Anatomical, neurophysiological, and neuroimaging evidence suggest that posterior cingulate cortex (CGp) contributes to both motivational and visuospatial processing (Mesulam 1999; Maddock 1999). New neurophysiological data indicate that CGp neurons not only report the spatial coordinates of visually-guided gaze shifts, but also represent their motivational value (McCoy et al. 2003). The timing arid reward-sensitivity of CGp responses, coupled with the observation that CGp dysfunction in humans is associated with both spatial disorientation and mood/anxiety disorders, invite the hypothesis that this limbic area binds motivational salience to visuospatial locations (Mesulam 1999; McCoy and Platt 2004). Furthermore, neurophysiological evidence suggests that motivational and visuospatial information may be computationally bound by CGp in a manner consistent with attention-based models in learning theory (cf. Pearce and hall 1980). This hypothesis raises several important questions. First, do CGp neurons encode the motivational value of visual targets independent of overt orienting? Second, are motivational signals in CGp referenced spatially to the eyes, head, world, or objects? Third, do motivational modulations of neuronal activity in CGp reflect external rewards or the subjective salience of visuospatial targets? And fourth, does activation of CGp functionally bind motivational salience to visuospatial locations? We propose to answer these questions through a combination of behavioral, neurophysiological, and microstimulation studies. Preliminary data suggest that CGp neurons signal the conditional motivational value of visual targets independent of saccades; CGp signals are referenced allocentrically to objects in the world; CGp responses are correlated with subjective preferences for a particular visual target location, rather than the history of rewards associated with that target; and microstimulation in CGp systematically biases orienting towards contralateral space. These data are consistent with the proposed hypothesis that CGp binds motivational salience to locations in visual space. Further microstimulation experiments will be aimed at determining whether the timing with which microstimulation is delivered to CGp systematically influences orienting choices as predicted by reinforcement learning theory. The proposed research is an important step in achieving the broader goal of understanding low the limbic system helps guide visuospatial processing areas to adapt attention dynamically to changing contexts and evolving behavioral goals. ? ?
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