On the Law for Minimal Discrimination of Intensities: I

Effects of Spatial Frequency Similarity and Dissimilarity on Contour Integration

We examined the effects of spatial frequency similarity and dissimilarity on human contour integration under various conditions of uncertainty. Participants performed a temporal 2AFC contour detection task. Spatial frequency jitter up to 3.0 octaves was applied either to background elements, or to contour and background elements, or to none of both. Results converge on four major findings. (1) Contours defined by spatial frequency similarity alone are only scarcely visible, suggesting the absence of specialized cortical routines for shape detection based on spatial frequency similarity. (2) When orientation collinearity and spatial frequency similarity are combined along a contour, performance amplifies far beyond probability summation when compared to the fully heterogenous condition but only to a margin compatible with probability summation when compared to the fully homogenous case. (3) Psychometric functions are steeper but not shifted for homogenous contours in heterogenous backgrounds indicating an advantageous signal-to-noise ratio. The additional similarity cue therefore not so much improves contour detection performance but primarily reduces observer uncertainty about whether a potential candidate is a contour or just a false positive. (4) Contour integration is a broadband mechanism which is only moderately impaired by spatial frequency dissimilarity.

Reconciling Fechner and Stevens: Toward a unified psychophysical law

How does subjective magnitude, S. increase as physical magnitude or intensity, I, increases? Direct ratings (magnitude scales; partition or category scales) can be fitted by the power function, S = aIb, in which S equals I raised to a power or exponent, b, and multiplied by a measure constant, a. The exponent is typically about twice as large for the magnitude scale (Stevens) as for the corresponding partition or category scale, but the higher exponent may be explained by the overly expansive way people use numbers in making magnitude estimations. The partition or category scale and the adjusted (for the use of number) magnitude scale for a given modality or condition generally agree with the neurelectric scale and the summated just noticeable difference (jnd) scale. A unified psychophysical law is proposed in which each jnd has the same subjective magnitude for a given modality or condition, subjective magnitude increases as approximately a power function of physical magnitude with the exponent ranging from near 0 to 1 (compressive function), and subjective magnitude depends primarily on peripheral sensory processes, that is, no nonlinear central transformations occur. An undue reliance on Weber's law blinded Fechner to the fact that the true psychophysical scale is approximately a power function. Rejecting Weber's law, which is not valid, means that we no longer have to choose between letting the summated jnd scale be a logarithmic function (Fechner's law) and introducing a nonlinear central transformation to make it into a power function (Brentano–Ekman-Teghtsoonian's law). Fechner and Stevens erred equally about the true psychophysical power function, whose exponent lies halfway between that of Fechner (an exponent approaching zero) and that of Stevens. To be reconciled, Fechnerians must give up the assumptions that Webers law is valid and that the jnd has the same subjective magnitude across modalities and conditions; Stevensians must give up the assumption that the unadjusted (for the use of number) magnitude scale is a direct measure of subjective magnitude.

In vitro expression of the HIV-2 genomic RNA is controlled by three distinct internal ribosome entry segments that are regulated by the HIV protease and the Gag polyprotein

The HIV-2 genomic RNA serves both as a messenger for protein synthesis and as a genome for viral assembly and particle production. Our previous work has shown that the HIV-2 genomic RNA encodes two additional Gag proteins that are N-terminal truncated isoforms of the p57 Gag polyprotein. In this study, by the use of mono- and bicistronic RNAs we show that translation at the three AUGs is driven by three distinct and independent internal ribosome entry segments both in vitro and ex vivo. Furthermore we used the recombinant Gag and HIV-2 protease to show that, in vitro, translation is tightly regulated by these two viral proteins. This regulation is exerted both at the level of protein production and also on the selection of the AUG initiation site which changes the ratio at which the three different Gag isoforms are produced.

The signal-to-noise characteristics of rod-cone interaction

1. The influence of rods on cone-mediated vision was assessed in eight human observers. To this end, increment threshold functions were obtained by determining thresholds of a cone-detected test flash (25 ms duration, 655 nm wave-length, 13' diameter) as a function of the illuminance of larger, 500 ms duration, rod-detected masking flashes. The type of photoreceptor influenced by each stimulus was carefully checked by means of a series of control procedures involving action spectra and selective rod adaptation.2. When the rod mask was 512 nm in wave-length, 40' in diameter, and less than one scotopic td in illuminance, increment threshold functions show that [Formula: see text], where I(Cth) is cone test threshold, I(R) is rod mask illuminance, and D is a dark noise term similar to that used by Barlow (1956). Further increases in I(R) have no additional influences on cone test threshold until threshold is influenced by the combined action of the mask on both rods and cones. If I(R) is expressed in terms of scotopic flux rather than illuminance, the functional relationship obtained with all rod masks </= 40' diameter and </= 580 nm wave-length is identical.3. Over the range of illuminance where a square-root relationship is obtained, probability of seeing functions show that a signal-to-noise mechanism limits the detectability of the cone test flash. These findings suggests a quantitative model in which cones produce a signal in a detector which is proportional to the illuminance of the cone test flash. Within a neural locus designated E (excitatory spatial summator), a response is produced which over at least a 40' diameter area, is proportional to the scotopic flux of the rod mask. E, however, feeds into a gain box, S, which saturates at illuminance levels at least 3 log(10) units less than usual estimates of rod saturation. Other than saturation, S behaves in a linear fashion.4. As diameter increases beyond 60', rod masks of equal scotopic illuminance have progressively less influence on cone test threshold; rod masks > 2 degrees have negligible influence on cone test threshold. We propose that I (inhibitory spatial summator), a neural locus which responds to scotopic flux provided over a very large area, attenuates the activity of E. The combined action of E and I is designated a rod channel. The response of cones and the rod channel summate at a detector. Within the detector, cone signals are distinguished from rod-related activity and intrinsic dark noise on the basis of signal-to-noise discriminations.5. The neural substrate for this rod channel most probably involves the combined action of several neurones which synapse within the inner plexiform layer of the retina. The relationship of this rod channel to other perceptual phenomena is discussed.