Reflection of an Orienting Reflex in the Phases of Evoked Potentials in the Rabbit Visual Cortex and Hippocampus during Substitution of Stimulus Intensity

Experiments on conscious rabbits were performed using the oddball paradigm, in which a rare (deviant) and common (standard) stimuli were of the same color but different intensities. Deviant stimuli were of lesser intensity. Recordings were made of evoked potentials induced by series of uniform deviant stimuli (without using standard stimuli), which were presented at the beginning and end of stimulation. Visual evoked potentials recorded in response to deviant stimuli in the visual cortex and hippocampus showed increases in the amplitudes of phases, shifted towards positivity as compared with responses to standard stimuli and uniform deviant stimuli at the beginning and end of stimulus blocks. Significant changes affected phases P1 and P2 of visual evoked potentials in the cortex and phases P1, N1, and P2 in the hippocampus. The most significant increase in evoked potentials in the cortex was seen for the P2 peak (P130). It is suggested that changes in responses to oddball-deviant stimuli result from an orienting reflex to rare, unexpected stimuli and that the P2 (P130) peak in the cortex is associated with transmission of information regarding changes in the intensity of the light. The amplitude of this peak was shown to be decreased in responses to uniform deviant stimuli at the beginning and end of stimulus blocks. It was also demonstrated that the clearest and most contrasting changes in visual evoked potentials in responses to deviant and standard stimuli were seen with the smallest differences in intensity between these types of stimulus, this reflecting increases in the orienting reflex at threshold differences.     

Effect of the phenol antioxidant katavidan on autooxidation of microsomes exposed to visible light

A study is performed of the effect of the phenol antioxidant katavidan on autooxidation of microsomes from rat liver exposed to visible light. It is shown that katavidan in a concentration of 10⁻³ M inhibits but in concentrations of 10⁻⁵–10⁻⁷ M stimulates autooxidation of microsomes. No stimulation is observed under conditions of dark incubation. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 118, № 10, pp. 393–394, October, 1994 Presented by I. P. Ashmarin, Member of the Russian Academy of Medical Sciences     

Synthesis and experimental study of a dextran-ferrite-based magnetically driven nanopreparation as an MRI contrast agent for tumor-feeding vessels

Dextran-ferrite (DF) nanoparticles were synthesized and the related DF sol was successfully tested in vivo as a negative contrast agent for enhancing early magnetic resonance imaging (MRI) detection of tumor, capsule, and tumor-feeding vessels. Intravenous injection of DF in combination with Magnevist and subsequent T₂,T₂^*-weighted 3D scanning gave enhanced MRI images that defined the arrangement and functional state of surface and other vessels feeding capsule and tumor in C57Bl/6j mice with a mammary adenocarcinoma (Ac755) model.     

Evoked Potentials in the Rabbit Visual Cortex Reflect Changes in Line Orientation and Intensity

Changes in the amplitudes of evoked potentials in the visual cortex of conscious rabbits in response to substitution of flashing lines of different orientations (0–90°) but constant intensity were studied, along with interneurons of different intensities but constant orientation, and complex stimuli with simultaneous changes in flash orientation and intensity. Factor analysis of the results showed that analysis of the N85 peak of evoked potentials produced by substitution of stimuli with different orientations but constant intensity identified a two-dimensional sensory space for orientations. An achromatic sensory space was also detected using substitution of lines of different intensities but constant orientation. Substitution of complex stimuli involved two versions of the experiment. In the first version, four stimuli in the initial orientations (0–38.58°) had an intensity of 5 cd/m², the other stimuli (with orientations of 51.44–90°) were presented at an intensity of 15 cd/m². On the plane of the sensory space formed by the first two significant factors, stimuli with different intensities were located in different quadrants of the circle, while within the quadrants themselves, the stimuli were located in accord with their orientations, from lower values to greater. It is suggested that in this version, an interaction between orientation and intensity attributes was seen on the single plane of the sensory space, with a clear predominance of the intensity factor. The other experimental version also included eight complex stimuli, each complex having its own orientation (one of eight over the range 0–90°) and intensity (also one of eight, in the range 5–21 cd/m²). In all experiments involving substitution of complex stimuli, factor analysis identified three to four significant factors. In the vast majority of cases, only the sensory space plane X1, X2 was found, this being formed by two significant factors. On this plane, the stimuli were located in order of changes in intensity. This may be associated with the fact that rabbits are crepuscular animals, such that stimulus brightness is the most important attribute. However, in some cases, potentials in the rabbit brain also demonstrated simultaneous processing of two visual stimulus attributes, i.e., intensity and orientation. This may be evidence indicating analysis of complex stimuli in the primary visual cortex.     

Computation of Color and Brightness Differences by Rabbit Visual Cortex Neurons

Extracellular recording of the activity of 54 neurons in the rabbit visual cortex in responses to substitutions of eight colored and eight monochromatic stimuli in pairs was studied. Stimuli were uniform flashes of light displayed on an SVGA monitor and illuminated the whole retina. The responses of phasic neurons showed an initial discharge (50–90 msec from the moment of the change in stimulus), associated with the brightness or color difference between the stimuli. These “discrimination discharges” were used to construct an 8 × 8 matrix for each neuron, showing the mean number of spikes per sec in responses to changes in different pairs of stimuli. Processing of the matrix by factor analysis identified the major factors determining the axes of the sensory space. A brightness space with only two dimensions, with darkness and brightness orthogonal axes, was seen for 30% of neurons. A four-dimensional color space was seen in 22% of neurons, with two color and two achromatic axes. The sensory space of these neurons was similar to the spaces obtained by analyzing the early components of visual evoked potentials in rabbits induced by changes in color stimuli and behavioral operant responses in conditioned reflex color differentiation. The fundamental coincidence of the sensory spaces obtained by different methods identifies the general nature of the principle of vector coding and the existence of special neuronal mechanisms for detection of color and brightness differences in the visual field. __________ Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 55, No. 1, pp 60–70, January–February, 2005.     

Limited plasticity of difference neurons in the visual cortex and hippocampus in rabbits during the oddball (random substitutions) test

The activity of 41 visual cortex neurons and 20 hippocampal field CA1 neurons was studied in rabbits during application of the oddball stimulation paradigm using color stimuli of different intensities. Among these cells, about one third were plastic cells (34% of cortical cells and 37% of hippocampal cells). These neurons showed significant increases in late responses, at times 200–500 and 200–1000 msec for visual cortex neurons and 300–550 msec for hippocampal neurons, to rare deviant stimuli of lesser intensity as compared with responses to the frequent standard stimuli of greater intensity. The initial peak of the response (40–120 msec), the “difference discharge,” remained stable in responses to deviant and standard stimuli throughout the experiment. It is suggested that the strengthening of the late components of neuron responses to rare deviant stimuli (limited plasticity) reflects inclusion of the mechanisms of the orientational reflex. __________ Translated from Zhurnal Vysshei Nervnoi Deyatel’nosti imeni I. P. Pavlova, Vol. 55, No. 3, pp. 360–367, May–June, 2005.