The human brain is roughly three times larger than it should be for a species of our body weight. In
addition, the evolution of the brain has not been uniform, with distinct cortical and sub-cortical regions being selected for in different species among mammals, and specifically within primates (T. W. Deacon, 1997; Finlay & Darlington, 1995; J. K. Rilling, 2006; Semendeferi & Damasio, 2000). For example, in primates, some have suggested there has been differential expansion of the frontal and temporal lobes relative to the other lobes and that these changes might reflect specific selection for cortical development in brain regions associated with complex cognition, including language (T. Deacon, 2004; J. K. Rilling & Seligman, 2002; Semendeferi, Armstrong, Schleicher, Zilles, & Van Hoesen, 2001; Semendeferi, Lu, Schenker, & Damasio, 2002).
One of the main challenges in the field of neuroscience is to understand the development and
evolution of the brain in relation to emergent behavioral and cognitive processes that define the human species relative to other primates. Related to this challenge is the quest to understand the role of genetic and non-genetic factors on the development and evolution of the brain in relation to specific behaviors of interest. The main focus of the proposed studies is to begin to address the relationship between the evolution of executive functions, broadly defined, in primates in the context of individual and phylogenetic changes in the brain, notably the prefrontal cortex and associated striatal and limbic system structures. The term "executive function" or "cognitive control" in human cognitive neuroscience reflects the ability to exert meta-control or decision making processes over a number of motivational, emotional and attentional systems. The notion of cognitive control implies that there are top-down systems that exert inhibitory control over more biologically driven motivational or emotional states. This top-down system underlies the ability to shut down or inhibit impulsive behaviors or provide for the ability to foresee reward in the future at the expense of immediate gains (sometimes referred to as delayed gratification) (E. K. Miller, 2000; E. K Miller, 2000), abilities some have suggested are highly advanced and possibly uniquely human (Roberts, 2002).
One heuristic conceptualization of this system has been referred to as the "hot-cool" system of delayed gratification (Metcalfe & Mischel, 1999). In this model, there is the hot "emotional go system" and the cool "know" system characterized as emotionally neutral, contemplative and the seat of self regulation and selfcontrol.
Accordingly, during hominin (and possibly hominoid) evolution, presumably there has been greater
selection for cognitive control (the cool system) over the "hot" emotional, impulsive system.
From a motivational and emotional standpoint, many have suggested that the prefrontal cortex, anterior cingulate and regions within the striatum (notably the caudate) play very important roles in the ability to exert self-control, or to suppress specific kinds of behavioral processes in the presence of pre-potent stimuli that exert strong stimulus control over the subjects' behavior. For example, in humans, there are significant developmental changes in delayed gratification that correspond to increasing maturation and connectivity in the prefrontal cortex, parietal lobe and striatum (Bunge & Wright, 2007; Casey, Getz, & Galvan, 2008).
Clinical studies further support the role of prefrontal cortex in cognitive or executive control. Individuals with lesions in prefrontal cortex and associated limbic system and striatum structures have been described as stimulus bound; that is, their behavior is captured by immediate prepotent stimuli that reflexively elicit strong reactions and they are unable to override these impulsive behaviors and engage in behaviors that result in reward at later points in time (Bechara, Tranel, & Damasio, 2000; E. K Miller, 2000; Sax et al., 1999).
Arguably, one of the most pronounced clinical manifestations of disrupted executive functions are
patients diagnosed as attention-deficit hyperactivity disorder (ADHD). Though executive functions are
broadly defined in the human neuropsychological literature, many have argued that the central behavioral problem of most ADHD individuals is a breakdown in behavioral inhibition (Baird, Stevenson, & Williams, 2000; R. A. Barkley, 1997; R. A. Barkley, 2001) that expresses itself behaviorally as poor self-control and impulsivity. There is considerable overlap in the neural correlates of executive function and in the comparison of ADHD and non-ADHD adult and child populations including prefrontal cortex, anterior cingulate and region of the striatum (caudate and putamen). For instance, meta-analyses of studies of structural brain differences between ADHD individuals and controls have revealed significant differences in the volume and lateralization of the prefrontal cortex, cerebellum, splenium of the corpus callosum and caudate (Casey et al., 1997; Mostofsky, Cooper, Kates, Denckia, & Kaufmann, 2002; Valera, Faraone,
Murray, & Seidman, 2007). It has been shown that damage to the right frontal cortex is associated with deficits in working memory as well as response inhibition, a pattern of results similarly observed in adults with ADHD (Clark et al., 2007). It is of note that ADHD is significantly more prevalent in males than females and a recent review of the studies on delay of gratification showed that women were significantly better than men (Silverman, 2003).