Academic journal article Cognitive, Affective and Behavioral Neuroscience

Striatal Outcome Processing in Healthy Aging

Academic journal article Cognitive, Affective and Behavioral Neuroscience

Striatal Outcome Processing in Healthy Aging

Article excerpt

Functional MRI of young adults has implicated the striatum in the processing of rewarding and punishing events. To date, only two published experiments (Samanez-Larkin et al., 2007; Schott et al., 2007) have explored similar phenomena in older adults, with both studies emphasizing the anticipation of monetary outcomes. To better understand older participants' striatal responses to delivered outcomes, we engaged 20 older adults and 13 younger adults in a card-guessing task that rewarded correct guesses with monetary gain and punished incorrect guesses with monetary loss. Overall, the older adults retained most of the typical features of the striatal response, so that activity in the caudate head showed reliable differentiation between rewards and punishments during the 6- to 9-sec postoutcome window. Comparison of the older and younger adults also pointed to some potential aging effects on outcome activity, including reductions in the magnitude and extent of striatal activation, and a trend for the older adults to show a decreased early punishment response. In sum, our data suggest that the signaling of outcome valence remains relatively stable into late adulthood, although more research is needed to understand some subtle changes that might occur across the life span.

Evidence from animals and humans has established the striatum as a key structure for processing motivationally relevant events. In rodents and primates, both appetitive and aversive stimuli modulate firing patterns in the striatum (Ravel, Legallet, & Apicella, 2003; Roitman, Wheeler, & Carelli, 2005; Schultz, Tremblay, & Hollerman, 2000; Williams & Eskandar, 2006) and in the dopamine-producing neurons that project to it (e.g., Hollerman & Schultz, 1998; Ungless, Magill, & Bolam, 2004). Likewise, humans in functional MRI experiments have shown striatal activation to a range of motivating outcomes (e.g., Beras, McClure, Pagnoni, & Montague, 2001, for juice; Delgado, Nystrom, Fissell, Noll, & Fiez, 2000, for money), as well as to outcome-predictive cues (e.g., Knutson, Westdorp, Kaiser, & Hommer, 2000; Wittmann et al., 2005). From a computational standpoint, these responses are often described as signaling an error in reward prediction (e.g., O'Doherty et al., 2004), so that the greatest striatal activity is driven by the least predictable cues and outcomes. This predictiondriven response pattern is assumed to reflect the involvement of the striatum in reinforcement-based learning, an assumption that is supported by reports of altered learning behavior when striatal dopamine is manipulated (e.g., Pessiglione, Seymour, Flandin, Dolan, & Frith, 2006).

Striatal reward- and punishment-signaling has been well documented in young adults, but the anatomical and behavioral correlates of aging call into question the stability of these phenomena across the life span. At the neuroanatomical level, declines in older adults' striatal volume (Raz, Torres, & Acker, 1995) and in dopamine Dl and D2 receptor availability (Volkow et al., 1998; Wang et al., 1998) suggest that the learning and motivational functions of the striatum may be affected as well. Similarly, at the behavioral level, altered striatal function may help explain why older adults have shown learning deficits on tasks that require the processing of positive and negative outcomes (e.g., Denburg, Recknor, Bechara, & Tranel, 2006; Marschner et al., 2005; MeIl et al., 2005; Weiler, Bellebaum, & Daum, 2008). Thus, there are compelling reasons to expect that reward and punishment processing might be disrupted in the aging striatum. Efforts to address this question with neuroimaging have just begun to emerge.

To date, two fMRI experiments have focused on reward and punishment activity in older adults (Samanez-Larkin et al., 2007; Schott et al., 2007); in each case, the researchers used customized versions of the monetary incentive delay (MID) task (Knutson et al. …

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