Biomedical subjects

A D Logvinenko

Publications and source records attributed to A D Logvinenko.

10 recordsLinked to original sources

PubMed2004

In search of an elusive hard threshold: a test of observer's ability to order sub-threshold stimuli.

The contrast transducer function (d' vs. contrast) for sine gratings was claimed to come up from some non-zero contrast value rather than from the origin. This implies that there is a point (a hard threshold) on the grating contrast axis below which observers could not distinguish between presentations containing gratings and those containing a homogeneous field. We studied the ability to order sub-threshold square wave gratings and found, to the contrary, that observers were able to do this no matter how low the contrasts. At the same time, the observers failed to order the sub-threshold gratings when they were of the same contrast. The latter is inconsistent with signal detection theory which predicts that an observer's judgements are based on the same ordered set of sensory states irrespective of whether the stimuli differ or are the same. On the other hand, these data can be reconciled with the notion of a threshold if the latter is thought of as a fuzzy rather than a sharp margin on the contrast axis.

Contrast Sensitivity↗

PubMed2001

High-spatial-frequency tritanopia: S-filling-in or S-filtering-out?

It has long been an accepted fact that a small test field presented against a large background may change its colour appearance because the test-field background contrast is attenuated by the receptor colour channels unequally (Willmer, 1944 Nature 153 774-775; Hartridge, 1947 Philosophical Transactions of the Royal Society of London, Series B 232 519-671). Such an effect is usually called small-field tritanopia. However, as shown in the present report, a similar colour illusion can be achieved with a large test field as well, provided its spatial-frequency content is high enough to reveal the differential drop of contrast sensitivity for the receptor colour channels (high-spatial-frequency tritanopia). A few demonstrations are presented which show that a traditional explanation of high-spatial-frequency tritanopia (including small-field tritanopia), based on the hypothetical process of filling-in, is not correct. An alternative account, based on spatial filtering within the receptor colour channels, is put forward.

Color Perception↗

PubMed2001

The role of vergence in the perception of distance: a fair test of Bishop Berkeley's claim.

Binocular eye movements were measured while subjects perceived the wallpaper illusion in order to test the claim made by Bishop Berkeley in 1709 that we perceive the distance of nearby objects by evaluating the vergence angles of our eyes. Four subjects looked through a nearby fronto-parallel array of vertical rods (28-35 cm away) as they binocularly fixated a point about 1 meter away. The wallpaper illusion was perceived under these conditions, i.e. the rods appeared farther away than their physical location. We found that although binocular fixation at an appropriate distance was needed to begin perceiving the wallpaper illusion (at least for naive observers), once established, the illusion was quite robust in the sense that it was not affected by changing vergence. No connection between the apparent localization of the rods and vergence was observed. We conclude that it is unlikely that vergence, itself, is responsible for the perceived distance shift in the wallpaper illusion, making it unlikely that vergence contributes to the perception of distance as Bishop Berkeley suggested. We found this to be true even when vergence angles were relatively large (more than 2 deg), the region in which the control of vergence eye movements has been shown to be both fast and effective.

Adult↗

PubMed1999

Lightness induction revisited.

Lightness induction is the classical visual phenomenon whereby the lightness of an object is shown to depend on its immediate surround. Despite the long history of its study, lightness induction has not yet been coherently and satisfactorily explained in all its variety. The two main theories that compete to explain it descend (i) from H von Helmholtz, who believed that lightness induction originates from some central mechanisms that take into account the whole viewing situation, with particular stress upon the apparent illumination of the object; and (ii) E Hering who argued in favour of more peripheral sensory mechanisms based on local luminance contrast. The balance between these theories has recently been shifted towards Helmholtz's position by E H Adelson who has provided additional evidence that lightness induction depends on perceptual interpretation and, particularly, on apparent transparency. I challenge Adelson's conclusions by introducing modified versions of his tile pattern that use luminance gradients. In the first of these new demonstrations there is a strong lightness induction even though no apparent transparency is experienced. In the second there is a clear impression of transparent strips, yet no lightness induction is present. And the third shows that breaking up the Adelson tile pattern, while it affects neither the impression of transparency nor the type of grey-level junctions, makes the lightness-induction effect vanish. This implies that Adelson's illusion can be accounted for by neither local contrast, nor the apparent transparency, nor the type of grey-level junctions. Presented here is an alternative look at lightness induction as a phenomenon of the pictorial (as contrasted to natural) vision, which rests on the lightness-shadow invariance, much as Gregory's 'inappropriate constancy scaling' theory of geometrical illusions rests on the apparent size-distance invariance.

Contrast Sensitivity↗

PubMed1998

On derivation of spectral sensitivities of the human cones from trichromatic colour matching functions.

Despite the recent advance made by using the direct methods of retinal densitometry, microspectrophotometry and suction electrophysiology, the psychophysical approach based on colour matching data still remains an important source of accurate information about the spectral sensitivity of the cone photoreceptors in the human visual system. However, the commonly used technique of estimating cone sensitivities, based on the assumption that dichromacy is caused by the lack of one of the three types of the cone photoreceptors, requires the colour matching functions not only from trichromatic observers but from dichromats as well. Here we evaluate an alternative approach, originally put forward by Bongard and Smimov, that derives cone spectral sensitivities from colour matching functions only; without resorting to colour deficiency or any other data. When applied to CIE standard colour matching functions, this method yields curves of spectral sensitivities that are close to the classical Smith-Pokorny fundamentals, though the long-wave cone is shifted towards the short-wave region of the spectrum by 5 nm, as compared with Smith and Pokorny's results.

Color Perception↗

PubMed1996

Convexity of a set of subthreshold stimuli implies a peak detector.

It has been shown that for every model of detection (whether single- or multi-channel, with linear or non-linear channels, and whatever decision rule), provided that it predicts a convex set of subthreshold stimuli, there is a psychophysically equivalent peak detector made up of a collection of linear analysers followed by a maximum-output decision rule. In this paper, the equivalent peak detector representations of some widely accepted detection models are calculated. The calculations rest on a general technique for deriving, from a given model, a formula which specifies the analyser most sensitive to any given stimulus.

Contrast Sensitivity↗

PubMed1996

On cardinal directions in spatial pattern space and falsifying multi-channel detection models.

It has been generally recognised that in its early stages the human visual system comprises a set of independent subsystems, or channels, acting in parallel. There is general agreement that the receptive fields of neurones constituting the channels overlap considerably, making the task of selectively stimulating individual channels nontrivial. Since such a task is important for estimating the spatio-temporal characteristics of these channels, a method for determining a set of spatial patterns which stimulate multiple channels independently, irrespective of how their receptive fields overlap, is presented here. As an example such patterns were calculated for Wilson and Bergen's model (Wilson and Bergen, 1979, Vision Res. 19, 19-32). Using the modification of the subthreshold summation technique (Logvinenko, 1995, Biol. Cybernet. 73, 547-552) it is shown that in reality these stimuli are not processed independently. It follows that Wilson and Bergen's model involves a set of channels which is inappropriate or incomplete or both.

Humans↗

PubMed1995

On deriving analyser characteristics from summation-at-threshold data.

It has been proved that a detection process may be accounted for by a simple two-state model consisting of a collection of linear analysers followed by a maximum-output decision rule provided that a set of all threshold stimuli is convex. A non-parametrical method to identify the analysers constituting such a model is proposed.

Humans↗

PubMed1994

Convergence as a cue for distance.

Binocular eye movements were measured in subjects experiencing the wallpaper illusion. It was found that a physical displacement of the fixation point by more than 1 m out of the plane of apparent localisation of the strips had no influence on the illusory location of these strips. Hence, illusory localisation in the wallpaper illusion is independent of the actual magnitude of the subject's convergence angle, once the illusion has come into existence. This result suggests that convergence does not serve as a source of information about apparent distance, at least in the wallpaper illusion.

Adult↗

PubMed1990

Nonlinear analysis of spatial vision using first-and-second-order Volterra transfer functions measurement.

The harmonic input method of nonlinear system identification is modified to allow the Volterra series approach to be used for psychophysical investigation of various aspects of human pattern vision in the spatial frequency domain. While it is well known that only one modulation transfer function provides a complete characterization of a linear system, a number of multidimensional transfer functions are needed to identify a nonlinear system. We have shown, that so far as the contrast sensitivity to sine-wave gratings may be used for an empirical estimate of the first-order modulation transfer function of the human visual system, the contrast sensitivity to difference harmonics may be used as an empirical estimate of the second-order modulation transfer function. A difference harmonic arises from a mixture of two sine-wave gratings resulting from the nonlinearity of the visual system. Difference harmonic, experienced as some periodic beatlike structure, may still be observed if frequencies of the component gratings are higher than the maximum visual acuity. The visibility of the low-frequency beatlike pattern produced by pairs of sine-wave gratings, which themselves are of spatial frequencies too high to be resolved, could be accounted for either by a difference frequency distortion product (Burton, 1973) or by a special beat detector (Derrington & Badcock, 1985). We found that increasing the contrast of one component grating may be compensated for by reducing the contrast of the other component grating, the beatlike pattern being at threshold. This is exactly what would be expected if the beatlike pattern is detected because of the difference harmonics produced by nonlinearity of the visual system. We have determined contrast thresholds for the difference harmonics which occur between two unresolved different spatial frequencies. The contrast sensitivity function for difference harmonics was found to have a marked similarity both in the shape and position of peak sensitivity to the contrast sensitivity function for single sine-wave gratings. Another important characteristic of the contrast sensitivity function for difference harmonics is that it depends only on the frequency difference, delta f = f1 - f2, rather than on the value of either f1 or f2. All this indicates that a difference harmonic arises from local nonlinearities in the visual system. More specifically, the visual system may be represented as a cascade system, composed of a linear system with transfer function O (f) followed by a nonlinear element, r(.), without spatial spread in cascade with another linear system with transfer function P (f).(ABSTRACT TRUNCATED AT 400 WORDS)

Contrast Sensitivity↗