Search

Abstract: The cortical representation of heading perception derives from several functional processes distributed across many cortical areas. The aim of the present study was to assess if the optic flow motion directions, expansion and contraction, differently modulate the firing activity of area PEc neurons. We determined the influence of the eye position and/or the spatial position of the focus of expansion (FOE) on this activity. Single neuron activity during radial optic flow stimulation was recorded in three behaving monkeys. The retinal FOE position and the spatial eye position were examined in order to study eye position''s influence upon the directional selectivity for the radial stimuli. We observed that the neurons able to discriminate the retinotopic FOE position are differently modulated by expansion and contraction. One class of neurons exhibited a different preferred FOE position during expansion and contraction. A second class showed the same preferred position with similar firing activity in the two stimuli. A third class showed the same preferred position but different firing activity. Eye position affected the directional selectivity of most PEc cells. The main result of this study is that there is a continuum in cell modulation by optic flow direction, and it can be modified by the angle of gaze with respect to the FOE. These results shed light on potential cellular integrative mechanisms of area PEc in heading perception. [Copyright &y& Elsevier]

Abstract: The visual perception of self-motion is mainly provided by optic flow. Eyes usually scan the environment during locomotion, and the gaze is not always directed to the focus of expansion (FOE) of the flow field. Such eye movements change the retinal FOE position with respect to the fovea. Here, we assess if optic flow selective neurons in parietal area PEc are modulated by eye position. We recorded single neuron activity during radial optic flow stimulation in two monkeys, varying eye and retinal FOE positions. We found that the majority of PEc neurons are modulated by the FOE retinotopic position with different tuning for expansion and contraction. Although many neurons did not show any gaze field without visual stimulation, the eye position modulated optic flow responses in about half of the cells. These novel results suggest that PEc neurons integrate both visual and eye position signals, and allow us to hypothesize their role in guiding locomotion as a part of a cortical network involved in FOE representation during self-motion. Visual and eye position interaction in this area could be seen as a contribution to the building of the invariant space representation necessary to motor planning. [ABSTRACT FROM AUTHOR]

Abstract: Neurons in area PEc in the superior parietal cortex encode signals from different modalities, such as visual, extraretinal and somatosensory, probably combining them to encode spatial parameter of extrapersonal space to prepare body movements. This study reports the characterization of the functional properties of PEc non-visual neurons that showed saccade-related activity. We analyzed the pre- and post-saccadic firing activity in 189 neurons recorded in five hemispheres of three behaving monkeys. Spiking activity of PEc single neurons was recorded while the monkeys performed visually-guided saccades in a reaction time task. We found that 84% of neurons recorded from area PEc showed pre-saccadic activity with directional tuning. In 26% of neurons, we found inhibition of activity in the pre-saccadic period. The onset of this “pause” always started before the saccade and, in 51% of neurons, it was invariant among different gaze directions. The post-saccadic activity in these cells was either a phasic response with directional tuning (77%) and/or an eye position tuning (75%). The analysis of the preferred direction did not show hemispheric preference, however, for the majority of neurons, the angular difference in the preferred direction, in the pre- and post-saccadic period, was more than 60°. By confirming, therefore, that PEc neurons carry information about eye position, these novel findings open new horizons on PEc function that, to date, is not well documented. The pre-saccadic activity may reflect an involvement in saccade control, whereas post-saccadic activity may indicate a role in informing on the new eye position. These novel results about saccade and eye position processing may imply a role of area PEc in gaze direction mechanisms and, possibly, in remapping visual space after eye movements. [Copyright &y& Elsevier]

Abstract: Deprivation of socio-sensory interactions during early life impairs brain function in adulthood. In previous investigations we showed that early isolation severely affects neuron development in several structures of the hippocampal region, including the entorhinal cortex. In the present study we investigated the effects of early isolation on signal processing along the entorhinal cortex–dentate–CA3–CA1 system, a major memory circuit of the hippocampal region. Male and female guinea-pigs were assigned at 6–7 days of age to either a social or an isolated environment. At 90–100 days of age the animals were anesthetized and field potentials were recorded from the entorhinal cortex–dentate–CA3–CA1 circuit, driven by dorsal psalterium commissural volleys. Analysis of the input–output function in the different structures showed that in isolated males there was a small reduction in the input–output function of the population excitatory postsynaptic potential and population spike evoked in layer II of the entorhinal cortex. No changes occurred in isolated females. In isolated males and females there was a reduction in the input–output function of the population excitatory postsynaptic potential and population spike evoked in the dentate gyrus, CA3 and CA1, but this effect was larger in males. In isolated males, but not in females, the population spike/population excitatory postsynaptic potential ratio was reduced in all investigated structures, indicating that in males the size of the discharged neuron population was reduced more than due to the decreased input. Results show that isolation reduces the synaptic function in the whole entorhinal cortex–dentate gyrus–CA3–CA1 system. While the entorhinal cortex was moderately impaired, the dentate–hippocampal system was more severely affected. The impairment in the signal transfer along the entorhinal cortex–dentate gyrus–CA3–CA1 system was heavier in males, confirming the larger susceptibility of this sex to early experience. This work provides evidence that malfunctioning of a major hippocampal network may underlie the learning deficits induced by impoverished surroundings during early life. [Copyright &y& Elsevier]

Previous studies showed that early environmental conditions severely affect the morphology of the granule cells in the hippocampal dentate gyrus and pyramidal neurons in fields CA3 and CA1. The aim of the present study was to determine whether early isolation affects neuron morphology in layer II of the entorhinal cortex, from which the perforant path to the dentate gyrus and CA3 takes its origin. Male and female guinea pigs were assigned at 6–7 days of age to either a control (social) or an isolated environment where they remained for 80–90 days. The brains were Golgi–Cox stained and neurons were sampled from layer II of the entorhinal cortex. Morphometric analysis was carried out on star cells, the most abundant neuron population. Isolated males had star cells with less dendritic branches, a shorter dendritic length and a smaller dendritic spine density than control males. In contrast, isolated females had more dendritic branches than control females, though this difference was of small magnitude. While isolated males had star cells with a smaller soma than control males, isolated females had a soma larger than control females. In both environments sex differences were found in the star cell morphology. In the control environment males had more dendritic branches, a greater dendritic length, a larger soma but a smaller spine density than females. In the isolated environment males had less branches, a shorter dendritic length, a smaller spine density and a smaller soma than females.The results indicate that early isolation affects the structure of the star cells in the entorhinal cortex and that males and females react to isolation in an opposite manner. A similar sexually dimorphic response to early isolation was previously observed in the dentate gyrus and fields CA3 and CA1. The presence of widespread effects of isolation in the entorhinal cortex and numerous hippocampal structures suggests that the outcome of early isolation might be a change in learning and memory functions requiring the hippocampal region. [Copyright &y& Elsevier]

The properties, settling rates, and the rheology of coal–water mixtures (CWM) made up from different coals were investigated. Test suspensions containing different concentrations of suspended particles were made up from each of four different pulverized parent coals: a Pittsburgh Seam No. 8 coal, an Illinois coal, and the product and feed from a flotation process using an Illinois coal. All the parent coals had broad particle-size distributions. In addition, the Pittsburgh Seam No. 8 coal was classified into four different size fractions using sieving, resulting in three coarser, narrow particle size cuts and the fine residual fraction passing 325-mesh, which had a broad size distribution. The property characterizations for all coals included the following: elemental analyses, heating values, particle size analyses, particle surface areas and pore sizes, solid heat capacities, and thermal conductivities. Tests on the suspensions included detailed supernatant ion analyses, and measurements of pH values and zeta potentials. For each test coal maximum-packing volume fractions were estimated using centrifugation, and the settling rates and directly measured yield stresses, using the vane method, were determined for every concentration of coal–water mixtures (CWM) used. The shear-stress/shear-rate dependences of the test suspensions, covering the shear-rate range from 0.1 to 105 s−1, were determined using a capillary rheometer. For some of the test suspensions, correction for a pronounced wall-slip effect was required. Apart from the directly measured yield stresses using the vane method, yield stresses were estimated indirectly by extrapolation and rheological model-fitting. Extrapolation of directly measured yield stresses to infinite limit was used to estimate maximum packing for comparison with those determined from the irreducible sediment volumes using centrifugation. The two-parameter power-law, Bingham plastic and Casson empirical rheological models, and the three-parameter Herschel-Bulkley and Sisko models were used to fit the shear-stress/shear-rate data. In general, the shear-stress/shear-rate dependence was found to be shear-thinning, power-law, over the lower ranges of shear, and to tend to Newtonian limit at high shear; a dependence which is best described by the three-parameter Sisko model. [Copyright &y& Elsevier]