Executive functions

Executive functions refer to higher order mental processes such as cognitive control, which includes the ability to coordinate thought and action, which the prefrontal cortex (PFC) is said to play an important role in (Miller & Cohen, 2001). Executive functions cover a broad range of behavioural areas including working memory and decision-making and reward behaviours which are explored further here.

1.3.1.1. Working memory

Working memory (WM) is the representation of new information and information retrieved from long-term memory (Curtis & D’Esposito, 2003). It is required for accessing memories (Rodriguez & Paule, 2009) in addition to being involved in the maintenance and manipulation of information (Mars & Grol, 2007). WM relies on an executive controller to supply auditory and visual information through interactions with short-term and long-term

required (Meyer & Lieberman, 2012) which is the ability to think about others intentions, beliefs and mental states (Frith & Frith, 2003). Mentalising therefore depends upon WM, in order to form a conclusion about the social situation the individual must be able to access, maintain and manipulate information about themselves and others involved (Meyer & Lieberman, 2012).

Neuroimaging studies have found that the PFC is crucial for WM (Baker, Rogers, Owen, Frith, Dolan, Frackowiak & Robbins, 1996; Curtis & D’Esposito, 2003), more specifically the dorsolateral prefrontal cortex (DLPFC; Curtis & D’Esposito, 2003). The DLPFC is believed to play a role in directing attention to internal representations of sensory and motor information (Curtis & D’Esposito, 2003). Several studies have found that DLPFC activation is directly linked to WM (Baker, et al., 1996; Belger, Puce, Krystal, Gore,

Goldman-Rakic & McCarthy, 1998; Courtney, Ungerleider, Keil & Haxby, 1997). It is

debated however, whether the DLPFC is involved in the maintenance of information stored in WM or if it is in fact responsible for response sections within WM (Rowe, Toni & Josephs, 2000). In a study by Rowe at al. (2000) participants took part in a delay-response task to access WM whilst undergoing an fMRI. Participants were tasked to remember three spatial locations for different items for eighteen seconds. After this time they were asked to identify the correct location for one of the items based on memory. Results revealed that prefrontal areas were activated during the maintenance of information however, the DLPFC was only activated when participants had to select between different items during the WM task. Therefore Rowe et al. (2000) concluded that the DLPFC has a specific role in WM; specifically in the selection of representations. Similarly, Pochon et al. (2001) provided

supporting results, and also found that DLPFC activation was greater when a response was selected at the beginning of the delay interval.

Working memory is also important for infants and children as it prepares them for social experiences by allowing them to attend and discriminate between multiple sources of information (Hoskyn, 2010). Literature on the emergence of working memory in infants and young children is not well understood (Hoskyn, 2010), furthermore neuroimaging research comparing brain activation between adults and children is limited (Klingberg, 2006). In an EEG study conducted by Bell and Wolfe (2007), brain activation was measured in 53 8- month old infants whilst they performed a task requiring working memory (A-not-B task). EEG measurements were also recorded for 43 of these infants at 4 years of age whilst they performed another task of working memory (Day-Night Stroop task). The authors found that the 8-month old infants had widespread brain activation during the working memory task but at 4 years, brain activation became increasingly localised to the prefrontal cortex. Bell and Wolfe (2007) concluded that brain regions involved in working memory become more localised as the child develops. However, an fMRI study conducted by Kaldy and Sigala (2004) found that by 6.5 months, ventrolateral and dorsolateral brain regions are employed by infants in simple tasks of working memory. Furthermore, by the first year, EEG activity in these regions has also been found to increase during working memory tasks (Cowan, 1995). Despite the limited neuroimaging research on working memory and infancy, the preliminary studies suggest the recruitment of brain regions similar to those seen in adults (Hoskyn, 2010).

1.3.1.2. Decision Making and Reward Behaviours

The prefrontal cortex has been identified as having a distinct role within reward- guided behaviour and decision making (Fellows & Farah, 2005; Rushworth, et al., 2011). Studies have found that individuals with ventromedial damage suffer from decision-making impairments (Bechara, Dolan, Denburg, Hindes, Anderson & Nathan, 2001; Fellows & Farah, 2005; Waters-wood, Xiao, Denburg, Hernandez & Bechara, 2012). Individuals with damage to the ventromedial prefrontal cortex (VMPFC) are prone to impulsive decision-making in real life and in laboratory based situations (Bechara, Damasio, Damasio & Anderson, 1994; Fellows & Farah, 2005). The majority of studies exploring decision-making impairments have used the Iowa gambling task (IGT). This measure is deemed as sensitive in detecting decision making impairments. It simulates real life decisions as it factors in uncertainty, reward and punishments into decision-making. The IGT involves participants playing a card game, selecting cards from one of four decks with the aim of winning money. Two decks are

deemed as risky decks as they are associated with big wins but also big losses. In contrast, the remaining two decks are associated with smaller wins but also smaller losses. As the game develops, typical participants learn to opt for the less risky decks in order to secure a long term larger win. However, the opposite is found for participants with VMPFC damage (Bechara et al., 1994). Bechara and colleagues (1994) found participants with VMPFC damage regularly opted for the riskier decks. The authors believed that these patients were unable to see the future consequences of their actions and were only guided by the prospect of an immediate win within the task. In addition, Bechara, et al. (2001) explored the impact of substance abuse on decision-making abilities. Individuals who are deemed as substance

dependent are believed to have a dysfunctional VMPFC. In this study, forty control

participants, five individuals with ventromedial damage and forty-one substance dependants participated. Individuals in the substance dependent group met criteria for dependence with either alcohol or stimulants. All participants took part in the IGT. Results revealed that performance on the gambling task was predicted by several factors including years of

substance abuse and number of relapses whilst in recovery. The authors explained this finding as impairments in decision-making being linked with dysfunctions in the VMPFC.

Additionally, VMPFC impairments are believed to be irreversible (Waters-wood, et al., 2012). Water-woods et al. (2012) conducted a study with patients with VMPFC damage to determine whether impairments in decision-making were stable over time or whether recovery of function over time was possible. Patients with VMPFC damage and a control group of participants had a regular administration of the IGT over six years. The control group showed repeated improvement due to practice effects. In contrast, the VMPFC group’s performance on the task remained impaired without improvement. As a result, it is believed that VMPFC impairments in decision-making are not subject to autonomous recovery.