Browsing by Author "Schiller, PH"

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    Express averaging saccades in monkeys.
    (Vision Res, 1999) Chou, IH; Sommer, MA; Schiller, PH
    When monkeys are presented simultaneously with multiple stimuli, they can make one of two types of response. Either they make averaging saccades, that land at intermediate locations between the targets, or target-directed saccades, that land close to one of the targets. The two types of saccades occur at different latencies and are thought to reflect different processes; fast reflexive averaging and slower target selection. We investigated the latency of averaging saccades in five monkeys, with particular emphasis on 'express' latency saccades, which are thought to be inhibited by target selection. Express averaging saccades were made prolifically by the two monkeys that made both express and regular latency saccades, but only when no specific instruction was given regarding the saccade target. When these monkeys had to choose one of the targets, on the basis of its color, they still made averaging saccades. However, the endpoints formed two distributions close to the targets as opposed to one single distribution centered between the targets, as was the case when targets were identical; also, express saccades were almost entirely absent. We conclude that express averaging saccades are a form of spatial and temporal optimization of gaze shifting.
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    Eye fields in the frontal lobes of primates.
    (Brain Res Brain Res Rev, 2000-04) Tehovnik, EJ; Sommer, MA; Chou, IH; Slocum, WM; Schiller, PH
    Two eye fields have been identified in the frontal lobes of primates: one is situated dorsomedially within the frontal cortex and will be referred to as the eye field within the dorsomedial frontal cortex (DMFC); the other resides dorsolaterally within the frontal cortex and is commonly referred to as the frontal eye field (FEF). This review documents the similarities and differences between these eye fields. Although the DMFC and FEF are both active during the execution of saccadic and smooth pursuit eye movements, the FEF is more dedicated to these functions. Lesions of DMFC minimally affect the production of most types of saccadic eye movements and have no effect on the execution of smooth pursuit eye movements. In contrast, lesions of the FEF produce deficits in generating saccades to briefly presented targets, in the production of saccades to two or more sequentially presented targets, in the selection of simultaneously presented targets, and in the execution of smooth pursuit eye movements. For the most part, these deficits are prevalent in both monkeys and humans. Single-unit recording experiments have shown that the DMFC contains neurons that mediate both limb and eye movements, whereas the FEF seems to be involved in the execution of eye movements only. Imaging experiments conducted on humans have corroborated these findings. A feature that distinguishes the DMFC from the FEF is that the DMFC contains a somatotopic map with eyes represented rostrally and hindlimbs represented caudally; the FEF has no such topography. Furthermore, experiments have revealed that the DMFC tends to contain a craniotopic (i.e., head-centered) code for the execution of saccadic eye movements, whereas the FEF contains a retinotopic (i.e., eye-centered) code for the elicitation of saccades. Imaging and unit recording data suggest that the DMFC is more involved in the learning of new tasks than is the FEF. Also with continued training on behavioural tasks the responsivity of the DMFC tends to drop. Accordingly, the DMFC is more involved in learning operations whereas the FEF is more specialized for the execution of saccadic and smooth pursuit eye movements.
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    What neural pathways mediate express saccades?
    (Behavioral and Brain Sciences, 1993-09) Sommer, MA; Schiller, PH; McPeek, RM