Contextual modulation in the V1 real motion cells

Retinal images move when the eyes move across a stationary object, or alternatively, when the object moves while the eyes are stationary. Orientation selective cells in V1 showed preference for these two types of retinal image slip. Furthermore, if an orientation cell preferred moving objects, the response to an element of a complex image was modulated by background stimuli placed outside the cell's receptive field. However, the response of cells, that showed no preference for a moving object, was hardly affected by the background. These results indicate that figure and ground are already segregated in the very early stage of visual processing.     

Dynamics of tubulovesicular recycling endosomes in hippocampal neurons.

Neurons are polarized cells, the activity of which relies on the morphological and functional differences between their axonal and somatodendritic domains. One mechanism for establishing and maintaining neuronal polarity is via the selective targeting of proteins to these domains. The endocytic pathway plays a major role in the generation and maintenance of cellular polarity by selectively sorting and recycling endocytosed plasma membrane proteins. In this study we first show that endogenous syntaxin 13 localizes to tubulovesicular organelles that are present in the somatodendritic and axonal domains of neurons. These organelles contain and actively recycle transferrin receptor and are sensitive to brefeldin A, suggesting that they are analogous to the tubulovesicular recycling endosomes in non-neuronal cells. We next use a syntaxin 13-GFP fusion protein transiently expressed in hippocampal neurons, together with time-lapse microscopy, to study the dynamics of the endosomal system in neurons. The analysis revealed the presence of two distinct classes of syntaxin 13-labeled endosomes: round-oval stationary organelles and highly mobile tubulovesicular structures. The dynamic population of tubulovesicular endosomes travels in both directions along microtubules in dendrites and axons. The mobile organelles appear to fuse with and bud from the stationary endosomes, possibly as a means of delivering and picking up their cargo.     

Visual responses of neurons in the nucleus of the basal optic root to stationary stimuli in pigeons

The nucleus of the basal optic root of the accessory optic system in pigeons is involved in generating optokinetic nystagmus, which stabilizes object images on the retina by compensatory eye movements. Previous studies have indicated that basal optic neurons are selective for the direction and velocity of motion. The present study shows that these optokinetic cells also respond to stationary stimuli and thereby could be categorized into three groups. The first group of cells (69.1%) responds to stationary gratings orthogonal to the preferred direction but not to gratings parallel to the preferred direction. They do not respond to stationary random-dot patterns without any orientational cues. The second group of cells (7.4%) almost equally discharges a series of bursts in response to stationary gratings with any orientations and to random-dot patterns as well. The third group of cells (23.5%) is responsive to motion but not to stationary gratings and random-dot patterns. The receptive field of basal optic cells is composed of an excitatory field and an inhibitory field, both of which overlap or occupy different regions in the visual field. The aforementioned properties may be attributed to the excitatory receptive field, whereas the inhibitory receptive field is functional when visual stimuli are moving in the direction opposite to the preferred direction of basal optic cells. The functional significance of visual responses of optokinetic neurons to stationary patterns is discussed.     

Deficits in visual motion processing following ibotenic acid lesions of the middle temporal visual area of the macaque monkey

Physiological experiments have produced evidence that the middle temporal visual area (MT) of the monkey is selectively involved in the analysis of visual motion. We tested this hypothesis by studying the effects of small chemical lesions of MT on eye movements made in response to moving as opposed to stationary visual targets. We observed two deficits for eye movements made to moving targets: a monkey's ability to match the speed of his smooth pursuit eye movements to the speed of the moving target was impaired, and a monkey's ability to adjust the amplitude of a saccadic eye movement to compensate for target motion was impaired. In contrast, saccades to stationary targets were unaffected by the MT lesions, suggesting that monkeys with MT lesions had more difficulty responding to moving than to stationary stimuli. These results provide the first behavioral evidence that neural processing in MT contributes to the cortical analysis of visual motion.     

Visual search for moving and stationary items in chimpanzees (Pan troglodytes) and humans (Homo sapiens)

Four visual search experiments were conducted using human and chimpanzee subjects to investigate attentional processing of movement, and perceptual organization based on movement of items. In the first experiment, subjects performed visual searches for a moving target among stationary items, and for a stationary target among moving items. Subjects of both species displayed an advantage in detecting the moving item compared to the stationary one, suggesting the priority of movement in the attentional processing. A second experiment assessed the effect of the coherent movement of items in the search for a stationary target. Facilitative effects of motion coherence were observed only in the performance of human subjects. In the third and fourth experiments, the effect of coherent movement of the reference frame on the search for moving and stationary targets was tested. Related target movements significantly influenced the search performance of both species. The results of the second, third, and fourth experiments suggest that perceptual organization based on coherent movements is partially shared by chimpanzees and humans, and is more highly developed in humans.     

MDReg‐Net: Multi‐resolution diffeomorphic image registration using fully convolutional networks with deep self‐supervision

We present a diffeomorphic image registration algorithm to learn spatial transformations between pairs of images to be registered using fully convolutional networks (FCNs) under a self‐supervised learning setting. Particularly, a deep neural network is trained to estimate diffeomorphic spatial transformations between pairs of images by maximizing an image‐wise similarity metric between fixed and warped moving images, similar to those adopted in conventional image registration algorithms. The network is implemented in a multi‐resolution image registration framework to optimize and learn spatial transformations at different image resolutions jointly and incrementally with deep self‐supervision in order to better handle large deformation between images. A spatial Gaussian smoothing kernel is integrated with the FCNs to yield sufficiently smooth deformation fields for diffeomorphic image registration. The spatial transformations learned at coarser resolutions are utilized to warp the moving image, which is subsequently used as input to the network for learning incremental transformations at finer resolutions. This procedure proceeds recursively to the full image resolution and the accumulated transformations serve as the final transformation to warp the moving image at the finest resolution. Experimental results for registering high‐resolution 3D structural brain magnetic resonance (MR) images have demonstrated that image registration networks trained by our method obtain robust, diffeomorphic image registration results within seconds with improved accuracy compared with state‐of‐the‐art image registration algorithms.     

No correlation between moving phosphene and motor thresholds: a transcranial magnetic stimulation study

The aim of this study was to investigate the temporal stability of moving phosphenes and to assess whether moving phosphene thresholds (PTs) correlate with motor thresholds (MTs). Small moving sensations, so-called moving phosphenes, are perceived when V5, an area important for visual motion analysis, is stimulated by transcranial magnetic stimulation (TMS). However, it is still a matter of debate if V5 phosphenes are stable sensations across measurements and if they are a reasonable index of the cortical excitability of V5. Currently, MT is more commonly used as an index of global cortical excitability. However, previous studies have indicated that stationary PTs are suitable alternatives when the primary visual cortex is stimulated by TMS. Using paired-pulse TMS, stationary and moving PTs and applying single pulse TMS, MTs were measured in 11 subjects. PTs were retested in nine subjects 5-7 days later. Stationary and moving PTs were stable within subjects across the two sessions and showed a high inter-correlation. Conversely, PTs and MTs did not correlate. Our results are in agreement with previous studies showing that excitatory measurements of one specific cortex cannot be generalized to the excitability of the whole cortex. Thus, we propose specific measures for cortices of interest: PT for visual experiments and MT for motor experiments.     

Human cortical auditory motion areas are not motion selective

The existence of a specialized mechanism supporting auditory motion processing in humans is a matter of debate in the psychophysical literature. Recent functional neuroimaging data appear to have resolved the debate in favor of a specialized motion system in that several studies have found cortical regions that seem to be motion selective. While all these studies contrast some form of moving auditory stimulation with a stationary stimulus, none have adequately controlled for the possibility that these areas are simply computing sound-source location and not motion per se: a moving stimulus varies in spatial location as well as motion, and so a system computing spatial location (and not motion) would be activated in response to both a moving and stationary sound source. To control for this possibility, ten subjects were scanned while listening to moving stimuli and while listening to stationary stimuli that varied randomly in spatial location. Consistent with previous imaging studies, we found that a motion stimulus when contrasted with rest (scanner noise) activated STG/planum temporale (bilaterally) and right parietal lobe. However, stationary stimuli presented at varying locations activated these regions equally well, arguing against the existence of specialized motion-processing areas in human cortex.     

Gamma-protocadherin homophilic interaction and intracellular trafficking is controlled by the cytoplasmic domain in neurons

Gamma-protocadherins (Pcdh-γs) are good candidates to mediate specificity in synaptogenesis but their role in cell-cell interactions is a matter of debate. We proposed that Pcdh-γs modify preformed synapses via trafficking of Pcdh-γs-containing organelles, insertion into synaptic membranes and homophilic transcellular interaction. Here we provide evidence in support of this model. We show for the first time that Pcdh-γs have homophilic properties and that they accumulate at dendro-dendritic and axo-dendritic interfaces during neuronal development. Pcdh-γs are maintained in a substantial mobile intracellular pool in dendrites and cytoplasmic deletion shifts the molecule to the surface and reduces the number and velocity of the mobile packets. We monitored Pcdh-γ temporal and spatial dynamics in transport organelles. Pcdh-γ organelles bud and fuse with stationary clusters near synapses. These results suggest that Pcdh-γ-mediated cell-cell interactions in synapse development or maintenance are tightly regulated by control of intracellular trafficking via the cytoplasmic domain.     

Reversal of rapidly transported protein and organelles at an axonal lesion

The time required for both rapid axonally transported organelles (vesicles and tubulo-vesicular structures) and proteins to undergo anterograde to retrograde reversal at a crush site was examined using sciatic nerve preparations obtained fromXenopus laevis. The transport and reversal of a pulse of newly synthesized³⁵S-labeled proteins was studied with a position-sensitive detector of ionizing radiation. Organelle transport and reversal were studied using video microscopy. Both protein and organelle reversal were assessed in two bathing media: a physiological saline and a medium that was compatible with the intracellular environment (internal medium). The time required for protein transport to reverse at a ligature was determined as a function of the time interval between the application of the ligature and the arrival of the pulse at the ligature (lesion time). In physiological saline, reversal times were greatest, about 3.5 h, when the lesion time was 1 h or less and decreased to approximately 1.5 h for lesion times of 4–12 h. When corrected for the approximately 2 mm length of degeneration caused by the saline, the results were similar to those obtained in internal medium and indicated a minimal reversal time for proteins of about 2 h. Organelle transport was examined close to narrow lesions in single myelinated axons. That the organelles moving away from the lesion represented organelles that had undergone reversed transport was suggested by observation of the reversal of individual organelles, and by a correlation between the flux of organelles towards and away from the lesion. Analysis of organelle flux within and adjacent to a segment of axon isolated by two lesions indicated that 70–80% of organelles moving away from a lesion represented reversed transport. Observations in internal medium were consistent with a reversal time of < 15 min, and in physiological saline < 30 min. The substantially smaller reversal time for organelle transport as compared to protein transport is consistent either with the existence of two types of organelles with different reversal times and hence different reversal times and hence different reversal mechanisms, or with the possibility that during reversal proteins are off-loaded from carrier organelles and subsequently up-loaded to different organelles.     

Inhibition of the rapid movement of optically detectable axonal particles colchicine and vinblastine.

The rapid saltatory motion of intra-axonal particles detected by dark-field microscopy in myelinated axons isolated from sciatic nerves of adult Xenopus laevis was inhibited by colchicine or vinblastine at a concentration of larger than or equal to 0.1 mM. Both the predominant somatopetal transport and the somatofugal transport of these round particles were inhibited. The reduction in numbers of moving particles was apparent first in the juxtanodal portions of the isolated axons within about 1 h. No particles could be detected moving by 3-5 h after application of the colchicine or vinblastine. During the phase of partial inhibition, those particles that were still progressing along the axon did so at apparently normal velocities while they were in motion, but remained stationary increasingly frequently and for progressively longer periods. Colchicine or vinblastine at a concentration of less than or equal to 10 micronM caused no observable inhibition within 4 h of application. Colchicine at a concentration of larger than or equal to 10 mM caused local accumulation of round particles, and vinblastine at a concentration of larger than or equal to 2.5 mM caused fragmentation of rod-shaped organelles, believed to be mitochondria. Electron microscopy of nerve fibers treated with 5 mM colchicine showed a progressive loss of microtubules from the axoplasm, such that approximately 70% of the microtubules had disappeared after 4h.     

Calmodulin regulates fast axonal transport of squid axoplasm organelles

The role of calmodulin (CaM) in organelle motility (fast axonal transport) in the axoplasm of the squid giant axon was evaluated directly using video-enhanced microscopy. Addition of 6 μM CaM to extruded squid axoplasm produced a 2.6-fold increase in the number of organelles moving per minute per unit area of axoplasm. When lower concentrations of CaM, including physiological concentration (2 μg/ml), were added to extruded axoplasm, the number of organelles moving was equally increased. CaM had no significant effect on the mean velocity of organelle translocations. The stimulatory effect of CaM was reduced significantly by the CaM inhibitors melittin (36 μM) and trifluoperazine (50 μM). Parvalbumin, a high-affinity calcium binding protein, did not stimulate motile activity. These results suggest that CaM is a positive regulator of fast axonal transport. At the molecular level, this regulation may involve microtubule-and/or actin-based motor proteins. Several possible molecular mechanisms are proposed.     

Persistence of Axonal Transport in Isolated Axons of the Mouse

We have examined the hypothesis, for the case of mouse axons, that isolating an axon from its cell body will lead to a rapid failure of fast axonal transport as anterogradely moving organelles vacate the axon in a proximo-distal direction, and retrogradely moving organelles vacate it in the opposite direction. We used CD1 and BALB/c mice and the Wallerian degeneration-resistant mutant C57BL/Ola. Sciatic nerves were cut high in the thigh; at various times up to 8 days later nerves were removed from the animal and individual myelinated axons from the segment distal to the cut were examined by video light microscopy to detect rapid organelle transport. Bidirectional fast organelle transport did decrease in amount with time but not nearly as rapidly as predicted, and anterograde and retrograde organelle velocities remained normal through time. In the C57BL/Ola mouse some structurally preserved axons contained organelles that transported at normal velocities in the anterograde and retrograde directions for as long as 8 days after axotomy. To test one of the possible origins of transported organelles in long-surviving axons we examined organelle transport very close to narrow lesions in axons bathed in a medium compatible with intracellular function. No organelles crossed the lesion but bidirectional organelle transport took place proximal and distal to the lesion; the amounts were compatible with the interpretation that ∼30% of organelles reversed transport direction on either side of the lesion. We propose that at least some of the organelles that undergo persistent transport in axons isolated from their cell bodies shuttle back and forth between the ends of the isolated segment.     

Simulated motion enhances neuronal selectivity for a sound localization cue in background noise.

In nature, sound sources move and signals are accompanied by background noise. Noting that motion helps the perception of visual stimuli, we tested whether motion similarly facilitates the detection of acoustic targets, at the neuronal level. Auditory neurons in the central nucleus of the barn owl's inferior colliculus (ICc), due to their selectivity for interaural phase difference (delta phi), are sharply tuned to the azimuth of sound sources and are arrayed to form a topographic map of delta phi. While recording from single ICc neurons, we presented tones that simulated either moving or stationary sound sources with and without background noise. We found that the tuning of cells in the ICc for delta phi was sharper for stimuli that simulated motion than for those that simulated stationary targets. The neurons signaled the presence of a tone obscured by noise better if the tone moved than if the tone remained stationary. The resistance to noise observed with moving stimuli could not be reproduced with the temporal modulation of the stimulus amplitude, suggesting that a change of position over time was required.     

Visual discrimination in the pigeon (Columba livia): effects of selective lesions of the nucleus rotundus.

The nucleus rotundus is a large thalamic nucleus in birds and plays a critical role in many visual discrimination tasks. In order to test the hypothesis that there are functionally distinct subdivisions in the nucleus rotundus, effects of selective lesions of the nucleus were studied in pigeons. The birds were trained to discriminate between different types of stationary objects and between different directions of moving objects. Multiple regression analyses revealed that lesions in the anterior, but not posterior, division caused deficits in discrimination of small stationary stimuli. Lesions in neither the anterior nor posterior divisions predicted effects in discrimination of moving stimuli. These results are consistent with a prediction led from the hypothesis that the nucleus is composed of functional subdivisions.     

G-protein effects on retrograde axonal transport

Movements of medium and large sized membranous organelles (0.5–1 μm in diameter) were visualized within segments of the crab walking leg nerve with Nomarski differential interference contrast optics and subjected to video contrast enhancement. Accessibility to the axoplasm was demonstrated by intra-axonal fluorescence following addition of rhodamine conjugated to 40 kDa dextran to the external medium. Perfusion of the axons with a 1 μM solution of the 20 kDa G-protein, cp20, but not control solutions, reduced the number of organelles moving in the retrograde direction per unit time, but not the number of organelles moving in the anterograde direction. Such alteration of organelle movement may contribute to memory-specific changes of neuronal morphology.     

Visual 'cortical-recipient' and 'tectal-recipient' pontine zones play distinct roles in cat visuomotor performance

In order to test the hypothesis that visual information reaching the cerebellum through the pontine nuclei is involved in the control of visually guided movements, the effects of bilateral kainic acid pontine lesions have been analysed in cats performing a reaching movement towards a spot of light that was either stationary or moving. In 4 cats, the lesion was restricted either to the ventromedian region (cortical-recipient zone) or to the dorsolateral nucleus (tectal-recipient zone) of the pons. A major and persistent impairment was seen when the cerebellum was deprived of the pontine information influenced by the colliculus. While cats displayed no impairment when reaching towards a stationary target, they exhibited a strong accuracy deficit associated with an increased reaction time when reaching towards a moving target. In contrast, lesioning the pontine zone influenced by the visual cortex induced a transient accuracy deficit with moving targets and a transient delay in movement onset whatever the mode of target presentation. These results emphasise the involvement of visual pontine regions in the guidance of movements; they also confirm previous results showing that tectal visual information plays a more important role than that originating in the visual cortex when movements are directed towards moving targets.     

Influences of Horizontal Connections on Visual Responses in Rabbit Striate Cortex

The goal of this study was to examine the role of horizontal connections in rabbit striate neurons. Anaesthetized rabbits were prepared in the usual fashion for single-cell recordings in area 17 of the visual cortex. We compared responses evoked by moving and stationary stimuli prior to, during and after recovery from lateral microinjection of either lidocaine ( n = 61), γ-aminobutyric acid (GABA, n = 18) or bicuculline ( n = 8) 2 mm from the recording site. This procedure allows evaluation of the contribution of neighbouring neurons in visual responses. Results showed that striate neurons are dependent on the adjacent cells¹ excitability. Modification of responses to stationary targets suggests that lateral interactions play a role in the generation of discharges to fixed stimuli. Lateral inactivation preferentially influenced non-directional over direction-selective units. This influence usually resulted in the non-directional neuron becoming directional by attenuation of the visually driven response in one direction. Simple and complex cells tended to be influenced differently by lateral inactivation. Simple cells became less responsive, whereas complex cells became more responsive. This dichotomy among cellular types suggests that simple cells receive mainly excitatory horizontal influences, while complex cells are contacted mostly by lateral inhibitory inputs.     

Influence of stationary and moving textured backgrounds on the response of visual neurons in toads (Bufo bufo L.)

Previous studies have shown that configurational prey recognition in common toads is performed by feature-analyzing functional units consisting of assemblies of connected neurons such as retinal (classes R2, R3), tectal [classes T5(1), T5(2), T5(3)], and pretectal (class TH3) cells. In the present paper, effects of textured backgrounds on the response of these neurons to a configurational moving stimulus have been tested quantitatively. (1) In all investigated neurons, neither the overall activation nor the respective stimulus-response relationships were significantly influenced by a stationary black/white-textured background as far as black stimulus objects are concerned. (2) The neuronal activity in response to a moving object (signal) could be inhibited (masked) if a black/white-textured background (noise) was moving simultaneously at the same speed. The strength (I) of this 'surround inhibition' (signal masking by the background) was different in the various classes of neurons, i.e. strongest for T5(2) and weakest for R3: IT5(2) greater than IT5(1) greater than IT5(3) greater than IR2 greater than ITH3 greater than IR3. These inhibitory effects were not correlated with the size of the neuronal excitatory receptive field (ERF), since T4 neurons (ERF = 180 degrees) in this context displayed response properties similar to T5(2) neurons (ERF less than 30 degrees). (3) It is suggested that the signal (prey)-masking effect of a moving textured background is brought about by pretecto (TH3)-tectal [T5(1), T5(2)] inhibitory connectivity which allows toads: (a) to select prey from nonprey; (b) to discriminate between prey and a textured background, and (c) to determine the origin of moving retinal images caused either by object movement or by self-induced motion.