Consider driving home from work in clear weather. Stopping at a traffic light, you see pedestrians waiting to cross the street. Effortlessly, you discern whether one of them is your spouse, your boss or a stranger, and connect the percept with the appropriate action, so you will either be waving frantically, greeting respectfully or taking another sip of coffee. During a rainstorm, however, the sensory input is noisier and you thus must look longer to gather more sensory data in order to make a decision about the person at the light and the appropriate behavioural response. This sort of decision-making process is crucial not only for such mundane situations as the one described above, but also for more biologically and socially important situations 1 . The process by which information that is gathered from sensory systems is combined and used to influence how we behave in the world is referred to as perceptual decision making. Typical experimental approaches that have been used to investigate the mechanisms of per- ceptual decision making are described in BOX 1. Recent advances in both neurophysiological studies in monkeys (for reviews, see REFS 1–5) and functional brain imag- ing methods have inspired studies of perceptual decision making in humans. Perceptual decision making is influenced not only by the sensory information at hand, but also by factors such as attention, task difficulty, the prior probability of the occurrence of an event and the outcome of the decision 6,7 . Although traditional psychological theories conjecture that the decision-making process consists of components that act in a hierarchical manner, with serial progression from perception to action (for example, see REF. 8; however, see also REF. 9), more recent neuroscientific findings indicate that some of the components of this process happen in parallel. The neural architecture for perceptual decision making can be viewed as a system that consists of four distinct but interacting processing modules. The first accumulates and compares sensory evidence; the second detects perceptual uncertainty or difficulty and signals when more attentional resources are required to process a task accurately; the third represents decision variables and includes motor and premotor structures; and the fourth is involved in performance monitoring, which detects when errors occur and when decision strategies need to be adjusted to maximize performance. A unique contribution of neuroimaging to the study of perceptual decision making is its ability to probe the interactions between these different brain systems, including sensory areas, and identify the role of higher-level decision-making structures. This is now possible because current neuroimaging tech- niques and data analysis methods have evolved to the point where we can directly link behavioural measures (that is, decisions) to signals in the human brain on a trial-by-trial basis. To motivate the four-module conceptualization of perceptual decision making and the studies that under- pin it, we begin this Review by providing a description of perceptual decision making, as gleaned from find- ings in monkey experiments that made use of single- unit recordings. Then we provide an overview of what is known about the neural systems that are involved *Neurocognition of Decision Making Group, Max Planck Institute for Human Development, Lentzeallee 94, 14195 Berlin, Germany. ‡ Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany. § Neuroscience Research Center & Berlin NeuroImaging Center, Charité University Medicine Berlin, Berlin, Germany. || Functional Magnetic Resonance Imaging Facility; ¶ Laboratory of Brain and Cognition, National Institute of Mental Health, Bethesda, Maryland, USA. Correspondence to H.R.H. e-mail: heekeren@mpib-berlin.mpg.de doi:10.1038/nrn2374 Published online 9 May 2008 Decision variable A quantity that is monotonically related to the relative likelihood of one alternative occurring versus another occurring. In perceptual decision-making tasks, the link between the sensory representation and the commitment to a choice is thought to involve the computation of a decision variable. The neural systems that mediate human perceptual decision making Hauke R. Heekeren* ‡§ , Sean Marrett || and Leslie G. Ungerleider ¶ Abstract | Perceptual decision making is the act of choosing one option or course of action from a set of alternatives on the basis of available sensory evidence. Thus, when we make such decisions, sensory information must be interpreted and translated into behaviour. Neurophysiological work in monkeys performing sensory discriminations, combined with computational modelling, has paved the way for neuroimaging studies that are aimed at understanding decision-related processes in the human brain. Here we review findings from human neuroimaging studies in conjunction with data analysis methods that can directly link decisions and signals in the human brain on a trial-by-trial basis. This leads to a new view about the neural basis of human perceptual decision-making processes. REVIEWS NATURE REVIEWS | NEUROSCIENCE VOLUME 9 | JUNE 2008 | 467 © 2008 Nature Publishing Group