In vivo 1 H MRS is rapidly developing as a clinical tool for diagnosing and characterizing breast cancers. Many in vivo and in vitro experiments have demonstrated that alterations in concentrations of choline-containing metabolites are associated with malignant transformation. In recent years, considerable efforts have been made to evaluate the role of 1 H MRS measurements of total cholinecontaining compounds in the management of patients with breast cancer. Current technological developments, including the use of high-field MR scanners and quantitative spectroscopic analysis methods, promise to increase the sensitivity and accuracy of breast MRS. This article reviews the literature describing in vivo MRS in breast cancer, with an emphasis on the development of highfield MR scanning and quantitative methods. Potential applications of these technologies for diagnosing suspicious lesions and monitoring response to chemotherapy are discussed.

When faced with decisions, rats sometimes pause and look back and forth between possible alternatives, a phenomenon termed vicarious trial and error (VTE). When it was first observed in the 1930s, VTE was theorized to be a mechanism for exploration. Later theories suggested that VTE aided the resolution of sensory or neuroeconomic conflict. In contrast, recent neurophysiological data suggest that VTE reflects a dynamic search and evaluation process. These theories make unique predictions about the timing of VTE on behavioral tasks. We tested these theories of VTE on a T-maze with return rails, where rats were given a choice between a smaller reward available after one delay or a larger reward available after an adjustable delay. Rats showed three clear phases of behavior on this task: investigation, characterized by discovery of task parameters; titration, characterized by iterative adjustment of the delay to a preferred interval; and exploitation, characterized by alternation to hold the delay at the preferred interval. We found that VTE events occurred during adjustment laps more often than during alternation laps. Results were incompatible with theories of VTE as an exploratory behavior, as reflecting sensory conflict, or as a simple neuroeconomic valuation process. Instead, our results were most consistent with VTE as reflecting a search process during deliberative decision making. This pattern of VTE that we observed is reminiscent of current navigational theories proposing a transition from a deliberative to a habitual decision-making mechanism.

The ability to change behavioural strategies in the face of a changing world has been linked to the integrity of medial prefrontal cortex (mPFC) function in several species. While recording studies have found that mPFC representations reflect the strategy being used, lesion studies suggest that mPFC is necessary for changing strategy. Here we examine the relationship between representational changes in mPFC and behavioural strategy changes in the rat. We found that on tasks with a forced change in reward criterion, strategy-related representational transitions in mPFC occurred after animals learned that the reward contingency had changed, but before their behaviour changed. On tasks in which animals made their own strategic decisions, representational transitions in mPFC preceded changes in behaviour. These results suggest that mPFC does not merely reflect the action–selection policy of the animal, but rather that mPFC processes information related to a need for a change in strategy.

The rodent prelimbic cortex has been shown to play an important role in cognitive processing, and has been implicated in encoding many different parameters relevant to solving decision-making tasks. However, it is not known how the prelimbic cortex represents all these disparate variables, and if they are simultaneously represented when the task requires it. In order to investigate this question, we trained rats to run the Multiple-T Left Right Alternate (MT-LRA) task and recorded multi-unit ensembles from their prelimbic regions. Significant populations of cells in the prelimbic cortex represented the strategy controlling reward receipt on a given lap, whether the animal chose to go right or left on a given lap, and whether the animal made a correct decision or an error on a given lap. These populations overlapped in the cells recorded, with several cells demonstrating differential firing to all three variables. The spatial and strategic firing patterns of individual prelimbic cells were highly conserved across several days of running this task, indicating that each cell encoded the same information across days.