Pieter J.A. Unema

Time course of information processing during scene perception: The relationship between saccade amplitude and fixation duration

The present study focuses on two aspects of the time course of visual information processing during the perception of natural scenes. The first aspect is the change of fixation duration and saccade amplitude during the first couple of seconds of the inspection period, as has been described by Buswell (1935), among others. This common effect suggests that the saccade amplitude and fixation duration are in some way controlled by the same mechanism. A simple exponential model containing two parameters can describe the phenomena quite satisfactorily. The parameters of the model show that saccade amplitude and fixation duration may be controlled by a common mechanism. The second aspect under scrutiny is the apparent lack of correlation between saccade amplitude and fixation duration (Viviani, 1990). The present study shows that a strong but nonlinear relationship between saccade amplitude and fixation duration does exist in picture viewing. A model, based on notions laid out by Findlay and Walkers (1999) model of saccade generation and on the idea of two modes of visual processing (Trevarthen, 1968), was developed to explain this relationship. The model both fits the data quite accurately and can explain a number of related phenomena.

The omnipresent prolongation of visual fixations: saccades are inhibited by changes in situation and in subject's activity

Presenting a distractor prolongs not only saccadic reaction times in paced tasks but also fixation durations in unpaced tasks. To investigate whether the effect of a distractor is a pure optomotor reflex, we used both visual and auditory distractors in an unpaced picture-viewing paradigm. Results show a distractor effect for both modalities. Analysis of data from previous studies showed similar effects, even in amodal shifts of attention. These findings challenge the hypothesis that the effect is modality-specific and suggest that the distractor effect may be another expression of the orienting reflex.

Towards gaze-mediated interaction: Collecting solutions of the Midas touch problem

For a development of truly user-centered interfaces we need to take into account not only generic characteristics of human beings but also actual dynamics of attention and intentions of persons involved in an interaction. Modern eyetracking methods are indispensable tools in such a development, as they allow the use of eye movement data for control of output devices, for gaze-contingent image processing and for desambiguation of verbal as well as nonverbal information. The main obstacle on the way to these applications is the so-called “Midas touch problem”: how to differentiate “attentive” saccades with intended goal of communication from the lower level eye movements that are just random or provoked by external stimulation? We report results of our investigations of the problem and present a solution based on a functional classification of fixations correlated with their duration. Several additional solutions are also considered together with the data on the trainability of the human oculomotor system.

Blinks, blanks and saccades: How blind we really are for relevant visual events

We report on a study in which subjects viewed color video stills of natural traffic situations while eye movements were recorded. A display change could occur randomly during three different occlusion modes--blinks, blanks and saccades--or during a fixation. These changes could be either relevant or irrelevant with respect to the traffic safety situation. Furthermore we contrasted insertions and deletions. All occlusion modes appeared equivalent concerning detection rate and detection time, and only differed from the fixation condition. The results also show that the detection of relevant changes was more likely and faster than that of irrelevant ones. However, even relevant insertions, which were almost always detected, were around 180 ms longer to report when they occurred during an occlusion. Furthermore, the detection of relevant changes was fairly stable across a wide range of the visual field, whereas irrelevant changes were less well detected, the further away from the fovea they occurred. We close with an outlook on a follow-up study where only relevant insertions and the blank occlusion were used in a driving simulator environment. Surprisingly, we found an advantage in change detection rate and time with blanks compared to the control condition. Change detection was also good during blinks, but not in saccades.