Area summation in human vision at and above detection threshold

The initial image-processing stages of visual cortex are well suited to a local (patchwise) analysis of the viewed scene. But the world's structures extend over space as textures and surfaces, suggesting the need for spatial integration. Most models of contrast vision fall shy of this process because (i) the weak area summation at detection threshold is attributed to probability summation (PS) and (ii) there is little or no advantage of area well above threshold. Both of these views are challenged here. First, it is shown that results at threshold are consistent with linear summation of contrast following retinal inhomogeneity, spatial filtering, nonlinear contrast transduction and multiple sources of additive Gaussian noise. We suggest that the suprathreshold loss of the area advantage in previous studies is due to a concomitant increase in suppression from the pedestal. To overcome this confound, a novel stimulus class is designed where: (i) the observer operates on a constant retinal area, (ii) the target area is controlled within this summation field, and (iii) the pedestal is fixed in size. Using this arrangement, substantial summation is found along the entire masking function, including the region of facilitation. Our analysis shows that PS and uncertainty cannot account for the results, and that suprathreshold summation of contrast extends over at least seven target cycles of grating.     

A complex relationship among chemical concentration, detection threshold, and suprathreshold intensity of bitter compounds

Detection thresholds and psychophysical curves were established for caffeine, quinine-HCl (QHCl), and propylthiouracil (PROP) in a sample of 33 subjects (28 female mean age 24 +/- 4). The mean detection threshold (+/-standard error) for caffeine, QHCl, and PROP was 1.2 +/- 0.12, 0.0083 +/- 0.001, and 0.088 +/- 0.07 mM, respectively. Pearson product-moment analysis revealed no significant correlations between detection thresholds of the compounds. Psychophysical curves were constructed for each bitter compound over 6 concentrations. There were significant correlations between incremental points of the individual psychophysical curves for QHCl and PROP. Regarding caffeine, there was a specific concentration (6 mM) below and above which the incremental steps in bitterness were correlated. Between compounds, analysis of psychophysical curves revealed no correlations with PROP, but there were significant correlations between the bitterness of caffeine and QHCl at higher concentrations on the psychophysical curve (P<0.05). Correlation analysis of detection threshold and suprathreshold intensity within a compound revealed a significant correlation between PROP threshold and suprathreshold intensity (r=0.46-0.4, P<0.05), a significant negative correlation for QHCl (r=-0.33 to -0.4, P<0.05), and no correlation for caffeine. The results suggest a complex relationship between chemical concentration, detection threshold, and suprathreshold intensity.     

A self-similar stack model for human and machine vision

A new model is proposed that not only exhibits the major properties of primate spatial vision but also has a structure that can be implemented efficiently in a machine vision system. The model is based on a self-similar stack structure with a spatial resolution that varies with eccentricity. It correctly reproduces the visual cortical mapping function, yet it has the important attribute that it can produce invariant responses to local changes in the size and position of image features. By proposing a novel purpose for cortical bar-detectors, the model can also produce invariance to more general distortions. The structure of the model allows efficient hierarchical search to be made and it naturally embraces the concept of attention area. Exploitation of this model has already confirmed these properties and has also revealed its robust ability to control the focus and gain of machine vision systems.     

Energy model for contrast detection: spatiotemporal characteristics of threshold vision

A model for contrast detection of spatiotemporal stimuli is proposed which consists of a spatiotemporal linear filter, an energy device and a threshold device. Assuming the existence of independent intrinsic noise, the probability of stimulus detection was approximated by a Weibull function of the response energy. With this assumption, the stimulus energy is a constant at fixed detection probability. This energy model for contrast detection satisfactorily accounted for the elliptical threshold contours of line pairs at stimulus separations within the range 2–30 min and at stimulus onset asynchronies within the range 20–140 ms. The threshold contour at a large stimulus onset asynchrony (300 ms) was in the form of a rounded square. This finding was explained by assuming that the probability of seeing the line pair was determined by the joint probability that at least one stimulus had been detected. With the energy model, the temporal and spatial autocorrelation functions of the response to a flashed line were evaluated. The autocorrelation functions thus determined were used to predict the temporal contrast sensitivity function to a flickering line stimulus and the spatial contrast sensitivity function to flashed gratings, which were in agreement with the experimental data. The data obtained were fitted adequately by an impulse response approximated by a spatiotemporal Gabor-like function. Received: 08 December 1997 / Accepted in revised form: 26 January 1999     

Modelling spatial vision at the threshold level.

Some available single- and multiple-channel models are reviewed. Multichannel models are generalized and tested against threshold data on various stimulus sets. Without using the explicit assumption of spatial probability summation, simple multichannel models are shown to provide good simultaneous predictions of threshold curves of sinewave gratings and other gratings. They fail in predicting threshold curves of disk-shaped stimuli. If global or local spatial probability summation within channels is incorporated into the models, correctly shaped threshold curves of disks can be predicted. However, the predicted curves appear still too low if compared to measured curves. The same holds for noise gratings. Possible extensions of the models, based on local summation between channel responses and/or models consisting of initial isotropic channels (retina) followed by anisotropic channels (cortex), are discussed.     

Effects of human pregnancy on cardiac autonomic function above and below the ventilatory threshold

This study examined the effectsof human pregnancy on heart rate variability (HRV), spontaneousbaroreflex (SBR) sensitivity, and plasma catecholamines at rest andduring exercise. Subjects were 14 healthy, physically active pregnantwomen (PG; mean gestational age = 33.9 ± 1.0 wk). Resultswere compared with an age-matched nonpregnant control group (NPG;n = 14) with similar characteristics. Theelectrocardiographic R-wave-R-wave interval and systolic blood pressure (via finger plethysmograph) were measured on a beat-to-beat basis at rest and during upright cycling at 60 and 110% of the ventilatory threshold (T_vent). Parasympathetic nervoussystem (PNS) modulation (as reflected by HRV high-frequency/total power and SBR slope) was significantly reduced at rest in the PG vs. the NPG.During exercise, PNS modulation decreased significantly in both groups,but the magnitude of PNS withdrawal from rest to 110%T_vent was smaller in the PG vs. NPG. Sympathetic nervous system (SNS) modulation (reflected by the low-frequencypower-to-high-frequency power ratio) increased above resting values at60 and 110% T_vent in the NPG. SNS modulation at 110%T_vent was significantly lower in the PG compared with theNPG. Plasma norepinephrine and epinephrine levels were also lower at110% T_vent in the PG. It was concluded that healthypregnant women exhibit lower PNS modulation at rest and blunted SNSmodulation during exercise above T_vent in late gestation.

     

A computational theory of human stereo vision.

An algorithm is proposed for solving the stereoscopic matching problem. The algorithm consists of five steps: (1) Each image is filtered at different orientations with bar masks of four sizes that increase with eccentricity; the equivalent filters are one or two octaves wide. (2) Zero-crossings in the filtered images, which roughly correspond to edges, are localized. Positions of the ends of lines and edges are also found. (3) For each mask orientation and size, matching takes place between pairs of zero-crossings or terminationss of the same sign in the two images, for a range of disparities up to about the width of the mask's central region. (4) Wide masks can control vergence movements, thus causing small masks to come into correspondence. (5) When a correspondence is achieved, it is stored in a dynamic buffer, called the 2 1/2-D sketch. It is shown that this proposal provides a theoretical framework for most existing psychophysical and neurophysiological data about stereopsis. Several critical experimental predictions are also made, for instance about the size of Panum's area under various conditions. The results of such experiments would tell us whether, for example, cooperativity is necessary for the matching process.     

Changes in perceived contrast of suprathreshold gratings as a function of orientation and spatial frequency

Orientation anisotropy for suprathreshold gratings of different spatial frequencies was measured using a contrast matching procedure. Observers matched the contrast of sine-wave gratings of various orientations to a vertical reference grating set at different reference contrasts. At threshold, the size of the anisotropy increased with spatial frequency, confirming previous results. When the reference grating contrast was set above threshold, the anisotropy declined, and eventually disappeared for gratings of medium spatial frequencies. At higher spatial frequencies, although the relative anisotropy became smaller, it did not disappear within the range of contrasts used in this study. For medium, but not for high spatial frequencies, the data are consistent with Kulikowski's (1976) model of effective contrast constancy.     

The function of alpha-crystallin in vision

The alpha-crystallins account for approximately one-third of the total soluble protein in the lens, contributing to its refractive power. In addition, alpha-crystallin also has a chaperone-like function and thus can bind unfolding lens proteins. Alpha B-crystallin is also found outside the lens, having an extensive tissue distribution. It is over-expressed in response to stresses of all kinds, where it is thought to serve a general protective function. Recently, it has been shown in humans that naturally occurring point mutations in the alpha-crystallins result in a deficit in chaperone-like function, and cause cataracts as well as a desmin-related myopathy. This review summarizes much of the past and current knowledge concerning the structure and functions of alpha-crystallin.     

A gating motif in the translocation channel sets the hydrophobicity threshold for signal sequence function

A critical event in protein translocation across the endoplasmic reticulum is the structural transition between the closed and open conformations of Sec61, the eukaryotic translocation channel. Channel opening allows signal sequence insertion into a gap between the N- and C-terminal halves of Sec61. We have identified a gating motif that regulates the transition between the closed and open channel conformations. Polar amino acid substitutions in the gating motif cause a gain-of-function phenotype that permits translocation of precursors with marginally hydrophobic signal sequences. In contrast, hydrophobic substitutions at certain residues in the gating motif cause a protein translocation defect. We conclude that the gating motif establishes the hydrophobicity threshold for functional insertion of a signal sequence into the Sec61 complex, thereby allowing the wild-type translocation channel to discriminate between authentic signal sequences and the less hydrophobic amino acid segments in cytosolic proteins. Bioinformatic analysis indicates that the gating motif is conserved between eubacterial and archaebacterial SecY and eukaryotic Sec61.     

A compliance-independent pressure transducer for biomedical device-tissue interfaces

The measurement of the interface pressure between a biomedical device and part of the human body is useful to improve the performance and safety of such devices during design. Testing of a selection of existing interface pressure transducers has demonstrated that many are dependent on device and tissue compliance. Such a transducer is useful only in an application where it has been calibrated for specific device-tissue compliance combinations. To overcome this limitation, the authors developed an interface pressure transducer whose output signal is not affected by changes in interface compliance. This enables the transducer to quantitatively measure pressure in many applications without the need to calibrate it for varying compliance conditions. Surgical retraction and surgical tourniquets were selected as demonstration applications for the developed transducer, because they represent a wide spectrum of device and tissue characteristics and properties, and are in common use.