway axons within the layers and CO blobs of owl monkey V1.

The lateral geniculate nucleus (LGN) of primates contains three classes of relay cells, the magnocellular (M), parvocellular (P), and koniocellular (K) cells. At present, very little is known about either the structure or function of the K relay cells in New or Old World monkeys (simian primates). In monkeys, K cells are located between the main LGN layers and adjacent to the optic tract. For convenience, these intercalated cell layers are numbered K1-K4 starting closest to the optic tract with K1. The objective of this study was to examine the details of K axon morphology in the primary visual cortex (V1) of owl monkeys and to determine if different K layers give rise to distinct axon types. For this purpose, injections of WGA-HRP or PHA-L were made into specific K LGN layers and the distribution and morphology of the resulting labeled axons were analyzed. Injections of fluorescent tracers also were made within the superficial layers of V1 to further document connections via analysis of the patterns of retrogradely labeled cells in the LGN. Our main finding is that K axons in owl monkeys terminate as delicate focused arbors within single cytochrome oxidase (CO) blob columns in cortical layer III and within cortical layer I. Overall, the morphology of the K axons in these monkeys is quite similar to what we described previously for K geniculocortical axons in the distantly related bush baby (prosimian primate), suggesting that the basic features of this pathway are common to all primates. Our results also provide evidence that the axon arbors from different K layers are morphologically distinct; axons from LGN layer K1 project mainly to cortical layer I, while axons from LGN layer K3 chiefly terminate in cortical layer III. Taken together, these results imply that the basic features of axons within the K pathway are conserved across primates, and that the K axons from different K layers are likely to differ in function based upon their different morphologies.

Visual field representation in striate and prestriate cortices of a prosimian primate (Galago garnetti).

Microelectrode mapping techniques were used to study the visuotopic organization of the first and second visual areas (V1 and V2, respectively) in anesthetized Galago garnetti, alorisiform prosimian primate. 1) V1 occupies approximately 200 mm2 of cortex, and is pear shaped, rather than elliptical as in simian primates. Neurons in V1 form a continuous (1st-order) representation of the visual field, with the vertical meridian forming most of its perimeter. The representation of the horizontal meridian divides V1 into nearly equal sectors representing the upper quadrant ventrally, and the lower quadrant dorsally. 2) The emphasis on representation of central vision is less marked in Galago than in simian primates, both diurnal and nocturnal. The decay of cortical magnification factor with increasing eccentricity is almost exactly counterbalanced by an increase in average receptive field size, such that a point anywhere in the visual field is represented by a compartment of similar diameter in V1. 3) Although most of the cortex surrounding V1 corresponds to V2, one-quarter of the perimeter of V1 is formed by agranular cortex within the rostral calcarine sulcus, including area prostriata. Although under our recording conditions virtually every recording site in V2 yielded visually responsive cells, only a minority of those in area prostriata revealed such responses. 4) V2 forms a cortical belt of variable width, being narrowest (approximately 1 mm) in the representation of the area centralis and widest (2.5-3 mm) in the representation of the midperiphery (>20 degrees eccentricity) of the visual field. V2 forms a second-order representation of the visual field, with the area centralis being represented laterally and the visual field periphery medially, near the calcarine sulcus. Unlike in simians, the line of field discontinuity in Galago V2 does not exactly coincide with the horizontal meridian: a portion of the lower quadrant immediately adjacent to the horizontal meridian is represented at the rostral border of ventral V2, instead of in dorsal V2. Despite the absence of cytochrome oxidase stripes, the visual field map in Galago V2 resembles the ones described in simians in that the magnification factor is anisotropic. 5) Receptive field progressions in cortex rostral to dorsal V2 suggest the presence of a homologue of the dorsomedial area, including representations of both quadrants of the visual field. These results indicate that many aspects of organization of V1 and V2 in simian primates are shared with lorisiform prosimians, and are therefore likely to have been present in the last common ancestor of living primates. However, some aspects of organization of the caudal visual areas in Galago are intermediate between nonprimates and simian primates, reflecting either an intermediate stage of differentiation or adaptations to a nocturnal niche. These include the shape and the small size of V1 and V2, the modest degree of emphasis on central visual field representation, and the relatively large area prostriata.