Human eye-head gaze shifts in a distracter task. II. Reduced threshold forinitiation of early head movements

This study was motivated by the observation of early head movements (EHMs) occasionally generated before gaze shifts. Human subjects were presented wi th a visual or auditory target, along with an accompanying stimulus of the other modality, that either appeared at the same location as the target (en hancer condition) or at the diametrically opposite location (distractor con dition). Gaze shifts generated to the target in the distracter condition so metimes were preceded by EHMs directed either to the side of the target (co n ect EHMs) or the side of the distracter (incorrect EHMs). During EHMs, th e eyes performed compensatory eye movements to keep gaze stable. Incorrect EHMs were usually between 1 and 5 degrees in amplitude and reached peak vel ocities generally <50 degrees/s. These metrics increased for more eccentric distracters. The dynamics of incorrect EHMs initially followed a trajector y typical of much larger bead movements. These results suggest that incorre ct EHMs are head movements that initially were planned to orient to the per ipheral distracter. 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 ana logous to the incorrect gaze shifts (IGSs) described in the previous paper and suggest that a motor program encoding a gaze shift to a distracter is c apable of initiating either an IGS or an incorrect EHM. A neural program no t strong enough to initiate an IGS nevertheless can initiate an incorrect E HM.