In the perceptual learning (PL) literature, researchers typically focus on improvements in accuracy, such as d’. In contrast, researchers who investigate the practice of cognitive skills focus on improvements in response times (RT). Here, we argue for the importance of accounting for both accuracy and RT in PL experiments, due to the phenomenon of speed-accuracy tradeoff (SAT): at a given level of discriminability, faster responses tend to produce more errors. A formal model of the decision process, such as the diffusion model, can explain the SAT. In this model, a parameter known as the drift rate represents the perceptual strength of the stimulus, where higher drift rates lead to more accurate and faster responses. We applied the diffusion model to analyze responses from a yes-no coherent motion detection task. The results indicate that observers do not use a fixed threshold for evidence accumulation, so changes in the observed accuracy may not provide the most appropriate estimate of learning. Instead, our results suggest that SAT can be accounted for by a modeling approach, and that drift rates offer a promising index of PL.

Phosphoinositides, the phosphorylated derivatives of phosphatidylinositol (PtdIns), are key regulators of many fundamental biological processes, including cell growth, proliferation, and motility. Here, we present a novel method for rapid, sensitive, and simultaneous profiling of phosphatidylinositol trisphosphate (PtdInsP3), phosphatidylinositol bisphosphate (PtdInsP2), and phosphatidylinositol phosphate (PtdInsP) of different fatty acid compositions. This method is based on a technique called "charged diacylglycerol fragment ion-specific multiple precursor ion scanning" (DAG(+)-specific MPIS), coupled with prior phosphate methylation. Using DAG(+)-specific MPIS, we were able to identify 32 PtdIns, 28 PtdInsP, 30 PtdInsP2, and 3 PtdInsP3 molecular species from bovine brain extracts or prostatic cancer cell lines in an efficient and time-saving manner. Our analysis revealed a large range of fatty acyl compositions in phosphoinositides not obtained previously from mammalian samples. We also developed a method that involves isotopic labeling of endogenous phosphoinositides with deuterated diazomethane (CD2N2) for quantitation of phosphoinositides. CD2N2 was generated in situ through acid-catalyzed H/D exchange and methanolysis of trimethylsilyl diazomethane (TMS-diazomethane). Phosphoinositides, extracted from a PC3 prostatic cancer cell line, were labeled either with CH2N2 or CD2N2 and mixed in known proportions for DAG(+)-specific MPIS-based mass spectrometry (MS) analysis. The results indicate that isotopic labeling is capable of providing accurate quantitation of PtdInsP3, PtdInsP2, and PtdInsP with adequate linearity as well as high reproducibility with an average coefficient variation of 18.9%. More importantly, this new methods excluded the need for multiple phosphoinositide internal standards. DAG(+)-specific MPIS and isotopic labeling based MS analysis of phosphoinositides offers unique advantages over existing approaches and presents a powerful tool for research of phosphoinositide metabolism.

We have developed a novel sheath-flow interface for low-flow electrospray ionization mass spectrometry (ESI-MS) and capillary electrophoresis/electrospray mass spectrometry (CE/ESI-MS). The interface is composed of two capillaries. One is a tapered fused-silica ESI emitter suitable for microliter and nanoliter flow rate electrospray and the other is a tail-end gold-coated CE separation column that is inserted into the emitter. A sheath liquid is supplied between the column and the emitter capillaries. The gold coating and the sheath liquid are used as the conducting media for ESI and the CE circuit. This novel design was initially evaluated by an infusion ESI-MS analysis of the most common antiretroviral dideoxynucleosides, followed by CE/MS coupling analysis of several antidepressant drugs. With infusion studies, the effects of the sheath liquid and the sample flow rates on detection sensitivity and signal stability were investigated. For an emitter with an internal diameter of 30 microm, the optimum flow rates for the sheath and the sample were 200 and 300 nL/min, respectively. The main improvement of this approach in comparison with conventional sheath liquid approaches using an ionspray interface is the gain in sensitivity. Sensitivities were three times better for dideoxynucleosides analyzed by infusion and 12 times higher for antidepressant drugs analyzed by CE/MS with this interface compared with ionspray. The emitter is durable, disposable, and simple to fabricate.

We report the use of capillary electrophoresis-electrospray ionization-mass spectrometry (CE-ESI-MS) for the determination of antiretroviral dideoxynucleosides (ddNs), their nucleotides, and a set of ribonucleosides and ribonucleotides. A CE system for separation of most commonly used antiretroviral ddNs has been developed based on a basic buffer with a volatile electrolyte suitable for ESI-MS detection in an untreated capillary column. Positive and negative ionization modes are investigated and compared for sensitive and stable electrospray performance. A 14-compound mixture of nucleosides and nucleotides is profiled in a single capillary zone electrophoresis separation with a distinct elution order: electroosmotic flow, ddNs, mononucleotides, dinucleotides, and trinucleotides in less than 18 min. The fragmentation pathways of the nucleosides and nucleotides in ESI-MS have been interpreted. Concentration limits of detection are 100 to 200 nM with an injection volume of approximately 10 nL. This technique has been used to detect naturally occurring nucleotides and to study the metabolism of lamivudine (3TC) in the human hepatoma cell line Hep G2. 3TC and its metabolites 3TC-monophosphate, 3TC-diphosphate, and 3TC-triphosphate were detected after 10 h of incubation of 3TC with the cells.

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