Integration of Sensory and Reward Information during Perceptual Decision-Making in Lateral Intraparietal Cortex (LIP) of the Macaque Monkey

Single neurons in cortical area LIP are known to carry information relevant to both sensory and value-based decisions that are reported by eye movements. It is not known, however, how sensory and value information are combined in LIP when individual decisions must be based on a combination of these variables. To investigate this issue, we conducted behavioral and electrophysiological experiments in rhesus monkeys during performance of a two-alternative, forced-choice discrimination of motion direction (sensory component). Monkeys reported each decision by making an eye movement to one of two visual targets associated with the two possible directions of motion. We introduced choice biases to the monkeys' decision process (value component) by randomly interleaving balanced reward conditions (equal reward value for the two choices) with unbalanced conditions (one alternative worth twice as much as the other). The monkeys' behavior, as well as that of most LIP neurons, reflected the influence of all relevant variables: the strength of the sensory information, the value of the target in the neuron's response field, and the value of the target outside the response field. Overall, detailed analysis and computer simulation reveal that our data are consistent with a two-stage drift diffusion model proposed by Diederich and Bussmeyer [1] for the effect of payoffs in the context of sensory discrimination tasks. Initial processing of payoff information strongly influences the starting point for the accumulation of sensory evidence, while exerting little if any effect on the rate of accumulation of sensory evidence

The binding of nucleotides to 3'-nucleotidase from wheat germ

The 3'-mononucleotidase (3'-ribonucleotide phosphohydrolase, EC 3.1.3.6) from wheat germ has been purified 2,000 fold. The enzyme has a molecular weight of approximatley 32,000, as judged by the use of G-100 gel filtration, and does not attack 2'- or 5'- nucleotides. In order to obtain some indications on the structural requirements for binding and reactivity, the purified enzyme has been subjected to kinetic analyses, including initial velocities with several 3'-ribomononucleotides, inhibition by 5'- nucleotides and nucleotide-analogues, and effect of pH and sulphydryl compounds. The data indicate one base binding site at the active site of the enzyme. This site appears to be the same involved in the binding of both substrates and inhibitors, with higher affinity for purine nucleotides than for pyrimidine compounds, in the order guanosine greater than adenosine greater than inosine greater than uridine greater than cytidine nucleotides

Ribose 1-phosphate and inosine activate uracil salvage in rat brain

The purpose of this study was to determine the mechanism by which inosine activates pyrimidine salvage in CNS. The levels of cerebral inosine, hypoxanthine, uridine, uracil, ribose 1-phosphate and inorganic phosphate were determined, to evaluate the Gibbs free energy changes (deltaG) of the reactions catalyzed by purine nucleoside phosphorylase and uridine phosphorylase, respectively. A deltaG value of 0.59 kcal/mol for the combined reaction inosine+uracil uridine+hypoxanthine was obtained, suggesting that at least in anoxic brain the system may readily respond to metabolite fluctuations. If purine nucleoside phosphorolysis and uridine phosphorolysis are coupled to uridine phosphorylation, catalyzed by uridine kinase, whose activity is relatively high in brain, the three enzyme activities will constitute a pyrimidine salvage pathway in which ribose 1-phosphate plays a pivotal role. CTP, presumably the last product of the pathway, and, to a lesser extent, UTP, exert inhibition on rat brain uridine nucleotides salvage synthesis, most likely at the level of the kinase reaction. On the contrary ATP and GTP are specific phosphate donors