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Biomedical subjects

X Pei

Publications and source records attributed to X Pei.

64 records · Page 4Linked to original sources
PubMed1992

[Bacterial microcolony and its primary applications].

A new rapid and useful microcolony technique was developed by using cellulose-acetate membrane as a supporter. The bacteria were inoculated on the membrane and cultivated for 3-6h. Then the membrane was rendered, fixed and colored. The microcolonies were observed and counted under the microscope. By using this technique the microcolonies of 139 strains of bacteria were observed. 72 strains of Staphylococcus were differentiated by means of observing the characteristics of the microcolonies. The correlation rate between the microcolony method and the routine method was 90.28%. This technique was also first used to determine the bacteria numbers in urine within 6h, and the advantages of it were significant compared with the plate count method.

Colony Count, Microbial↗
PubMed1991

The neurophysiological correlates of colour and brightness contrast in lateral geniculate neurons. I. Population analysis.

The colour of an object is changed by surround colours so that the perceived colour is shifted in a direction complementary to the surround colour. To investigate the physiological mechanism underlying this phenomenon, we recorded from 260 neurons in the parvo-cellular lateral geniculate nucleus (P-LGN) of anaesthetized monkeys (Macaca fascicularis), and measured their responses to 1.0-2.0 degrees diameter spots of equiluminant light of various spectral composition, centered over their receptive field (spectral response function, SRF). Five classes of colour opponent neurons and two groups of light inhibited cells were distinguished following the classification proposed by Creutzfeldt et al. (1979). In each cell we repeated the SRF measurement while an outer surround (inner diameter 5 degrees, outer diameter 20 degrees) was continuously illuminated with blue (452 nm) or red (664 nm) light of the same luminance as the center spots. The 1.0-1.5 degree gap between the center and the surround was illuminated with a dim white background light (0.5-1cd/m2). During blue surround illumination, neurons with an excitatory input from S- or M-cones (narrow- and wide-band/short-wavelength sensitive cells, NS- and WS-cells, respectively) showed a strong attenuation of responses to blue and green center spots, while their maintained discharge rate (MDR) increased. During red surround illumination the on-minus-off-responses of NS- and WS-cells showed a clear increment. L-cone excited WL-cells (wide-band/long-wavelength sensitive) showed a decrement of on-responses to red, yellow and green center spots during red surround illumination and, in the majority, also an increment of MDR. The response attenuation of narrow-band/long-wave-length sensitive (NL)-cells was more variable, but their on-minus-off-responses were also clearly reduced in the average during red surrounds. Blue surround illumination affected WL-cell responses little and less consistently than those of NL-cells, but often broadened the SRF also in the WL-cells towards shorter wavelengths. The M-cone excited and S-cone suppressed WM-cells were strongly suppressed by blue but only little affected by red surround illumination. The changes of spectral responsiveness came out clearly in the group averages of the different cell classes, but showed some variation between individual cells in each group. The zero-crossing wavelengths derived from on-minus-off-responses were also characteristically shifted towards wavelengths complementary to those of the surround.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

The neurophysiological correlates of colour and brightness contrast in lateral geniculate neurons. II. Adaptation and surround effects.

We report on experiments which were undertaken in an attempt to clarify mechanisms underlying the contrast effects of chromatic surround illumination on spectral responsiveness of cells in the parvocellular layers of the LGN (P-LGN-cells), that had been demonstrated under standard conditions in the preceding companion paper. The experiments were done in anesthetized macaques (Macaca fascicularis). In some neurons, S-potentials were recorded together with the post-synaptic action potentials, and all effects seen in P-LGN-cells were present already in their retinal afferents indicating their retinal origin. The responsiveness of the cells for center stimuli of different wavelengths and during illumination of the receptive field center or the outer surround was determined. Continuous outer surround illumination altered maintained discharge rate (MDR), sensitivity and gain of P-LGN and retinal ganglion cells in the same way and empirically not distinguishable from direct illumination of the receptive field. Responses to surround flashes showed the same dependence on spectral composition as those to center flashes. Adaptation and excitation caused by outer surround illumination (inner diameter 5 degrees, outer diameter 20 degrees) were, in the average, ten times weaker than those exerted by light of the same spectral composition shone directly into the receptive field. Surround effects decreased proportional to r-2. Excitation by outer surround flashes was reduced by adaptation of the receptive field center in the same manner as responses to center flashes. The findings indicate that outer surround light has a direct excitatory and adaptive effect on the excitatory or inhibitory cones feeding into the receptive field. This indicates that straylight from the surround into the center could be responsible for the adaptive and excitatory effects of surround illumination. The straylight fraction from the remote surround into the receptive field must be higher, however, than that estimated from the psychophysically determined point spread function. It comes closer to earlier direct straylight measurements in excised eyes, but may be enhanced by chromatic aberration. If a surround of excitatory colour is flashed simultaneously with an excitatory center stimulus, additivity of center and surround excitation is observed only at low center intensities, while at higher center intensities the gain for center excitation is reduced similar to adaptive gain control. This could be explained by lateral interaction through horizontal connections in the retina, which decays within seconds, while adaptation of the cones feeding into the receptive field center is fully effective only after about 3 s.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Ocular↗
PubMed1991

Role of the extensive area outside the X-cell receptive field in brightness information transmission.

Stimulus area-response functions of retinal ganglion cells show an extensive disinhibitory region (DIR) outside the classical receptive field (RF). The DIR has a wide summation area but low sensitivity. Spatial responses of the retinal ganglion cells have been simulated in a model which takes into account also the properties of the DIR. By scanning the RF and its DIR with a visual image and reconstructing the transferred image for single cells, it is shown that these properties of the DIR are beneficial in the transmission of area brightness and image grey scales.

Action Potentials↗
PubMed1991

Whole cell recording and conductance measurements in cat visual cortex in-vivo.

Long and stable recordings of post-synaptic, action and membrane potentials from visual cortical neurons in-vivo, are possible with the patch-clamp technique. These are comparable to the whole-cell configuration, but with an incomplete seal. EPSPs and IPSPs of normal time course and up to several mV can be recorded. DC potentials ranged from - 30 to - 60 mV and input resistances from 50 to 150 M omega. Injected currents have the same effect as if applied intracellularly. Membrane conductance after electrical stimulation of the lateral geniculate nucleus is increased during the first 20 ms, but decreases from 60 to about 130 ms, during return of the membrane potential to its resting level. The recording method is compared to other intracellular recording techniques in-vivo and in-vitro.

Animals↗
PubMedDate not supplied

Noise-induced impulse pattern modifications at different dynamical period-one situations in a computer model of temperature encoding.

We used a minimal Hodgkin-Huxley type model of cold receptor discharges to examine how noise interferes with the non-linear dynamics of the ionic mechanisms of neuronal stimulus encoding. The model is based on the assumption that spike-generation depends on subthreshold oscillations. With physiologically plausible temperature scaling, it passes through different impulse patterns which, with addition of noise, are in excellent agreement with real experimental data. The interval distributions of purely deterministic simulations, however, exhibit considerable differences compared to the noisy simulations especially at the bifurcations of deterministically period-one discharges. We, therefore, analyzed the effects of noise in different situations of deterministically regular period-one discharges: (1) at high-temperatures near the transition to subthreshold oscillations and to burst discharges, and (2) at low-temperatures close to and more far away from the bifurcations to chaotic dynamics. The data suggest that addition of noise can considerably extend the dynamical behavior of the system with coexistence of different dynamical situations at deterministically fixed parameter constellations. Apart from well-described coexistence of spike-generating and subthreshold oscillations also mixtures of tonic and bursting patterns can be seen and even transitions to unstable period-one orbits seem to appear. The data indicate that cooperative effects between low- and high-dimensional dynamics have to be considered as qualitatively important factors in neuronal encoding.

Computer Simulation↗
PubMedDate not supplied

Dynamics of the orientation tuning of postsynaptic potentials in the cat visual cortex.

We evaluated the dynamic aspects of the orientation tuning of the input to cat visual cortical neurons by analyzing the postsynaptic potentials (PSPs) evoked by flashing bars of light. The PSPs were recorded using in vivo whole-cell technique, and we analyzed the orientation tuning during subsequent temporal windows after stimulus onset and offset. Our results show that the amplitudes of the postsynaptic potential are reliably tuned to orientation and matching that of the spike responses only during certain temporal windows. During the first 100 ms after stimulus presentation, orientation tuning of the membrane potential underwent regular changes. Within particular intervals, orientation tuning of the input was much sharper than that estimated according to the whole response. In most cells, optimal orientation was usually stable over the whole period. In several cells which had a second hump of EPSPs in the response, this second hump was tuned to the same orientation as the first one, but always showed sharper tuning. Estimation of the integration time revealed sufficient delay between the appearance of EPSPs and spikes, to let inhibition influence spike generation. These results show that orientation selectivity of the input to cortical cells is a dynamic function, and also indicate the possibility of temporal coding in the visual system.

Animals↗
PubMedDate not supplied

Excitation and inhibition in orientation selectivity of cat visual cortex neurons revealed by whole-cell recordings in vivo.

One striking transformation in response properties that occurs in the geniculo-cortical pathway is the appearance of a high degree of orientation selectivity in the cortex. This property may be conceived as arising purely from the excitatory inputs to the cell, as being structured largely by the inhibition a cortical cell receives or could be due to a combination of the two. We have studied the contributions of excitatory and inhibitory inputs to cortical cells' orientation selectivity by analyzing the postsynaptic potentials evoked in cat striate neurones by flashing stimuli of different orientations. We made these recordings using the in vivo whole-cell technique (Xing Pei et al., 1991), which provides more stable and reliable results than classical intracellular recording methods. Our results show that the cat striate cortex exhibits a variety of mechanisms to achieve orientation selectivity. Orientation selectivity of a particular cell can be created by excitatory, by inhibitory, or by a combination of both mechanisms.

Animals↗
PubMedDate not supplied

Low-dimensional dynamics in sensory biology 2: facial cold receptors of the rat.

We report the results of a search for evidence of unstable periodic orbits in the sensory afferents of the facial cold receptors of the rat. Cold receptors are unique in that they exhibit a diversity of action potential firing patterns as well as pronounced transients in firing rate following rapid temperature changes. These characteristics are the result of an internal oscillator operating at the level of the membrane potential. If such oscillators have three or more degree of freedom, and at least one of which also exhibits a nonlinearity, they are potentially capable of complex activity. By detecting the existence of unstable periodic orbits, we demonstrate low-dimensional dynamical behavior whose characteristics depend on the temperature range, impulse pattern, and temperature transients.

Action Potentials↗