Stefan Everling

The antisaccade: a review of basic research and clinical studies

The ability to suppress reflexive responses in favor of voluntary motor acts is crucial for everyday life. Both abilities can be tested with an oculomotor task, the antisaccade task. This task requires subjects to suppress a reflexive prosaccade to a flashed visual stimulus and instead to generate a voluntary saccade to the opposite side. This article reviews what is currently known about the neural structures and processes which are involved in the performance of this task. Current data show that a variety of brain lesions, neurological diseases and psychiatric disorders result in errors, i.e. prosaccades towards the stimulus, in this task. Brain imaging studies have shown that a widely distributed cortical and subcortical network is active during the generation of antisaccades. These findings are discussed and the potential of the antisaccade task for diagnostic purposes is evaluated.

Contribution of the Primate Superior Colliculus to Inhibition of Return

The phenomenon of inhibition of return (IOR) has generated considerable interest in cognitive neuroscience because of its putative functional role in visual search, that of placing inhibitory tags on objects that have been recently inspected so as to direct further search to novel items. Many behavioral parameters of this phenomenon have been clearly delineated, and based on indirect but converging evidence, the widely held consensus is that the midbrain superior colliculus (SC) is involved in the generation of IOR. We had previously trained monkeys on a saccadic IOR task and showed that they displayed IOR in a manner similar to that observed in humans. Here we recorded the activity of single neurons in the superficial and intermediate layers of the SC while the monkeys performed this IOR task. We found that when the target was presented at a previously cued location, the stimulus-related response was attenuated and the magnitude of this response was correlated with subsequent saccadic reaction times. Surprisingly, this observed attenuation of activity during IOR was not caused by active inhibition of these neurons because (a) they were, in fact, more active following the presentation of the cue in their response field, and (b) when we repeated the same experiment while using the saccadic response time induced by electrical micro-stimulation of the SC to judge the level of excitability of the SC circuitry during the IOR task, we found faster saccades were elicited from the cued location. Our findings demonstrate that the primate SC participates in the expression of IOR; however, the SC is not the site of the inhibition. Instead, the reduced activity in the SC reflects a signal reduction that has taken place upstream.

On your mark, get set: Brainstem circuitry underlying saccadic initiation

Saccades are rapid eye movements that are used to move the visual axis toward targets of interest in the visual field. The time to initiate a saccade is dependent upon many factors. Here we review some of the recent advances in our understanding of the these processes in primates. Neurons in the superior colliculus and brainstem reticular formation are organised into a network to control saccades. Some neurons are active during visual fixation, while others are active during the preparation and execution of saccades. Several factors can influence the excitability levels of these neurons prior to the appearance of a new saccadic target. These pre-target changes in excitability are correlated to subsequent changes in behavioural performance. Our results show how neuronal signals in the superior colliculus and brainstem reticular formation can be shaped by contextual factors and demonstrate how situational experience can expedite motor behaviour via the advanced preparation of motor programs.

Event-related potentials and saccadic reaction times: effects of fixation point offset or change

Abstract Previous studies have shown that saccadic reaction times (SRTs) are reduced if the initial fixation point (FP) disappears 200 ms (gap period) before a peripheral target is presented. This gap saccade task is associated with a negative cortical potential at the end of the gap period. To determine whether the neural processes underlying this potential account for the reduction of SRTs during gap saccade tasks, we recorded event-related potentials (ERPs) in 19 subjects performing a gap saccade task (gap duration 200 ms), a warning saccade task (the color of the FP changed 200 ms prior to target appearance) and an overlap task (the FP remained visible during the trial). SRTs were shortest during the gap task, longest during the overlap task and intermediate during the warning task. The gap and warning tasks were accompanied by the same widespread negative cortical potential with a maximum at the time of stimulus presentation. These findings indicate that the warning effect mediated by the disappearance of the FP during gap saccade tasks is responsible for the gap negativity which was observed by several authors. Our findings of shorter SRTs during the gap task than the warning task, however, suggest that the gap has an additional effect that probably depends on subcortical mechanisms.

Neuronal Activity in Monkey Superior Colliculus during an Antisaccade Task

It well known that the primate superior colliculus (SC) is involved in the generation of visually guided saccadic eye movements (for review see Sparks and Hartwich-Young 1989). Its intermediate layers contain neurons which display motor bursts for saccades within the response field of the neuron. These neurons project directly to preoculomo-toneurons in paramedian pontine reticular formation and the rostral interstitial nucleus of the medial longitudinal fasciculus, which provide the input to the extraocular muscle motoneurons (for review see Moschovakis et al. 1996).