Abstract
Purpose: :
An increased probability of a target appearing in a random sequence reduces the saccadic latency for looking towards that target when it appears (Nature, 1995; 377(6544):59-62)). Although the oculomotor system is sensitive to long-term appearance probabilities, it is not clear whether it is also sensitive to the rapid probability changes that accompany repeating sequences of targets. We therefore measured saccadic latency in repeating sequences, and also assessed the consequences of momentarily disrupting such sequences.
Methods: :
In Experiment 1, subjects (n=4) performed repeated runs of 120 presentations in which targets either appeared to the left (L) or right (R) at random with probabilities of 0.25:0.75 or 0.75:0.25 (stochastic runs), or appeared as a repeating sequence (RRRL or LLLR) (deterministic runs). Experiment 2 (n=5) used similar repeating sequences that were occasionally (p=0.03) interrupted by a randomly selected target (L or R) followed by a recommencement of the sequence at a randomly selected starting point. We presented targets on a calibrated computer monitor, and measured saccadic latencies from the output of an infrared oculometer.
Results: :
In Experiment 1, latencies were significantly longer (8.5 ± 1.6 (SEM), p=0.02, paired t-test) for the low probability condition in stochastic runs than in deterministic runs, indicating the oculomotor system is sensitive to short-term, deterministic probabilities in a repeating sequence. In Experiment 2, interruption of the sequence resulted in a significant increase in latency (9.3 ± 2.1 ms, p=0.01). Immediately following this interruption, latencies were longer if the low probability stimulus in the sequence had not yet reappeared, when compared to latencies measured after the reappearance of the low probability stimulus (average difference 7.2 ± 1.3 ms, p=0.01).
Conclusions: :
Saccadic latencies not only reflect long term probabilities, but also rapid changes in expectation accompanying deterministic sequences. Our results indicate that the oculomotor system can make strategic use of low-probability - yet high information - events, consistent with higher-level decision-making processes.
Keywords: eye movements: saccades and pursuits