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We studied quantitatively the receptive-field properties of 74 units recorded from the representation of the central visual fields in the cat's lateral suprasylvian (LS) visual cortex. In agreement with previous workers, we found that LS receptive fields tended to be large and to lack discernible spatial structure. They resembled the complex receptive fields of areas 17 and 18 in their general organization. We examined the responses of these neurons to moving optimally oriented sinusoidal gratings that varied in spatial and temporal frequency of drift. Most LS neurons were selective for the spatial frequency of sinusoidal gratings; 7% responded to all spatial frequencies below a cutoff value. In agreement with previous reports, the optimal spatial frequencies for LS neurons covered a wider range than is seen in either area 17 or 18 alone (0.05–1 cycle/deg), but are certainly included in the range covered by both these afferent areas. Individual neurons in LS responded to a range of spatial frequencies broader than is typical for neurons in areas 17 and 18. The effect of varying the drift rate of otherwise optimal gratings was similar in LS to that reported for areas 17 and 18. Most neurons were optimally responsive to drift rates between 0.5 and 4 Hz, and resolved frequencies as high as 10–30 Hz. A few neurons had optima higher than 6 Hz and resolved frequencies in excess of 30 Hz. We conclude that the receptive fields of LS neurons reflect rather closely the properties of their afferents from areas 17 and 18. Apart from the increased incidence of directional selectivity in LS and the increase in receptive-field size seen there, we find no evidence for a significant reorganization of visual signals.
We used X-ray spectroscopy as a diagnostic tool for investigating the properties of laser-cluster interactions at the stage in which non-adiabatic cluster expansion takes place and a quasi-homogeneous plasma is produced. The experiment was carried out with a 10 TW, 65 fs Ti:Sa laser focused on CO2 cluster jets. The effect of different laser-pulse contrast ratios and cluster concentrations was investigated. The X-ray emission associated to the Rydberg transitions allowed us to retrieve, through the density and temperature of the emitting plasma, the time after the beginning of the interaction at which the emission occurred. The comparison of this value with the estimated time for the “homogeneous” plasma formation shows that the degree of adiabaticity depends on both the cluster concentration and the pulse contrast. Interferometric measurements support the X-ray data concerning the plasma electron density.
We report and discuss experimental results on the propagation of CPA pulses of moderately relativistic intensity in gas: they evidence the effects of the precursor pedestals of the main pulse. Details of great interest were observed for the first time with high quality femtosecond 90-degree interferometry. The interferometric data are also correlated with imaging and spectroscopy data of laser pulse transmitted through the gas. The most relevant physical features are confirmed by a numerical code which simulates the laser pulse propagation self-consistently with the ionization of the gas. We found that in this regime, the propagation of the intense femtosecond pulse is basically stable apart from very weak refractive effects. In order to allow propagation at fixed intensity along an optical path larger than the Rayleigh range, we performed a first successful attempt at producing hollow plasma channels able to guide the pulse.
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