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Original Research ARTICLE

Time course of target recognition in visual search

Andreas Kotowicz^1,2*, Ueli Rutishauser¹ and Christof Koch^1,3

Computation and Neural Systems, California Institute of Technology, Pasadena, CA, USA

Institute of Neuroinformatics, University of Zurich and ETH Zurich, Zurich, Switzerland

Division of Biology and Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA

Visual search is a ubiquitous task of great importance: it allows us to quickly find the objects that we are looking for. During active search for an object (target), eye movements are made to different parts of the scene. Fixation locations are chosen based on a combination of information about the target and the visual input. At the end of a successful search, the eyes typically fixate on the target. But does this imply that target identification occurs while looking at it? The duration of a typical fixation (∼170 ms) and neuronal latencies of both the oculomotor system and the visual stream indicate that there might not be enough time to do so. Previous studies have suggested the following solution to this dilemma: the target is identified extrafoveally and this event will trigger a saccade towards the target location. However this has not been experimentally verified. Here we test the hypothesis that subjects recognize the target before they look at it using a search display of oriented colored bars. Using a gaze-contingent real-time technique, we prematurely stopped search shortly after subjects fixated the target. Afterwards, we asked subjects to identify the target location. We find that subjects can identify the target location even when fixating on the target for less than 10 ms. Longer fixations on the target do not increase detection performance but increase confidence. In contrast, subjects cannot perform this task if they are not allowed to move their eyes. Thus, information about the target during conjunction search for colored oriented bars can, in some circumstances, be acquired at least one fixation ahead of reaching the target. The final fixation serves to increase confidence rather then performance, illustrating a distinct role of the final fixation for the subjective judgment of confidence rather than accuracy.

Keywords:

eye movements, object recognition, psychophysics, top-down attention, visual search, confidence judgement

Citation:

Kotowicz A, Rutishauser U and Koch C (2010) Time course of target recognition in visual search. Front. Hum. Neurosci. 4:31. doi: 10.3389/fnhum.2010.00031

Received:

06 August 2009;

Paper pending published:

17 October 2009;

Accepted:

25 March 2010;

Published online:

13 April 2010.

Edited by:

Maurizio Corbetta, Washington University, USA

Reviewed by:

Martin Paré, Queen's University, Canada
Matt Peterson, George Mason University, USA
Miguel Eckstein, University of California, Santa Barbara, USA

© 2010 Kotowicz, Rutishauser and Koch. This is an open-access article subject to an exclusive license agreement between the authors and the Frontiers Research Foundation, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are credited.

*Correspondence:

Andreas Kotowicz, Institute of Neuroinformatics, University of Zurich and ETH Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland. e-mail: kotowicz@ini.phys.ethz.ch