Visual discrimination and cognitive flexibility are two related cognitive processes to some extent. Visual discrimination is the ability to accurately identify and distinguish certain visual information, including character profiles of objects and spatial location among complex environments (Taylor and Rodriguez, 2024). Cognitive flexibility refers to the ability to adapt responses based on previously acquired visual stimuli (Turner et al., 2017). Both of the cognitive domains are crucial for the adaptation and survival of all species (Bryce and Howland, 2015). Specifically, good visual discrimination and cognitive flexibility are essential for appropriate decision-making and modification of behaviors in response to variable environments (Horner et al., 2013). However, both pre-clinical and clinical evidences reveal that impairment in visual discrimination and/or cognitive flexibility are present in various neurological and psychological disorders, such as Alzheimer's disease (Jiang et al., 2024; Rai et al., 2020; Van Den Broeck et al., 2019), HIV-associated neurocognitive disorder (Kesby et al., 2018), schizophrenia (Klaus et al., 2021; Liao et al., 2022; Løchen et al., 2023), substance addiction (Cox et al., 2016; Tanaka et al., 2023) and depression (Maramis et al., 2021; Sasaki et al., 2010; Tran et al., 2016). Hence, improving visual discrimination and cognitive flexibility helps enhance life quality and social recovery for patients with mental illness.

Recently, touchscreen-based visual discrimination and reversal learning tasks have emerged as common methods for evaluating visual discrimination and cognitive flexibility in rodents, both in healthy and disease conditions. This task contains two stages: the visual discrimination stage and the reversal learning stage. During the visual discrimination stage, rodents learn to establish association between a specific image and food reward. In the reversal learning stage, rodents must shift the previously reward image to a new non-reward image to receive the food reward. Compared to traditional open mazes (i.e., Y-maze and T-maze), the touchscreen-based platform offers the advantages as follows (Bussey et al., 2012). First, its automation enables the simultaneous testing of multiple animals. Second, it is non-aversive and less stressful for animals. Third, it is translational across species from rodents to humans, which efficiently predicts drug validity (Wallace et al., 2015).

Pharmacological treatment is the primary therapeutic strategy for improving cognition, known as cognitive enhancers. Among various cognitive enhancers, d-amphetamine was the first drug used for both pathological (Cherkasova et al., 2014; Pardo et al., 2022) and physical (Dolder et al., 2018; Pigeau et al., 1995) conditions in recent decades. However, the serious risk of abuse dramatically limits its clinical application (Cao et al., 2016). Recently, lisdexamfetamine, the prodrug of d-amphetamine, has gained considerable attention. Lisdexamfetamine is covalently synthesized by linking d-amphetamine to l-lysine (Hutson et al., 2014). After absorption into bloodstream, it is metabolized by red blood cells to yield d-amphetamine by rate-limited, enzymatic hydrolysis (Rowley et al., 2012). Due to the rate-limited step, lisdexamfetamine produces lower peak and significantly delays duration of free amphetamine base than that of d-amphetamine (Ermer et al., 2016). This unique in vivo pharmacokinetics (PK) profile offers lisdexamfetamine a lower risk of addiction compared to d-amphetamine in both rodents and humans (Heal et al., 2013; Kaland and Klein-Schwartz, 2015). Currently, lisdexamfetamine has been approved for the treatment of attention-deficit hyperactivity disorder (Childress et al., 2022; Frampton, 2016) and binge eating disorder (Heo and Duggan, 2017; Schneider et al., 2021). Furthermore, our previous researches demonstrated that lisdexamfetamine also improved sustained attention and working memory in rats under non-disease conditions, suggesting that it may be regarded a novel cognitive enhancer (Chen et al., 2024; Chen et al., 2022). Indeed, the manipulation of visual discrimination and cognitive flexibility requires several cognitive domains, including attention, working memory, and pattern separation (Horner et al., 2013). Therefore, based on our previous studies, we wondered whether lisdexamfetamine promotes visual discrimination and cognitive flexibility.

Therefore, the present study aimed to evaluate the effects of lisdexamfetamine (per os [p.o]) on visual discrimination and cognitive flexibility. In line with our previous results (Chen et al., 2024; Chen et al., 2022), we also explored the effects of d-amphetamine (intraperitoneal [i.p.]) to clarify the relationship between certain PK and cognitive performance. As N-methyl-D-aspartic acid receptors (NMDARs) plays a role in synaptic plasticity, learning and memory process (Wang and Peng, 2016), as well as visual discrimination and reversal learning (Radke et al., 2019). Hence, we performed Western Blotting to examine the distribution of NMDARs in the medial prefrontal cortex (mPFC) and hippocampus, aiming to uncover the molecular mechanisms behind lisdexamfetamine's pharmacological action. The current research has offered the potential benefits of lisdexamfetamine in enhancing visual discrimination and cognitive flexibility, which could help address certain cognitive deficits in neuropsychiatric disorders.