Human Eye-Head Gaze Shifts in a Distractor Task. II. Reduced Threshold for Initiation of Early Head Movements

Abstract

This study was motivated by the observation of early head movements (EHMs) occasionally generated before gaze shifts. Human subjects were presented with a visual or auditory target, along with an accompanying stimulus of the other modality, that either appeared at the same location as the target (enhancer condition) or at the diametrically opposite location (distractor condition). Gaze shifts generated to the target in the distractor condition sometimes were preceded by EHMs directed either to the side of the target (correct EHMs) or the side of the distractor (incorrect EHMs). During EHMs, the eyes performed compensatory eye movements to keep gaze stable. Incorrect EHMs were usually between 1 and 5 degrees in amplitude and reached peak velocities generally <50 degrees /s. These metrics increased for more eccentric distractors. The dynamics of incorrect EHMs initially followed a trajectory typical of much larger head movements. These results suggest that incorrect EHMs are head movements that initially were planned to orient to the peripheral distractor. Furthermore gaze shifts preceded by incorrect EHMs had longer reaction latencies than gaze shifts not preceded by incorrect EHMs, suggesting that the processes leading to incorrect EHMs also serve to delay gaze-shift initiation. These results demonstrate a form of distraction analogous to the incorrect gaze shifts (IGSs) described in the previous paper and suggest that a motor program encoding a gaze shift to a distractor is capable of initiating either an IGS or an incorrect EHM. A neural program not strong enough to initiate an IGS nevertheless can initiate an incorrect EHM.

Dynamic and Opposing Adjustment of Movement Cancellation and Generation in an Oculomotor Countermanding Task

“…There was no difference in the propensity to generate head-only movements in Experiments 2 and 3 (two-way t test, t (16) ϭ 0.23, p ϭ 0.82), but head-only movements were significantly less likely on a percentage basis in Experiment1 compared to Experiments 2 and 3 (two-way t test, t (24) ϭ 3.7, p ϭ 0.001). This difference relates to the use of the staircasing algorithm in Experiments 2 and 3, which tends to sample intermediate SSDs at which head-only movements are more likely (Corneil and Elsley, 2005). Regardless, we obtained a considerable yield of head-only movements from all experiments.…”

“…Antagonist muscle recruitment is observed during noncancelled gaze shifts that do not attain the target (Goonetilleke et al, 2010). Large eye-head gaze shifts can be truncated in midflight in this and related tasks Corneil and Elsley, 2005), suggesting that processing of the stop signal continues during move-ment generation. In this and our previous countermanding work, we analyze antagonist muscle recruitment only during head-only errors, rather than truncated noncancelled gaze shifts, to avoid confounds that may be associated with gaze shift initiation.…”

“…1 A). Our previous work has documented the frequency of these three movement sequences, how their propensity changes across SSD, and the profile of bilateral neck muscle recruitment during each sequence (Corneil and Elsley, 2005;Goonetilleke et al, 2010Goonetilleke et al, , 2012. Headonly errors are uniquely associated with a burst of neck EMG activity on the antagonist muscle (Fig.…”

“…The circuits governing the initiation of orienting head movements are more permissive than those governing gaze shifts (for review, see Corneil, 2011); head movements can be both initiated and actively stopped even though gaze remains stable. Head-only movements can be considered as a partial response of the saccadic system (Corneil and Munoz, 1999;Corneil and Elsley, 2005), paralleling small muscle twitches, finger movements, or changes in force production that are occasionally made during otherwise successfully cancelled manual movements (Osman et al, 1986;De Jong et al, 1990;Franks, 1997, 2003;McGarry et al, 2000;van Boxtel et al, 2001;Scangos and Stuphorn, 2010;Ko et al, 2012). Considering head-only movements as a type of partial response that are arrested midflight does not lessen the significance of our findings, so long as the initiation of antagonist recruitment arises from the processing of the stop signal.…”

“…When attempting to cancel an impending eye-head gaze shift, human subjects occasionally generate a small head-only movement toward the target even though the gaze shift is cancelled successfully (Corneil and Elsley, 2005). Recordings of neck muscle activity demonstrate that this small head movement is actively braked by a pulse of antagonist neck muscle activity (Goonetilleke et al, 2010).…”