Effective connectivity of dorsal and ventral visual pathways in chunk decomposition

Chunk decomposition is defined as a cognitive process which breaks up familiar items into several parts to reorganize them in an alternative approach. The present study investigated the effective connectivity of visual streams in chunk decomposition through dynamic causal modeling (DCM). The results revealed that chunk familiarity and perceptual tightness made a combined contribution to highlight not only the “what” and the “where” streams, but also the effective connectivity from the left inferior temporal gyrus to the left superior parietal lobule.     

Honeybees' speed depends on dorsal as well as lateral, ventral and frontal optic flows

Flying insects use the optic flow to navigate safely in unfamiliar environments, especially by adjusting their speed and their clearance from surrounding objects. It has not yet been established, however, which specific parts of the optical flow field insects use to control their speed. With a view to answering this question, freely flying honeybees were trained to fly along a specially designed tunnel including two successive tapering parts: the first part was tapered in the vertical plane and the second one, in the horizontal plane. The honeybees were found to adjust their speed on the basis of the optic flow they perceived not only in the lateral and ventral parts of their visual field, but also in the dorsal part. More specifically, the honeybees' speed varied monotonically, depending on the minimum cross-section of the tunnel, regardless of whether the narrowing occurred in the horizontal or vertical plane. The honeybees' speed decreased or increased whenever the minimum cross-section decreased or increased. In other words, the larger sum of the two opposite optic flows in the horizontal and vertical planes was kept practically constant thanks to the speed control performed by the honeybees upon encountering a narrowing of the tunnel. The previously described ALIS ("AutopiLot using an Insect-based vision System") model nicely matches the present behavioral findings. The ALIS model is based on a feedback control scheme that explains how honeybees may keep their speed proportional to the minimum local cross-section of a tunnel, based solely on optic flow processing, without any need for speedometers or rangefinders. The present behavioral findings suggest how flying insects may succeed in adjusting their speed in their complex foraging environments, while at the same time adjusting their distance not only from lateral and ventral objects but also from those located in their dorsal visual field.     

Responses of dorsal and ventral thalamic neurons to visual stimulation in Emys orbicularis tortoises]

Extracellular studies have been made on the background activity and reactions to visual stimuli in neurons of nucleus rotundus and nucleus suprapeduncularis of the thalamus in the tortoise E. orbicularis. Reactions of on-off type to diffuse light flashes were recorded predominantly in both of the nuclei, whereas specific reactions to movement stimuli were found in the neurons of n. rotundus. The receptive area of the neurons in both of the nuclei involves all the visual field of one or both of the eyes. The structure of the receptive area of the neurons of n. suprapeduncularis is relatively homogeneous, whereas the receptive area of the neurons in n. rotundus exhibits a zone with an increased excitability under the horizontal meridian. After a damage of tectum opticum, the heterogeneity of the receptive areas and the specific responses to movement in cells of n. rotundus disappeared, while thresholds of the reactions increased. In the neurons of n. suprapeduncularis homogeneous structure of the receptive area remained unaffected, the thresholds of the reactions being increased as well.     

Syntactic processing depends on dorsal language tracts

Frontal and temporal language areas involved in syntactic processing are connected by several dorsal and ventral tracts, but the functional roles of the different tracts are not well understood. To identify which white matter tract(s) are important for syntactic processing, we examined the relationship between white matter damage and syntactic deficits in patients with primary progressive aphasia, using multimodal neuroimaging and neurolinguistic assessment. Diffusion tensor imaging showed that microstructural damage to left hemisphere dorsal tracts--the superior longitudinal fasciculus including its arcuate component--was strongly associated with deficits in comprehension and production of syntax. Damage to these dorsal tracts predicted syntactic deficits after gray matter atrophy was taken into account, and fMRI confirmed that these tracts connect regions modulated by syntactic processing. In contrast, damage to ventral tracts--the extreme capsule fiber system or the uncinate fasciculus--was not associated with syntactic deficits. Our findings show that syntactic processing depends primarily on dorsal language tracts.     

Protein synthesis in dorsal and ventral regions of Xenopus laevis embryos in relation to dorsal and ventral differentiation

Two-dimensional gel electrophoresis has been used to analyze protein synthesis in dorsal and ventral regions in embryonic stages of Xenopus laevis. Proteins specific either to dorsal or to ventral regions are synthesized for the first time at gastrulation, concomitant with morphological differentiation. The reliability of these proteins as markers of dorsal and ventral differentiation was tested by examining their synthesis in Uv-irradiated embryos, which have severely reduced capacity for dorsal development, reflected in reduced levels of the neuromuscular-specific enzyme acetylcholinesterase, but which continue to synthesize the great majority of proteins at normal rates. Synthesis of dorsal indicator proteins should be reduced or absent in these embryos, whereas ventral indicators should be synthesized at least to the same extent as in control embryos. Some of the putative dorsal and ventral indicators failed this test, but the majority were confirmed as reliable markers of dorsal and ventral differentiation, thus providing a connection between morphology and gene expression in the establishment of the dorsal-ventral axis in X. laevis.     

Two visual pigment opsins,one expressed in the dorsal region and another in the dorsal and ventral regions,of the compound eye of a dragonfly,Sympetrum frequens

This paper describes the primary structure of two visual pigment opsins (DfRh1 and DfRh2) in the regionalized compound eye of a dragonfly,Sympetrum frequens. The amino acid sequences were deduced from the nucleotide sequences of cDNAs isolated from a cDNA library of the dragonfly retina. The two opsins both consist of 379 amino acids with 81.3% identity. Analysis of hydropathy indicated that the sequences have seven transmembrane domains like those of previously described opsins. Expression analysis using RT-PCR revealed that DfRh1 was present only in the dorsal region whereas DfRh2 was detected in both the dorsal and the ventral regions of the eye.     

Keratocytes pull with similar forces on their dorsal and ventral surfaces

As cells move forward, they pull rearward against extracellular matrices (ECMs), exerting traction forces. However, no rearward forces have been seen in the fish keratocyte. To address this discrepancy, we have measured the propulsive forces generated by the keratocyte lamella on both the ventral and the dorsal surfaces. On the ventral surface, a micromachined device revealed that traction forces were small and rearward directed under the lamella, changed direction in front of the nucleus, and became larger under the cell body. On the dorsal surface of the lamella, an optical gradient trap measured rearward forces generated against fibronectin-coated beads. The retrograde force exerted by the cell on the bead increased in the thickened region of the lamella where myosin condensation has been observed (Svitkina, T.M., A.B. Verkhovsky, K.M. McQuade, and G. G. Borisy. 1997. J. Cell Biol. 139:397-415). Similar forces were generated on both the ventral (0.2 nN/microm(2)) and the dorsal (0.4 nN/microm(2)) surfaces of the lamella, suggesting that dorsal matrix contacts are as effectively linked to the force-generating cytoskeleton as ventral contacts. The correlation between the level of traction force and the density of myosin suggests a model for keratocyte movement in which myosin condensation in the perinuclear region generates rearward forces in the lamella and forward forces in the cell rear.     

Differential effects of viewpoint on object-driven activation in dorsal and ventral streams

Using fMRI, we showed that an area in the ventral temporo-occipital cortex (area vTO), which is part of the human homolog of the ventral stream of visual processing, exhibited priming for both identical and depth-rotated images of objects. This pattern of activation in area vTO corresponded to performance in a behavioral matching task. An area in the caudal part of the intraparietal sulcus (area cIPS) also showed priming, but only with identical images of objects. This dorsal-stream area treated rotated images as new objects. The difference in the pattern of priming-related activation in the two areas may reflect the respective roles of the ventral and dorsal streams in object recognition and object-directed action.     

Coding of visual objects in the ventral stream

How are objects represented in the brain? Two facets of this question are currently under investigation. First, are objects represented by activity in a relatively small number of neurons that are each selective for the shape or identity of a specific object (a 'sparse code'), or are they represented by a pattern of activity across a large number of less selective neurons (a 'population code')? Second, how are the neurons that code for an object distributed across the cortex: are they clustered together in patches, or are they scattered widely across the cortex? The results from neurophysiology and functional magnetic resonance imaging studies are beginning to provide preliminary answers to both questions.     

Invariance and selectivity in the ventral visual pathway.

Pattern recognition systems that are invariant to shape, pose, lighting and texture are never sufficiently selective; they suffer a high rate of "false alarms". How are biological vision systems both invariant and selective? Specifically, how are proper arrangements of sub-patterns distinguished from the chance arrangements that defeat selectivity in artificial systems? The answer may lie in the nonlinear dynamics that characterize complex and other invariant cell types: these cells are temporarily more receptive to some inputs than to others (functional connectivity). One consequence is that pairs of such cells with overlapping receptive fields will possess a related property that might be termed functional common input. Functional common input would induce high correlation exactly when there is a match in the sub-patterns appearing in the overlapping receptive fields. These correlations, possibly expressed as a partial and highly local synchrony, would preserve the selectivity otherwise lost to invariance.     

The photopic spectral sensitivity of the dorsal and ventral retinae of the chicken

Summary The relative spectral sensitivities of the dorsal and ventral chicken (Gallus gallus) retinae were determined under photopic conditions by means of electroretinography and compared with data from the pigeon (Columba livid). Differences in spectral sensitivity between the dorsal and ventral chicken retinae appear only in the short wavelength range. In the chicken the dorsal retina is more sensitive to near UV light than the ventral retina relative to long wavelengths (beyond 470 nm), but both retinal areas are less sensitive to near UV than in the pigeon. The variation in relative near UV sensitivity is discussed in relation to recent data on the retinal distribution of different types of oil droplets in birds. The adaptive significance of near UV sensitivity is also discussed.     

Cognitive and perceptual functions of the visual thalamus

The thalamus is classically viewed as passively relaying information to the cortex. However, there is growing evidence that the thalamus actively regulates information transmission to the cortex and between cortical areas using a variety of mechanisms, including the modulation of response magnitude, firing mode, and synchrony of neurons according to behavioral demands. We discuss how the visual thalamus contributes to attention, awareness, and visually guided actions, to present a general role for the thalamus in perception and cognition.     

Cognitive mechanisms of nicotine on visual attention

Understanding nicotine's neurobiological and cognitive mechanisms may help explain both its addictive properties and potential therapeutic applications. As such, functional MRI was used to determine the neural substrates of nicotine's effects on a sustained attention (rapid visual information-processing) task. Performance was associated with activation in a fronto-parietal-thalamic network in both smokers and nonsmokers. Along with subtle behavioral deficits, mildly abstinent smokers showed less task-induced brain activation in the parietal cortex and caudate than did nonsmokers. Transdermal nicotine replacement improved task performance in smokers and increased task-induced brain activation in the parietal cortex, thalamus, and caudate, while nicotine induced a generalized increase in occipital cortex activity. These data suggest that nicotine improves attention in smokers by enhancing activation in areas traditionally associated with visual attention, arousal, and motor activation.     

Dependence of stability of the visual cognitive set on motivation

In order to reveal the character of motivation influence on the stability of the unconscious visual set, young adult healthy subjects were promised to be rewarded for correct solution of the cognitive task. A substantial increase in set stability was observed under conditions of augmented motivation of a subject to the result of his performance. Positive and negative aspects of the impact of the increased motivation of a subject on his cognitive activity at the stages of set formation and actualization are discussed.