Research reportFixation errors and timing in sequences of memory-guided saccades
Introduction
It is widely believed that visual information plays the dominant role in error correction in saccadic eye movements (e.g. [2]). If, however, no visual information is available, what happens? To answer this question, we examined sequences of memory-guided saccades that had to be performed in the dark. To perform sequences of saccadic eye movements in the absence of visible targets, information about the desired movements has to be stored. It is possible that either a sequence of motor commands could be stored during memorization, or the target positions themselves.
There are different ways to regard the execution of eye movement sequences. Execution could be considered an open loop mode in which the single movements are performed with a fixed amplitude, regardless of the actual eye position. Alternatively, it could be viewed as a closed loop mode in which feedback information about the actual eye position is considered when the next component of the sequence has to be executed, thus providing the possibility of correcting errors. Such a tight linkage between amplitude and position control, and open and closed loop modes is only true, if the information that has to be passed to the burst generator(s) to make the eyes move, is indeed the desired displacement. This idea is a key feature of most models of the saccadic system (overview in [2]). Theoretically, an open loop position control could also be possible [6].
Our study addressed the following questions:
- 1.
Which information is stored in order to perform sequences of saccades to previously presented target positions: saccadic amplitudes or target positions?
- 2.
Is the execution based on pre-established motor programs or can feedback information about what has happened thus far influence the performance?
Since the oculomotor system is an imprecise system, i.e. the actual movement amplitude can differ somewhat from the intended amplitude, fixation errors occur. Certain predictions can be made about these errors. If motor commands (movement amplitudes) are stored, there should be a positive correlation between successive position errors due to error propagation. If target positions are stored and information about the actual eye position is used, the oculomotor system could correct position errors by using this information to generate the next saccadic command. Such a mechanism would lead to a compensatory correlation between position errors and the deviations of the amplitude of the following saccade from the nominal value as defined by the target step.
Some of the literature has focussed on position errors. In a previous study, Bock et al. [3] investigated error propagation during sequences of memory-guided saccades. The subjects repeatedly had to perform a fixed sequence of saccades without visual feedback. Assuming that a homogeneous model, i.e. a model with a single uniform mechanism, can explain the phenomenon, they pooled their data, calculated the correlation between successive position errors, and found a linear regression coefficient of about 0.4. They concluded that about 40% of a position error would be propagated and about 60% would be corrected. They also assumed that an extraretinal signal providing information about the actual eye position is used to correct position errors. Bridgeman et al. [4] conducted experiments on estimating the position of visual targets and pointing to perceived positions. They found a gain of 0.87 for the extraretinal signal. These findings suggest that a better error correction should have been possible. The reason for this discrepancy is, however, unclear.
Other researchers have examined the saccade sequence itself. Zingale et al. [11], for example, discovered that sequences of saccades have systematic characteristics in their time structure. The most striking pattern is a longer pause before the last three saccades of a sequence that separates different groups of saccades. Consequently, the fixation times in sequences of saccades are not homogeneously distributed. This suggests that a homogeneous model is not appropriate for describing the processes underlying the execution of sequences of saccades. These studies prompted us to investigate whether spatial accuracy is related in some way to timing, especially, grouping effects.
Section snippets
Experimental paradigm
Ten healthy subjects (mean age: 28, range: 24–37) were seated in a darkened room and told to memorize sequences of target positions while fixating a central laser spot. The targets were presented on a white, unstructured, half-cylindrical screen (distance from subject: 1.2 m) using a second laser projection system. All targets and the fixation spot lay on an imaginary horizontal line. The positions were randomly chosen from a range of 25° to the left of the fixation point to 25° to the right of
General observations
Twenty-four percent of the total 838 sequences presented to the subjects had to be discarded due to eye movements during the memorization period or because of missing responses (less saccades than targets). Thus, 641 sequences were analyzed. Although most subjects had the impression that they were not able to repeat the longer sequences correctly, the analysis showed that the subjects performed even the sequences consisting of five targets rather well. Since varying the presentation,
Error components
These errors that we term position errors, which can be measured, are not only errors caused by the oculomotor system. They can also have other causes (e.g. perception and memorization errors). Unfortunately, it is not possible to separate these components. Nevertheless, the correlations and regressions calculated here predominantly represent the correction or propagation of oculomotor errors. Since we know that feedback information can play a role in oculomotor control, we expect the
Acknowledgements
We wish to thank our subjects for their patience and Stefan Glasauer, Kevin O'Regan, and Gert Hauske for their helpful comments on the topic. Furthermore, we would like to thank Judy Benson and Michelle Seiche for their help with improving the English. The study was supported by the BMBF (FKZ 01KL9001) and the DFG (GRK 267/1-96).
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