Research Interests

Overview
Adolescents are known to take risks that can impact their health and well-being. Prominent ‘dual-systems’ models of adolescent risk-taking propose that maturation of the mesolimbic dopamine system, which supports behavioral motivation and reward, temporally precedes maturation of prefrontal cortical regions supporting cognitive control. The resultant imbalance in these systems during adolescence contributes to ‘immature’ decision-making (relative to adults), and often risk taking, as the cognitive control (“braking”) system often cannot override reward-seeking impulses. It was from this broad framework that my research program first emerged and grew. My work uses behavioral measures and multi-modal functional neuroimaging to examine (1) the normative neurodevelopment of reward and control-related brain systems during adolescence and young adulthood, (2) how the interaction of these systems contribute to health-related behaviors and illness, including consummatory behavior and obesity, smoking cigarettes, and depression and anhedonia, and (3) how initially reward or goal-directed behaviors evolve into habits. Below, I highlight key findings and their importance from these primary lines of research.
 
(1) Normative Development of Reward and Control  Dual-systems models often focus on the top-down inhibition of reward drives, but little attention is paid to how rewards influence cognitive control. One major focus of my work is to characterize how reward motivation enhances control systems in the brain during adolescence, including inhibitory control and working memory (WM), key components of decision-making. This innovative approach suggests alternative mechanisms underlying risk taking (e.g., where salient rewards may drive decision processes as opposed to inhibitory processes failing to regulate impulses) and has implications for the promotion of positive or adaptive risk taking in youth (e.g., highlighting the rewarding value of trying out for a sports team). To this end, I developed a robust experimental paradigm (‘rewarded antisccade task’) in which participants earn rewards by correctly inhibiting a reflexive eye movement (saccade) to a peripherally presented stimulus (Geier et al., 2010; Padmanabhan et al 2011; Chung et al., 2011; Geier et al. 2012). Relative to adults, adolescents’ neural response to the initial indication of reward availability is at first attenuated, but later heightened during reward anticipation/response preparation (Geier et al., 2010). This finding helped clarify a major debate in the field at the time about whether adolescent reward systems are broadly characterized as over- vs. under-active relative to adults by adding a much-needed temporal perspective (i.e., it matters when during a trial reward processing is assessed). Follow-up studies controlling for the subjective value of rewards across ages and revealed key developmental differences in sensitivity to the magnitude of incentive contingencies (Geier & Luna, 2012). Of note, this work appeared in an Amicus Curiae brief read by the U.S. Supreme Court to inform them on brain maturation in a case involving sentencing juveniles to life in prison without the chance of parole (e.g., Miller vs. Alabama).

These early studies laid the groundwork for a series of longitudinal investigations that further clarified how rewards and cognitive control interact across adolescence (e.g., Paulsen et al., 2015; Hawes et al., 2017; Hallquist, Geier, & Luna, 2018). Across these studies, we demonstrated age-related changes in how striatal and amygdala circuitries support inhibitory control (Paulsen et al., 2015) and that the neural mechanisms (ventral striatum activation) supporting individual differences in sensation seeking change from early to late adolescence (Hawes et al., 2017). Hallquist, Geier, and Luna (2018) added a critical network perspective by using Independent Component Analysis (ICA) to clarify how developmental changes in task-evoked functional networks related to differences in behavioral performance across time.

Of note, dual systems models of adolescent neurodevelopment can be useful in describing adolescents in general, but may not adequately account for the neurobiology or behavior of any given adolescent. That is, the extent to which motivation and control systems are ‘imbalanced’ shows considerable variability across individuals, both within- and between-persons. However, this variability has not yet been well-characterized. Our recent work, in collaboration with Drs. Lisa Gatzke-Kopp and Nilam Ram, seeks to characterize within and between-subject variability in the extent to which rewards can both facilitate and hinder cognitive (inhibitory) performance in typically developing individuals as a function of age, pubertal maturation, and environmental context (Petrie, Fry, Roberts, Gatzke-Kopp, & Geier, 2019 - Flux presentation).
 
(2)  Reward and Control in Health and Illness   Throughout my career, I have deeply valued and practiced multidisciplinary and team approaches to solving scientific challenges. Over the past decade, I have extended my basic investigations of reward and control systems into other domains in collaboration with researchers across disciplines. In one such line of work, I examine how nicotine (smoking cigarettes) affects reward and inhibitory control systems. Chronic smoking has multiple effects in the brain, including altering reward processing. Theories of nicotine addiction posit that nicotine/smoking cues (e.g., ashtray) evoke a greater brain response than non-drug rewards (money), particularly during periods of abstinence. This imbalance leads to decisions biased towards drug seeking. Despite theoretical predictions, few empirical studies have actually shown these dual effects in smokers. We have examined this issue in several ways. First, abstinent adult smokers and non-smoker controls underwent fMRI while simultaneously performing a rewarded antisaccade (AS) task to characterize non-drug (money) reward processing deficits (Geier et al., 2014). Use of this task also enabled us to examine how altered reward responses may affect inhibitory control processing, an important consideration given that a central feature of smoking abstinence is inhibiting the strong urge to smoke. We demonstrated that activation in the ventral striatum is greatly reduced in abstinent smokers, as are widespread areas involved in inhibitory control, particularly during reward anticipation. This work complemented work, conducted in collaboration with Dr. Maggie Sweitzer and Dr. Eric Donny, showing both heightened responses to smoking (drug) cues and diminished responses to non-drug (monetary) rewards during smoking abstinence in the same individuals (Sweitzer et al., 2013). Moreover, we showed that attenuated brain reward responses during abstinence are predictive of smoking relapse after a real-world quit attempt (Sweitzer et al., 2016a, b). We have also demonstrated attenuated reward responses in abstinent smokers and subsequent effects of altered rewards on working memory (Geier et al., 2014; Lydon et al., 2015; Geier et al., 2018), as well as how sensation seeking, impulse control, and smoking covary during adolescence and young adulthood (Lydon & Geier, 2017). Collectively, these studies, along with a body of collaborative work with Dr. Stephen Wilson (PSU Psychology) (e.g., Lydon et al. 2015; Lydon et al., 2015), adds to our understanding of the neural mechanisms underlying addiction. Ultimately, this fundamental knowledge may help better tailor smoking cessation efforts (e.g., choosing a contingency management strategy only for those individuals who do not show reduced reward responding) and ease the burden of addiction.

Another population in which I study reward and control systems is typically developing children and adolescents of varying weight and BMI statuses, and in youth at-risk for developing obesity. In a series of studies conducted in collaboration with Dr. Kathleen Keller (PSU Nutrition), funded by an NIH R01, we used fMRI to evaluate how the developing brain responds to images indicating food, money, or neutral outcomes as well as how brain function in response to these stimuli may be related to the amount of food and calories consumed in test meals and lead to obesity (Adise et al., 2018; Adise et al., 2019; Keller et al., 2020; Pearce et al. 2020; Fuchs et al., 2021; Adise et al., 2021). As part of this work, I have mentored Nutrition and HDFS grad students on 5 grants supported by the USDA Childhood Obesity Training Prevention Program, resulting in 3 publications first-authored by students. Overall, this line of work adds to our understanding of the neural mechanisms of food cues and may guide prevention and intervention efforts to curb the development of obesity.       

Finally, examining reward and control system interaction is central for understanding the etiology of depression and its treatment. I work with and serve as a senior faculty mentor to Dr. Dahlia Mukherjee (PSU Psychiatry) on data collection and analysis of multiple neuroimaging studies aimed at characterizing the neural basis of anhedonia in individuals with treatment-resistant depression (e.g., conference presentations: Mukherjee, Marr, & Geier, 2022; Mukherjee, Geier, et al., 2019), with an eye towards identifying neurobiological targets for intervention. Notably, this work investigates the role of both the reward and prefrontal control systems across multiple time scales (e.g., within an imaging session, across sessions, and across weeks) and is currently funded by a grant from the PA Tobacco Settlement Fund.  
 
(3)  Neurodevelopment of Habits   Currently, I lead a series of studies which examine behavioral and neurodevelopmental changes in habit formation during adolescence and into adulthood, funded by the Dr. Frances Keesler Graham Early Career Professorship in Developmental Neuroscience. This line of work is a natural extension of prior research as habits gradually develop from goal-directed behavior repeated across time. Habitual behaviors constitute a large portion of one’s daily behavioral repertoire and, as such, play a key role in one’s overall health and well-being. However, the nature of habit formation across development, particularly during adolescence, has not yet been studied in humans, limiting our ability to intervene and positively impact habitual behavior. I am collecting task and resting state neuroimaging data from adolescents and young adults as they perform multiple validated habit formation tasks in the scanner over 3 consecutive days. As a means to capture the dynamics of habit formation, my lab, in collaboration with Drs. Sy-Miin Chow and Zack Fisher, apply advanced statistical methods (e.g., GIMME, time-varying vector autoregressive (VAR) models, etc.) to our fMRI data to characterize both within-person and group-level changes in brain effective connectivity, across time, among task-relevant regions of interest (Petrie et al., 2021, Fisher et al., under review; Park et al., under review).

Collaborators

Dr. Beatriz Luna
Dr. Kathleen Keller 
Dr. Suzy Scherf
Dr. Stephen Wilson
Dr. C. Daryl Cameron
Dr. Jose Soto
Dr. Robert Roeser 
Dr. Dahlia Mukherjee 
​Dr. Zack Fisher

... and many more!  

"Fundamentally, our work is focused on understanding the links between brain development and behavior during adolescence and young adulthood. Some examples of broad research questions we tackle in the lab include: How does neural development during adolescence contribute to risky decision making and behaviors? How does exposure to nicotine through smoking cigarettes affect adolescent and adult reward and cognitive control systems? How does goal-directed behavior evolve into habitual behavior during adolescence?" "How can we model within- and between-person variability in brain connectivity?"
​-- Dr. Charles Geier​