Tahei Kitamura

Effect of noise on the contrast detection threshold in visual perception.

It has recently been shown that noise can improve the detection of stimuli in several sensory modalities. We herein investigated whether visual contrast detection sensitivity can be improved by adding a certain amount of noise. The contrast detection thresholds of a light changing brightness periodically were measured either with or without overlapping noise in 22 normal participants. Sinusoidal modulating light at 1 or 15 Hz was used as a signal. White noise was used to produce random flickering light as the noise. Participants were required to detect any changes in the brightness of the signal with or without noise. The contrast detection threshold, which was measured using a psychophysical method, decreased at around the threshold level of the noise intensity. The maximum facilitatory effect was obtained at a noise intensity of 5 dB. This effect was consistently observed regardless of the frequency of the signal (1 and 15 Hz). These findings indicate that noise can improve the signal detection in human visual perception.

Suprathreshold stochastic resonance in visual signal detection

The present study aimed to demonstrate stochastic resonance (SR) for a visual signal above the threshold by using a two-alternative forced-choice protocol. The percent correct response enhanced at non-zero intensity of noise, while decreased with more intense noise. SR for the suprathreshold signals was found for signals with relatively low intensity above the threshold, and disappeared for more intense signals. These findings show the suprathreshold SR in the visual perception.

Subthreshold noise facilitates the detection and discrimination of visual signals

It has recently been shown that noise can facilitate signal detection in the sensory systems. We have previously shown that contrast detection sensitivity in the visual perception is improved in the presence of noise by measuring the detection threshold (Sasaki et al. Effect of noise on the contrast detection threshold in visual perception, Neurosci. Lett. 408 (2006) 94-97). In the present study, we reexamined the previous findings by measuring the percent correct detection rates in a two-alternative forced-choice (2AFC) paradigm and examined further whether the addition of noise improved the discrimination of the visual patterns. In the first experiment, the addition of weak noise facilitated the detection rate, thus confirming the previous findings. Furthermore, a significant correlation between the control detection rate without noise and the noise intensity that induced the maximum facilitation of the detection rate was found. In the second experiment, the addition of a subthreshold noise facilitated the discrimination of visual patterns. These results showed that the effect of noise that induces facilitation in signal detection is robust regardless of the methods used for measuring the detection sensitivity, and that pattern discrimination can also be improved by the addition of noise.

Experimental study of formation kinetics in a discharge-pumped F2 laser

Abstract In a discharge-pumped F 2 laser, it is known that the major mechanisms for formation of the population density of the laser upper level (F 2 ∗ (D′)) are the neutral channel and the ion channel reactions. In order to study the degree of contribution of the neutral channel or the ion channel reactions, the population densities of the fundamental species, such as the excited F and He atoms and a F − ion and an electron, have been measured by means of the optical interferometric and absorption methods. On the basis of these experimental results, the population density of the laser upper level is estimated with the aid of simple rate equations, and the formation processes of laser upper level molecules have been discussed.

Small-signal gain measurements in a discharge-pumped F2 laser

In a discharge-pumped F2 laser, time-resolved net-small-signal gain has been measured to study the population inversion by employing an oscillator-amplifier configuration. Characteristics of the gain and the fluorescence are studied under various conditions in two cases of He and Ne buffer gases. The peak of the net-small-signal gain in He/F2 and Ne/F2 show values of about 17%/cm and 9%/cm, respectively. The formation processes of the upper laser level F2(D′) are discussed on the basis of these experimental results.

Gain measurements of Ar 2* excimer formed by high-pressure homogeneous discharge using plasma electrode

Highly homogeneous discharges have been obtained in pure Ar gas at high pressures of 10 - 20 atm by using a discharge device in which the surface discharge plasma from a dielectric surface is used as a preionizer and an electrode, the plasma electrode, aiming at an Ar2* excimer laser. The plasma electrode consists of a ceramic pipe with a length of 290 mm. The same electrode is used as a cathode and an anode. The electrode separation is 2 mm to generate the main discharge. The fluorescence of Ar2* excimer radiation has been investigated at a wavelength of around 126 nm. Time-resolved electron density and gain measurements have been carried out with nanosecond time-resolution. The electron density was over 16th power of ten at 10 atm. The optical gain at 126 nm has been observed at an Ar gas pressure of 15 atm, and the peak gain was 8.6%/cm.

Noise-induced increase in visual spatial resolution in humans

Maturation of motor function is essential in postnatal development of mammals. Execution of voluntary movement relies on the basal ganglia neuronal circuitry, especially direct and indirect pathways from the striatal matrix compartment. However, maturation of two antagonistic pathways is poorly understood. We hence visualized single striatofugal neurons of the matrix compartment in postnatal developing rats with membrane-targeted GFP by intrastriatal injection of a recombinant Sindbis virus. At postnatal day (P) 4, both direct and indirect pathway neurons had short dendrites and scarcely branched axons. During P8-12, axon collaterals around somata and in target nuclei became obvious and dendrites elongated. At P16, dendritic and axonal arbors seemed almost mature, except that dendritic spines were rare. During P24–P32 dendrites were covered with dense spines. These findings together lay out a timetable for basal ganglia circuit maturation, and suggest concurrent development of the direct and indirect pathways.

Effect of noise on pattern discrimination

The lateral geniculate nucleus (LGN) of the thalamus conveys visual information to the primary visual cortex. Because activity of LGN neurons is elicited by retinal inputs, amount of LGN activity are expected to represent strength of visual inputs. Also, visual deprivation is assumed to decrease the LGN activity and leads to experience-driven cortical plasticity in young animals. However, it is not clear whether neural activity of LGN faithfully represents the input strength in awake condition. In order to test this hypothesis in behaving animals, we implanted a wire electrode to the LGN of rat chronically and recorded neural activity in behaving condition. Strobe flash stimuli effectively elicited visual responses of LGN neurons. Unexpectedly, however, we found no quantitative difference in mean firing rate of ongoing activity between the light and dark environment. In addition, abrupt increase of activity of short duration was accompanied by exploratory whisking of rats. These results suggest that neural activity of LGN does not simply represent the quantity of visual inputs in behaving rats.