NYU/CNS : Faculty : Associates of CNS :Clayton Curtis

The Prefrontal Cortex
The prefrontal cortex (PFC) is thought to be the most important area for higher order cognition and sits at the apex of the motor hierarchy. It is critical for the planning, selection, and execution of willed behavior. Despite its appreciation as a critical component of some of our most uniquely human behaviors, we know very little about the mechanisms that it uses to bias behavior and very little even about its gross functional subdivisions. The rationale behind many of our current studies is to treat a small portion of the PFC, the frontal eye field (FEF), as a model system of the PFC. The experiments we are conducting are primarily designed to characterize the mechanisms that the FEF uses for planning, attention, memory, and selection in the context of well-controlled oculomotor tasks. We hope that working within a better-defined and constrained system like the oculomotor system may quickly lead to mechanistic accounts of these functions that may be less tenable in a more complicated and less understood system like the PFC as a whole. Like the PFC in general, the FEF has been implicated in planning, attention, memory, and selection making it an ideal model for PFC function. Our initial experiments aim to define and characterize the human homologue of the monkey FEF with the use of event-related functional magnetic resonance imaging (fMRI) techniques. Additionally, they will test between hypotheses that propose that different FEF subregions have separable functions and hypotheses that propose that similar mechanisms are employed by the FEF to support diverse functions.

Working Memory
Delayed response tasks have been used for decades now to allow for the disassociation of stimulus sensory cues and motor responses made contingent upon those cues. The imposition of an unfilled delay between the cue and response separates these two events and requires the maintenance across time of the stimulus-response contingency. Persistent neural activity is considered by neuroscientists to directly reflect the maintenance of information in working memory (Curtis & D'Esposito, TiCS, 2003). In a series of studies we've attempted to understand what is actually being represented or coded for in working memory. With the use of oculomotor delayed response tasks, we demonstrated two different, but overlapping, frontal-parietal networks whose activity was specifically tied to the maintenance period of the task. One was mainly composed of oculomotor areas and was active when the forthcoming memory-guided saccade was known throughout the delay. The other was mainly composed of higher-level areas that have been implicated in spatial attention and was active when the forthcoming response was unknown until after the delay. We proposed that these two networks maintain different representational codes, one reflecting a prospective motor memory code and the other a retrospective sensory memory code (Curtis et al., J Neurosci, 2004). Importantly, we have shown that the FEF seem to contribute to spatial working memory by selecting locations as potential saccade goals and maintaining these motor intentions throughout the memory delay (Curtis & D'Esposito, SFN 2004). Indeed, there exists a map of space is coded in an eye-movement coordinate framework in the FEF.

Voluntary Control
Humans, like all other animals, often act on reflexes. Although a reflex is an incredibly strong force for action, our will allows us to act with volition in the face of reflexive tendencies. The dynamic interplay between automatic and voluntary determinants of behavior is one of the most general organizing principles of brain function, but about which we know very little. We use antisaccade (Curtis & D'Esposito, J Cogn Neurosci, 2003) and countermanding (Curtis et al., Cerb Cortex, in press) tasks to study voluntary control, with an emphasis on the inhibition of prepotent movements. Withholding an action that is normally elicited by a stimulus or canceling an already planned action is a critical demonstration of voluntary control. Our studies have shown that medial frontal brain areas, like the supplementary eye fields (SEF) and a region just anterior to the SEF (pre-SEF) seem to play a special role in the evaluation and resolution of conflicting oculomotor plans (ie, the simultaneous activation of the intentions to make a movement and withhold a movement). For example, when warned during a preparation interval that one will have to inhibit making an eye movement to a visual stimulus once it appears causes the pre-SEF to ramp-up in activity compared to trials in which subjects are asked to simply make a saccade to the visual stimulus. Moreover, because we always monitor the position of subject’s eye during scanning, we showed that this preparatory activity was critical to saccade suppression; it predicted whether or not the saccade was later successfully inhibited (Curtis & D'Esposito, J Cogn Neurosci, 2003). Recently, we’ve shown that control exerted farther down-stream in the perception-action cycle, after the motor plan has been generated, can be seen in the pattern of activity in the human FEF and SEF (Curtis et al., Cerb Cortex, in press). Activity related to saccade initiation and performance monitoring in these regions can sufficiently account for success and failure of voluntary saccadic control.

Email: clayton.curtis@nyu.edu

Publications
See the Curtis Lab Website for a complete list of publications and presentations.