RESUMO
This technical note deals with a priori estimation of efficiency of functional magnetic resonance imaging (fMRI) designs. The efficiency of an estimator is a measure of how reliable it is and depends on error variance (the variance not modeled by explanatory variables in the design matrix) and the design variance (a function of the explanatory variables and the contrast tested). Changes in the experimental design can induce changes in the variance of estimated responses. This translates into changes in the standard error of the response estimate or equivalently into changes in efficiency. One consequence is that statistics, testing for the same activation in different contexts (i.e., experimental designs), can change substantially even if the activation and error variance are exactly the same. We demonstrate this effect using an event-related fMRI study of single word reading during blocked and randomized trial presentations. Furthermore, we show that the error variance can change with the experimental design. This highlights a problem with a priori comparison of efficiency for two or more experimental designs, which usually assumes identical error variance.
Assuntos
Córtex Cerebral/fisiologia , Modelos Lineares , Imageamento por Ressonância Magnética/estatística & dados numéricos , Reconhecimento Visual de Modelos/fisiologia , Leitura , Adulto , Atenção/fisiologia , Mapeamento Encefálico , Imagem Ecoplanar/estatística & dados numéricos , Eficiência , Feminino , Humanos , Masculino , Computação Matemática , Distribuição Aleatória , Tempo de Reação/fisiologia , Projetos de Pesquisa , Processos EstocásticosRESUMO
In this study we demonstrate that, even in blocked design fMRI, an event-related analysis may provide a more accurate model of the hemodynamic responses than an epoch-related analysis. This is because the temporal shape of the predicted response differs between the event-related and the epoch model, with the former reaching its peak sooner and returning to baseline later than the latter. We present data from a blocked design fMRI study of single word reading alternated with rest. Conventionally, such a design would be analyzed using an epoch analysis with boxcar regressors. However, here we used a combined model in which trials were modeled as both single events and epochs. This allowed us to estimate the variance in the BOLD signal that was explained by either the event-related or the epoch regressors having discounted the effect of the other. We found that, in a number of language regions, the event-related model explained changes in activity that were not accounted for by the epoch model. In addition, we show that the advantage of the event-related over epoch model was engendered by its early onset rather than its late offset, relative to the epoch model.