Plasticity and stability of the visual system in human achiasma

The absence of the optic chiasm is an extraordinary and extreme abnormality in the nervous system. The abnormality produces highly atypical functional responses in the cortex, including overlapping hemifield representations and bilateral population receptive fields in both striate and extrastriate visual cortex. Even in the presence of these large functional abnormalities, the effect on visual perception and daily life is not easily detected. Here, we demonstrate that in two achiasmic humans the gross topography of the geniculostriate and occipital callosal connections remains largely unaltered. We conclude that visual function is preserved by reorganization of intracortical connections instead of large-scale reorganizations of the visual cortex. Thus, developmental mechanisms of local wiring within cortical maps compensate for the improper gross wiring to preserve function in human achiasma.     

The corticofugal pathways of the visual system

Cortical neurons belonging to the same topological ensemble send axons to thalamic and mesencephalic structures and also to contra and ipsilateral cortical areas. The projections are called the corticofugal system. This review addresses the organization and the functions of the efferent cortical fibers within the visual network. For example, the cortico-geniculate fibers participate in shaping the structure of the concentric receptive fields of geniculate cells. Namely, the size of the surround area depends on descending impulses from the cortex. By contrast, cortico-mesencephalic fibers have a more global influence on visual responses. Following the interruption of cortical activity all responses to visual stimuli decline; although in rodents and lagomorphs cortical inactivation does not eliminate those visual responses that are sent to the superior colliculus or pretectum directly from the retina. In each hemisphere it has been demonstrated that contra-lateral cortico-cortical fibers participate in the continuity of the two visual hemi-fields, as the interruption of the callosal impulses results in a truncated field in which the contralateral part of the receptive field is missing., overlaps the vertical meridian is missing. Finally, ipsilateral cortico-cortical fibers allow a consolidation of visual properties of cortical cells. It must be added that there are considerable differences among species in the organization of cortico-cortical relationships. However, this survey seems to indicate that all corticofugal axons are excitatory.     

Weighted cue integration in the rodent head direction system

How the brain combines information from different sensory modalities and of differing reliability is an important and still-unanswered question. Using the head direction (HD) system as a model, we explored the resolution of conflicts between landmarks and background cues. Sensory cue integration models predict averaging of the two cues, whereas attractor models predict capture of the signal by the dominant cue. We found that a visual landmark mostly captured the HD signal at low conflicts: however, there was an increasing propensity for the cells to integrate the cues thereafter. A large conflict presented to naive rats resulted in greater visual cue capture (less integration) than in experienced rats, revealing an effect of experience. We propose that weighted cue integration in HD cells arises from dynamic plasticity of the feed-forward inputs to the network, causing within-trial spatial redistribution of the visual inputs onto the ring. This suggests that an attractor network can implement decision processes about cue reliability using simple architecture and learning rules, thus providing a potential neural substrate for weighted cue integration.     

Plasticity in the visual system: role of neurotrophins and electrical activity

We report recent results concerning the action of neurotrophins on the development and plasticity of the visual system of mammals and in particular of their visual cortex. It has been demonstrated that NGF prevents all the effects of monocular deprivation during the critical period. BDNF, that in part also prevents the effects of monocular deprivation, has the interesting additional property of accelerating the development of inhibitory processes. In transgenic mice overexpressing BDNF only in the cortex, the critical period for plasticity initiates a week earlier and presents a precocious closure. Visual acuity also develops much before than in normal animals. These phenomenological observations are paralleled by a precocious increase of inhibitory synapses and inhibitory currents in pyramidal neurons. LTP, tested by stimulation of the white matter, recording in layers 2 and 3 of the visual cortex, presents modifications correlated with the alterations observed in the critical period. Last we report the finding from in vitro and in vivo experiments that MAPkase (Erg 1 and 2) is the molecular chain of events driven both by light and neurotrophins, likely at the bases of the phenomena of plasticity observed during the critical period.     

Are direction and speed coded independently by the visual system? Evidence from visual search.

Three visual search experiments examined whether motion is coded as two separate features, speed and direction. Increasing the heterogeneity of the directions in which stimuli moved disrupted detection of a target defined by speed (fast among medium and slow nontargets), suggesting that speed is coded integrally with direction. However, heterogeneity in speed did not disrupt detection of a target moving in a particular direction among nontargets with different directions. This suggests that direction is coded independently of speed. The apparent paradox raised by these contrasting conclusions is consistent with neurophysiological and computational models of motion-detection, which suggest that low-levels of the visual system contain direction-detectors insensitive to speed, while speed is coded at higher levels by detectors which are also sensitive to direction. Evidence consistent with the existence of the latter conjunction detectors was obtained in a final experiment which found search for a conjunction of speed and direction to be parallel.     

The circadian system of Drosophila melanogaster and its light input pathways

The fruit fly Drosophila melanogaster has been a grateful object for circadian rhythm researchers over several decades. Behavioral, genetic, and molecular studies in the little fly have aided in understanding the bases of circadian time keeping and rhythmic behaviors not only in Drosophila, but also in other organisms, including mammals. This review summarizes our present knowledge about the fruit fly's circadian system at the molecular and neurobiological level, with special emphasis on its entrainment by environmental light-dark cycles. The results obtained for Drosophila are discussed with respect to parallel findings in mammals.     

Modeling convergent ON and OFF pathways in the early visual system

For understanding the computation and function of single neurons in sensory systems, one needs to investigate how sensory stimuli are related to a neuron’s response and which biological mechanisms underlie this relationship. Mathematical models of the stimulus–response relationship have proved very useful in approaching these issues in a systematic, quantitative way. A starting point for many such analyses has been provided by phenomenological “linear–nonlinear” (LN) models, which comprise a linear filter followed by a static nonlinear transformation. The linear filter is often associated with the neuron’s receptive field. However, the structure of the receptive field is generally a result of inputs from many presynaptic neurons, which may form parallel signal processing pathways. In the retina, for example, certain ganglion cells receive excitatory inputs from ON-type as well as OFF-type bipolar cells. Recent experiments have shown that the convergence of these pathways leads to intriguing response characteristics that cannot be captured by a single linear filter. One approach to adjust the LN model to the biological circuit structure is to use multiple parallel filters that capture ON and OFF bipolar inputs. Here, we review these new developments in modeling neuronal responses in the early visual system and provide details about one particular technique for obtaining the required sets of parallel filters from experimental data.     

A theoretical account of cue averaging in the rodent head direction system

Head direction (HD) cell responses are thought to be derived from a combination of internal (or idiothetic) and external (or allothetic) sources of information. Recent work from the Jeffery laboratory shows that the relative influence of visual versus vestibular inputs upon the HD cell response depends on the disparity between these sources. In this paper, we present simulation results from a model designed to explain these observations. The model accurately replicates the Knight et al. data. We suggest that cue conflict resolution is critically dependent on plastic remapping of visual information onto the HD cell layer. This remap results in a shift in preferred directions of a subset of HD cells, which is then inherited by the rest of the cells during path integration. Thus, we demonstrate how, over a period of several minutes, a visual landmark may gain cue control. Furthermore, simulation results show that weaker visual landmarks fail to gain cue control as readily. We therefore suggest a second longer term plasticity in visual projections onto HD cell areas, through which landmarks with an inconsistent relationship to idiothetic information are made less salient, significantly hindering their ability to gain cue control. Our results provide a mechanism for reliability-weighted cue averaging that may pertain to other neural systems in addition to the HD system.     

Photosensory input pathways in the medicinal leech

Summary The medicinal leech,Hirudo medicinalis possesses two types of photosensory organs: five bilateral pairs of eyes embedded in two longitudinal rows in the dorsal surface of the head, and seven bilateral pairs of sensilla situated in both the dorsal and the ventral surface of each of the 21 body segments. The photoreceptor cells of each eye or sensillum project their axons centrally via a characteristic cephalic or segmental nerve which carries the photosensory input to the brain or to the segmental ganglion. In response to a pulse of light the photoreceptors produce a train of impulses whose frequency first rises to anearly peak and then declines to asteady state plateau at which it remains until the end of the pulse. The amplitude of the early peak response and the level of the steady state plateau rise linearly with the log of the light pulse intensity, but the dynamic range of the early peak response is much narrower than that of the plateau. Both ocular and sensillar photoreceptors adapt to the intensity of interpulse background illumination; the ocular receptors adapt so completely that their level of background activity is nearly independent of the background light intensity, whereas the ventral sensillar photoreceptors adapt incompletely, so that their background activity rises with the background light intensity. Ocular and sensillar photoreceptors make their maximal response to green light at a wavelength of about 540 nm. They are almost insensitive to red and violet light at both extremes of the visible spectrum. The photosensory response of a single eye is directionally selective, whereas that of a single sensillum has much less directional selectivity. Several higher order sensory neurons were identified in the segmental ganglion that receive photosensory input from the sensilla. One of these neurons has the sensillum in the ipsilateral dorso-medial body wall of the same segment as its receptive field and another neuron the bilateral set of ventral sensilla in the body wall of the next posterior segment.We are indebted to Frank S. Werblin for valuable advice and discussions. We thank Kenneth L. Carlock for designing and constructing much of the special electronic equipment used in this study. We also thank Alexander Petruncola for his helpful suggestions regarding the computational analysis of the experimental results and for writing the computer programs used in the processing of the data.This research was supported by Grant No. GB 31933X from the National Science Foundation, and NIH research grant No. GM 17866 and Training Grant No. GM 00829 from the Institute for General Medical Sciences.     

The light sensitivity of the human visual system depends on the direction of view

The near-stable North-South orientation of the natural geomagnetic field provides an ideal basis for navigation. Sailors have used it since ancient times, animals for much longer. Various mechanisms have developed for this purpose. Experiments have pointed to a connection between orientation in the geomagnetic field and light perception. Such observations are supported by theoretical considerations. The underlying interaction should also modulate the light sensitivity of the visual system. Recently we demonstrated the effect of an oscillating field. Here we report the existence of a weak influence of the static field on visual sensitivity in man. By comparison with control experiments, if the directions of view line and field vector coincide the perception threshold of a light stimulus is slightly but significantly increased. This significance is lost if the view line deviates by 10 degrees from the field direction.     

淡水蜗牛视觉系统的输入与输出通路(英文)

通过神经生物素在视神经上的逆行性传输对淡水蜗牛(Planorbarius corneus)视网膜及中央神经节的输入、输出神经元进行标记。由于没有发现突触联系,所以至少一部分光感受细胞的轴突可被视为直接参与形成视神经。这些神经元的轴突进入大脑神经节形成密集的细传入神经纤维束-视神经堆。传出神经元则存在于除颊部以外的所有神经节。一些上行轴突在大脑神经节处分叉,通过脑-脑联合,到达对侧眼并在眼杯处形成分枝。部分传出神经元的轴突也投射于不同的外周神经,如:n.n.intestinalis,pallialis dexter,pallialis sinister internus et externus。五羟色胺能纤维和FMRF-酰胺能纤维均存在于视神经上,且这些纤维隶属于只投射在同侧眼的中央神经元。它们形成了位于眼杯处的丰富曲张结构及视网膜核心层,并且可能有助于调节视网膜对光的敏感性。     

Joint coding of visual input and eye/head position in V1 of freely moving mice

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Dissecting Nck/Dock signaling pathways in Drosophila visual system

The establishment of neuronal connections during embryonic development requires the precise guidance and targeting of the neuronal growth cone, an expanded cellular structure at the leading tip of a growing axon. The growth cone contains sophisticated signaling systems that allow the rapid communication between guidance receptors and the actin cytoskeleton in generating directed motility. Previous studies demonstrated a specific role for the Nck/Dock SH2/SH3 adapter protein in photoreceptor (R cell) axon guidance and target recognition in the Drosophila visual system, suggesting strongly that Nck/Dock is one of the long-sought missing links between cell surface receptors and the actin cytoskeleton. In this review, I discuss the recent progress on dissecting the Nck/Dock signaling pathways in R-cell growth cones. These studies have identified additional key components of the Nck/Dock signaling pathways for linking the receptor signaling to the remodeling of the actin cytoskeleton in controlling growth-cone motility.     

Structure of visual pathways in the nervous system of freshwater pulmonate molluscs

Central nervous system of freshwater pulmonate molluscs Lymnaea stagnalis and Planorbarius corneus was stained using retrograde transport of neurobiotin in the optic tract fibers. In both species, perikarya and fibers of the stained neurons are found in all ganglia except the buccal ones. Afferent fibers of the optic nerve form dense sensory neuropil located in relatively small volume of cerebral ganglia. Typical neuronal groups sending their processes into the optic nerves of ipsilateral and contralateral body halves are described. Among them, neurons of visceral and parietal ganglia innervating both eyes concurrently as well as sending projections into peripheral nerves are revealed. These neurons, supposedly, have a function to integrate sensory signals, which may be a basis for regulation of light sensitivity of retina and functioning of peripheral organs. Bilateral links of the molluscan eye with the pedal ganglia cells and statocysts are found, which is, likely, a structural basis of certain known behavioral patterns related to stimulation of visual inputs in the studied gastropod molluscs.     

Descending pathways connecting the male-specific visual system of flies to the neck and flight motor

Summary During sexual pursuit, male flies Sarcophaga bullata, stabilize the image of a pursued target on the dorso-frontal acute zone of their compound eyes. By retinotopic projection, this region is represented in the upper frontal part of the lobula where it is sampled by ensembles of male-specific motion- and flicker-sensitive interneurons. Intracellular recordings of descending neurons, followed by biocytin injection, demonstrate that male-specific neurons are dye-coupled to specific descending neurons and that the response characteristics of these descending neurons closely resemble those of male-specific lobula neurons. Such descending neurons are biocytin-coupled in the thoracic ganglia, revealing their connections with ipsilateral frontal nerve motor neurons supplying muscles that move the head and with contralateral basalar muscle motor neurons that control wing beat amplitude. Recordings from neck muscle motor neurons demonstrate that although they respond to movement of panoramic motion, they also selectively respond to movement of small targets presented to the male-specific acute zone. The present results are discussed with respect to anatomical and physiological studies of sex-specific interneurons and with respect to sex-specific visual behavior. The present study, and those of the two preceding papers, provide a revision of Land and Collett's hypothetical circuit underlying target localization and motor control in males pursuing females.     

Structure of visual pathways in the nervous system of fresh water pulmonary molluscs

The central nervous system of freshwater pulmonary molluscs Lymnaea stagnalis and Planorbarins corneus was stained by the method of neurobiotin retrograde transport along optic nerve fibers. In the animals of both species, bodies and fibers of stained neurons are found in all ganglia except for the buccal ones. Afferent fibers of the optic nerve form a dense sensor neuropil located in a small volume of cerebral ganglia. Characteristic groups of neurons sending their processes into optic nerves both of ipsi- and of contralateral half of the body are described. Revealed among them are neurons of visceral and parietal ganglia, which simultaneously innervate both eyes as well as give projections into peripheral nerves. It is suggested that these neurons can perform function of integration of sensor signals and, on its base, regulate photosensitivity of retina as well as activity of peripheral organs. There is established the presence of bilateral connections of the mollusc eye with cells of pedal ganglia and statocysts, which seems to be the structural basis of manifestation of the known behavior forms associated with stimulation of visual inputs of the studied gastropod molluscs.     

Plasticity in the cockroach neuromuscular system

The retrograde transport of wheat germ agglutinin-conjugated horseradish peroxidase extracellularly injected into a leg muscle was used to identify the regenerating cockroach motor neurons that have grown an axonal branch into that muscle. At least 66% of the animals with crushed nerve roots eventually reform the original innervation pattern of this muscle with no mistakes. In spite of this apparent specificity the cockroach neuromuscular system can express plasticity as evidenced by the correction of mistakes made at early stages of regeneration. These mistakes are corrected through elimination during the time interval between 40 and 60 days after nerve crush. In addition, when the distal segments of the leg are removed, thus depriving some motor neurons of their normal target muscles, many of them form stable inappropriate axonal branches in denervated as well as fully innervated muscles. These observations are discussed in terms of possible mechanisms responsible for the specificity of the cellular interactions and in terms of their relevance to understanding the development of vertebrate neuromuscular systems.