Effects of caudate microstimulation on spontaneous and 1 purposive saccades

26 Electrical stimulation has been delivered to the basal ganglia (BG) to treat intractable 27 symptoms of a variety of clinical disorders. However, it is still unknown how such 28 treatments improve behavioral symptoms. A difficulty of this problem is that artificial 29 signals created by electrical stimulation interact with intrinsic signals before influencing 30 behavior, thereby making it important to understand how such interactions between 31 artificial and intrinsic signals occur. We addressed this issue by analyzing the effects of 32 electrical stimulation under the following two behavioral conditions that induce different 33 states of intrinsic signals: (1) subjects behave spontaneously without task demands; (2) 34 subjects perform a behavioral paradigm purposefully. We analyzed saccadic eye 35 movements in monkeys while delivering microstimulation to the head and body of the 36 caudate nucleus, a major input stage of the oculomotor BG. When monkeys generated 37 spontaneous saccades, caudate microstimulation biased saccade vector endpoints toward 38 the contralateral direction of stimulation sites. However, when caudate microstimulation 39 was delivered during a purposive prosaccade (look toward a visual stimulus) or an 40 antisaccade (look away from a stimulus) paradigm, it created overall ipsilateral biases by 41 suppressing contralateral saccades more strongly than ipsilateral saccades. These results 42 suggest that the impact of BG electrical stimulation changes dynamically depending on 43 the state of intrinsic signals that vary under a variety of behavioral demands in everyday 44 life. 45