Faculty Bibliography

Behavioral studies suggest that making a decision involves representing the overall desirability of all available actions and then selecting that action that is most desirable. Physiological studies have proposed that neurons in the parietal cortex play a role in selecting movements for execution. To test the hypothesis that these parietal neurons encode the subjective desirability of making particular movements, we exploited Nash's game theoretic equilibrium, during which the subjective desirability of multiple actions should be equal for human players. Behavior measured during a strategic game suggests that monkeys' choices, like those of humans, are guided by subjective desirability. Under these conditions, activity in the parietal cortex was correlated with the relative subjective desirability of actions irrespective of the specific combination of reward magnitude, reward probability, and response probability associated with each action. These observations may help place many recent findings regarding the posterior parietal cortex into a common conceptual framework.

The substantia nigra pars reticulata (SNr), a major output nucleus of the basal ganglia, has been implicated anatomically, pharmacologically and physiologically in the generation of saccadic eye movements. However, the unique contribution of the SNr to saccade generation remains elusive. We studied the activity of SNr neurons while rhesus monkeys made saccades from different initial orbital positions, to determine what effects, if any, eye position had on SNr neuronal activity. We found that there was no effect of eye position on SNr neuronal responses. We also examined the responses of SNr neurons during memory-guided saccades to determine whether SNr discharges were affected by whether the target of the upcoming saccade was visible. We found that there was no change in response properties during memory saccade trials as compared to otherwise identical visually guided trials. SNr neurons appear to carry no information about either eye position or whether a movement is guided by a visible or remembered target. These results suggest that nigral signals are encoded in the same coordinate frame as those in the SC and FEF, but that unlike neuronal responses in these areas, SNr activity is not influenced by whether the saccade target remains visible until the movement is executed.

Introduction: It has been shown that subjects are faster and more accurate at detecting or discriminating stimuli when they are more certain of where a stimulus will appear. We have shown that a probability paradigm, in which observers use the probability of where a stimulus is likely to occur, can direct the allocation of resources and improve accuracy with increasing probability, or spatial certainty (Ciaramitaro et al, 2001). We now study the temporal dynamics of attention across probability transitions, to investigate how quickly observers track shifts in probability, as the certainty of where to attend changes. Method: Six observers performed an orientation discrimination task, viewing an extrafoveal stimulus (102ms) that was followed by a mask (102ms) after a delay of 17, 35 or 52ms. The probability of stimulus occurrence in the left or right hemifield switched between 20% and 80% in several blocks of ∼200 trials each. Behavioral data were convolved with a gaussian to derive a trial-by-trial running estimate of fluctuations in performance over time. Results & Conclusion: When probability transitions were signaled, observers' overall performance improved as probability increased across blocks, whereas when transitions were not signaled, and observers may have been less certain of the probability condition, their overall performance was not well matched to changes in probability. On a trial-by-trial basis, performance within a block was not always stable, potentially obscuring overall differences between blocks, and performance changes across blocks often showed rapid transitions, suggesting that observers learned the new probability quickly. Quantifying the dynamics of changes in behavior over time is an important step if we ultimately want to link such changes to dynamic changes at the neuronal level as we switch attention to different locations.

Over the past two decades significant progress has been made toward understanding the neural basis of primate decision making, the biological process that combines sensory data with stored information to select and execute behavioral responses. The most striking progress in this area has been made in studies of visual-saccadic decision making, a system that is becoming a model for understanding decision making in general. In this system, theoretical models of efficient decision making developed in the social sciences are beginning to be used to describe the computations the brain must perform when it connects sensation and action. Guided in part by these economic models, neurophysiologists have been able to describe neuronal activity recorded from the brains of awake-behaving primates during actual decision making. These recent studies have examined the neural basis of decisions, ranging from those made in predictable sensorimotor tasks to those unpredictable decisions made when animals are engaged in strategic conflict. All of these experiments seem to describe a surprisingly well-integrated set of physiological mechanisms that can account for a broad range of behavioral phenomena. This review presents many of these recent studies within the emerging neuroeconomic framework for understanding primate decision making.

Behavioral ecologists argue that evolution drives animal behavior to efficiently solve the problems animals face in their environmental niches. The ultimate evolutionary causes of decision making, they contend, can be found in economic analyses of organisms and their environments. Neurobiologists interested in how animals make decisions have, in contrast, focused their efforts on understanding the neurobiological hardware that serves as a more proximal cause of that same behavior. Describing the flow of information within the nervous system without regard to these larger goals has been their focus. Recent work in a number of laboratories has begun to suggest that these two approaches are beginning to fuse. It may soon be possible to view the nervous system as a representational process that solves the mathematically defined economic problems animals face by making efficient decisions. These developments in the neurobiological theory of choice, and the new schema they imply, form the subject of this article.

Imagine the decisions you might make while playing a simple game like 'matching pennies'. At each play, you and your opponent, say the mathematician John vonNeumann, each lay down a penny heads or tails up. If both pennies show the same side, vonNeumann wins, if not, you win. Before each play, you have the subjective experience of deciding what to do: of choosing whether to play heads or tails. Although decisions like these are not yet understood at a physiological level, progress has been made towards understanding simple decision making in at least one model system: the primate neural architecture that uses visual data and prior knowledge about patterns in the environment to select and execute saccades. Both the visual system and the brainstem circuits that control saccadic eye movements are particularly well understood, making it possible for physiologists to begin to study the connections between these sensory and motor processes at a level of complexity that would be impossible in other less well understood systems.