Human brain signals associated with perceptual processes have been shown to be useful for visual target image search. For the purpose of online training, we develop a novel mixed effects evoked response detector, which is capable of combining individual random effects and population fixed effects, for the analysis of neural signatures associated with targets. To avoid numerical problems in high dimensional matrix computations, we develop equivalent dimension reduced expressions for the mixed models. We construct the mixed effects evoked response model using principal component analysis to provide bases for the population model and linear discriminant analysis (LDA) to provide bases for the individual models. In addition, the LDA is adopted for Elecroencephalography channel dimensionality reduction. Data collected at different time and experimental conditions from two subjects performing image search tasks are utilized to assess the quality of the models. We also compare the proposed model with the support vector machine (SVM). The results demonstrate that the mixed models approach the SVM and provide reliable inference on cross session evaluation for the single-trial evoked response detection.