The production of new hippocampal neurons in adulthood has been well documented in rodents. Recent studies have extended these findings to other mammalian species, such as tree shrews and marmoset monkeys.

Moore, Tirin and Mazyar Fallah. Microstimulation of the frontal eye field and its effects on covert spatial attention. J Neurophysiol 91: 152-162, 2004. First published September 17, 2003 10.1152/jn.00741.2002. Many studies have established that the strength of visual perception and the strength of visual representations within visual cortex vary according to the focus of covert spatial attention. While it is clear that attention can modulate visual signals, the source of this modulation remains unknown. We have examined the possibility that saccade related mechanisms provide a source of spatial attention by studying the effects of electrical microstimulation of the frontal eye fields (FEF) on spatial attention. Monkeys performed a task in which they had to detect luminance changes of a peripheral target while ignoring a flashing distracter. The target luminance change could be preceded by stimulation of the FEF at current levels below that which evoked saccadic eye movements. We found that when the target change was preceded by stimulation of FEF, the monkey could detect smaller changes in target luminance. The increased sensitivity to the target change only occurred when the target was placed in the part of the visual field represented by neurons at the stimulation site. The magnitude of improvement depended on the temporal asynchrony of the stimulation onset and the target event. No significant effect of stimulation was observed when long intervals (Ͼ300 ms) between stimulation and the target event were used, and the magnitude of the increased sensitivity decreased systematically with increasing asynchrony. At the shortest asynchrony, FEF stimulation temporally overlapped the target event and the magnitude of the improvement was comparable to that of removing the distracter from the task. These results demonstrate that transient, but potent improvements in the deployment of covert spatial attention can be obtained by microstimulation of FEF sites from which saccadic eye movements are also evoked. I N T R O D U C T I O NSpatial attention refers to the selective processing of stimuli at behaviorally relevant locations at the expense of stimuli at other locations. Saccadic eye movements are the primary way by which visual resources (i.e., the foveas) are brought to bear on relevant spatial locations and are thus key exemplars of spatial attentive behavior. Much has been made of the fact that objects can be selected for attention without actually being fixated (Sperling and Melchner 1978), i.e., selected covertly, but a wealth of evidence suggests that the mechanisms of overt and covert attention are similar, if not the same (Corbetta et al.

Covert spatial attention produces biases in perceptual performance and neural processing of behaviorally relevant stimuli in the absence of overt orienting movements. The neural mechanism that gives rise to these effects is poorly understood. This paper surveys past evidence of a relationship between oculomotor control and visual spatial attention and more recent evidence of a causal link between the control of saccadic eye movements by frontal cortex and covert visual selection. Both suggest that the mechanism of covert spatial attention emerges as a consequence of the reciprocal interactions between neural circuits primarily involved in specifying the visual properties of potential targets and those involved in specifying the movements needed to fixate them.

Several lines of evidence suggest that planning eye movements and directing visuospatial attention share overlapping brain mechanisms. This study tested whether spatial attention can be enhanced by altering oculomotor signals within the brain. Monkeys performed a spatial attention task while neurons within the frontal eye field, an oculomotor area within prefrontal cortex, were electrically stimulated below the level at which eye movements are evoked. We found that we could improve the monkey's performance with microstimulation when, but only when, the object to be attended was positioned in the space represented by the cortical stimulation site.