Search PubMed⌕ Search

Biomedical subjects

B Moulden

Publications and source records attributed to B Moulden.

At least 37 records · Page 2Linked to original sources
PubMed1989

Modelling visual detection: luminance response non-linearity and internal noise.

Two experiments that investigate the effect of various display factors on the detectability of a thin line signal in random visual noise are described. Three statistical decision models are described, together with their ability to account for the results. The first is an "ideal detector" model, the second an "energy integrator" model, and the third a model based upon the operation of retinal ganglion cells which incorporates a gain control mechanism. The ideal detector model fails to give a good account of human performance, whereas the other two models provide a good fit to the data. The digital Laplacian with gain control model has the slight advantage over the energy integrating model in being able to account for a small superiority in the detection of dark as opposed to bright signals. Finally, both models require the inclusion of an estimate of the internal noise of the human visual system to account for the pattern of performance observed under changing conditions of display contrast.

Adult↗
PubMed1989

Corner effect in induced hue: evidence for chromatic band-pass filters.

An experiment is described which investigates the spatial determinants of the apparent difference in hue between the central grey patches of chromatic 'H' pattern pairs, an effect similar to that first demonstrated by Wright (1969, The Measurement of Colour, Hilger, London) in coloured gratings. The hue difference is shown to be analogous to the brightness difference in achromatic 'H' patterns demonstrated by Moulden and Kingdom (1989, Vision Res. 29, 1245-1259). The origin of both effects is argued to be the presence of the corner intersections in the 'H' patterns, which are powerful stimuli for cells with circularly-symmetric, centre-surround organization. It is suggested that the results of the experiment with the chromatic 'H' patterns implicates the operation of cells with a spectrally double-opponent, rather than single-opponent receptive field organization.

Color Perception↗
PubMed1988

A simple analogue teaching device for demonstrating visual-receptive-field properties.

A cheap (under 100 pounds at current prices) and simple analogue device is described which permits one to demonstrate, according to an adjustable configuration of the photoreceptive elements, some of the main properties of (i) circularly-symmetrical, (ii) 'simple' elongated opponent-flank, and (iii) multiple-discharge-centre visual receptive fields. The design and construction of the device are described, together with some suggested demonstrations.

Electricity↗
PubMed1988

Border effects on brightness: a review of findings, models and issues.

This paper presents a summary of experimental findings, theoretical models and unresolved issues regarding border effects on brightness, of which the Cornsweet illusion (Cornsweet, 1970 Visual Perception. Academic Press: New York) is the best-known example. It is argued that no current theoretical model completely accounts for the wide variety of effects described. Contrast sensitivity function (CSF) models can explain many low-contrast, but not high-contrast, border effects. Lightness integration models based on Land and McCann's retinex theory (Land and McCann, 1971. J. Opt. Soc. Am. 61, pp. 1-11) have the advantage over CSF models in that they predict transitivity of border effects where they are found to occur. However, they fail to predict the appearance of a variety of Cornsweet-like figures, have never been tested with relatively high contrast versions of those figures, and have only been implemented by qualitative demonstration. It is argued that edge-detector models are potentially the most promising theoretical candidates but, as with lightness-integration models, they have invariably relied on qualitative demonstrations and have only dealt with low-contrast border effects. A computational edge-detector model which predicts the appearance of both high and low contrast Cornsweet figures is proposed and its advantages over other models, as well as its current limitations, are discussed. The final section discusses the neural locus for border effects in brightness.

Animals↗
PubMed1987

Effect of the number of grey levels on the detectability of a simple line signal in visual noise.

The number of grey levels, G, contained in a digitized image of an external event must affect the fidelity of reproduction of that event for physical reasons. The question arises as to whether there is a separate perceptual effect of G. Three experiments are described which investigate the effect of G on the visibility of a straight-line signal in visual noise using a signal detection analysis to separate the physical and perceptual effects of G. The results show that, for the type of displays employed, and for the specific task of detection of lines in visual noise, there was no effect of G on efficiency, which suggests that G had no separate perceptual effect.

Color Perception↗
PubMed1987

Model for the detection of line signals in visual noise.

Two experiments are described that investigate the detectability of signals embedded in spatially uncorrelated visual noise. The signals were thin lines extending vertically from the top to the bottom of the noise background and were essentially of two classes. One class consisted of signals whose pixel luminances were spatially uncorrelated, the other of signals whose pixel luminances were sinusoidally modulated along their length. In both classes the space-average luminance of the signals was greater than that of the noise. In the first experiment it was found that the modulated signals were more visible than the unmodulated, even though the space-average luminance of both types was equal. For the modulated signals visibility was proportional to the amplitude of modulation and inversely proportional to the spatial frequency of modulation. Neither an ideal-observer model nor an energy-integrator model was found to give a good account of the results. On the other hand, a model referred to as the maximum filter output model gave an excellent account of the data. The model employed a simple energy-integrating filter approximately 0.8 deg in length and recorded the filter's maximum output for every potential signal, indicating the signal as the line that gave the largest output. In a second experiment, modulated signals that contained added random luminance perturbation were used, and again the model was found to give a good fit to the data. The findings are discussed in the context of known properties of cortical-bar detectors.

Adult↗
PubMed1986

The Münsterberg figure and twisted cords.

The Münsterberg tilt effect can be explained by the effects of band-pass filtering in the visual system, supplemented by the principle that peaks and troughs in the filtered image are represented separately at the stage of orientation encoding. Circularly symmetric band-pass filtering of the Münsterberg figure reveals that both peaks and troughs along the mortar are oriented along lines that are slightly tilted with respect to the horizontal. The result is a Fraser twisted cord, which underlies the Münsterberg effect. We use the principle of the twisted cord to derive a new version of the Münsterberg effect.

Filtration↗
PubMed1986

Digitized images: what type of grey scale should one use?

One of the issues faced by engineers when designing a system which records an external event and represents it in the form of a digitized image on VDU screen is which type of grey scale to use. An experiment is described which compares, in a simulated digitized image, the effect of a linear and a logarithmic grey scale on the detectability of a straight-line signal embedded in visual noise. It was found that both bright and dark signals were detected more easily with the linear scale. A signal detection theory analysis was carried out to compare human performance with that of an 'ideal' observer who performed the detection task with a filter spatially matched to the signal. It was found that the model of performance for this ideal observer accounted well for the results provided the assumption of a linear transformation of luminance was made. The analysis showed that the superiority of the linear over the logarithmic grey scale was simply due to the higher signal-to-noise ratio of the signals in the former.

Adult↗
PubMed1986

Some tests of the Marr-Ullman model of movement detection.

Several psychophysical experiments are described which test and uphold predictions derived from the Marr-Ullman model of movement detection. First, we demonstrate the existence of adaptation which is specific not merely to the direction of movement of an edge, but also to its contrast polarity. Second, it is shown that adaptation to a spatially homogeneous field whose luminance is modulated according to a temporal sawtooth waveform produces predictable changes in sensitivity to the movement of an edge; these changes, too, are specific to particular conjunctions of direction and edge polarity. Third, similar changes in sensitivity are demonstrated to occur when the luminance of an edge is physically perturbed at the moment of its displacement. Finally, it is shown that, as predicted, the sudden onset of an edge can itself give rise to a momentary impression of movement, the apparent direction of which depends upon the change in luminance that accompanies the onset of the edge.

Adaptation, Physiological↗
PubMed1984

Intensity-response nonlinearities and the theory of edge localization.

If an edge having a particular blur profile is exchanged for an edge at the same location but having a different blur profile, apparent movement between the two edges may be seen. The spatial extent of this movement was measured by a cancellation technique. Implications for the theory of edge location are discussed, including the possibility that spatial filtering is preceded by a nonlinear transduction stage.

Filtration↗
PubMed1984

Two channels for flicker in the human visual system.

The human visual system contains a large number of narrowly-tuned spatial-frequency-specific channels. Does it contain an analogous set of channels tuned to a narrow range of temporal frequency? On the basis of data gathered with the use of a threshold elevation technique it is argued that human sensitivity to flicker can be accounted for by assuming the existence of just two filters, one a low-pass filter peaking gently at around 6 Hz and one a band-pass filter peaking at around 9 Hz. Similar data gathered from studies of interocular transfer suggest that at least some of the mechanisms involved are binocular, rather than being purely monocular as has previously been suggested.

Adaptation, Physiological↗
PubMed1980

After-effects and the integration of patterns of neural activity within a channel.

Prolonged inspection of an adapting simulus changes the appearance of a subsequent test stimulus. There are five distinct viewing conditions under which such 'after-effects' may be generated. There are MON-MON (inspect with one eye, same eye), BIN-BIN (inspect with both eyes, test both eyes), BIN-MON (inspect with both eyes, test only one eye), MON-BIN (inspect with one eye, test with both) and TRANSFER (inpect with one eye, test with the other eye). A model based upon the assumption of the linearly additive effects of adaptation generated in 'dominance classes' or cortical units that are driven either by one eye, or the other eye, or by either or both eyes together, is described. This model generates predictions concerning the expected ralative magnitudes of after-effects generated under the five viewing modes described above, and experiments are described that confirm these predictions. The model can be extended to gaenerate predictions about other experimental conditions. A more complex version of the model is consistent with electrophysiologically derived estimates of the proportion of cortical units in each dominance class.

Afterimage↗
PubMed1979

The Münsterberg illusion and 'irradiation'.

The Münsterberg illusion is a compelling phenomenon for which there is no generally accepted explanation. It is suggested that two different contour-shifting effects operate in this illusion, and these are named the 'symmetrical effect' and the 'corner effect' respectively. It is further suggested that both of these effects are caused by 'irradiation', and an attempt is made to cast some light upon the phenomenon of irradiation, which is itself only poorly understood.

Humans↗