Deep brain stimulation (DBS) is an effective treatment for movement disorders, but the mechanisms are unclear. DBS generates inhibition of neurons surrounding the electrode while simultaneously activating the output axons of local neurons. This dual effect does not explain two hallmarks of DBS effectiveness: symptom relief is dependent on using a sufficiently high-stimulation frequency, and clinical effects are analogous to those produced by lesion. The effect of DBS at different frequencies on the output of intrinsically active neurons was studied using computational models. DBS produced frequency-dependent modulation of the variability of neuronal output, and above a critical frequency stimulation resulted in regular output with zero variance. The resulting loss of information offers an explanation for the two hallmarks of DBS effectiveness.

Objective-Deficits in visual perception and working memory are commonly observed in neuropsychiatric disorders and have been investigated using functional MRI. However, interpretation of differences in brain activation may be confounded with differences in task performance between groups. Differences in task difficulty across conditions may also pose interpretative issues in studies of visual processing in healthy subjects.Method-In order to address these concerns, the present study characterized brain activation in tasks which were psychometrically matched for difficulty. Functional magnetic resonance imaging (fMRI) was used to assess brain activation in ten healthy subjects during discrimination and working memory judgments for static and moving stimuli. For all task conditions, performance accuracy was matched at 70.7%.Results-Areas associated with V2 and V5 in the dorsal stream were activated during motion processing tasks and V4 in the ventral stream were activated during form processing tasks. Frontoparietal areas associated with working memory were also statistically significant during the working memory tasks.Conclusions-Application of psychophysical methods to equate task demands provides a practical method to equate performance levels across conditions in fMRI studies, and to compare healthy and cognitively impaired groups at comparable levels of effort. These psychometrically matched tasks can be applied to patients with a variety of cognitive disorders to investigate dysfunction of multiple a priori defined brain regions. Measuring the changes in typical activation (Drzezga, 2008;Sperling, 2007;Wierenga & Bondi, 2007), and Parkinson's disease (Dagher & Nagano-Saito, 2007;van Eimeren & Siebner, 2006). A major aim of this approach is to characterize differences in the amount, location, or connectivity of brain activity between the control and patient groups. However, the interpretation of such differences is often confounded by performance differences between groups, since patients with brain dysfunction often perform more poorly than healthy subjects on cognitive tasks. Individual differences in performance may reflect differences in perceptual processing, attention, learning efficiency, effort, or cognitive strategies. More generally, comparison of fMRI activation between conditions may also be influenced by individual differences in performance. In the visual modality, for example, parametric manipulation of stimulus detectability, decision certainty or effort can have marked effects on activation. Huang et al. (Huang, Xiang, & Cao, 2006) varied the visibility of a target stimulus in a masking paradigm, and found that activation in V1 decreased with decreased stimulus visibility. Volz et al. (Volz, Schubotz, & von Cramon, 2005) showed that increasing uncertainty in a decision making task increased activation within the posterior frontal median cortex. Haynes et al. (Haynes, Driver, & Rees, 2005) reported that the psychometric visibility for target stimuli in a metacontrast paradigm affected...