Nadejda Bocheva

Perceived location of two-dimensional patterns

The study attempted to test the possibility that the center of gravity of two-dimensional patterns is the cue used by a human observer for their localization. Four experiments were carried out. The first, using a matching procedure, required the localization of the center of irregular dot patterns, contour and filled polygons which varied in size and orientation. In the other three experiments the subjects had to point to briefly exposed dot patterns in which overall shape (convex and concave in Expts 2 and 3) and dot density (Expt 4) were manipulated. The performance of these direct localization tasks was found to be as accurate as the performance in previous studies of indirect localization or regular patterns. The results consistently supported the claim that information about position of the center of gravity is used for the localization of visual objects.

Age-related changes in fine motion direction discriminations

The present study used an equivalent noise method to characterize the sources of reduced performance in fine discrimination of motion with age. We varied the density of the displays, the speed and speed variability and the temporal correlation of dots’ motion in successive frames to assess their effect on the sensitivity to motion direction. The results showed that, in all experimental conditions, the older observers had higher levels of internal noise. Both age groups used the stimulus information less efficiently at slow speed and most efficiently when the moving elements were uncorrelated across frames. The older observers were less efficient than the younger observers in all conditions except at high speed where the efficiency of the two age groups was the same. We fitted two biologically plausible models to the experimental data: a modified version of the local-to-global direction encoding model (Dakin et al. in Vis Res 45:3027–3049, 2005) and a model for pooling of motion information in medial temporal area (MT) where the neuronal responses were correlated (Huang and Lisberger in J Neurophysiol 101:3012–3030, 2009). The modeling results indicate that the correlation in neuronal responses is essential to characterize the influence of speed and speed variability on the sensitivity to direction information. For the younger observers, a single set of parameters can account for the effect of noise and the spatio-temporal parameters of the stimuli, while, for the older observers, a change in the correlation of neuronal activity and the directional tuning bandwidth with the levels of external noise is needed. The findings are discussed with respect to optimal use of the dynamic information to overcome the negative effect of aging.

Different Mechanisms may Underlie the Performance in Relative Localization Tasks

Two different groups of subjects had to adjust two-dimensional stimuli, differing in size, shape and type (dot patterns or irregular contour figures), within a reference circle. The two groups performed under two different instructions. The first instruction stressed matching the centres of the stimulus and the circle, while the second required simply positioning the test stimulus in the middle of the reference circle. In two control experiments the subjects had to determine the position of the centres of each stimulus and of the reference circle. Under the first instruction the accuracy of performance, estimated by the variance of the responses, depended on the stimulus size, shape and type in agreement with previous results and models of relative localization. Under the second instruction, however, accuracy remained invariant. Possible mechanisms of relative localization that might differ at their first stages of localization of the separate stimuli are considered.

Detection of motion discontinuities between complex motions

Three main experiments were performed to evaluate the ability of human observers to detect non-homogeneity in a motion field caused by the presence of two adjacent complex motions, having a common motion component. The detection performance varied significantly depending on the common motion component in the motion field. The highest detection rate was observed when the common motion component was radial or rotational flow. The results imply that the selectivity to the presence of a complex motion in the optic flow depends both on the sensitivity of specialized mechanisms tuned to different complex motions and on inhibition of the units tuned to similar motions.

Echo State Network for Classification of Human Eye Movements During Decision Making

The paper develops further a recently proposed author’s approach for classification of dynamic data series using a class of Recurrent Neural Network (RNN) called Echo state network (ESN). It exploits the Intrinsic Plasticity (IP) tuning of ESN reservoir of neurons to fit their dynamics to the data fed into the reservoir input. A novel approach for ranking of a data base of dynamic data series into groups using the length of the multidimensional vector of reservoir state achieved after consecutive feeding of each time series into the ESN is proposed here. It is tested on eye tracker recordings of human eye movements during visual stimulation and decision making process. The preliminary results demonstrated the ability of the proposed technique to discriminate dynamic data series.

Effects of Surface Markings on Judgments of Motion Direction

The effects of surface markings on perceived motion direction were examined for a rotating sphere in a structure-from-motion display. The markings were dot patterns representing separate line segments or intersecting line segments (crosses) covering the surface of the sphere. The orientation of the surface markings and their intersection angles affected the perceived direction of motion, suggesting that the markings were not interpreted as geodesics or planar cuts on the surface. The perceived direction of motion was biased towards the mean orientation of the markings over the visible area of the surface. A similar bias was observed for translating planar stimuli covered with crosses, suggesting that the bias is not specific to curved surfaces or motion in depth. The deviation between the simulated motion direction and the external horizontal and vertical axes also affected the perceived motion direction. These results suggest that the average orientation of surface contours with respect to an external reference frame influences the perceived direction of motion.