Domagoj Švegar

Modelling the statistical processing of visual information

Recent psychophysical investigations showed that humans have the ability to compute the mean size of a set of visual objects. The investigations suggest that the visual system is able to form an overall, statistical representation of a set of objects, while the information about individual members of the set is lost. We proposed a neural model that computes the mean size of a set of similar objects. The model is a feedforward, two-dimensional neural network with three layers. Computer simulations showed that the presented model of statistical processing is able to form abstract numerical representation and to compute the mean size independently from the visual appearance of objects. This is achieved in a fast, parallel manner without serial scanning of the visual field. The mean size is computed indirectly by comparing the total activity in the input layer and in the third layer. Therefore, the information about the size of individual elements is lost. An extended model is able to hold statistical information in the working memory and to handle the computation of the mean size for surfaces with empty interiors.

The effects of facial expressions and tears on the speed of sex recognition

The goal of this research was to examine the effects of facial expressions on the speed of sex recognition. Prior research revealed that sex recognition of female angry faces was slower compared with male angry faces and that female happy faces are recognized faster than male happy faces. We aimed to replicate and extend the previous research by using different set of facial stimuli, different methodological approach and also by examining the effects of some other previously unexplored expressions (such as crying) on the speed of sex recognition. In the first experiment, we presented facial stimuli of men and women displaying anger, fear, happiness, sadness, crying and three control conditions expressing no emotion. Results showed that sex recognition of angry females was significantly slower compared with sex recognition in any other condition, while sad, crying, happy, frightened and neutral expressions did not impact the speed of sex recognition. In the second experiment, we presented angry, neutral and crying expressions in blocks and again only sex recognition of female angry expressions was slower compared with all other expressions. The results are discussed in a context of perceptive features of male and female facial configuration, evolutionary theory and social learning context.

A model of the illusory contour formation based on dendritic computation

We proposed a new model of illusory contour formation based on the properties of dendritic computation. The basic elements of the network are a single-excitatory cell with two dendritic branches and an inhibitory cell. Both dendritic branches behave as an independent linear unit with a threshold. They sum all excitatory input from the nearby collinear cells, and the inhibition from one collateral of the corresponding inhibitory cell. Furthermore, the output of dendritic branches multiplicatively interacts before it is sent to the soma. The multiplication allows the excitatory cell to be active only if both of its branches receive enough excitation to reach the threshold. Computer simulations showed that the presented model of the illusory contour formation is able to perform perceptual grouping of nonadjacent collinear elements. It shows a linear response relationship with the input magnitude because dendritic inhibition counteracts recurrent excitation. The model can explain why illusory contours are stronger with irregular placement of inducing elements rather than regular placement and why top-down influences may prevent the illusory contour formation.

In vitro assessment of the accuracy of three electronic apex locators

Aim of this in vitro study was to compare accuracy of three electronic apex locators (EALs) (ES-02, ES-03 and ProPex) in root canals of extracted human teeth utilizing two meter readings and different conditions of root canals (RC). RC lengths of 46 single-rooted teeth were visually determined using a K-file # 15. Teeth and labial connector were then embedded in alginate. Two sets of measurements were performed for each device - root canal condition combination (dry root canal, 2.5% NaOCl, 17% EDTA, RC prep, Glyde) according to readings “-0.5” and “0.0”. The differences were evaluated using descriptive and nonparametric statistics. The mean difference between real and measured values at meter reading “0.0” was 0.10 mm with ProPex, 0.31 mm with ES-02 and 0.71 mm with ES-03. At meter reading “-0.5“ that difference was 0.28 mm with ProPex, 0.41 mm with ES-02 and 0.87 mm with ES-03. In vitro testing showed ProPex to be the most accurate EAL in most of combinations, followed by ES-02, and ES-03 (p<0.05). Most accurate measurements were performed in canal conditions as follows: dry canals, Glyde, Rc-Prep, NaOCl and EDTA solutions (p<0.05). ProPex was the most accurate at meter reading “0.0”. At reading “-0.5” ES-02 and ProPex were equal, whereas ES-03 was the least accurate. All tested EALs should be used with meter reading “0.0” as a reference because it is more accurate than using the meter reading “-0.5” (p<0.001).

TRAIT ANXIETY MODERATES THE EFFECT OF DENTAL SURGERY PHASES ON VAGAL ACTIVATION

Decreases in parasympathetic (PNS) activation are a normative reaction to stressful situations. By the Porges’ Polyvagal theory (Porges, 1995) and Thayer and Lane’s (2000) model of neurovisceral integration, PNS inflexibility is related to poorer functioning of emotional and other psychological regulatory mechanisms. The aim of the present study was to investigate the differences in PNS activation between high and low trait anxiety individuals throughout two phases of dental surgery the period of waiting for the beginning of the surgery and the phase after the injection of anesthesia which was expected to be more stressful. PNS activation defined as high frequency heart rate variability was measured in 11 female and 10 male participants during two phases of dental surgery. Participants that were higher on trait anxiety had lower parasympathetic activation during the expectation of the surgery compared to low trait anxiety participants. High anxiety participants also did not show parasympathetic reactivity (i.e. vagal withdrawal) during the next phase, which was evident only in low trait anxiety participants. Such a pattern was not observed for the other components of trait of neuroticism. The results were interpreted in the context of lower psychophysiological flexibility and poorer coordination of different emotional systems in high trait-anxiety individuals.