Alvydas Šoliūnas

Interaction of bottom-up and top-down processes in the perception of ambiguous figures.

Ambiguous figures reverse their appearance during prolonged viewing and can be perceived in two (or more) available interpretations. Both physical stimulus manipulations and cognitive control influence the perception of ambiguous figures, but the underlying mechanisms remain poorly understood. In the current study, the perception of an ambiguous figure was manipulated by adaptation to unambiguous figures and/or placing the ambiguous figure into a context of unambiguous figures. Our results indicate that both adaptation and context can effectively modulate perception of the ambiguous figure. When manipulated together, adaptation and context processes showed additive effects upon the perception of the ambiguous figure implying the independent mechanisms. Thus, top-down and bottom-up processes seem to influence the perception of the ambiguous figures independently and neither seems to be uniquely responsible for the generation of perceptual changes.

Hexagonal Approach and Modeling for the Visual Cortex

In this paper, the hexagonal approach was proposed for modeling the functioning of cerebral cortex, especially, the processes of learning and recognition of visual information. This approach is based on the real neurophysiological data of the structure and functions of cerebral cortex. Distinctive characteristic of the proposed neural network is the hexagonal arrangement of excitatory connections between neurons that enable the spreading or cloning of information on the surface of neuronal layer. Cloning of information and modification of the weight of connections between neurons are used as the basic principles for learning and recognition processes. Computer simulation of the hexagonal neural network indicated a suitability and prospectiveness of proposed approach in the creation, together with other modern concepts, of artificial neural network which will realize the most complicated processes that take place in the brain of living beings, such as short-term and long-term memory, episodic and declarative memory, recall, recognition, categori- sation, thinking, and others. Described neural network was realized with computer program written on Delfi 3 language named the first order hexagon brainware (HBW-1).

Dviejų vienu metu pateiktų vaizdų tarpusavio panašumo įtaka jų atpažinimo tikimybei

We investigated the influence of the degree of similarity of two simultaneously presented geometrical figures on their recognition in tachistoscopical experiments on human beings. T he test stimuli w ere 16 nonverbal figures consisting of six vertical and horizontal line-segments. There were five groups of figure pairs (16 pairs in each group) that differed according the ,number of coincident line segments between two figures: two, three, four, five, or six (in the case of identical figures) . A pair of test figures was presented for 50 ms and after an individually determined interstimulus interval it w as masked by two rectangular frames of 12 line segments. On the first experimental situation, a special marker indicating which test figure must be drawn appeared after masking the patterns. ln the second experimental situation, the subjects were required to draw both test figures. Identical figures were recognized with higher probability than figures with a different degree of similarity. T he recognition probability of two simulultaneously presented figures also depended upon the practice level of the subjects. For experienced subjects we obtained a U-shaped function of recognition probability on the similarity level. Most of the inexperienced subjects did not show any recognition dependence on the similarity level. Different instructions in the second situation - to draw both figures - altered the experimental results. We concluded that the results in the situation. in which the subject was required to draw both figures reflect not the recognition accuracy of the two figures but the accuracy of restoration of the second figure from working memory.

Nežinomų ir žinomų vaizdų atpažinimo ypatumai

Recognition of unknown meaningless figures was investigated in psychophysical experiment with the aim to clarify which stage of known figure recognition, facture extraction or extracted facture matching with the long-term memory representations, involves successive information processing. Five groups of test figures, consisting of four, five, six, seven, and eight horizontal and vertical line-segments, were composed. Each trial consisted of the following sequence: test figure for 10 ms; individually determined interstimulus interval; masking figure; delay of definite duration (8 values between 200 and 4700 ms); three figures for comparison. The task for subject was to find and indicate which one of three figures corresponds to the test figure. Recognition probability of unknown figures did not depend on the number of figure clements. This fact, together with the previously obtained reverse dependence of the recognition probability of known figures on their complexity, cd to the conclusion that the extraction of factures is the simultaneous process while the matching of extracted features with the long-term memory representations is the successive process. Retention of information in the short-term memory up to 5 s had no effect on the task performance.

Identification of partially presented meaningless patterns: effect of completeness and distinctiveness

The role of parts versus that of wholes in a visual perception has been debated for a century as two opposite approaches, namely, an analytic and holistic. In two psychophysical experiments we investigated whether the stimulus completeness or distinctiveness is essential for identification of the partially presented patterns under brief presentation conditions. For this purpose, a special class of stimuli was constructed in such a way that the patterns could be divided into informative and redundant parts. The first experiment clearly demonstrated the importance of the redundant part for effective pattern identification for the majority of subjects. The second experiment revealed the direct dependence of identification accuracy of the patterns on their completeness (2, 3, 4, 5, or 6 elements). Familiarisation of subjects with the test stimuli influenced the strength of this dependence.

DVIEJŲ VIENU METU PATEIKTŲ FIGŪRŲ PANAŠUMO ĮTAKA JŲ SUVOKIMUI

Relatedness of the analysis of a few simultaneously presented objects in the visual field was investigated in the psychophysical experiment on human subjects. Test stimuli were plane nonsemantic geometrical figures composed on a basis of 4 x 3 matrix. Five groups of figure pairs with different degree of reciprocal similarity were presented for recognition under the backward masking conditions. Exposition duration of test figures varied individually from 400 to 3000 ms. Masking of test figures was followed by a special marker that indicated which one of two test figures must be detected in the subsequently presented table that included all 12 test figures. Experimental data revealed two groups of subject. The same recognition accuracy for the pairs of identical and different figures as well as no effect of similarity on the recognition of different figures characterised the results of the first group of subjects. The second group of subjects recognised identical figures more accurately than different ones and the recognition of different figures had direct dependence on their similarity - the more similar the figures were, the more accurately they were recognised. Experimental results were interpreted on the basis of different analysis strategies applied by different subejcts.