The Section on Neurophysiology studies the frontal cortex and related parts of the brain. In the recent past, we have focused on the use of symbols to guide action, a process that we call symbolic mapping. Making decisions based on symbols is a fundamental feature of daily life, and diseases such as schizophrenia, attention deficit-hyperactivity disorder (ADHD), obsessive-compulsive disorder (OCD), and others result, at least in part, from inappropriate selection and control of actions, often in relation to symbolic stimuli such as objects and language. In symbolic mapping, the choice of an action depends on the behavioral context provided by a symbol. This is the basis for learning the meaning of most words, linking meaning to speech, and for the wide variety of symbol-guided and signal-guided behavior that underlies most advanced human behavior. Previous work on this project has shown that symbolically guided action depends upon the proper functioning of specific parts of the frontal cortex, the hippocampal system and the basal ganglia. Our research revealed that the one-trial learning of symbolic mappings depends on Hebbian mechanisms (Brasted et al., 2005) and that the putamen, a part of the basal ganglia, retains the information needed for learning longer during the learning process than does the premotor cortex and, importantly, it does so until feedback arrives (Buch et al., 2006). In the past year, we have used symbolic stimuli to instruct both motor plans and the allocation of attentional resources in order to test the premotor theory of attention. According to the premotor theory of attention, reorienting selective spatial attention to some location necessarily involves planning a movement to that location. One prediction of this hypothesis is that the same neurons should be recruited whether one is covertly attending to some location or planning a saccadic eye movement to fixate that location. To test this prediction, we spatially dissociated these cognitive operations by training subjects to attend to a visual stimulus at one location while planning a saccade to a stimulus at a second location. Four identical visual stimuli were located up, down, left and right from a central fixation point. Selective spatial attention was required to detect a subtle increase in the brightness of one of these stimuli, which served as the go signal for the saccade. Both the attended location and the saccade target varied from trial to trial and were instructed by a symbolic cue at the fixation point. The cue consisted of two colored parts: a circle that instructed the attended location, surrounded by an annulus that instructed the saccade target. The color of each part uniquely specified one of the four targets according to a common color-to-place mapping. On 20% of the trials, the attention cue was omitted. On these catch trials, subjects became less reliable at detecting the go signal (76-78% from as baseline of 85% correct), and saccadic reaction times increased by 15 ms for correct trials. These findings indicate that subjects attended to a location when cued to do so. We recorded from isolated neurons in prefrontal and premotor cortex, including the supplementary eye field. For each neuron, mean firing rates during the 800 ms period preceding the go signal were analyzed. Of 412 cells, 93 had effects of either the attended location, the movement goal, or both. To evaluate spatial tuning, we estimated the spatial preference of these neurons as a vector average. Contrary to the prediction of the premotor theory of attention, only a minority of these neurons (12/93, 13%) were affected by both the planned movement and the attended location, and of these about half had different spatial preferences for attention and saccade planning. In addition to these neurons, 68 cells (73%) were tuned only to the target of movement, and 13 (14%) were tuned only to the attended location. These findings suggest that, for the most part, distinct neuronal populations underlie these two aspects of spatial cognition in frontal cortex and strongly contradict the premotor theory of attention. Future work on this project will involve preparing a report on the results just described, testing the differential role of the ventral prefrontal cortex (PFv) and the orbital prefrontal cortex (PFo) on symbolic visuomotor learning and the learning of symbolic pairs (paired associate learning).
| Buch, Ethan R; Brasted, Peter J; Wise, Steven P (2006) Comparison of population activity in the dorsal premotor cortex and putamen during the learning of arbitrary visuomotor mappings. Exp Brain Res 169:69-84 |
