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Biomedical subjects

G A Oliva

Publications and source records attributed to G A Oliva.

8 recordsLinked to original sources
PubMed1990

Eye movements and reversal rates of ambiguous patterns.

To study the influence of eye movements on the time course of perceptual alternation of ambiguous patterns, we recorded the reversal signals and eye movements of four subjects, during prolonged observations of a Necker cube, under three experimental conditions: unrestricted normal viewing, instruction-restricted normal viewing, and imperfectly stabilized viewing. An important decrease in the frequency of saccadic eye movements and a significant increase in the reversal rates were shown by all subjects under imperfectly stabilized viewing over the normal unrestricted viewing. Moreover, three out of four subjects showed a significant decrease in their reversal rates when changing from the normal to instruction-restricted viewing. Results are discussed in terms of the role and time cost of scanning eye movements in perceptual processes.

Adult↗
PubMed1984

Limit cycle oscillations of the human eye.

The altered feedback technique is very suited to display nonlinearities of the human smooth pursuit system. In fact, when the gain of the retinal feedback path is raised, for the horizontal channel, above its normal unitary negative value, a threshold is met beyond which sustained horizontal self-excited smooth oscillations of the eye can be observed, which point out the existence of a stable limit cycle. Furthermore, the characterizing features of both the transient and steady state show a well defined dependence on the total feedback factor K. In particular, the analytical dependence on K of the amplitude and frequency of limit cycle oscillations can be derived. Implications of the experiment with respect to the mathematical modelling of the system are discussed.

Eye Movements↗
PubMed1984

Effect of luminance on perceptual alternation of ambiguous patterns.

The perceptual alternation of the Necker cube, in its stationary phase, is studied as a function of the stimulus luminance down to the extreme condition in which the cones are completely inactivated so that the pattern of neural excitation reaching the primary visual cortex is very different from the normal one. No evident effect of luminance was found under passive observation either on the reversal rate or on the features of data distributions. Moreover, the complementary situation in which perception is based only on the cones does not affect the phenomenon either. These results permit the assumption that the perceptual alternation in its stationary phase, is a passive and automatic process, that is affected neither by a satiation of the first levels of the visual system nor by the psychological attitude of the subject consequent on dazzling the retina.

Discrimination Learning↗
PubMedDate not supplied

[Mark's disease: V. Experimental behavior of 3 isolates taken place in the country].

Three experiments were designed to determine the parameters of virus infection, antibodies and mortality with three different MD isolates inoculated in one day old birds from commercial origin. The animals were divided in three inoculated lots (1-2-3) and three control groups (4-1, 4-2, 4-3) and were followed weekly from hatching through 17 weeks. The former were inoculated respectivelly with FOV-6, FCV-8 and FCV-9. Each day old bird received between 50-75 FPU/bird by I.P. route. Samples were taken from circulating blood of five birds by cardiac puncture with an heparinized syringe (20 U/ml), were centrifuged and the white cells inoculated to 5-15 four day old embryos by yolk salk route for virus detection; plasma was assayed by immunodiffusion against MD antigen in order to detect precipitating antibodies, and mortality was recorded after microscopical examination. Infection appeared to persist indefinitelly in the host chicken flock and coexist with (100%) precipiting antibodies, (Fig. 1-2-3). First virus isolation was accomplished after 4 weeks post-inoculation and the 100% porcentage of antibodies, was found only 1-3 weeks after the first peak of viraemia. With the most pathogenic isolate FCV-6 (ig. 1) the antibody response was significantly delayed. Maternal antibodies decreased more rapidly in inoculated than in control birds. Accumulative mortality showed isolate FCV-6 and FCV-8 as pathogenic strains and FCV-9 strain as less pathogenic. Mortality begun 1-3 weeks after the first peak of virus detection in the flock, independently of the pathogenic pattern of the isolate, but frecuencies of death were markedly different.

Animals↗
PubMedDate not supplied

[Morphological characteristics of lytic plaques produced by the SP1 strain of the equine herpes virus type 1].

Lytic plaques produced by three different strains of EHV 1 were morphologically compared. The local isolated SP1 strain was compared with the tissue culture adapted Kentucky B (Ky Bcc) strain, and the through hamster passages modified Kentucky B (Ky Ba) strain, both reference strains. Ky Bcc produced small, irregular in size and shape, lytic plaques of around 1 mm. In the case of Ky Ba, the plaques were uniform in size and shape, with a diameter of 2 to 3 mm. SP1 strain originated lytic plaques similar to those produced by Ky Bcc. It is concluded that SP1 is a wild abortigenic strain, similar to Ky Bcc regarding its lytic properties.

Animals↗