Ilpo Kojo

Effect of stimulus contrast on performance and eye movements in visual search

According to the visual span control hypothesis, eye movements are controlled in relation to the size of visual span. In reading, the decrease of contrast reduces visual span, saccade sizes, and reading speed. The purpose of the present study is to determine how stimulus contrast affects the speed of two-dimensional visual search and how changes in eye movements and visual span could explain changes in performance. The task of the observer was to search for, and identify, an uppercase letter from a rectangular array of characters in which the other items were numerals. Threshold search time, i.e. the duration of stimulus presentation required for search that is successful with a given probability, was determined by using a multiple-alternative staircase method. Eye movements were recorded simultaneously by using a video eye tracker. Four different set sizes (the sizes of stimulus array) (3x3-10x10), and five different contrasts (0.0186-0.412) were used. At all set sizes, threshold search time decreased with increasing contrast. Also the average number of fixations per search decreased with increasing contrast. At the smallest set size (3x3), only one fixation was needed except at the lowest contrast. Average fixation duration decreased and saccade amplitudes increased slightly with increasing contrast. The reduction of the number of fixations with increasing contrast suggests that visual span, i.e. the area from which information can be collected at one fixation, increases with increasing contrast. The reduction of the number of fixations together with reduced fixation duration result in reduced search times when contrast increases.

Using hidden Markov model to uncover processing states from eye movements in information search tasks

We study how processing states alternate during information search tasks. Inference is carried out with a discriminative hidden Markov model (dHMM) learned from eye movement data, measured in an experiment consisting of three task types: (i) simple word search, (ii) finding a sentence that answers a question and (iii) choosing a subjectively most interesting title from a list of ten titles. The results show that eye movements contain necessary information for determining the task type. After training, the dHMM predicted the task for test data with 60.2% accuracy (pure chance 33.3%). Word search and subjective interest conditions were easier to predict than the question-answer condition. The dHMM that best fitted our data segmented each task type into three hidden states. The three processing states were identified by comparing the parameters of the dHMM states to literature on eye movement research. A scanning type of eye behavior was observed in the beginning of the tasks. Next, participants tended to shift to states reflecting reading type of eye movements, and finally they ended the tasks in states which we termed as the decision states.

Eye movements in the visual search of word lists

The word identification span refers to the area of the visual field in which words can be identified during a single fixation. The purpose of the study was to estimate the vertical word identification span in a visual word search task, in which words were arranged in a vertical list. In addition, we studied the effect of list layout (orientation, length, and line spacing) on the speed of search and eye movements. The task of the observer was to identify a target word in a word list, where the other words were distracters. Threshold search time, that is, stimulus presentation time for correct identification at a probability level of 0.79, was determined by using a multiple alternative staircase method. Eye movements were recorded simultaneously. The results showed that, in vertical lists, 4-5 words could be identified during a single fixation. Thus, the vertical word identification span was 4-5 character spaces, whereas according to previous studies the horizontal word identification span is about 10 character spaces, which corresponds to 1-2 words. There were fewer fixations and the saccade amplitudes were smaller for vertical than for horizontal lists of the same length. However, search times did not depend on list orientation. This was due to longer fixation times for vertical lists. Further, since average fixation duration for vertical lists was longer than for horizontal lists, processing time seems to depend on the number of items within the span.

The role of visual spatial attention in audiovisual speech perception

Auditory and visual information is integrated when perceiving speech, as evidenced by the McGurk effect in which viewing an incongruent talking face categorically alters auditory speech perception. Audiovisual integration in speech perception has long been considered automatic and pre-attentive but recent reports have challenged this view. Here we study the effect of visual spatial attention on the McGurk effect. By presenting a movie of two faces symmetrically displaced to each side of a central fixation point and dubbed with a single auditory speech track, we were able to discern the influences from each of the faces and from the voice on the auditory speech percept. We found that directing visual spatial attention towards a face increased the influence of that face on auditory perception. However, the influence of the voice on auditory perception did not change suggesting that audiovisual integration did not change. Visual spatial attention was also able to select between the faces when lip reading. This suggests that visual spatial attention acts at the level of visual speech perception prior to audiovisual integration and that the effect propagates through audiovisual integration to influence auditory perception.

Reading vertical text from a computer screen

In Western languages, text is traditionally presented in horizontal lines. However, reading of vertically arranged text might be more efficient because of the elimination of horizontal eye movements. We investigated the effect of vertical arrangement upon reading text presented on a computer screen. Even though vertically aligned text was read at a slower rate than horizontally aligned text, the difference was smaller than in previous studies. Analysis of eye-movement data revealed that there were no differences in fixation numbers and numbers of regressions between vertical formats and the standard-text format. But fixation durations were shorter for the standard-text format than for the vertical formats. Taken together, the results indicate that reading vertically presented text from a display device may be nearly as efficient as reading normal horizontal text. Therefore, the fact that text is normally vertically arranged in small-screen devices is not a usability problem.

Detection of a Gabor patch superimposed on an illusory contour

Interactions between visual stimuli have been found to be specific to the spatial frequency, orientation and phase of the interacting stimuli. We asked if there are any interactions between luminance-defined Gabor patches and Kanizsa-type illusory contours. In psychophysical experiments we studied whether induction of a vertical illusory line affects detection thresholds for a Gabor patch superimposed on this line and whether these effects depend on the orientation, spatial frequency and phase of the Gabor elements. Employing a 2AFC method with a staircase procedure we measured contrast detection thresholds and varied the orientation, spatial frequency and phase of the test Gabor patch and the separation between the two pacmen in four experimental series. The results show that in a situation where the two inducers generate perception of an illusory line, the contrast detection of the Gabor patch is facilitated relative to a control condition where the rotated pacmen do not induce illusory contours. This facilitation was more pronounced for test Gabor signals that were collinear to the illusory line, but the observers performance was not altered by changes in the spatial frequency or phase of the Gabor stimuli. With increasing spatial separation of the two pacmen (and, consequently, with a decreasing support ratio), the difference between performance in the test and control conditions diminished. From the data obtained we cannot infer that we have measured some neural interactions between Gabor patches and Kanizsa-type illusory contours, and nor can we draw a unique conclusion about what causes the facilitation of detection of the test Gabor patch in the experimental situation that allows induction of the illusory line. We discuss possible mechanisms of the facilitation, such as contextual influences or a reduction of uncertainty about spatial location of the test Gabor patch.

Neon colour spreading in three-dimensional illusory objects in humans

We studied whether neon spreading can be induced within three-dimensional illusory triangles. Kanizsa triangles were induced by black pacman disks consisting of red sectors with curved sides. Viewing our stimuli monocularly produced two-dimensional illusory contours and surfaces as well as neon spreading in each figure. Triangles appeared concave or convex under stereoscopical viewing. Neon colour spreading was induced within illusory figures bending in three-dimensional space, suggesting that neural contour completion and surface filling-in interact across depth. Surprisingly, neon spreading was induced above the intervening surface even when the inducers were below the surface. Neon colour and illusory configuration were preserved behind the intervening surface only when it appeared transparent.

Integrating complex information with object displays: psychophysical evaluation of outlines

The interactive use of visual interface tools has diversified the use of visualisations. This article reviews the relevant aspects of interaction and challenges the sufficiency of traditional evaluation criteria developed for static graphs. Traditionally, the problem for statisticians has been to maintain perceptual discriminability of details, when quantities of data increase. Currently, however, even non-professional users need to integrate qualitatively different kinds of information. The review of task requirements indicates the use of a visual outline: (1) visual tools can facilitate parallel separation of individual data entities and integration of their features and (2) more focused comparisons require visual memory due to eye movements. The article reports psychophysical experiments that measure performance accuracy and response latency conditioned by the above task requirements. The impact of shape and colour on performance interacted with display times; the times were shorter (100 ms) or longer (1 s) than the duration of typical gaze fixation. The features of graphs in the experiments were derived from a popular internet service. Thus, we describe methods for evaluating visual components of real services and provide general guidelines for visual design of human–computer interaction.

Object displays for identifying multidimensional outliers within a crowded visual periphery

This article discusses the human ability to detect, locate, or identify objects and their features using peripheral vision. The potential of peripheral vision is underused with user interfaces probably due to the limits of visual acuity. Peripheral preview can guide focused attention to informative locations, if the visual objects are large enough and otherwise within the limits of discrimination. Our experiments focused on the task of identifying an outlier and implicated another limiting factor, crowding, for integration of object features. The target object and the corresponding data dimension were located from an object display representation used for integrating multidimensional data. We measured performance on a peripheral vision task in terms of reaction times and eye movements. Subjects identified the target item from 480 alternatives within 100ms. Therefore, the identification process would not slow down the natural gaze sequence and focused attention during monitoring and data mining tasks.

Visual completion of three-dimensional, chromatic, moving stimuli in humans

We studied whether visual completion can be produced within three-dimensional (3-D), moving, chromatic objects. Rotating thin and thick discs with red sectors, forming the corners of Kanizsa triangle, were presented on a display. Observers also rotated a hand-held real thin Kanizsa triangle. An illusory, moving contour encompassing an illusory colourful triangle was generally observed. This indicates that the visual system is capable of filling in complex (3-D, moving, chromatic) percepts.