Low-frequency Channel Research Articles

Stereo matching precedes dichoptic masking

Stereo matching can intervene to prevent dichoptic masking. In a dichoptic masking paradigm we measured the contrast threshold for a bar target, presented to one eye, as a function of the contrast of an identical masking bar, presented at retinal correspondence in the other eye. Confirming previous studies of dichoptic masking with sinusoidal gratings, the test bar thresholds rose proportionally with increasing masking contrast. This threshold elevation was almost nullified when an extra bar was presented to the eye seeing the test stimulus. Release from masking occurred when the disparity between the masking bar and extra bar was < 20 min arc over a range of contrast levels (8–45%), and for bars containing either broad spatial frequency spectra or bars with only high spatial frequencies (peak = 12 c/deg). The latter result rules out an explanation for the release from masking based on contrast discrimination in low spatial frequency channels. The extra bar was effective in releasing the test bar from masking as long as the extra bar's contrast was greater than about one-fifth the contrast of the mask, a result that suggests that there is a contrast threshold for stereo matching. We interpret our findings to indicate that a stage of stereo matching occurs prior to the neural site limiting dichoptic contrast discrimination.

Correction circuit and method for improving the transient behavior of a two-way loudspeaker system

A circuit for improving the transient behavior of a two-way loudspeaker system includes a crossover circuit with high selectivity, amplitude and phase correction circuitry for separately correcting the amplitude and phase responses of the high and low frequency drivers in their mounting environment, and correction circuitry for correcting the composite amplitude and phase response of the overall loudspeaker system after insertion of the crossover. A further phase offset technique and circuit provides for introducing frequency dependent phase shift in the loudspeaker system's high or low frequency channels for offsetting the phase responses of the high and low frequency drivers within the crossover frequency range. According to the phase offset technique of the invention, phase shift is added, preferably in the high frequency channel, until composite amplitude response curves observed on-axis and at different vertical angles off-axis are forced to be consistent. After consistency is achieved the deterioration of the amplitude response resulting from the phase offset is corrected to a flat response by means of a forced series amplitude correction circuit inserted before the crossover. The result is improved transient response off-axis as well as on-axis.

University of california at Santa Barbara Anisotropy Program: degree scale results from the South Pole 1990-1991.

We report on the preliminary result of a search for anisotropy in the cosmic background radiation (CBR). Our receiver operates with four equally spaced channels from 25 to 35 GHz with a beam size of approximately 1.5 degrees full width at half maximum. The system operated successfully for 500 hr at the South Pole during 1990-1991 austral summer. The data from one region, representing 25 hr after editing, are presented here. A strong signal is present in the lower-frequency channels with a spectrum unlike CBR fluctuations. The highest-frequency channel has the smallest contribution from this signal and has been used to set a 95% confidence level upper limit DeltaT/T </= 1.4 10-5.

The effect of transiency on perceived velocity of visual patterns: a case of “temporal capture”

We measured the points of subjective equality of velocity for dynamic unidirectionally moving random-dot patterns with different amounts of transiency. The transiency was changed by varying the time a dot would move before being randomly replotted within the stimulus. The perceived velocity of patterns moving at intermediate velocities (4 or 6 deg/sec) was increased by decreasing the point lifetime while no speedup was observed at high velocities (12 deg/sec). A speedup was also observed when a few stationary points of short lifetime were introduced into a stimulus. The non-directional transiency generated by these flickering points seems to be captured by the moving pattern and biases the velocity estimate. We term this phenomenon “temporal capture”. The results are in agreement with models that determine velocity by comparing the activity in lower and higher temporal frequency channels. Our stimuli would selectively increase activity in high temporal frequency channels and thus lead to an increase in perceived velocity.

Texture analysis and classification with tree-structured wavelet transform

A multiresolution approach based on a modified wavelet transform called the tree-structured wavelet transform or wavelet packets is proposed. The development of this transform is motivated by the observation that a large class of natural textures can be modeled as quasi-periodic signals whose dominant frequencies are located in the middle frequency channels. With the transform, it is possible to zoom into any desired frequency channels for further decomposition. In contrast, the conventional pyramid-structured wavelet transform performs further decomposition in low-frequency channels. A progressive texture classification algorithm which is not only computationally attractive but also has excellent performance is developed. The performance of the present method is compared with that of several other methods.

Peripheral temporal frequency channels code frequency and speed inaccurately but allow accurate discrimination

Perceived temporal frequency is vastly underestimated in the peripheral visual field, as all temporal frequencies above 10 Hz are perceived as flickering at 10 Hz even after scaling for acuity. Varying the contrast and spatial frequency of the peripheral pattern four-fold have negligible effects on the perceived flicker rate. Speed and auditory matching experiments also support this finding. Despite the saturation of perceived temporal frequency, frequency discrimination beyond 10 Hz was as accurate as in the fovea. By using a temporal masking paradigm, we obtained threshold elevation data that could be accounted for by three overlapping, broadly tuned temporal channels peaking at 5.5, 12 and 22 Hz. Based on these temporal frequency channels, we proposed that the visual system uses a line-element scheme for mediating temporal frequency discrimination, but adopts a weighted-average method for determining perceived temporal frequency. In the peripheral visual field, the weight assigned to the highest temporal channel is much larger than those assigned to the lower frequency channels.

A degree-scale measurement of anisotropy of the cosmic background radiation

We report on the preliminary result of a search for anisotropy in the cosmic background radiation (CBR) with a beam size of about 1.5 deg FWHM over a wavelength range of 8-12 mm. The system operated successfully for 500 hr at the South Pole during the 1990-1991 austral summer. The data from one region, representing 25 hr after editing, are presented here. A strong signal is present in the lower frequency channels with a spectrum unlike CBR fluctuations, and is probably due to foreground emission. The highest frequency channel has the smallest contribution from this signal and has been used to set a 95-percent confidence level upper limit of not greater than 1.4 x 10 exp -5 for fluctuations with a Gaussian autocorrelation function at a coherence angle of 1.2 deg. This is significantly more sensitive than previous experiments at this angle scale.

Precipitation observed by the airborne advanced microwave moisture sounder (AMMS)

Airborne radiometric measurements at frequencies near 92 GHz and 183 GHz were conducted over two precipitation events near Wallops Island, Virginia during February, 1986. The measured brightness temperatures are compared with those from calculations to estimate the snowfall and rainfall rates for both events. The estimated rates over water surface are within a factor of two of those derived from the concurrent measurements by the SPANDAR radar at the NASA Wallops Flight Facility. These estimated rates, however, suggest that both snowfall events are light and close to the threshold of radiometric detection especially over land surface. Observations of additional snowfall events with lower frequency channels are needed to demonstrate the approach and to account for cloud effects.

Perception and interpretation of high-pass filtered images

Digital high-pass filtering is used frequently to enhance details in scientific, industrial, and military images. High-pass filtered (HPF) images also are used both to illustrate and test models of visual perception. The visual system appears to interpret HPF images in the context of a multiplicative model of high-frequency reflectance and low-frequency illumination whenever possible. HPF images can be treated as a form of two-dimensional amplitude modulation signals. The low-frequency information, which is coded in the modulation envelope, disappears with the carrier if low-pass filtered. The envelope may be retrieved (demodulated) using one of many possible nonlinear operations followed by a low-pass filter. The compressive nonlinearity ofthe visual system is shown to suffice for demodulating such images. Simulations show that HPF images cannot be used to reject the hypothesis that illusions and grouping phenomena are due to low-frequency channels.

Multiphoton processes in an intense laser field. VI. Two-color ionization with incommensurable frequencies.

We present results of Floquet calculations of rates and yields for ionization of atomic hydrogen, in its ground state, in the presence of a relatively weak high-frequency field and a relatively strong low-frequency (1064-nm) field. We assume that the two frequencies are incommensurable, or, if they are not, we average over the relative phase. We explore three processes: (i) Resonance-enhanced ionization, via levels that are ac-Stark mixed or/and very strongly shifted, (ii) laser-induced continuum structure, and (iii) nonresonant ionization in a regime where, despite the fact that the infrared field significantly shifts the ground state relative to the continuum, it only redistributes the photoelectrons over the low-frequency channels. We also illustrate the sensitivity of the ionization yield to the spatiotemporal configuration of the two fields.

Metacontrast with masks varying in spatial frequency and wavelength

In a metacontrast masking paradigm, adult subjects were required to detect briefly presented target lines followed at various delays by a flanking mask varying in spatial frequency and wavelength. Detection accuracy, the dependent measure, was recorded as a function of the delay of the mask. The results showed that long wavelength masks produced maximum masking at a relatively short delay, while short wavelength masks produced maximum masking at a relatively long delay. Results are discussed within the framework of transient/sustained (magnocellular/parvocellular) theory of visual processing, and suggest that low spatial frequency channels respond with shorter latency and/or faster rise time to short wavelength stimuli, and high spatial frequency channels respond with greater sensitivity to long wavelength stimuli.

Electronic hearing aid with gain control means for eliminating low frequency noise

Electronic circuitry is described which enhances the intelligibility of a speech signal corrupted by low-frequency noise and certain flaws of miniaturized hearing aids are overcome by packaging the circuitry in a housing to be worn on the body of the hearing-impaired person and electrically coupling the output of the circuit to a high quality of a type to be worn in the ear speaker. The circuitry includes a low-frequency channel which passes components of the speech signal between about 600 Hz and about 1.2 KHz, and a parallel high-frequency channel which passes speech signal components between about 1.7 KHz and 4.6 KHz. The high-frequency channel includes a gain control amplifier for compressing the dynamic range of high-frequency components and means for adjusting the level of the compressed high-frequency signal relative to the level of signals passed by the low-frequency channel. The low-frequency and high-frequency signals are combined in a summing amplifier, the output level of which is controlled by a volume control and then amplified in power sufficiently to drive a wide dynamic range external speaker.

The ear as a speech analyzer

The representation of speech in the auditory periphery is evaluated from the perspectives of physiology and psychoacoustics. Current models fall into one of four broad classes: rate/place, synchrony/place, synchrony/quasi-place and synchrony/place-independent. Each representational type is evaluated in terms of its ability to account for the perceptual features of speech and on the basis of physiological plausibility. The optimum representational form may vary as a function of the acoustic environment. It is suggested that the rate/place representation operates primarily at low sound-pressure levels and for encoding the gross spectral characteristics of the high-frequency portion of aperiodic signals, such as stops and fricatives. The synchrony/place and synchrony/quasi-place representations may also play an important role, primarily at low sound-pressure levels and under conditions of low signal-to-noise ratio in the high-frequency auditory channels. The place-independent synchrony pattern may provide the basis for the representation of voiced sounds (particularly vowels) at moderate-to-high sound-pressure levels and for conditions where the signal-to-noise ratio in the low-frequency channels is particularly poor. The “onset” units of the posteroventral cochlear nucleus may act to integrate such across-channel synchronous activity to provide a stable of representation of the signal spectrum in terms of both rate and temporal activity.

Limits of binocular fusion in the short wave sensitive (“blue”) cones

Stereoscopic depth perception is possible when the short wave sensitive (SWS or “Blue”) cones are isolated using a yellow adapting field. We have measured the maximum disparity that can be fused (the diplopia threshold) as a function of the separation between pairs of dots or lines. Under all conditions, these diplopia thresholds are the same for the isolated SWS cones as for the entire visual system. In addition, SWS diplopia thresholds vary as a linear function of dot or line separation, so that they exhibit disparity scaling. Further experiments show that disparity scaling is dependent upon the presence of low spatial frequencies in the stimulus and not upon the retinal eccentricity of stimulation. These data indicate that the SWS cones provide information to the disparity processing system through more than one low spatial frequency channel but not through high frequency ones.

Spatial frequency channels and perceptual grouping in texture segregation

The literature suggests that both spatial frequency components and grouping processes affect texture segregation. The experiments we report investigated texture segregation in a three-part (tripartite) pattern in which each part contains equal numbers of two different squares that are arranged in vertical stripes in the top and bottom parts and in a checkerboard in the center part. We found: (1) A difference in sign of contrast yields strong texture segregation. (2) Texture segregation is a negatively accelerated function of the ratio of the Rayleigh contrasts of the low-luminance and high-luminance squares. (3) Size and contrast are not independent attributes but can cancel each other. High spatial frequency differences are not sufficient to cause segregation in the presence of contradictory information from the low spatial frequency channels. (4) Hue is a weak feature relative to contrast. Hue differences are not sufficient to cause texture segregation in the presence of contradictory contrast information. The results of the experiments support the argument that the higher order processes in texture segregation have access to information corresponding to the outputs of the spatial frequency channels. They are qualitatively consistent with the hypothesis that tripartite segregation is primarily a function of spatial frequency components and not of grouping processes.

The role of spatial-frequency channels in the perception of local and global structure.

Adaptation and reaction-time techniques were used to examine the role of different spatial-frequency channels in the perception of local and global structure. Subjects were shown figures consisting of a large C composed of smaller Cs and asked to identify the orientation of either the global C or its local elements. Prior to performing the task subjects were adapted to different spatial frequencies and the effect on subsequent performance was assessed. Two main results were found. First, the adapting frequency that most affected the global task was often lower than that most affecting the local task, suggesting that high and low frequencies independently code the structure of an image. Second, reaction time to global figures was often faster than to local figures at all levels of detectability, again suggesting a role of low-frequency channels in global processing.

Spontaneous release of transmitter from the growth cones of Xenopus neurons in vitro: The influence of Ca 2+ and Mg 2+ ions

To determine whether spontaneous release of transmitter from the growth cones of neurons exhibits properties similar to the spontaneous release which occurs from the neurons at the neuromuscular junction, release of transmitter from the growth cones of Xenopus neurons in culture was monitored in salines containing varying calcium and magnesium concentrations. Release was monitored by use of an outside-out piece of muscle membrane attached to a patch clamp electrode. Spontaneous release of transmitter from the growth cones in standard saline (2 m M CaCl 2, 1 m M MgCl 2) produces clusters of single-channel openings in the muscle membrane. Clusters are seen to consist of two types: a series of high-frequency channel openings, called “bursts,” and clusters of low-frequency channel openings called “singles.” The bursts were identified and examined for their possible relationship to MEPP-producing release, and the singles were identified and examined for their possible relationship to “leak” release of the neuromuscular junction. When the external saline contains high calcium (10 m M CaCl 2, 1 m M MgCl 2) or high magnesium (2 m M CaCl 2, 9 m M MgCl 2), the frequencies of both “bursts” and “singles” was greatly reduced. This reduction in release persists if the neurons are grown in the high-calcium or high-magnesium solutions. When the saline is a low-calcium solution (0 m M CaCl 2, 3 m M MgCl 2) the growth cones release transmitter at rates similar to those from standard saline. These results indicate that although the spontaneous release from the growth cone shares one characteristic with the leak release, neither the burst nor the singles release from the growth cones share exact relationship with either the MEPP producing release or the leak release. This suggests that further development of the mechanisms for spontaneous release of neurotransmitter occurs after nerve-muscle contact.

Satellite Passive Microwave Rain Rate Measurement over Croplands during Spring, Summer and Fall

Rain-rate algorithms for spring, summer and fall that have been developed from comparisons between the brightness temperatures measured by the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) and rain rates derived from operational WSR-57 radars over land are described. Data were utilized from a total of 25 SMMR passes and 234 radars, resulting in about 12,000 observations of about 1600 sq/km areas. Multiple correlation coefficients of 0.63, 0.80 and 0.75 are achieved for the spring, summer and fall algorithms, respectively. Most of this information is in the form of multifrequency contrast in brightness temperature, which is interpreted as a measurement of the degree to which the land-emitted radiation is attenuated by the rain systems. The SMMR 37-GHz channel has more information on rain rate than any other channel. By combining the lower frequency channels with the 37-GHz observations, variations in land and precipitation thermometric temperatures can be removed, leaving rain attenuation as the major effect on brightness temperature. Polarization screening at 37 GHz is found to be sufficient to screen out cases of wet ground, which is only important when the ground is relatively vegetation free. Heavy rain cases are found to be significant part of the algorithms' success, because of the strong microwve signatures (low-brightness temperatures) that result from the presence of precipitation-sized ice in the upper portions of heavily precipitating storms. If IR data are combined with the summer microwave data, an improved (0.85) correlation with radar rain rates is achieved.

Role of contrast, lettercase, and viewing conditions in a lateralized word-naming task.

Because of the asymmetric functional organization of the brain, the process of naming a visually presented word necessarily engages, in its end stage, the structures of the left anterior hemisphere, forcing information received by the right hemisphere to be transferred across the corpus callosum. On the basis of this functional organization, a task requiring speeded naming of a laterally presented word was designed to examine the nature of the information transferred from the right to the left hemisphere and the type of process initially performed by the right hemisphere before transfer. Three categories of variables, each affecting particular components of the stimulus were manipulated in a within-subject design: (1) the viewing conditions, with the target word presented under flicker masking, which desensitizes the transient low-frequency channels while increasing the sensitivity of the low-frequency sustained channels, with backward pattern masking, which interferes with the integration of high spatial frequencies, or without masking; (2) the contrast of the word stimulus, which determines stimulus energy; and (3) the lettercases, which influence the relative salience of the component and configural properties of the word. Analysis of latencies showed a right-visual-field main effect and interactions between visual field and contrast and between visual field and lettercase. Right visual field superiority increased with a reduction in stimulus energy, and the right hemisphere benefited more than the left hemisphere from the additional information provided by the configural property of the lowercase word. The viewing conditions influenced the strength of these effects but did not modify their general pattern. The results are discussed with respect to interhemispheric transfer of visual information and the processing competence of the cerebral hemispheres.

Visual grouping without low spatial frequencies

Gestalt psychologists have stressed the perceptual importance of grouping. On the basis of multichannel theory of early visual processing, grouping has been attributed to the operation of channels tuned to low spatial frequencies. If this explanation is correct and sufficient, grouping should not occur when looking at highpass filtered images, because they do not excite low spatial frequency channels. However this paper brings evidence that visual grouping also occurs in such a situation. Hence, a more comprehensive explanation of visual grouping is needed.