The postnatal development of geniculocortical axon arbors in owl monkeys

The Axonal Actin Filament Cytoskeleton: Structure, Function, and Relevance to Injury and Degeneration

Early investigations of the neuronal actin filament cytoskeleton gave rise to the notion that, although growth cones exhibit high levels of actin filaments, the axon shaft exhibits low levels of actin filaments. With the development of new tools and imaging techniques, the axonal actin filament cytoskeleton has undergone a renaissance and is now an active field of research. This article reviews the current state of knowledge about the actin cytoskeleton of the axon shaft. The best understood forms of actin filament organization along axons are axonal actin patches and a submembranous system of rings that endow the axon with protrusive competency and structural integrity, respectively. Additional forms of actin filament organization along the axon have also been described and their roles are being elucidated. Extracellular signals regulate the axonal actin filament cytoskeleton and our understanding of the signaling mechanisms involved is being elaborated. Finally, recent years have seen advances in our perspective on how the axonal actin cytoskeleton is impacted by, and contributes to, axon injury and degeneration. The work to date has opened new venues and future research will undoubtedly continue to provide a richer understanding of the axonal actin filament cytoskeleton.

Neurochemical comparison of synaptic arrangements of parvocellular, magnocellular, and koniocellular geniculate pathways in owl monkey (Aotus trivirgatus) visual cortex

As in other primates, the lateral geniculate nucleus (LGN) of owl monkeys contains three anatomically and physiologically distinct relay cell classes, the magnocellular (M), parvocellular (P), and koniocellular (K) cells. M and P LGN cells send axons to the upper and lower tiers of layer IV, and K cells send axons to the cytochrome oxidase (CO) blobs of layer III and to layer I of primary visual cortex (V1). Our objective was to compare the synaptic arrangements made by these axon classes. M, P, and K axons were labeled in adult owl monkeys by means of injections of wheat germ agglutinin-horseradish peroxidase into the appropriate LGN layers. The neurochemical content of both pre- and postsynaptic profiles were identified by postembedding immunocytochemistry for gamma-aminobutyric acid (GABA) and glutamate. Our key finding is that the synaptic arrangements made by M, P, and K axons in owl monkey exhibit more similarities than differences. They are exclusively presynaptic, contain glutamate and form asymmetric synapses mainly with glutamate-positive dendritic spines. The majority of the remaining axons synapse with glutamatergic dendritic shafts. There are also differences between LGN pathways. M and P terminals are significantly larger and more likely to make multiple synapses than K axons, although M and P axons do not differ from each other in either of these characteristics. Of interest, a larger percentage of M and K axons than P axons make synapses with GABAergic dendritic shafts. Cells directly postsynaptic to M and K axons are known to exhibit orientation selectivity and, in some cases, direction selectivity. Cells postsynaptic to P axons do not show these properties, but instead tend to reflect their LGN inputs more faithfully; therefore, it is possible that these physiologic differences seen in the cortical cells postsynaptic to different LGN pathways reflect the differential involvement of inhibitory circuits.

Vascularization in the primate visual cortex during development

We studied the relationship between vascularization and neuronal activity in the visual cortex during postnatal development in the primate. Analyses were focused on layer IVC that displays a sequential pattern of maturation for the magno- and parvocellular systems in separate sublayers, respectively IVC alpha and IVC beta. Cytochrome oxidase and endogenous alkaline phosphatase histochemistry was used to analyse, on the same sections, the laminar patterns of cortical activity and vessel density in the primary visual cortex of the marmoset (Callithrix jacchus). Experiments were carried out in five young and two adult animals. We showed that the temporal pattern of angiogenesis differs in layer IVC alpha and IVC beta. During the first postnatal month, vessel density is higher in IVC alpha than in IVC beta and runs parallel to cytochrome oxidase intensity. In 2-month-old animals, both vessel densities and cytochrome oxidase activity are similar in IVC alpha and IVC beta. In adults, the vessel densities in IVC alpha and IVC beta are the reverse of those observed during the first postnatal month. Vessel diameter does not account for this evolution in vascular patterns. In the discussion, we suggest that such a developmental time-course of angiogenesis might be linked to the synaptogenesis requirements that proceed differently for the magno- and parvocellular systems in the primate striate cortex.

Infant temporal contrast sensitivity functions (tCSFs) mature earlier for luminance than for chromatic stimuli: evidence for precocious magnocellular development?

In order to investigate the development of luminance and chromatic temporal contrast sensitivity functions (tCSFs), we obtained chromatic and luminance contrast thresholds from individual 3- and 4-month old infants, and compared them to previously obtained functions in adults. Stimuli were moving sinusoidal gratings of 0.27 cyc/deg, presented at one of five temporal frequencies: 1.0, 2.1, 4.2, 9.4 or 19 Hz (corresponding speeds: 3.8, 7.7, 15, 34, 69 deg/s). Previous studies, including our own, have shown that adult tCSFs are bandpass for luminance stimuli (peaking at 5-10 Hz), yet lowpass for chromatic stimuli (sensitivity falling at > 2 Hz), and that the two functions cross one another near 4-5 Hz when plotted in terms of cone contrast. In the present study, we find that the shapes and peaks of the luminance tCSF in both 3- and 4-months-olds appear quite similar to those of adults. By contrast, chromatic tCSFs in infants are markedly different from those of adults. In agreement with our earlier report (Dobkins, K. R., Lia, B., & Teller, D. Y. (1997). Vision Research, 37(19), 2699-2716), the chromatic function in 3-month-olds is rather flat, lacking the sharp high temporal frequency fall-off characteristic of the adult function. In addition, the luminance tCSF in 3-month-olds is elevated above the chromatic tCSF, and the two functions do not exhibit an adult-like cross-over within the range of temporal frequencies tested. By 4 months of age, substantial development of chromatic contrast sensitivity takes place at the lowest temporal frequencies. Although still immature, the 4-month-old chromatic tCSF has begun to adopt a more adult-like shape. In addition, similar to adults, luminance and chromatic tCSFs in 4-month-olds cross one another near 5 Hz. In adults, magnocellular (M) and parvocellular (P) pathways are thought to underlie the bandpass luminance and lowpass chromatic tCSF, respectively (e.g. Lee, B. B., Pokorny, J., Smith, V. C., Martin, P. R., & Valberg, A. (1990). Journal of the Optical Society of America (a), 7(12), 2223-2236). Based on this correspondence between psychophysical and neural responses in adults, our results suggest that the relatively slow development of the chromatic tCSF in infants may reflect immature chromatic responses in the P pathway and/or reliance on chromatic responses originating in the M pathway.

Morphology of M-cell axon arbors in striate cortex of monkeys reared with monocular aphakia

Although the effects of visual deprivation on the development of ocular dominance columns have been well described in primates, nothing is known in primates about the impact of the deprivation on the axonal profiles that make up the ocular dominance columns. We now show that the effects of monocular deprivation on the morphology of geniculostriate axons involve not only shifts in terminal arbor sizes, much as would be expected from the ocular dominance data, but also changes in the proliferation of terminal arbor branches. In macaque monkeys reared from birth with unilateral lens removal (aphakia), terminal arbors of geniculostriate axons were bulk-filled with horseradish peroxidase (HRP) in brain-slice preparations and reconstructed from serial sections through striate cortex (area 17). Our focus was on the arbors that terminate in the upper tier of layer IV, the target of cells in the magnocellular (M) layers of the LGN. Of the 26 M-cell arbors reconstructed from three aphakic monkeys, eight were unique in having few very simple terminal arbor branches. These also tended to be smaller in total extent than the average M-cell axons reconstructed from 1 normal monkey. In contrast, eight arbors had very rich terminal branching patterns, and seven of these were larger than any of those from the normal monkey. We propose that the small, sparse axon arbors are related to the deprived eye, and the large, dense arbors are related to the non-deprived eye. These morphological changes reflect abnormalities in the growth patterns of geniculostriate inputs that undoubtedly have important persisting consequences for visual performance.

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.

Infant color vision: temporal contrast sensitivity functions for chromatic (red/green) stimuli in 3-month-olds

In order to investigate the development of temporal contrast sensitivity functions (tCSFs) for chromatic (red/green) stimuli, we obtained chromatic contrast thresholds from 3-month-old infants and adults using behavioral techniques. Stimuli were moving or counterphase-reversing sinusoidal gratings of 0.25 c/deg. Five temporal frequencies were used: 0.7, 2.1, 5.6, 11 and 17 Hz (corresponding speeds = 2.8, 8.4, 22, 44 and 67 deg/sec). In order to compare chromatic results with those obtained under luminance-defined conditions, luminance tCSFs were also obtained from adults, and previously obtained infant luminance tCSFs were used (from Dobkins & Teller, 1996a). In accordance with previous studies, adults exhibited bandpass luminance tCSFs with peaks near 5 Hz and lowpass chromatic tCSFs that declined rapidly at temporal frequencies greater than 2 Hz, and the two curves crossed one another near 4 Hz. By contrast, infants exhibited bandpass rather than lowpass chromatic tCSFs with peaks near 5 Hz. These chromatic curves were quite similar in peak frequency and general shape to previously obtained infant tCSFs for luminance stimuli. Moreover, both chromatic and luminance tCSFs in infants were found to be quite similar in peak and shape to luminance tCSFs observed in adults. These findings point to the possibility that, for 3-month-old infants, both chromatic and luminance stimuli are detected by the same underlying mechanism under these conditions. We propose that such a mechanism is probably a physiological pathway dominated by magnocellular input. Earlier studies of infant color vision are discussed in this context.

Postnatal development of binocular disparity sensitivity in neurons of the primate visual cortex

In macaque monkeys, the age at which neurons in the primary visual cortex (V1) become sensitive to interocular image disparities, a prerequisite for stereopsis, is a matter of conjecture. To resolve this fundamental issue in binocular vision development, we measured the responsiveness of individual V1 neurons in anesthetized and paralyzed infant monkeys as a function of the relative, interocular, spatial phase of dichoptic sine-wave gratings. We found that an adult-like proportion of units were sensitive to interocular image disparity as early as the sixth postnatal day, several weeks before the onset age for stereopsis in monkeys. The ocular dominance distributions of cells in infant monkeys were also indistinguishable from those of adults. Thus, at or only a few days after birth, V1 neurons are capable of combining neural signals from the two eyes as in adults and are sensitive to interocular image disparities. However, the monocular spatial-frequency response properties of these disparity-sensitive units were immature, and their overall responsiveness was far lower than that in adults. During the first 4 postnatal weeks, both the spatial frequency response properties and the peak response amplitude rapidly improved, which resulted in a corresponding increase in the absolute sensitivity of individual units to interocular disparity. The results demonstrate that early binocular vision development in monkeys is not constrained by a paucity of disparity-sensitive V1 neurons but, instead, by the relative immaturity of the spatial response properties and the overall unresponsiveness of existing disparity-sensitive neurons.

Infant motion: detection (M:D) ratios for chromatically defined and luminance-defined moving stimuli

In order to assess the relative contributions of chromatic vs luminance information to motion processing in infants, we employed a motion:detection (M:D) paradigm. Stimuli consisted of 27 deg by 40 deg, 0.25 c/deg sinusoidal gratings moving at 22 deg/sec (5.6 Hz), and were either chromatically defined or luminance-defined. Contrast thresholds for direction-of-motion (M) were obtained using a directional eye movement technique. Contrast thresholds for detection (D) were obtained using forced-choice preferential looking. M:D threshold ratios were obtained for individual infant subjects, and results were compared to those of adults. As expected, adult M:D threshold ratios were near 1:1 for luminance-defined stimuli, but greater than 1:1 for chromatically defined stimuli. This suggests that, for adults, luminance-defined, but not chromatically defined, stimuli are detected by mechanisms labeled for direction of motion. By contrast, infant M:D ratios for chromatically and luminance-defined stimuli were approximately equal and close to 1:1, suggesting that, for infants, luminance- as well as chromatically defined stimuli are detected by mechanisms that are labeled for direction of motion.

Interhemispheric connections in neonatal owl monkeys (Aotus trivirgatus) and galagos (Galago crassicaudatus)

Interhemispheric connections were studied by injecting a mixture of horseradish peroxidase (HRP) and wheatgerm agglutinin conjugated with horseradish peroxidase (WGA-HRP) into multiple sites in dorsolateral occipital and parietal cortex of one cerebral hemisphere of three galagos (Galago crassicaudatus) and two owl monkeys (Aotus trivirgatus) within seven days of birth. Cortex was either separated from the rest of the brain, flattened and cut parallel to the surface to aid reconstructing surface-view patterns of labeled neurons and processes, or cut in standard coronal or parasagittal planes to better reveal laminar patterns of connections. In both primate species, the surface-view pattern of callosal connections in infants was remarkably adult-like. In infant owl monkeys, callosal connections were concentrated along the margin of area 18 with area 17, and only a few labeled cells were found within area 17. Other visual areas including the second visual area, V-II, and the middle temporal visual area, MT, had patchy distributions of labeled neurons that extended over large parts of the visual field representations. Primary motor, auditory, and somatosensory fields also had patchy distributions of labeled neurons, with regions of areas 3b and adjoining somatosensory fields having few callosal connections in portions that appeared to correspond with representations of the hand and foot. Results were very similar in galagos, except that newborn galagos, as in adults, had a patchy distribution of callosally projecting neurons that extended well within area 17. Furthermore, the labeled neurons were concentrated in patches that aligned with the cytochrome oxidase blobs of area 17. Finally, callosal connections were concentrated in cytochrome oxidase poor regions of area 3b.