No abstract

Two proposals have been made to account for the generation of saccadic eye movements. The first assumes that when the eyes move is under the control of a fixation gating system. The second attributes the decisions of both when and where the eyes move to the interplay between short-range excitatory and long-range inhibitory interactions within the motor map of the superior colliculus (SC). To distinguish both views, three behavioral experiments conducted on human participants tested the respective contributions of stimulus eccentricity and interstimulus distance on the effects of remote and proximal distractors on the latency and accuracy of saccades. Experiment 1 showed that the saccade-latency increase that results from the presentation of a remote distractor in the contralateral, nontarget hemifield varies with the ratio of distractor-to-target eccentricity, but not the interstimulus distance in visual or collicular space, thus indicating that the effect is not due to long-range inhibition. Experiments 2a and 2b showed that short-range excitation does not underlie the effect of proximal, ipsilateral distractors. Proximal distractors do not systematically shorten saccade latency, but rather show a range of effects (from a latency increase to no effect and then facilitation) as the ratio of distractor-to-target eccentricity increases, while deviating the eyes to gradually larger extents. The present findings strongly challenge the neural-field account, while suggesting that when a saccade is initiated depends mainly on the activity of a fixation gating system.

The idea that covert mental processes such as spatial attention are fundamentally dependent on systems that control overt movements of the eyes has had a profound influence on theoretical models of spatial attention. However, theories such as Klein’s Oculomotor Readiness Hypothesis (OMRH) and Rizzolatti’s Premotor Theory have not gone unchallenged. We previously argued that although OMRH/Premotor theory is inadequate to explain pre-saccadic attention and endogenous covert orienting, it may still be tenable as a theory of exogenous covert orienting. In this article we briefly reiterate the key lines of argument for and against OMRH/Premotor theory, then evaluate the Oculomotor Readiness account of Exogenous Orienting (OREO) with respect to more recent empirical data. These studies broadly confirm the importance of oculomotor preparation for covert, exogenous attention. We explain this relationship in terms of reciprocal links between parietal ‘priority maps’ and the midbrain oculomotor centres that translate priority-related activation into potential saccade endpoints. We conclude that the OMRH/Premotor theory hypothesis is false for covert, endogenous orienting but remains tenable as an explanation for covert, exogenous orienting.