Learning what matters: A neural explanation for the sparsity bias.

The visual environment is filled with complex, multi-dimensional objects that vary in their value to an observer's current goals. When faced with multi-dimensional stimuli, humans may rely on biases to learn to select those objects that are most valuable to the task at hand. Here, we show that decision making in a complex task is guided by the sparsity bias: the focusing of attention on a subset of available features. Participants completed a gambling task in which they selected complex stimuli that varied randomly along three dimensions: shape, color, and texture. Each dimension comprised three features (e.g., color: red, green, yellow). Only one dimension was relevant in each block (e.g., color), and a randomly-chosen value ranking determined outcome probabilities (e.g., green > yellow > red). Participants were faster to respond to infrequent probe stimuli that appeared unexpectedly within stimuli that possessed a more valuable feature than to probes appearing within stimuli possessing a less valuable feature. Event-related brain potentials recorded during the task provided a neurophysiological explanation for sparsity as a learning-dependent increase in optimal attentional performance (as measured by the N2pc component of the human event-related potential) and a concomitant learning-dependent decrease in prediction errors (as measured by the feedback-elicited reward positivity). Together, our results suggest that the sparsity bias guides human reinforcement learning in complex environments.

An elemental model of retrospective revaluation without within-compound associations.

When retrospective revaluation phenomena (e.g., unovershadowing: AB+, then A-, then test B) were discovered, simple elemental models were at a disadvantage because they could not explain such phenomena. Extensions of these models and novel models appealed to within-compound associations to accommodate these new data. Here, we present an elemental, neural network model of conditioning that explains retrospective revaluation apart from within-compound associations. In the model, previously paired stimuli (say, A and B, after AB+) come to activate similar ensembles of neurons, so that revaluation of one stimulus (A-) has the opposite effect on the other stimulus (B) through changes (decreases) in the strength of the inhibitory connections between neurons activated by B. The ventral striatum is discussed as a possible home for the structure and function of the present model.

Steroid-sparing effect of corticorelin acetate in peritumoral cerebral edema is associated with improvement in steroid-induced myopathy.

PURPOSE: To compare the safety and efficacy of corticorelin acetate (CrA) and placebo in patients with malignant brain tumors requiring chronic administration of dexamethasone (DEX) to control the signs and symptoms of peritumoral brain edema (PBE). PATIENTS AND METHODS: Prospective, randomized, double-blind study of 200 patients with PBE on a stable dose of DEX. Initially, DEX dose was decreased by 50% over a 2-week period and then held at this level for 3 weeks. The primary end point was the proportion of patients who responded to treatment-patients who achieved a ≥ 50% DEX reduction from baseline and achieved stable or improved neurologic examination score and Karnofsky performance score at week 2, and then continued to respond at week 5. RESULTS: One hundred patients received subcutaneous injections of 1 mg twice per day of CrA and 100 patients received placebo for the duration of the study period. Although results did not attain statistical significance (at the P < .05 level), a clinically important difference in the proportion of responders between the CrA group (57.0%) and the placebo group (46.0%; P = .12) was observed. In addition, the maximum percent reduction in DEX dose achieved during the double-blind 12-week study was significantly greater in the CrA group (62.7%) than in placebo group (51.4%; P < .001). Patients receiving CrA demonstrated an improvement in myopathy and were less likely to develop signs of Cushing syndrome. CONCLUSION: CrA enables a reduction in steroid requirement for patients with PBE and is associated with a reduction in the incidence and severity of common steroid adverse effects, including myopathy.