Biomedical subjects

Christian Casanova

Publications and source records attributed to Christian Casanova.

7 recordsLinked to original sources

PubMed2006

Overlapping visual response latency distributions in visual cortices and LP-pulvinar complex of the cat.

The visual system of the cat is considered to be organized in both a serial and parallel manner. Studies of visual onset latencies generally suggest that parallel processing occurs throughout the dorsal stream. These studies are at odds with the proposed hierarchies of visual areas based on termination patterns of cortico-cortical projections. In previous studies, a variety of stimuli have been used to compute latencies, and this is problematic as latencies are known to depend on stimulus parameters. This could explain the discrepancy between latency and neuroanatomical based studies. Therefore, the first aim of the present study was to determine whether latencies increased along the hierarchy of visual areas when the same stimuli are used. In addition, the effect of stimulus complexity was assessed. Visual onset latencies were calculated for area 17, PMLS, AMLS, and AEV neurons. Latencies were also computed from neurons in the lateral posterior (LP)-pulvinar complex given the importance of this extrageniculate complex in cortical intercommunication. Latency distributions from all regions overlapped substantially, and no significant difference was present, regardless of the type of stimulus used. The onset latencies in the LP-pulvinar complex were comparable to those seen in cortical areas. The data suggest that the initial processing of information in the visual system is parallel, despite the presence of a neuroanatomical hierarchy. Simultaneous response onsets among cortical areas and the LP-pulvinar suggest that the latter is more than a simple relay station for information headed to cortex. The data are consistent with proposals of the LP-pulvinar as a center for the integration and distribution of information from/to multiple cortical areas.

Animals↗

PubMed2005

Pattern-motion selectivity in the human pulvinar.

On the basis of anatomical and physiological data obtained on animal models, we recently proposed that neurons in the main visual extrageniculate nuclei complex, the pulvinar, are actively involved in higher-order visual processing. Pulvinar neurons have been shown to integrate the component signals of a plaid pattern into a coherent global percept (pattern-motion selectivity). Using positron emission tomography (PET), we have investigated the possibility that the human pulvinar is also involved in plaid-defined higher-order motion integration. Plaid patterns were presented to normal observers in two conditions (coherent vs. transparent) created by varying the relative spatial frequency of the two gratings comprising the plaid. Regions of interest analysis revealed a significant activation of the pulvinar in the coherent condition supporting the notion that the human pulvinar nucleus is involved in higher-order motion processing. Plaid pattern activation was also observed in the medial temporal gyrus (area MT/V5), a motion area with strong anatomical connections to the pulvinar. These data provide the first direct evidence that the human pulvinar is involved in complex motion integration, as previously shown in animal models, and further support the existence of cortico-thalamo-cortical computational networks involved in higher-order visual processing.

Adult↗

PubMed2005

The photopic ERG of the albino guinea pig (Cavia porcellus): a model of the human photopic ERG.

Altricial rodents such as rats and mice are probably the most widely used animal model in the electroretinogram (ERG) literature. However, while the scotopic responses of these rodents share obvious similarities with that of humans, their photopic electroretinograms are strikingly different. For instance, the photopic ERGs of rats and mice include, when measurable, a minimal a-wave, while the b-wave is of much larger amplitude than that of humans. The purpose of this study is to present the albino guinea pig which is like humans, is a precocial animal, and is a better rodent model of the human photopic ERG. In order to investigate the above, photopic electroretinograms and oscillatory potentials, obtained from guinea pigs and human subjects, were compared. Furthermore, in a subset of animals we injected, intravitreally, selective blockers of the ON- (L-2-amino-4-phosphonobutyric acid: L-AP-4; 10 mM) or OFF- (kynurenic acid: KYN; 50 mM) retinal pathways in order to mimic similar retinal disorders found in human. Based on our results, we believe that, compared to rats and mice, the photopic (cone-mediated) ERG of the guinea pig clearly represents a superior rodent model of the human photopic ERG.

Animals↗

PubMed2004

Retinal projections to the lateral posterior-pulvinar complex in intact and early visual cortex lesioned cats.

In intact cats, it is generally considered that the lateral posterior-pulvinar complex (LP-pulvinar) does not receive direct retinal terminals, with the exception of the retino-recipient zone known as the geniculate wing. There is, however, some evidence that early lesions of the visual cortex can occasionally induce the formation of novel retinal projections to the LP nucleus. Given the importance of knowing the connectivity pattern of the LP-pulvinar complex in intact and lesioned animals, we used the B fragment of cholera toxin, a sensitive anterograde tracer, to reinvestigate the retinal projections to the LP-pulvinar in normal cats and in cats with early unilateral lesions of the visual cortex (areas 17 and 18). Immunohistochemical localization of the toxin was performed to show the distribution and morphology of retinofugal terminals. A direct bilateral but predominantly contralateral retinal projection reached the caudal portion of LPl and LPm in the form of patches located mainly along its dorsomedial surface and many scattered terminals. The distribution of retinal projections to LP-pulvinar in intact and operated cats did not differ. Contrary to what had been previously reported, we found no evidence for lesion-induced sprouting of retinal axons in these higher-order thalamic nuclei. Retinal input to the LP-pulvinar might modulate visual responses driven by primary visual cortex or superior colliculus.

Animals↗

PubMed2003

On the use of isoflurane versus halothane in the study of visual response properties of single cells in the primary visual cortex.

Halothane is a widely used anesthetic in research. It produces several alterations in organs, especially in the brain. Recently, isoflurane emerged in neuroscience laboratories. For many reasons it appears to be better than halothane for animal brain research (e.g. isoflurane induces lower intracranial pressure, and is less detrimental on the cardiovascular system). However, no one is in a position to recommend it in electrophysiology research because its effects on specific brain functions are relatively unknown. Given that both anesthetics yield different actions on gross brain activity (EEG, VEP), it is likely that they differentially affect single neuron activity. The goal of this study is to determine whether halothane or isoflurane use is best suited to study the receptive field properties of neurons in the cat's primary visual cortex. Extra-cellular recordings were made for both anesthetics in area 17 of adult cats under different levels of anesthesia. Results indicate that various cell parameters differ under halothane anesthesia when compared with isoflurane. The main difference between the two anesthetics is the greater depression of the cell optimal visual response amplitude induced by isoflurane at equipotent concentration. Due to its stronger depressive effects, isoflurane may not be the ideal anesthetic for single-cell recordings in the primary visual cortex.

Action Potentials↗

PubMed2003

Dark adaptation is faster in pigmented than albino rats.

PURPOSE: Previous reports have raised the possibility that, compared to pigmented rats, albino rats might be night blind. The purpose of this study was to reinvestigate this issue by comparing the dark-adaptation process of the pigmented Long-Evans (LE) and albino Sprague Dawley (SD) rats. METHODS: Scotopic ERGs obtained from LE and SD rats were recorded following periods of dark adaptation 0.5, 3 and 12 h. Intensity response functions were generated with flashes of white light spanning over a 7 log-unit range with a maximal intensity of 8 cd x s x m(-2) in energy. RESULTS: SD rats showed a gradual increase in the amplitude of the scotopic b-wave Vmax (maximal 'saturated' rod b-wave amplitude) and retinal sensitivity (k) as the duration of the dark-adaptation period increased. In contrast, LE rats did not demonstrate any further significant gain in retinal function (Vmax or k) beyond 30 min of dark-adaptation. Thus for periods of dark-adaptation of 30 min or less, the rod function of the LE rats is superior to that of the SD rats while both strains have comparable retinal functions following 3 h or more of dark-adaptation. CONCLUSIONS: Our results indicate that LE and SD rats differ in their rapidity to dark-adapt, a finding that could explain the previous claim that SD rats were night blind. The reduced bio-availability of calcium ions in eyes lacking melanin could explain this difference. Calcium was previously shown to play a key role in retinal adaptation processes.

Albinism, Oculocutaneous↗

PubMed2002

Central and peripheral interactions in the perception of optic flow.

The purpose of this work was to evaluate the effects of central and peripheral stimulation on the perception of optic flow over large spatial extents. Coherence thresholds were measured for RDKs simulating observer translation and radial motion. Experiments 1 and 3a measured sensitivity to a range of speeds for a circular central region, for several annular regions of increasing eccentricity, and for a full-field stimulus (80 degrees diameter). Results suggest that the spatial extent over which signals are integrated may vary in order to maximize the information available for perceptual representations. Experiments 2 and 3b evaluated central and peripheral interactions in a direction discrimination task, by comparing the effects of different signal strengths and directions in one of the two regions. The presence of noise dots (0% coherence) in either center or periphery led to a performance decrease from baseline measures. A similar decrease was observed when dots in the two regions moved in opposite directions. When dots in both regions moved in the same direction, a stronger peripheral signal led to facilitation of direction discrimination, whereas a stronger central signal did not. These findings suggest that central and peripheral inputs are not separable in the integration of optic flow, that they contribute equally to the percept under normal conditions (equal signal strength), and that peripheral stimulation seems important under ecologically relevant conditions such as poor visibility.

Discrimination, Psychological↗