We explored the human observer's ability to detect and discriminate sine-wave and square-wave gratings that were sampled at intervals varying from 4.7 to 9.4 arcmin. To study the effect of sampling irregularity on visual performance, we varied the position of each line sample on the basis of a Gaussian probability distribution, the standard deviation of which varied from 0 (regular sampling) to 4.7 arcmin (highly irregular sampling). The results indicate that irregular sampling has no systematic effect on the observer's ability merely to detect the presence of a sine- or square-wave grating. In contrast, sampling irregularity strongly impairs the subject's ability to discriminate between these waveforms. A model based on the convolution of difference-of-Gaussians-type weighting profiles predicted that sampling irregularity should have little to no effect on the output of a channel tuned to the third harmonic of the square-wave grating. The findings thus suggest the existence of a sampling scheme in the visual system. This scheme is based on local feature-selective mechanisms, probably edge detectors, that are highly sensitive to the relative position of the sample points in the space domain.

Discrimination, Psychological↗

PubMed1988

Transient and sustained components in spatio-temporal visual perception.

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Biomechanical Phenomena↗

PubMed1988

Apparent contrast of spatially and temporally sampled gratings.

We have measured the apparent contrast of spatially and temporally sampled gratings having the spatial frequency of 2, 4 and 8 c/deg. A contrast matching task was used in two different sampling conditions which allowed us to stimulate selectively the sustained and transient mechanisms in spatial and temporal vision. The results demonstrate the visual interpolation characteristics in the two domains and suggest a better response linearity for spatial than for temporal interpolation. The strongest nonlinear responses were obtained for temporally sampled medium spatial frequency gratings, suggesting separate temporal processing mechanisms for medium and high spatial frequencies

Humans↗

PubMed1986

Fatal epithelioid haemangioendothelioma presenting in the lung and liver.

Three patients with epithelioid haemangioendothelioma (EHE) are described. Two patients presented with pulmonary infiltrates and one with a hepatic tumour. All had a metastatic disease ending fatally, and all were autopsied. The diagnosis was confirmed either by immunohistological or ultrastructural analysis. All three tumours were cytokeratin-negative and vimentin-positive, while only two contained cells reacting with the antibody of factor VIII-related antigen. Electron microscopy of the third tumour revealed features indicating endothelial differentiation. A short literature review is also presented demonstrating that the outlook of EHE is worse than previously thought.

Adolescent↗

PubMed1985

Visual illusion mimicking dyslexia. A case history.

A 14-year-old girl with reading difficulties was found to have an unusual illusory movement of objects in the central visual field. The movement could be blocked by using a 26 mm Visolette magnifier with an arrow pasted on its lower surface to function as a reference point. After having read with help of this arrangement for 19 months the girl suddenly could see normally and has now been able to read without difficulties for over 3 years. We report the first documented case of visual illusion as the cause of reading difficulty which disappeared at the age of 15 years, thus years after the maturation of visual functions is thought to be completed. Although visual illusion like the one presented here must be rare, it should be diagnosed, because the treatment is totally different from that of usual dyslexia.

Adolescent↗

PubMed1977

Long-lasting afterimages caused by neural adaptation.

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Adaptation, Ocular↗

PubMed1976

Characteristics of single mechanoreceptive fibres innervating hairy skin of the human hand.

Unit activity was recorded percutaneously by microelectrodes from 69 nerve fibres innervating the hairy skin of the back of the hand in 6 awake human subjects. Twelve fibres had no receptive field on the skin, twenty-one fibres could be classified as rapidly adapting (RA) and thiry-six fibres as slowly adapting (SA) mechanoreceptive fibres. Four SA fibres has a spontaneous discharge in absence of stimulation. Mechanical thresholds of tra fibres ranged from 0.3 to 1.7 g and those of SA fibres from less than 0.1 to 1.7 g. Conduction velocities of the fibres were 22-41 m/s (RA fibres) and 10-73 m/s (SA fibres). Intensity functions of SA fibres could be described by power functions with exponents varying between 0.31-1.23. Most RA and SA fibres were entrained by vibratory stimuli. The results indicate that the receptors in the human hairy skin do not differ considerably in their characteristics from the receptors in the human glabrous skin or from animal receptors in the hairy skin.

Adolescent↗

PubMedDate not supplied

Mismatch negativity (MMN) for sequences of auditory and visual stimuli: evidence for a mechanism specific to the auditory modality.

ERPs to sequences of standard and deviant sinusoidal 100 msec tone pips, high-contrast sinusoidal gratings and to their simultaneously presented combinations were recorded. Mismatch negativity (MMN), an ERP component elicited by deviant stimuli, was estimated for the different stimulus sequences in order to find out whether it reflects modality-specific processes or non-specific attentive phenomena. In addition to the auditory modality, we studied whether the mismatch response could be evoked by a deviant visual stimulus in a visual sequence or by a deviant stimulus in either modality. The results show that only auditory stimuli produced the mismatch response, suggesting that MMN is not a manifestation of a general attentional mechanism but is probably specific to the auditory modality.

Acoustic Stimulation↗