Black box analysis of the skin senses as a multiple communication channel.

The skin senses have been investigated as a multichannel parallel information system. In psychophysical experiments using two cold two warmth and two vibration stimuli the capability of these senses have been determined for transmitting independent parallel information. A black box model is used to describe interactions within the system between pairs of signals causing two different response components in a multiple response after multimodal activation. The theory distinguishes two kinds of interaction, those between internal noise sources and between internal signal sources. It appears that the skin senses show only a very small interaction of their respective noise sources whereas their signal sources interact extensively. The repercussions for the skin as a parallel information transmitter is discussed shortly in the context of a development of an optimum "skin vision" system, which should project an environment onto the skin, employing both the sense of touch and the sense of temperature.

Recognition of organs in CT-image sequences: a model guided approach.

A method is proposed for segmentation of organs in CT-image sequences. An important feature of the method is the use of search areas to guide the segmentation process. Time consuming, data-directed operations are restricted to these areas, instead of being applied to the whole image. A search area for a particular anatomical structure is determined by constraints, derived from a model of the imaged scene and by results already obtained in the recognition process. The method has been successfully applied on recognition of the spleen in abdominal X-ray CT scans.

Afterimages: a collective term for percepts of different origin.

Exposure of the eye to a strong photoflash results in a so-called "afterimage", which may last for 20 min or longer. In contrast, the true afterimage, which fluctuates in brightness and is best seen in complete darkness, lasts only a few minutes. This true afterimage can be attributed to the strong oscillatory neuronal responses immediately initiated by the flash. Thereafter dark and light regions, insensitivity percepts, are observable against bright and dark backgrounds, respectively. These percepts can be adequately explained by a reversal of the response behaviour of rod and cone driven ganglion cells situated along the contour of the flash-exposed area. The slow recovery of the rods explains why insensitivity percepts can be seen for many minutes.

Apparent size and receptive field properties.

A light square against a dark background besides an equisized dark square against a light background shows a difference in apparent size such that the light square seems larger than the dark one. This illusion was studied for squares of 109 x 109 min arc. Their centres were located 90 min arc out of the centre of the fovea. The apparent size appeared to be dependent on the contrast between the square and the background and not on the mean luminance in the vicinity of the contour. The addition of a small white or dark line to the luminance step that constituted the border of the square changed the illusion dramatically. It is argued that this explains the observations where an inversion of the illusion for very small contrast values was found. The results are explained on the basis of a model with receptive fields of different sizes overlapping the same retinal location and having different sensitivities.

Perturbing line pictures for identification of visual features and their syntax.

Experiments are reported in which line pictures were perturbed by omission or displacement of a combination of single pixels, fragments of lines, contours, and whole figures. Different effects of perturbation were expected by selectively violating visual syntactic rules or by impeding the contribution of certain feature detectors. The deterioration of the perturbed picture was measured according to standard psychophysical methods by rating on a 5-point scale. Multivariate methods were used to single out the relative effects of perturbation by, respectively, a set of single pixels, line fragments, contours and whole figures. Lines, as opposed to loose pixels, are clearly powerful descriptors of the pictures; contours or whole figures do not add significantly to what lines already describe. Different effects were observed if perturbations were dislocations rather than removals. Then contours and whole figures showed a typical disrupting effect compared to line fragments. These results have consequences for the development of a syntax of visual form perception. The perturbation method seems appropriate for identifying features or syntactic rules, although the results are dependent on a number of environmental and contextual factors.

Identification of figural elements in a visual domain.

Identification of figural elements is based upon comparison of one of several formal pattern descriptions with subjective similarity judgements. The best description appears to contain figural elements which are heavily dependent on the visual domain chosen for the experiment. Generally those figural elements seem to be involved which distinguish optimally between the reference patterns used to mark off the visual domain. An algorithm is proposed for searching figural elements which are building stones of optimal descriptions. The present study indicates that grammars generating structures based upon features, higher order figures and rules of composition may be tools for visual research.

A volume conductor study of compound action potentials of nerves in situ: the forward problem.

A computer model for the stimulation of compound nerve action potentials, based on superposition of volume conducted single nerve fibre potentials, is presented. The model assumes that the intracellular fibre potential, the fibre diameter distribution and the electrical conductivities of different tissues are known. Volume conductor fields are calculated in the spatial frequency domain. The influence of important parameters in the model is evaluated numerically. It is shown that it is necessary to give up the usual assumption of homogeneity and isotropy in the extracellular medium. In the present model parameters are introduced which allow an overall description of the complex morphological and physiological structure of the nerve trunk. Simulation results indicate that the model is a rather promising tool in studying the main properties of compound action potentials which up till now have not been sufficiently well understood.