Coordination of hand movements and saccades: evidence for a common and a separate pathway

Summary We studied the reaction times and initial directions of hand movements and saccades of human subjects who fixated and pointed as quickly as possible at eccentric targets which were presented unexpectedly. The targets were positioned on a horizontal bar which was placed in front of the subject. Different stimulus conditions were used in the experiments. Knowledge of the target position or the presence of an auditory co-stimulus slightly affected the reaction times of saccades in response to visual stimuli. Auditory co-stimuli reduced the reaction times considerably when the targets were presented after a delay of 200 ms after extinction of the central fixation point. Similar reductions were observed in the reaction times of the hand movements. However, these reductions were seen in hand responses to undelayed as well as delayed target presentations. The saccades were always made in the correct direction when the target was presented without delay. When the target was delayed about 50% of the saccades were made in the wrong direction. Even for undelayed targets the hand sometimes made mistakes. The number of mistakes increased to 35% when the target presentation was accompanied by the sound pulse. For delayed targets the proportion of wrong hand movements was about 50%. For such targets saccades and hand movements were practically always made in the same direction. If visual information is available, saccades and hand movements are generated independently of each other. However, if visual information is not present at the appropriate time and the target position has to be guessed, saccades and hand movements are generated on the basis of shared information. We suggest that saccades can be generated by two different mechanisms. One mechanism uses only visual information while the other one uses visual as well as cognitive information. The first mechanism is exclusively used for the generation of saccades while the second one has a more general purpose and is used for the generation of saccades as well as hand movements.