To understand how neuronal activity patterns give rise to behavior, we have adopted a new technology that extends a two-photon microscope to stimulate both single neurons and patterns of individual neurons. This approach opens the door to studying how animals' choices depend on patterns of neuronal activity. We have achieved stable psychophysical performance in mice in the laboratory, and collected data in which we use the instruments to evoke stimulation patterns. We hypothesize that the brain regards as similar, for behavioral decisions, many different patterns that share similar statistical structure. However, it may be possible that particular neurons or groups of neurons are exclusively observed by the brain during the decision process. Our research is directed at determining which of these hypotheses is true. The experiments will shed light on how our brains interpret the thousands or millions of neurons whose activity is constantly changing in response to our perceptions and actions. Determining how the brain uses these activity patterns, which seem to be central to much of brain function, will be an important step forward in understanding how our brains work, both in health and disease.