Hemodynamic and Behavioural Changes in Older Adults During Cognitively Demanding Dual-Tasks


 Background: Executive functions play a fundamental role in walking by integrating information from cognitive-motor pathways. Subtle changes in brain activation and behaviour may help identify older adults who are more susceptible to executive function deficits with advancing age due to prefrontal cortex deterioration. This study aims to examine how older adults mitigate executive demands while walking during cognitively demanding tasks.Methods: Twenty healthy older adults (M = 71.8 years, SD = 6.4) performed simple reaction time (SRT), go/no-go (GNG), n-back (NBK) and double number sequence (DNS) cognitive tasks of increasing difficulty while walking (i.e., dual-task). Functional near-infrared spectroscopy (fNIRS) was used to measure the hemodynamic response (i.e., oxy- [HbO2] and deoxyhemoglobin [HbR]) changes in the prefrontal cortex (PFC) during dual- and single-tasks (i.e., walking alone). In addition, performance was measured using gait speed (m/s), response time (s) and accuracy (% correct). Results: Using repeated measures ANOVAs, neural findings demonstrated a main effect of task such that ∆HbO2 (p = 0.047) and ∆HbR (p = 0.040) decreased between single- and dual-tasks. An interaction between task and cognitive difficulty (p = 0.014) revealed that gait speed decreased in the DNS between single- and dual-tasks. A main effect of task in response time indicated that the SRT response time was faster than all other difficulty levels (p < 0.001). Accuracy performance declined between single- and dual-tasks (p = 0.028) and across difficulty levels (p < 0.001) but were not significantly different between the NBK and DNS.Conclusion: Findings suggest that a healthy older adult sample might mitigate executive demands using an automatic locomotor control strategy such that shifting conscious attention away from walking during the dual-tasks resulted in decreased ∆HbO2 and ∆HbR. However, decreased prefrontal activation was inefficient at maintaining response time and accuracy performance and may be differently affected by increasing cognitive demands.


Background
Declines in cognition are more common as people age and have been supported by studies examining changes in brain activation between older and younger adults [1][2][3]. Neuroimaging ndings suggest that compensatory neural mechanisms exist to counteract decline and to allow for the maintenance of cognition over time [4,5]. One example is the revised Scaffolding Theory of Aging and Cognition (STAC-r) which outlines compensatory scaffolding as an adaptive measure for older adults to generate and recruit additional neural resources to replace those that have deteriorated over time [5]. This theory can account for greater brain activation in older versus younger adults when behavioural measures are similar between both groups [4].
Behavioural measures of performance such as gait speed have also been used to evaluate cognition [6].
Early research has demonstrated that some older adults are unable to walk and talk at the same time and those that stopped walking to talk were more prone to falling [7]. While walking alone did not lead to any gait changes, slowing down or stopping may be an involuntary strategy exhibited by older adults to prioritize gait and ensure safe ambulation [8,9]. Alternatively, higher functioning and cognitively healthy older adults may resemble younger adults in that they exhibit an automatic locomotor control strategy to manage walking and talking simultaneously (i.e., dual-tasking) [10]. Automatic control is e cient in that steady state walking can be achieved under minimal conscious attention thereby freeing up executive resources for a secondary task [11,12]. However, studies have demonstrated that greater task di culty may lead to a loss of automaticity and greater reliance on the prefrontal cortex (PFC) due to the attentional demands associated with maintaining gait performance [11,13]. This is known as the executive control of walking, which operates under a limited processing capacity, but may be recruited when dual-tasks require greater executive resources [14,15].
The PFC is responsible for mediating complex cognitive processes namely planning, attention and coordination which are involved in everyday tasks such as walking or dual-tasking [4]. In fact, the dualtask paradigm measures changes in executive functioning by comparing brain activation and performance between single-and dual-tasks [16]. Reviews in the literature demonstrate inconsistent ndings as to whether prefrontal activation and behaviour should increase, decrease or stay the same between single-and dual-tasks [17,18]. This may be due to diverse cognitive tasks such as verbal uency [13,19,20] and counting backwards [6,21] which differentially engage executive functions and the PFC.
Therefore, it may important to account for differences in cognitive task di culty between studies [22]. One approach to mitigate this concern is a study design that targets the examination of executive functioning across multiple task di culties. This may also allow for the identi cation of easier cognitive tasks that are not sensitive enough or do not challenge older adults su ciently to detect changes in single-versus dual-tasks. More speci cally, this may reveal whether executive control is only evoked under greater cognitive demands and whether STAC-r compensatory mechanisms are e cient enough to preserve performance.
In order to simultaneously examine the neural and behavioural mechanisms underlying executive functioning, functional near-infrared spectroscopy (fNIRS) can be used to monitor cerebral oxygenation (∆HbO2) and deoxygenation (∆HbR) changes in the PFC. FNIRS is advantageous over other functional neuroimaging techniques most notably for its non-invasive and portable nature that doesn't limit an individual's mobility [23]. In its application to walking, it tolerates motion artifacts better than other techniques and can be used on people of all ages with no adverse health consequences [23]. FNIRS exploits the transient nature of biological tissue to near-infrared light as well as the distinct absorption spectra of oxygenated (HbO2) and deoxygenated (HbR) hemoglobin in the near-infrared region [24]. In theory, the PFC requires an in ux of HbO2 and e ux of HbR as cognitive demands increase. Therefore, during dual-tasks, the increased cerebral blood ow and metabolic demand of oxygen can be coupled in a process known as neurovascular coupling [24]. This process can then serve as a neurophysiological marker for fNIRS to detect changes in cerebral oxygenation during dual-task walking studies [25,26].
Furthermore, various behavioural measures can be used to quantify the shift from performance maintenance to decline. Firstly, gait speed is a commonly used measure to assess locomotor control [15,27,28]. Studies have demonstrated a strong relationship between poor executive functioning and slower gait speed especially during dual-tasks involving a challenging locomotor component [19,21,29]. This is in line with the executive processing of gait which is recruited when tasks are unlearned or too challenging to be automatically processed [11]. Other behavioural measures such as response time and accuracy have been reported in the literature but with greater variability across different task types and di culty levels. For example, by using a cognitive-auditory response time task, Rosso et al., [30] found slower response times in dual-compared to single-tasks but no differences in accuracy. In contrast, studies examining neural inhibition and working memory have demonstrated that performance declines in older adults in dual-compared to single-tasks [1,31]. This may be due to the complex processing steps involved in discerning relevant from irrelevant stimuli during an inhibition task and temporarily storing and manipulating information during a working memory task both of which are particularly challenging for older adults [31,32]. As such, the present study is unique in that it will evaluate various executive processing domains by manipulating cognitive demands according to an easy processing speed task, a medium level neural inhibition task and two di cult working memory tasks.
The purpose of this study was to examine how older adults mitigate the demands of dual-tasking through changes in brain activation and behaviour. The rst aim was to determine the changes in cerebral oxygenation (∆HbO2 and ∆HbR) using fNIRS and performance (gait speed, cognitive response time and accuracy) in single-versus dual-tasks and across four levels of cognitive task di culty. Greater cerebral oxygenation changes were expected during the dual-tasks in comparison to single-tasks and these changes were expected to increase with each successive di culty level. Performance was expected to decrease between single-and dual-tasks with the most signi cant change occurring during the working memory tasks. The second aim was to correlate cerebral oxygenation and behaviour to determine whether increased brain activation would be associated with poorer performance during the dual-tasks. Understanding neural and behavioural changes in healthy older adults may help reveal whether declines are only associated with speci c executive function domains.

Participants
Twenty healthy older adults (M = 71.8 years, SD = 6.4 years, 10 females) were recruited from community centres across Ottawa, Canada. Participant eligibility was determined using a phone screening (Table 1) whereby participants were included if they were right-handed according to the Edinburgh Handedness Inventory [33] and did not have a diagnosed hearing impairment or hearing aid. Participants also had to be comfortable walking 15 meters without assistance and without neuromuscular or physical complaints that could affect walking (i.e., severe arthritis). Cognitive status was determined using the Montreal Cognitive Assessment (MoCA) where participants were required to score ≥ 26 to ensure that they were cognitively healthy [34]. This study was ethically approved by the University of Ottawa Research Ethics Board and all participants provided written informed consent before participating in the study.

FNIRS equipment
Participants were tted with a wearable OctaMon fNIRS device (Artinis, The Netherlands) to measure prefrontal ΔHbO2 and ΔHbR. The distance between the nasion and inion was measured for each participant to ensure the fNIRS device was placed along the PFC according to the modi ed International EEG 10-20 system [35]. The OctaMon uses continuous wave near-infrared spectroscopy, which measures near-infrared light absorption at two distinct wavelengths (760 and 850 nm). This device also uses eight light emitting diode (LED) channels and two detectors with an interoptode distance of 35 mm ( Fig. 1).

Experimental protocol
Participants were presented with four runs in a randomized order each evaluating one of four levels of cognitive demands. A run was comprised of 12 counterbalanced blocks with an equal number of single cognitive (SC), single motor (SM) and dual-tasks (DT) blocks (Fig. 2). In the SC condition, participants performed the cognitive task while standing and staring straight ahead at a target. The SM block had participants walk without a cognitive task at their self-selected pace along a 10 m walkway. During the DT condition, participants were asked to perform both the cognitive and motor task simultaneously and were instructed to pay equal attention to both tasks. To gain a better understanding of the subjective emphasis dedicated to the dual-tasks, participants were asked to report how much attention (out of a possible 100%) they attributed to the cognitive and motor task following the DT blocks. Each 33 s block was preceded by a 10 s baseline of quiet standing and was followed by a 15 s rest period to allow the hemodynamic response to revert to the baseline in between blocks. Throughout the experiment, participants were given breaks as needed and upon request.

Cognitive task di culty levels
E-Prime software (version 2.0) was used to create different cognitive task sequences. The experimenter delivered all instructions to the participants using a microphone which could be heard through wireless headphones worn by the participant. Four cognitive-auditory tasks: simple reaction time (SRT), go/no-go (GNG), n-back (NBK) and double number sequence (DNS), were chosen from previous work in our labs, to represent processing speed (SRT), neural inhibition (GNG) and working memory tasks (NBK and DNS) [1,36]. During a short practice session, participants familiarized themselves with the cognitive tasks until they were able to correctly respond to 70% of the SC stimuli. The SRT task represented the simplest cognitive demand and had participants respond to a random sequence of beeps (2850 Hz at 99 dB) by saying the word "top" as quickly as possible following each stimulus. GNG was the medium level task and had participants listen to both high-(2850 Hz at 99 dB) and low-pitched (970 Hz at 95 dB) beeps but only respond "top" to the high-pitched beeps. The next level task was the NBK and had participants listen to a continuous sequence of single-digit numbers (1-9) and respond with the number they heard two numbers back. Lastly, the DNS task represented the highest cognitive demand and had participants listen to a sequence of three-digit numbers. At the end of the block, they reported the total number of times they heard two target digits within the entire sequence [37]. Two working memory tasks (NBK and DNS) were chosen because working memory is the executive domain known to be most affected by cognitive aging [32].

Behavioural measures
Three behavioural measures were chosen to evaluate performance differences between single and dualtasks as well as across cognitive task di culty. The rst measure, gait speed (m/s), was calculated by dividing the distance the participants walked by the xed duration of the block. Response times (s) were recorded using a voice recorder and imported into Audacity (version 2.3.1) to measure the time from stimulus onset until the participant's response. Response times were recorded during the SRT, GNG and NBK di culty levels. No response time was measured during the DNS condition because it is a non-verbal working memory task that has participants withhold their response until the end of the block. Finally, experimenters calculated accuracy scores (% correct) for correct responses to the cognitive tasks. In the SRT di culty level, correct responses were recorded when the participant responded to a beep by saying the word "top" while incorrect responses were noted when the participant did not respond to a beep. Correct responses in the GNG condition were calculated when the participant correctly responded to the high-rather than the low-pitched beep. Errors were noted when participants either missed the high-beep or responded to the low beep. During the NBK, correct responses involved participants correctly responding with the number they heard two numbers back. Errors were given when participants responded with the incorrect number or did not respond at all. Finally, correct responses in the DNS were calculated based on the participant's nal tally of each target digit compared to the total possible correct responses.

Test battery
Following the experiment, participants were asked to complete a battery of neuropsychological and physical tests. The purpose of these tests was to ensure good cognitive and physical function, low fear of falling and no depression which may in uence study outcomes. The neuropsychological tests included the Montreal Cognitive Assessment (MoCA) [34], Digit Forward and Backward [39], Digit Symbol Substitution Test [39] and Trail Making Test (TMT) Part A and B [40]. The MoCA is a screening tool used to assess cognitive impairment. Individuals who score ≥ 26 out of 30 re ect healthy cognition [34]. Digit Forward and Backward are used to assess working memory and points were awarded for correctly repeating a growing list of numbers in either the forward or reverse direction. The Digit Symbol Substitution Test measures processing speed as individuals ll-in as many symbols as possible within 90 s based on a key provided at the top of the worksheet. The Trail Making Tests are timed tests (s) used to measure task switching and executive functioning. It is divided into two parts whereby Part A has participants draw lines connecting 25 ascending numbers while Part B has participants draw lines alternating between ascending numbers and letters. A shorter time to complete these tests indicates better performance. Furthermore, physical status and fear of falling were assessed using the Short Physical Performance Battery (SPPB) and the Falls E cacy Scale-International (FES-I), respectively. The SPPB measures lower extremity functioning in older adults and is scored out of 12, where 12 is equivalent to no de cits in functioning [41]. FES-I uses a 4-point Likert scale to assess an individual's fear of falling [42]. It is scored out of 64 whereby a higher score indicates a greater fear of falling. The Geriatric Depression Scale was also used to assess depression in older adults as it is known to have effects on the PFC [43]. It is scored out of 30 and a lower score within the range of 0-9 indicates no depression.

Data processing of fNIRS signal
Neural data was collected in Oxysoft (version 3.0.97.1) and sampled at a frequency of 10 Hz. After visually inspecting the signal quality, the Modi ed Beer-Lambert law was applied to the raw HbO2 and HbR intensities using a differential pathlength factor set to 6.61 for all older adults [44]. The concentrations were then preprocessed o ine using a custom MATLAB (version R2018a) script. The script eliminated motion artifacts by removing outliers that were 2.5 SD from the mean and replaced them with a zero value. Additionally, in line with the literature, a Butterworth bandpass lter set between 0.01-0.14 Hz was used to reduce physiological noise (heartbeat and breathing) within the signal [3,20,21]. An average ∆HbO2 and ∆HbR value were then calculated in µM for each task (SC, SM, DT) and each di culty level (SRT, GNG, NBK, DNS) from the changes in signal between the baseline and active conditions.
Assessments of behavioural response time were tested with a 2 × 3 repeated measures ANOVA to measure the interaction between task (SC, DT) and di culty (SRT, GNG, NBK). Note that the DNS task had participants respond at the end of the block, therefore, no response time was calculated. Signi cant differences in gait speed and accuracy were evaluated with 2 × 4 repeated measures ANOVAs to measure the interaction between task (SC/SM vs. DT) and di culty (SRT, GNG, NBK, DNS).
A one-way ANOVA was conducted on the subjective emphasis responses to test whether there were signi cant differences between how much attention the participants dedicated to walking versus the cognitive tasks across each di culty level (SRT, GNG, NBK, DNS).
For all repeated measures ANOVAs, if Mauchly's Test of Sphericity was violated, a Greenhouse-Geisser pvalue was reported. In addition, Bonferroni post-hoc analysis was used to determine the location of signi cance where statistical signi cance was set at p < 0.05. Means (M) and standard deviations (SD) are reported in the results and when a distinction between di culty levels is needed, the di culty level is identi ed in subscript (i.e., M SRT = Mean value for SRT di culty level). Means and standard deviations were calculated for all participant demographics and neuropsychological assessments.
No signi cant differences were observed in terms of cerebral oxygenation between channels or hemispheres (p-values > 0.05). Therefore, brain activation was analyzed across the whole PFC by averaging the concentration output from each channel. In addition, we veri ed if there were signi cant changes in cerebral oxygenation within task (e.g., the four SM blocks in SRT) and there were no signi cant differences (p-values > 0.90). As such, an average of each task type was calculated for analyses.
A Pearson correlation was used to examine the relationship between cerebral oxygenation (ΔHbO2 and ΔHbR) and performance (gait speed, response time and accuracy) during the dual-tasks.

Discussion
The current study applied fNIRS imaging to assess whether older adults demonstrated changes in prefrontal cerebral oxygenation and behaviour while walking with cognitive tasks of increasing di culty. The aims of this study were two-fold. Firstly, to analyze neural and behavioural measures to better understand neural compensation mechanisms during dual-tasks of different di culty levels. Secondly, to determine whether there was a correlation between neural and behavioural outcomes such that increases PFC activation may be associated with better performance, or vice versa, in older adults. In doing so, this may reveal how older adults mitigate their attention capacity through prefrontal executive involvement or adopt compensatory neural strategies to meet the demands of di cult dual-tasks.

Neural
According to our initial hypothesis, ∆HbO2 was expected to increase from single-to dual-tasks based on the principles of STAC-r [5]. This prediction was based on the neuroimaging literature which suggests that older adults exhibit more widespread and bilateral activation in the PFC during dual-versus single-tasks and, therefore, greater dependency on executive control compared to younger adults [1,3]. Contrary to this expectation, this study demonstrated a signi cant decrease in ∆HbO2 and ∆HbR between walking alone (i.e., single-task) and walking with a cognitive task (i.e., dual-task). These ndings are in line with several reports that observed a decrease of prefrontal cerebral oxygenation and an alternative strategy to mitigate the demands of dual-task walking [18,45]. One possibility is an automatic locomotor control strategy which would be bene cial in dual-task situations to minimize interference with other controlled processes [11,46]. The PFC's contributions to walking include managing the attentional demands and motor planning associated with safe and e cient displacement [11,15]. However, executive resources are limited and may be reorganized depending on task demands. Studies have shown that decreased PFC activation is associated with automatically controlled tasks and walking, in particular, is amenable to automation because it is well learned [47,48]. Therefore, increased prefrontal activation may only be observed in individuals who show a loss of automaticity such as in people with neurological disorders or frail older adults [8,14,29,49]. Based on the data presented in Table 2, the older adults in this study demonstrated high scores in cognitive function, walk speed (i.e., > 1 m/s) and no frailty, amongst other factors, which are typically associated with decreased executive functioning. These measures suggest that our participant group was high functioning and could rely on an automatic locomotor strategy to free up cognitive resources in the PFC. Participants were also asked to subjectively rate how much attention they paid towards the cognitive versus walking task. Their responses re ected an automatic control strategy in that they reported focusing < 39% on walking during all the cognitive tasks. The cognitive tasks may have also served as an external focus which has been known to facilitate automatic processing [10,12]. This has been outlined in the "constrained action hypothesis" which suggest that focusing on the outcome of a movement (i.e., external focus), rather than the movement itself (i.e., internal focus), minimizes interference with other consciously controlled tasks [50,51]. Similarly, diverting attention away from a postural task (i.e., to a cognitive task) even when cognitive demands are low may provide an external focus to improve motor performance [52]. As such, compared to walking alone, responding to the various stimuli during the dualtasks may have helped draw attention away from walking and allowed for greater stability without greater recruitment of the PFC. Conversely, in the absence of a cognitive task, attention could be drawn to both internal and external sources thereby engaging greater executive control.
Healthy individuals inherently shift between automatic and executive control strategies to mitigate cognitive demands [11,15]. However, studies have also demonstrated age-related decreases in cerebral blood ow (CBF) to the PFC due to changes in brain structure [53]. The reorganization of locomotor control pathways and a reduction of CBF with age may, therefore, contribute to an overall reduced availability of prefrontal oxygenation. Dietrich's [54] theory of hypofrontality suggests that there is a redistribution of metabolic resources from prefrontal brain regions to motor regions during tasks such as walking due to the complex integration of sensory, motor and autonomic processes. In other words, the brain is limited by a nite supply of metabolic resources that must be strategically allocated based on the most critical demands [54]. Taken together with automaticity, hypofrontality may cause a downregulation of metabolic resources in the PFC which can be redistributed to other brain regions to supplement motor control. Regions outside the PFC could not be measured within the scope of this study, however, studies have shown heightened brain activation in motor areas such as the premotor [55] and supplemental motor area [55][56][57] during dual-task walking. These brain regions should be further examined simultaneously with the PFC to determine whether a decrease in prefrontal cerebral oxygenation from single-to dual-task corresponds with changes in motor regions when walking more automatically.
We must also acknowledge certain study parameters including the (i) cognitive and (ii) motor tasks that differentiate this study from others in the literature. (i) Cognitive tasks: Verbal uency [13,19,20] and counting backwards [21,25] are the most commonly used tasks in dual-task studies that demonstrate increased or no change in cerebral oxygenation between single-and dual-tasks [18]. Our study used processing speed, neural inhibition and working memory tasks which continuously prompted responses and engaged participants based on a random sequence of stimuli. This differs from verbal uency and counting tasks in that participants were not provided with a starting cue (i.e., a letter or number) after which they could respond at their own pace. The external focus of the cognitive tasks and unpredictable pattern of stimuli may have helped recruit automatic control pathways by ensuring that the full duration of the task was attention-demanding [10,58]. (ii) Motor task: Walking trajectories vary signi cantly across studies due to equipment and space constraints. As evidenced by studies examining obstacle negotiation, the interruption of steady state walking caused increased PFC activation and may equally impede automaticity [8,19,29]. Our study provided participants with a 10 m pathway to maximize straight-line walking which is considerably longer than studies examining gait along electronic walkways [3,20,21,59]. Therefore, our walking task provided longer stretches of steady state walking and a greater opportunity to automatize gait than studies using shorter walkways.
Lastly, in addition to the ΔHbO2 decrease, there was also a decrease in ΔHbR between the single-and dual-tasks. ΔHbR is a reliable measure of neural activation but is less commonly reported in the literature.
This is due to its low signal amplitude making signi cant changes between baseline and task conditions more di cult to obtain [60]. The low signal amplitude also means that HbR is less likely to be contaminated with physiological artifacts and also results in a lower signal to noise ratio [60]. As such, capturing a signi cant HbR change that mirrors the HbO2 ndings further supports a decrease in brain activation between single-and dual-tasks.

Behavioural
Examining gait speed in older adults alongside behavioural measures such as response time and accuracy may offer insights into the cognitive-motor interactions underlying dual-task walking. Gait speed changes in older adults have been well documented in the literature such that increasing attentional demands while walking may affect walking performance [15,27,28]. Findings from the present study partially support this in that gait speed decreased but only during the most di cult cognitive task. Gait speed maintenance across the rst three levels of task di culty may be explained by an automatic locomotor control strategy, as described in the neural ndings. However, this strategy may not have been su cient to mitigate the demands of the DNS dual-task. As suggested in the "posture rst hypothesis," older adults subconsciously prioritize gait over cognitive performance to ensure safe ambulation [8,9,61]. Slowing gait speed may, therefore, be a combination of prioritization and compensation strategies to ensure older adults can function safely under complex task demands. It is worth noting that older adults commonly decrease their gait speed < 1.0 m/s during dual-tasks which is also a cut-off used to identify individuals who are at a greater risk of falls [20,28,62,63]. When the older adults in this study decreased their gait speed during the most di cult task, it still remained on average > 1.0 m/s. This may further indicate the physical status of the participants which could have an impact on performance as compared to other studies in the literature [13,64].
Decreased response time and accuracy performance may also be a consequence of gait prioritization.
Our ndings demonstrated increased response times from the easiest to the most demanding task. More speci cally, the response times in the SRT task were signi cantly faster than the GNG and NBK tasks. However, the GNG and NBK tasks were not signi cantly different from one another. This was expected in that compared to the SRT task, the GNG and NBK tasks involved more complex processing steps. For example, the simple reaction time task required a response after each stimulus whereas the GNG task forced the older adults to rst discriminate between a "go" and "no-go" stimulus before responding [31].
Similarly, the NBK working memory task involved maintaining and updating information before responding to the stimuli [6]. Based on these ndings, more complex processing steps require more processing capacity. This was evident during the more di cult tasks as the older adults slowed their response times signi cantly during the inhibition and the working memory tasks compared to the SRT task. Further, the older adults responded less accurately as the di culty level increased. However, there were no differences between the working memory tasks. These ndings support our di culty manipulation such that participants were most accurate during the processing speed task and least accurate during the working memory tasks.
In line with the literature, increasing task di culty was expected to result in lower accuracy [1,65,66].
Interestingly, participants maintained their accuracy > 80% throughout all the dual-tasks. This suggests that a high level of performance is achievable with increasing cognitive demands when cognitive resources are allocated effectively. However, participants were less accurate during the dual-versus single-tasks. This has been demonstrated in the literature whereby participants make more errors during dual-tasks due to the competing demands of performing two tasks simultaneously [65,67].

Correlation between cerebral oxygenation and behaviour
There were no signi cant correlations between the changes in cerebral oxygenation and behavioural performance. More speci cally, the changes in cerebral oxygenation across task and di culty were not associated with gait speed, response time or accuracy performance. This could be due to the small sample of older adults in this study. However, interpreting neural and behavioural ndings together revealed that the redistribution of metabolic resources in the PFC may have contributed to insigni cant differences in gait speed across the rst three levels of task di culty. The same cannot be said for response time and accuracy performance in which decreased cerebral oxygenation in the PFC did not result in behavioural gains. Future studies should examine automaticity and neural e ciency across task di culty in regions outside the PFC as certain regions of interest may increase or decrease activity with the maintenance and decline of different performance measures. Follow-up studies should be conducted to determine how this impacts cognition in the long-term. This may equally reveal whether individuals exhibiting decrements in behaviour due to neural ine ciency may be at a greater risk of cognitive decline.

Limitations
Gait parameters were only quanti ed using gait speed. Gait speed is commonly used in the literature because it is easily collected in clinical settings, it requires minimal equipment and is a good indicator of motor performance in older adults [13]. However, other measures that capture gait variability including stride length or stride time could complement gait speed measures and may provide greater insight into subtle changes in dual-task performances, different age groups, and different clinical populations. In addition, the choice of fNIRS device limited our data acquisition to the PFC (Artinis, The Netherlands).
This device facilitated setup and caused minimal discomfort for the participants, however, we can only speculate as to which other brain regions were involved in dual-tasking and the potential executiveautomatic processing shift in walking with increasing di culty. Despite this, fNIRS has a high temporal resolution compared to other techniques such as fMRI and is a reliable tool for measuring cerebral oxygenation in the PFC [23].

Conclusion
Executive functions are known to decline with age and can signi cantly affect the way older adults divide their attention between two simultaneous tasks. Many older adults adapt to these changes by using compensatory neural strategies to accomplish tasks exceeding their cognitive capacity. The neural ndings of this study suggest that an automatic locomotor control strategy can decrease the recruitment of executive resources in the PFC during dual-versus single-tasks. Behaviourally, this allowed for gait speed maintenance until the most di cult working memory task after which older adults slowed down to mitigate the cognitive task demands. Consequently, prioritizing gait led to slower response times and worse response accuracy across task di culty.
Findings from this study helped reveal the PFC's role in allocating cognitive resources during processing speed, neural inhibition and working memory tasks while walking. Future studies can develop an even better understanding of this relationship as neuroimaging becomes more portable, more extensive (i.e., covering the entire brain) and adaptable to different environments. In particular, assessing dual-task walking in real-life situations such as crossing the street while talking on the phone may generate more novel approaches to understanding executive and controlled processes within the scope of cognitive aging. Declarations 6.1 Ethics approval and consent to participate  Description of a sample run including single cognitive (SC; responding to the cognitive task), single motor (SM; normal walking) and dual-task (DT; walking with a cognitive task) blocks. Each 33 s block is preceded by a 10 s baseline and followed by a 15 s rest period. The approximate duration of a run is 11 minutes and is repeated for each cognitive task di culty level.

Figure 3
Description of a sample run including single cognitive (SC; responding to the cognitive task), single motor (SM; normal walking) and dual-task (DT; walking with a cognitive task) blocks. Each 33 s block is preceded by a 10 s baseline and followed by a 15 s rest period. The approximate duration of a run is 11 minutes and is repeated for each cognitive task di culty level. Mean hemodynamic response across all participants in the single motor and dual-task blocks. The blue and red lines represent HbR and HbO2, respectively, such that the hemodynamic signal is normally distributed across the block and ∆HbO2 is greater than ∆HbR. Response time (ms) changes (mean ± SE) between cognitive task di culty levels (SRT), go/no-go (GNG) and n-back (NBK). Response times in the GNG and NBK were signi cantly slower than the SRT (p < 0.001). (*) indicates signi cance p < 0.001. Figure 6 a) Accuracy (% correct) decrease (mean ± SE) between single cognitive (SC) and dual-task (DT) blocks.