Authors: Hanneke Liesker Eli Brenner Jeroen B J Smeets
Publish Date: 2009/10/09
Volume: 201, Issue: 2, Pages: 221-227
Abstract
When searching for a target with eye movements saccades are planned and initiated while the visual information is still being processed so that subjects often make saccades away from the target and then have to make an additional return saccade Presumably the cost of the additional saccades is outweighed by the advantage of short fixations We previously showed that when the cost of passing the target was increased by having subjects manually move a window through which they could see the visual scene subjects still passed the target and made return movements with their hand When moving a window in this manner the eyes and hand follow the same path To find out whether the hand still passes the target and then returns when eye and hand movements are uncoupled we here compared moving a window across a scene with moving a scene behind a stationary window We ensured that the required movement of the hand was identical in both conditions Subjects found the target faster when moving the window across the scene than when moving the scene behind the window but at the expense of making larger return movements The relationship between the return movements and movement speed when comparing the two conditions was the same as the relationship between these two when comparing different window sizes We conclude that the hand passing the target and then returning is not directly related to the eyes doing so but rather that moving on before the information has been fully processed is a general principle of visuomotor controlIt is known that saccades are initiated before the visual information is fully processed This implies that eye movement will not always be appropriate Indeed when reading 14 of the saccades are regressions Starr and Rayner 2001 and during visual search the eyes often in 5–55 of the trials move away from the target and then immediately return Hooge and Erkelens 1996 We recently reported that when hand movements are used in conjunction with eye movements in a search task not only the eyes but also the hand makes return movements Liesker et al 2008 We concluded that the hand movement is planned before visual information processing is completed and that the visual information has not been fully processed by the last moment at which the hand movement to the next item can still be cancelledWhen the hands are moving to process tactile items they never pass the target Overvliet et al 2007 One might conclude that hand movements are only initiated after tactile information processing is complete However the two studies did not only differ with respect to sensory information the eye and hand had to move together in the study by Liesker et al 2008 whereas eye movements were irrelevant in the study by Overvliet et al 2007Our eyes and our hands often move together to perform everyday tasks For instance when making tea or preparing sandwiches our eyes often move ahead of our hands Land and Hayhoe 2001 In a pointing task Neggers and Bekkering 2000 even found that subjects were unable to make a saccade towards a new target before their hand had reached the initial target location However Rotman et al 2004 found that subjects did not make saccades to the position they tapped but kept pursuing the disk with their eyes when tapping targets flashed near a moving disk Smeets et al 1996 studied subjects’ head movements while executing various natural manual tasks requiring gaze shifts The movements of hand and gaze were uncoupled to a large extent allowing them to conclude that the head not only followed the gaze but also the hand Also when searching with the eyes and hand there is evidence that the effectors can move independently Liesker et al 2009 Thus the exact details of a task determine whether eye and hand movements are coupled Does how fast hand movements are planned depend on whether or not the eyes move to the same positionsa The experimental setup The stimulus was projected onto a projection surface Participants saw this stimulus via a mirror making it appear to coincide with the surface of a graphics tablet Participants moved a pen over the graphics tablet and indicated that they had found the target by pressing the space bar on the keyboard b c The stimulus The dark grey spots indicate the item positions The white circle is the window through which an item is visible Image B is an example of the condition in which moving the pen over the graphics tablet moved the window over the scene so the pen was ‘linked’ to the window Image C is an example of the condition in which moving the pen over the graphics tablet moved the scene behind the window so the pen was ‘linked’ to the sceneWhen moving the window over the scene we can expect the eyes to either follow the moving window and the hand or to make saccades between item locations and wait at each location for the window to arrive When moving the scene behind the window the eyes are forced to move in a different way from the hand The eyes could either remain fixated on the window at the screen centre or they could saccade away from the target that was processed which was visible in the window to the next item which was visible as a grey spot and track this spot as it was moved into the window The two methods of moving the visible part of the scene require the same hand movements stepwise movements along a circle and in both cases the eyes may make saccades and pursue the target or window that is being moved by the hand but the relationship between what is visible and the eye and hand movements is different if the window is moved the saccades will be in the same direction as the hand movements whereas if the scene is moved the saccades will be in the direction opposite to the hand movementsIf there is an intrinsic benefit in moving eye and hand together then moving the window over the scene will be easier than moving the scene behind the window resulting in shorter viewing times If the hand makes a return movement to the target because it was following the eyes then moving the window over the scene will also lead to more or larger return movements Because the number of return movements to the target depends on the viewing time Liesker et al 2008 we varied the difficulty of the task by varying the window size and the contrast between the items and the background to see how these variables affect viewing time hand movement velocity and the extent of return movements and in particular whether the two tasks differ in the relationship between these variablesTwelve participants five male and seven female aged between 19 and 32 years participated in this experiment All participants were right handed and had normal or corrected to normal vision None of the participants were aware of the goal of the study
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