Movement and novelty of a square wave display affect 2-deoxyglucose uptake in the rat visual system

Tracing the route to path analysis in neuroimaging

This article provides a personal perspective of the adoption of path analysis (structural equation modeling) to neuroimaging. The paper covers the motivation stemming from the need to merge functional measures with neuroanatomy and early innovations in its application. The use of path analysis as a means to test directional hypotheses about networks is presented along with the development of the complementary method, partial least squares. A method is useful when it provides insights that were previously inaccessible, and reflecting this, the paper concludes with a synopsis of the theoretical developments that arose for the routine use of methods like path analysis.

Functional impairment of the rat superior colliculus after kainic acid intraocular injection: A 2-Deoxyglucose study

Long Evans rats monocularly injected with the kainic acid (KA), were exposed to "tonic" (diffuse steady light, stationary pattern, total darkness) and "phasic" (flashing, moving pattern) stimulations. By means of the autoradiographic 2-deoxyglucose (2DG) technique we assessed the functional activity of the Superior Colliculus (SC) contralateral to the injected eye as compared to the normal eye SC. In the control SC all "tonic" stimulations determined low 2DG uptake not modified by the intraocular KA injection. On the contrary, "phasic" stimulations elicited a strong 2DG consumption in the normal SC, with a peculiar pattern of distribution depending on the kind of stimulus. Considering the total 2DG uptake as the added intrinsic and afferent metabolism, KA was able to affect only the latter, decreasing two-fold that expected for the afferent input loss. These findings can suggest a possible KA effect on off-line ganglion cells and, on the other side, they confirm the role of the SC in discriminating "phasic" and sudden phenomena from "tonic" and continuous ones.

Quantitative validation of voxel-wise statistical analyses of autoradiographic rat brain volumes: application to unilateral visual stimulation

PET scanners devoted to in vivo functional study have recently been developed, but autoradiography remains the reference technique for assessing cerebral glucose metabolism (CMRGlu) in rodents. Autoradiographs are conventionally subjected to region of interest (ROI) analysis, which is intrinsically hypothesis-driven and therefore not suitable for whole-brain investigation. Voxel-wise statistical methods of analysis have long been used to determine differences in brain activity during in vivo functional neuroimaging experiments. They have also recently been applied to 3D reconstructed autoradiographic volume images from rat brains. We present here a fully automated analysis for autoradiographic data combining (1) computerized procedures for the acquisition and 3D reconstruction of postmortem volume images and (2) spatial normalization followed by classical whole-brain voxel-wise statistical analysis. We also describe an additional procedure for characterizing functional differences between the right and left hemispheres of the brain. We compared two spatial normalization techniques and evaluated how the effect of choosing a particular normalization technique impacted on the statistical analysis. We also propose a small volume correction analysis to address the problem of multiple statistical comparisons. Lastly, we investigated the reliability of such analyses, by comparing their results qualitatively and quantitatively with those previously obtained with our semiautomated ROI-based analysis [Dubois, A., Dauguet, J., Herard, A.-S., Besret, L., Duchesnay, E., Frouin, V., Hantraye, P., Bonvento, G., Delzescaux, T., 2007. Automated three-dimensional analysis of histologic and autoradiographic rat brain sections: application to an activation study. J. Cereb. Blood Flow Metab. 27 (10), 1742-1755.]. Both voxel-wise statistical analyses led to the detection of consistent interhemispheric differences in CMRGlu. This work demonstrates the potential value and robustness of voxel-wise statistical methods for analyzing autoradiographic data sets.

Automated three-dimensional analysis of histological and autoradiographic rat brain sections: application to an activation study

Besides the newly developed positron emission tomography scanners (microPET) dedicated to the in vivo functional study of small animals, autoradiography remains the reference technique widely used for functional brain imaging and the gold standard for the validation of in vivo results. The analysis of autoradiographic data is classically achieved in two dimensions (2D) using a section-by-section approach, is often limited to few sections and the delineation of the regions of interest to be analysed is directly performed on autoradiographic sections. In addition, such approach of analysis does not accommodate the possible anatomical shifts linked to dissymmetry associated with the sectioning process. This classic analysis is time-consuming, operator-dependent and can therefore lead to non-objective and non-reproducible results. In this paper, we have developed an automated and generic toolbox for processing of autoradiographic and corresponding histological rat brain sections based on a three-step approach, which involves: (1) an optimized digitization dealing with hundreds of autoradiographic and histological sections; (2) a robust reconstruction of the volumes based on a reliable registration method; and (3) an original 3D-geometry-based approach to analysis of anatomical and functional post-mortem data. The integration of the toolbox under a unified environment (in-house software BrainVISA, http://brainvisa.info) with a graphic interface enabled a robust and operator-independent exploitation of the overall anatomical and functional information. We illustrated the substantial qualitative and quantitative benefits obtained by applying our methodology to an activation study (rats, n=5, under unilateral visual stimulation).

Diffuse light increases metabolic activity in the lateral geniculate nucleus, visual cortex, and superior colliculus of the cone-dominated ground squirrel visual system

Ground squirrels were monocularly exposed to either steady- or flashing-diffuse light for 45 min following an injection of 14C 2-deoxyglucose (2-DG). Autoradiographic analysis indicated greater metabolic activity in the lateral geniculate nucleus, visual cortex and superior colliculus (SC) of the hemisphere lying contralateral to and receiving input from the diffusely stimulated eye (covered by a white mask), than in the corresponding regions of the other hemisphere receiving input from the occluded eye (black mask). The diffuse light results for the cortex and colliculus of the diurnal ground squirrel are different from those for the nocturnal rat. In the rat visual cortex, there is no difference between metabolic activity under conditions of diffuse light (steady or flashing) and under darkness. In the rat SC, although flashing-diffuse light increases metabolic activity (as is the case for the squirrel), steady-diffuse light decreases it to a level below that which occurs in darkness. The cortex and colliculus differences in 2-DG response to diffuse light between the ground squirrel and rat were attributed to differences in the operations of their respective cone- and rod-dominated visual systems.

Metabolic activity in rat visual system during exposure to high and low intensities of patterned and diffuse light

An examination of rat visual system activity, during exposure to either "pattern" (black and white stripes) or "diffuse" (eye covered by white mask) visual stimulation at high or low illumination intensities (1600 and 1 lux at cornea, respectively), was carried out using the 2-deoxyglucose (2-DG) autoradiographic technique. Pattern elevated 2-DG uptake in the dorsal and ventral lateral geniculate nuclei, in the lateral posterior nucleus, and in area 17, but was less effective at the high than at the low light intensity. Diffuse light also elevated 2-DC uptake in the thalamic nuclei but the increase was less impressive and the same at both intensities. Diffuse light at either high or low intensity had no effect on cortex. Like thalamus and cortex, pattern was a less effective stimulus for the colliculus at the high than at the low intensity, but, in contrast to thalamus and cortex, high intensity diffuse light suppressed 2-DG uptake in the colliculus to a level below that produced by darkness; low intensity diffuse light had no effect. These 2-DG findings are discussed in terms of how forebrain and midbrain divisions of the rat's rod-dominated visual system maintained their respective spatial processing and change-detecting functions over a considerable range of illumination intensity.

Structural modeling of functional visual pathways mapped with 2-deoxyglucose: effects of patterned light and footshock

This paper describes the first application of structural modeling to the visual system. Structural modeling, or path analysis, is a mathematical method that allows for the quantification of the functional strengths of anatomical connections between the structures that form a neural system. The objective was to demonstrate how structural modeling can be used to determine the functional interrelationships between brain structures that form the visual system and how these interrelationships change under different conditions. Data were obtained from measures of 2-deoxyglucose uptake in the visual system of rats presented with either patterned light or darkness. The effects of arousing footshock on visual system operations were also investigated. Models based on the anatomical connections and the interregional correlations between metabolic activity data were used to determine path coefficients representing the magnitude of the influence of each directional path. Statistical evaluation of the models revealed that the dominant positive influences on visual system activity in the darkness were the tectocortical subsystem and the descending connections from secondary visual cortex. In the patterned light model, the total influence of the geniculocortical subsystem was higher than in the dark, and the tectocortical pathways showed both a reduction and a shift in the direction of effects. The models also revealed that the effects of footshock-induced arousal on visual system operations depended upon the visual environment and on extra-visual influences. The footshock led to an increase in the interaction of the two main subsystems at the level of connections between primary visual cortex and the lateral posterior nucleus, and a descending negative influence from the secondary visual cortex became dominant. The models are discussed in the context of conventional analyses to show how structural modeling allows for the determination of much more information about the functional interactions within the visual system of subjects under different experimental conditions.

Effects of flashing-diffuse light on [2-14C]deoxyglucose uptake in the visual system of the black-hooded rat

What components of the visual system process diffuse light information? A [2-14C]deoxyglucose (2-DG) autoradiographic analysis revealed that exposure of freely moving rats (wearing light-diffusing masks) to flashing-diffuse light consistently elevated 2-DG uptake in the lateral geniculate nucleus and superior colliculus to levels rivalling those occurring in rats exposed to flashing-gratings. Uptake in visual cortex (area 17) in response to flashing-diffuse light, however, varied as a function of early contour experience, i.e. lower than that produced by darkness in rats reared with high contrast patterns, higher than darkness in rats which had been lid-sutured from the time of eye opening, and falling between these two extremes in 'ordinary' cage-reared rats. The findings point to subcortical mediation of discriminations based on diffuse light information. Cortex might participate in the processing of diffuse light information in the special case of animals lacking contour experience during development.

Metabolic activation of the rat visual system by patterned light and footshock

Autoradiography with [14C]2-deoxy-D-glucose was used to examine metabolic changes in the visual system of hooded rats exposed to patterned light or to darkness following footshock. Primary retinorecipient structures (superficial layer of the superior colliculus and the dorsal lateral geniculate nucleus) showed a response to light but not to shock. Higher visual sites showed two different shock effects. First, in darkness the intermediate grey layer of the superior colliculus was suppressed by the shock. Second, in the lateral posterior nucleus and primary visual cortex, the footshock led to significant enhancement of the metabolic responses to the patterned light. The findings suggest that footshock-induced arousal has significant modulatory effects on the operations of higher visual centers of behaving rats.

Structural modeling of functional neural pathways mapped with 2-deoxyglucose: effects of acoustic startle habituation on the auditory system

This paper describes the first application of structural modeling to neuroscience. Structural modeling (also known as path analysis) is a method to assess the relative impact of directional links in a system and how these interrelations may change under different conditions. The objective was to demonstrate how structural modeling can be used to determine the functional interrelationships between brain structures that form the auditory system. Using structural modeling, changes in auditory system 2-DG uptake were examined during long- and short-term habituation of the acoustic startle reflex. Models were based on the anatomical connections between central auditory system structures. Using functional 2-DG data, the correlations between these structures were calculated and numerical weights were computed for each anatomical link. The analysis revealed that the lemniscal path was dominant during short-term habituation, while during long-term habituation this influence was modified through extra-lemniscal pathways. The models are discussed in the context of previous findings to demonstrate how structural modeling can not only complement, but also extract more information from 2-DG mapping experiments.