• Halle Quang, Campbell Le Heron, Bernard Balleine, Tuong-Vu Nguyen, Truc-Quynh Nguyen, My-Ngan Nguyen, Fiona Kumfor, and Skye McDonald.
• University of New South Wales, School of Psychology, Sydney, NSW 2052, Australia. Electronic address: halle.quang@unsw.edu.au.
• Neuroscience. 2023 Sep 15; 528: 263626-36.

BackgroundAltered reward processing is increasingly recognised as a crucial mechanism underpinning apathy in many brain disorders. However despite its clinical relevance, little is known about the mechanisms of apathy following moderate-to-severe traumatic brain injury (TBI). In real-life situations, reward representations encompass both foreground (gains from current activity) and background (potential gains from the broader environment) elements. This latter variable provides a crucial set-point for switching behaviour in many naturalistic settings. We hypothesised apathy post-TBI would be associated with disrupted background reward sensitivity.MethodsWe administered a computer-based foraging task to 45 people with moderate-to-severe TBI (20 with apathy, 39 males) and 37 matched controls. Participants decided when to leave locations (patches) where foreground reward rates depleted at differing rates, to pursue greater rewards from other patches in the environment, which had either a high or low background reward rate. Primary analysis was performed using linear mixed effects models, with patch leaving time the dependent variable.ResultsFindings showed a significant interaction between apathy and background reward sensitivity, driven by apathetic TBI participants not altering patch-leaving decisions as environmental reward rate changed. In contrast, although TBI was associated with reduced sensitivity to changing foreground rewards, this did not vary as a function of apathy.ConclusionsThese results provide the first evidence directly linking disrupted background reward processing to apathy in any brain disorder. They identify a novel mechanism for apathy following moderate-to-severe TBI, and point towards novel interventions to improve this debilitating complication of head injury.Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.

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