Aftereffects of Saccades Explored in a Dynamic Neural Field Model of the Superior Colliculus

When viewing a scene or searching for a target, an observer usually makes a series of saccades that quickly shift the orientation of the eyes. The present study explored how one saccade affects subsequent saccades within a dynamic neural field model of the superior colliculus (SC). The SC contains an oculocentric motor map that encodes the vector of saccades and remaps to the new fixation location after each saccade. Our simulations demonstrated that the observation that saccades which reverse their vectors are slower to initiate than those which repeat vectors can be explained by the afore-mentioned remapping process and the internal dynamics of the SC. How this finding connects to the study of inhibition of return is discussed and suggestions for future studies are presented

Investigating a two causes theory of inhibition of return

It has recently been demonstrated that there are independent sensory and motor mechanisms underlying inhibition of return (IOR) when measured with oculomotor responses (Wang et al. in Exp Brain Res 218:441-453, 2012). However, these results are seemingly in conflict with previous empirical results which led to the proposal that there are two mutually exclusive flavors of IOR (Taylor and Klein in J Exp Psychol Hum Percept Perform 26:1639-1656, 2000). The observed differences in empirical results across these studies and the theoretical frameworks that were proposed based on the results are likely due to differences in the experimental designs. The current experiments establish that the existence of additive sensory and motor contributions to IOR do not depend on target type, repeated spatiotopic stimulation, attentional control settings, or a temporal gap between fixation offset and cue onset, when measured with saccadic responses. Furthermore, our experiments show that the motor mechanism proposed by Wang et al. in Exp Brain Res 218:441-453, (2012) is likely restricted to the oculomotor system, since the additivity effect does not carry over into the manual response modality.10 page(s

Sensory and motor mechanisms of oculomotor inhibition of return

We propose two explicit mechanisms contributing to oculomotor inhibition of return (IOR): sensory and motor. Sensory mechanism: repeated visual stimulation results in a reduction in visual input to the superior colliculus (SC); consequently, saccades to targets that appear at previously stimulated retinotopic locations will have longer latencies than those that appear at unstimulated locations. Motor mechanism: the execution of a saccade results in asymmetric activation in the SC; as a result, saccades that reverse vectors will have longer latencies than those that repeat vectors. In the IOR literature, these two mechanisms correspond to IOR effects observed following covert exogenous orienting and overt endogenous orienting, respectively. We predict that these two independent mechanisms will have additive effects, a prediction that is confirmed in a behavioral experiment. We then discuss how our theory and findings relate to the oculomotor IOR literature.13 page(s

Averaging saccades are repelled by prior uninformative cues at both short and long intervals

When two spatially proximal stimuli are presented simultaneously, a first saccade is often directed to an intermediate location between the stimuli (averaging saccade). In an earlier study, Watanabe (2001) showed that, at a long cue-target onset asynchrony (CTOA; 600 ms), uninformative cues not only slowed saccadic response times (SRTs) to targets presented at the cued location in single target trials (inhibition of return, IOR), but also biased averaging saccades away from the cue in double target trials. The present study replicated Watanabe's experimental task with a short CTOA (50 ms), as well as with mixed short (50 ms) and long (600 ms) CTOAs. In all conditions on double target trials, uninformative cues robustly biased averaging saccades away from cued locations. Although SRTs on single target trials were delayed at previously cued locations at both CTOAs when they were mixed, this delay was not observed in the blocked, short CTOA condition. We suggest that top-down factors, such as expectation and attentional control settings, may have asymmetric effects on the temporal and spatial dynamics of oculomotor processing.23 page(s

Oculomotor inhibition of return : how soon is it "recoded" into spatiotopic coordinates?

When, in relation to the execution of an eye movement, does the recoding of visual information from retinotopic to spatiotopic coordinates happen? Two laboratories seeking to answer this question using oculomotor inhibition of return (IOR) have generated different answers: Mathôt and Theeuwes (Psychological Science 21:1793-1798, 2010) found evidence for the initial coding of IOR to be retinotopic, while Pertzov, Zohary, and Avidan (Journal of Neuroscience 30:8882-8887, 2010) found evidence for spatiotopic IOR at even shorter postsaccadic intervals than were tested by Mathôt and Theeuwes (Psychological Science 21:1793-1798, 2010). To resolve this discrepancy, we conducted two experiments that combined the methods of the previous two studies while testing as early as possible. We found early spatiotopic IOR in both experiments, suggesting that visual events, including prior fixations, are typically coded into an abstract, allocentric representation of space either before or during eye movements. This type of coding enables IOR to encourage orienting toward novelty and, consequently, to perform the role of a foraging facilitator.9 page(s

Examining the dissociation of retinotopic and spatiotopic inhibition of return with event-related potentials

Inhibition of return (IOR) is thought to reflect a mechanism that biases orienting which, under some circumstances, reduces perceptual processing at previously processed locations. Studies using event-related potentials (ERPs) have generally revealed that IOR is accompanied by an amplitude reduction of early sensory ERP components (e.g., P1). While behavioral studies suggest that IOR may be represented in both spatiotopic and retinotopic coordinates, all previous ERP studies have used the prototypical spatial cueing paradigm and have thus confounded retinotopic and spatiotopic reference frames. Because of this confound it is unknown whether the P1 reduction that has been associated with IOR will be observed in retinotopic or spatiotopic coordinates when these are dissociated. The current experiment investigated whether the P1 component would be modulated by IOR when the retinotopic and spatiotopic reference frames were dissociated by an eye movement between cue and target onset. Strong spatiotopic IOR was found to be accompanied by a negative difference (Nd) in the 200-300 ms time window, while a P1 reduction was absent, suggesting that P1 reductions do not provide an accurate reflection of IOR.5 page(s

Simulating oculomotor inhibition of return with a two-dimensional dynamic neural field model of the superior colliculus

Sensory adaptation and oculomotor inhibition of return (IOR) have been extensively modeled using a onedimensional dynamic neural field (DNF) model of the superior colliculus (SC). However, a great deal of paradigms are incapable of being simulated in a single dimension, limiting the generality of previous implementations. Here, we expand on previous work by implementing the inhibitory cueing mechanisms underlying IOR in a two-dimensional DNF. With such a model, we were able to reproduce the results reported in our previous work, validating the use of two-dimensional DNF models in future theoretical investigations. We discuss a number of new findings in the literature that should be simulated in two dimensions to further our understanding of inhibitory cueing mechanisms and saccade dynamics, such as the center of gravity effect of IOR.5 page(s