Prefrontal activation and pupil dilation during n-back task performance: A combined fNIRS and pupillometry study

Highlights

• There were few changes in PFC activity and pupil size during the 0-back task.

• There were large changes in PFC activity and pupil size during the 3-back task.

• Changes in PFC activity and pupil size positively correlated during the 0-back task.

• Changes in PFC activity and pupil size did not correlate during the 3-back task.

• The link between changes in lateral PFC activity and pupil size is load dependent.

Abstract

The n-back task is one of the most commonly used working memory (WM) paradigms in cognitive neuroscience. Converging evidence suggests activation in the lateral prefrontal cortex (PFC) and pupil dilation [a proxy for locus coeruleus (LC) activation] during this task. However, it remains unclear whether the lateral PFC and the LC are functionally associated during n-back task performance. This study's aim was to examine the relationship between changes in lateral PFC activity and the pupil diameter and to evaluate the effect of WM load on such relationship during the n-back task. Thirty-nine healthy young adults (10 males, 29 females) underwent a number n-back paradigm with 0- and 3-back conditions. Their prefrontal hemodynamics and changes in pupil size during task performance were simultaneously measured using a 16-channel functional near-infrared spectroscopy (fNIRS) device and a wearable eye tracker. Young adults exhibited significant activation in the bilateral lateral PFC and significant increases in pupil size when the WM load was high (i.e., 3-back) but not low (i.e., 0-back) compared with the resting period. Interestingly, significant positive correlations were found between changes in lateral PFC activity and pupil size during the 0-back task only. These correlations tended to be stronger during the 0-back than the 3-back condition. Thus, the functional relationship between the lateral PFC and the LC may vary at different load levels during the n-back task. Our findings have important implications for neuropsychiatric research and support concurrent fNIRS and pupillometric measurements for a better understanding of the mechanisms underlying WM processing.

Introduction

Human short-term memory has a limited capacity (Baddeley, 2012; Miller, 1956). Therefore, the ability to replace old, irrelevant information with information that is relevant to the current context is a core aspect of executive function essential for the pursuit of goals (Miyake et al., 2000). In cognitive neuroscience, the n-back task has been one of the most widely used working memory (WM) paradigms (Kirchner, 1958). Studies using this task have shown that accuracy and reaction time (RT) worsen with increasing WM load across ages and materials (Boisgueheneuc et al., 2006; Bopp and Verhaeghen, 2020). Additionally, meta-analyses of the positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) literature have identified activity in the bilateral dorsolateral and ventrolateral prefrontal cortex (PFC), premotor cortex, supplementary motor area, and parietal cortex that increases with increasing WM load during the n-back task (Mencarelli et al., 2019; Owen et al., 2005; Rottschy et al., 2012; Yaple et al., 2019). Studies using functional near-infrared spectroscopy (fNIRS) have also generated converging evidence of the involvement of the bilateral lateral PFC during this task (Ito et al., 2011; Koike et al., 2013; Kopf et al., 2013; Yeung et al., 2016, 2019). It is generally assumed that activation in various PFC subregions during the n-back task reflects the active operation of the cognitive processes associated with the executive control of WM, such as the storage and manipulation of internal representations, monitoring incoming information, the implementation of stimulus response mapping, and the use of organizational strategies (Mencarelli et al., 2019; Owen et al., 2005).

Accumulated evidence indicates that the pupil size of the eye increases in response to an increasing cognitive load during a variety of tasks (Beatty, 1982; Kahneman and Beatty, 1966; van der Wel and van Steenbergen, 2018). In the context of the n-back task, the pupil size increases with an increasing WM load (Belayachi et al., 2015; Karatekin et al., 2007; Niezgoda et al., 2015; Scharinger et al., 2015). It is well known that the pupil size is influenced by the sympathetic and parasympathetic branches of the autonomic nervous system, which is modulated by the locus-coeruleus (LC) norepinephrine system (Joshi et al., 2016; McDougal and Gamlin, 2011; Murphy et al., 2014; Samuels and Szabadi, 2008). The LC has been implicated in the regulation of arousal and vigilance (Sara and Bouret, 2012). Therefore, the task-evoked pupillary response has been and can be employed as a proxy measure for arousal and mental effort (i.e., LC activity; Bradley et al., 2008; Mathôt, 2018). In the present study, arousal refers to the degree of vigilance and alertness during periods of wakefulness, manifesting as motor activation, responsiveness to sensory inputs, emotional reactivity, and enhanced cognitive processing (Carter et al., 2013). Arousal is expected to occur during the n-back task because individuals have to detect incoming sensory stimuli, operate on the information, and generate an efferent response in a timely manner.

The LC extensively innervates the cerebral cortex of all hemispheric lobes, including the dorsolateral and the dorsomedial PFC (Arnsten and Goldman-Rakic, 1984; Aston-Jones and Cohen, 2005). Neurons in this small pontine nucleus are the primary source of cortical norepinephrine (Samuels and Szabadi, 2008). Thus, the LC is implicated in the regulation of arousal and mental effort across many tasks, playing a general, modulatory role in neocortical functions. In contrast, the lateral PFC is implicated in executive control and goal-directed behavior (Petrides, 2005). This region represents a primary site of WM and plays a crucial role in n-back task performance (Barbey et al., 2013; Levy and Goldman-Rakic, 2000; Tsuchida and Fellows, 2009). Although lateral PFC activation and pupil dilation have been separately observed during n-back task performance, whether they are functionally related to each other remains elusive. In addition, the effect of WM demand on the relationship between activity in the lateral PFC and the LC during the n-back task, if any, remains unclear. A high WM load may enhance the functional integration of the two systems due to the increased general, modulatory effects exerted by the LC system on the cortex. Alternatively, it may reduce the functional coupling between the lateral PFC and the LC, or remove the co-dependence of the two systems, due to the specialized role of the lateral PFC in WM and to a LC or norepinephrine-induced increase in local neural communications within network during task performance (Eldar et al., 2013).

The aim of this study was to examine the relationship between changes in lateral PFC activity and the pupil diameter (a proxy for LC activity) during n-back task performance. In addition, the effect of WM load on such relationship was investigated by evaluating the correlation between the moment-to-moment changes in lateral PFC activity and the pupil size at two WM load levels (i.e., 0- and 3-back conditions). A significant positive correlation between the observed [oxy-Hb] changes and the [oxy-Hb] changes predicted from the pupil signal would suggest that lateral PFC activity and pupil dilation were subtended by common mechanisms. In contrast, a nonsignificant zero correlation between the observed and the predicted [oxy-Hb] changes would indicate that the two proxies might be underpinned by distinct mechanisms, and more evidence is needed to elucidate the relationship. Clarifying the functional relationship between the lateral PFC and the LC during the n-back task would give us insights into the meaning of lateral PFC activity (e.g., lateral PFC activity correlating with arousal level) during this task.

There is growing recognition that fNIRS is relatively tolerant to motion and is free from environmental interference (Ferrari and Quaresima, 2012). Thus, fNIRS is a suitable tool to use in conjunction with eye trackers, and it is an excellent tool for monitoring brain activity in a natural setting (Hosseini et al., 2017; İşbilir et al., 2019). For these reasons, we used fNIRS to probe prefrontal hemodynamic changes. Due to the use of a blocked design and fast trial presentation to increase power, the distinction between a shift in tonic and phase LC activity (Aston-Jones and Cohen, 2005) in response to increased WM load would not and could not be made in the present study.