Vivien. A. Casagrande

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.

Most cells in the retina and lateral geniculate nucleus (LGN) of primates have a concentric center/surround receptive-field organization. Details of the relationship between center and surround often can be used to predict how cells respond to visual stimuli. Models of the receptive-field organization and center/surround relationships also are useful when comparing cell classes. In the present study, we used the difference-of-Gaussians (DOG) model to quantitatively examine and compare the receptive-field center/surround organization of koniocellular (K), magnocellular (M), and parvocellular (P) LGN cells of owl monkeys. We obtained estimates of receptive-field center size (r_c) and center sensitivity (K_c), and surround size (r_s), and surround sensitivity (K_s) from 62 K, M, and P LGN cells by fitting their spatial-frequency responses with a DOG function (Rodieck, 1965; Croner & Kaplan, 1995). The DOG function not only accounted for the responses of P and M cells, but also provided a good description of K-cell responses. We found that at matched eccentricities of less than 15 deg, K cells had the largest r_c and r_s among the three cell classes. K cells also had the lowest K_c and K_s.

The koniocellular (K) pathway is one of three pathways from the lateral geniculate nucleus (LGN) to primate visual cortex (V1). K pathway projections to the cytochrome oxidase (CO) blobs of V1 suggest involvement in chromatic processing given reports that the CO blobs in diurnal primates contain cells selective for color. K LGN layers and CO blobs, however, are also well developed in nocturnal primates such as owl monkeys, which are likely to be color blind. Thus, the K pathway plays either different roles in different species or some as yet unidentified common role(s). Because synaptic arrangements underlie functional mechanisms, the purpose of this investigation was to compare the synaptic circuitry related to the K pathway within the CO blobs of two diurnal primates (macaque monkeys and squirrel monkeys) and one nocturnal primate (owl monkey). Presynaptic K axons were labeled with wheat germ agglutinin-HRP, and presynaptic and postsynaptic profiles in CO blobs were identified with post-embedding immunocytochemistry for GABA and glutamate. In all three species, K axon terminals are glutamatergic and larger than local axon terminals, suggesting that they have a greater impact on postsynaptic CO blob targets than signals arriving via layer IV from the P or M pathways. A greater proportion of K axons, however, synapse with larger glutamatergic shafts in the diurnal monkeys than in the nocturnal owl monkey, perhaps reflecting the importance of color within the K pathway of these diurnal species. Alternatively, the loss of color vision in the owl monkey could impact K pathway circuitry earlier in the pathway. The basic similarities between K axon circuitry within the CO blobs of the three primate species examined also could indicate that this pathway plays some common role or roles across species.

There is considerable controversy over the existence of orientation and direction sensitivity in lateral geniculate nucleus (LGN) neurons. Claims for the existence of these properties often were based upon data from cells tested well beyond their peak spatial frequencies. The goals of the present study were to examine the degree of orientation and direction sensitivity of LGN cells when tested at their peak spatial and temporal frequencies and to compare the tuning properties of these subcortical neurons with those of visual cortex. For this investigation, we used conventional extracellular recording to study orientation and direction sensitivities of owl monkey LGN cells by stimulating cells with drifting sinusoidal gratings at peak temporal frequencies, peak or higher spatial frequencies, and moderate contrast. A total of 110 LGN cells (32 koniocellular cells, 34 magnocellular cells, and 44 parvocellular cells) with eccentricities ranging from 2.6 deg to 27.5 deg were examined. Using the peak spatial and temporal frequencies for each cell, 41.8% of the LGN cells were found to be sensitive to orientation and 19.1% were direction sensitive. The degree of bias for orientation and direction did not vary with eccentricity or with cell class. Orientation sensitivity did, however, increase, and in some cases orientation preferences changed, at higher spatial frequencies. Increasing spatial frequency had no consistent effect on direction sensitivity. Compared to cortical cell orientation tuning, the prevalence and strength of LGN cell orientation and direction sensitivity are weak. Nevertheless, the high percentage of LGN cells sensitive to orientation even at peak spatial and temporal frequencies reinforces the view that subcortical biases could, in combination with activity-dependent cortical mechanisms and/or cortical inhibitory mechanisms, account for the much narrower orientation and direction tuning seen in visual cortex.

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.

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 4α (4Cα), 4β (4Cβ) and 4ctr send primary projections to layers 3C (4B), 3Bβ (4A) and 3Bα (3B), respectively. Examination of laminar structure with Nissl staining supports a tripartite organization of layer 4. The cortical output layer above layer 3Bα (3B) (e.g. layer 3A) does not appear to receive any direct connections from layer 4 but receives heavy input from layers 3Bα (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 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.