Phasic alerting increases visual attention capacity in younger but not in older individuals

Baseline levels of alertness influence tES effects along different age-related directions

Normal aging is usually accompanied by several structural and functional physiological changes of the brain, which are closely related to alterations of cognitive functions (e.g., visual short-term memory). As the average age of the population increases, it has become crucial to identify cognitive-behavioural interventions to maintain a healthy level of cognitive performance. Among a variety of approaches, the targeting of specific intrinsic alertness mechanisms has shown a solid rationale and beneficial effects in both healthy and pathological ageing. In a similar vein, the use of non-invasive transcranial electrical stimulation (tES) represents another promising approach to induce an alerting state that can produce advantages in the information processing in the brain and therefore behaviour. Here, we investigated whether time-locked bursts of tES (i.e., transcranial random noise stimulation) were effective in inducing behavioural and physiological changes, consistently with an alertness increase, in both young and older healthy adults. Namely, we expected to find a beneficial alerting effect on visual short-term memory performance as a function of stimulus perceptual salience and tES. The initial results showed that the performance of younger adults was not affected by tES, while older adults scored lower correct responses for high-salience stimuli during real tES with respect to sham stimulation. However, after including a baseline measure of subjective level of alertness in the analyses, a tES-induced memory improvement did emerge in the less alerted younger adults, while only the more alerted older adults were subject to the worsening effect by tES. We discuss these results in consideration of the evidence on critical age-related differences as well as the interaction between neurostimulation and baseline alerting mechanisms.

Target value and prevalence influence visual foraging in younger and older age

The prevalence and reward-value of targets have an influence on visual search. The strength of the effect of an item's reward-value on attentional selection varies substantially between individuals and is potentially sensitive to aging. We investigated individual and age differences in a hybrid foraging task, in which the prevalence and value of multiple target types was varied. Using optimal foraging theory measures, foraging was more efficient overall in younger than older observers. However, the influence of prevalence and value on target selections was similar across age groups, suggesting that the underlying cognitive mechanisms are preserved in older age. When prevalence was varied but target value was balanced, younger and older observers preferably selected the most frequent target type and were biased to select another instance of the previously selected target type. When value was varied, younger and older observers showed a tendency to select high-value targets, but preferences were more diverse between individuals. When value and prevalence were inversely related, some observers showed particularly strong preferences for high-valued target types, while others showed a preference for high-prevalent, albeit low-value, target types. In younger adults, individual differences in the selection choices correlated with a personality index, suggesting that avoiding selections of low-value targets may be related to reward-seeking behaviour.

Phasic alerting increases visual processing speed in amnestic mild cognitive impairment

External warning cues temporarily increase the brain's sensitivity for upcoming events, helping individuals to flexibly adapt their reactions to the requirements of complex visual environments. Previous studies reported that younger and cognitively normal older adults profit from phasic alerting cues. Such an intact phasic alerting mechanism could be even more relevant in individuals with Alzheimer's disease who are characterized by reduced processing capacities. The present study employed a theory of visual attention based verbal whole report paradigm with auditory cues in order to investigate phasic alerting effects in amnestic mild cognitive impairment (aMCI). Patients with aMCI were also compared to a previously reported sample of cognitively normal older adults. In patients with aMCI, visual processing speed was higher in the cue compared to the no-cue condition. Further, visual processing speed was reduced in patients with aMCI compared to cognitively normal older adults. Taken together, the results suggest that the processing system of patients with aMCI exhibits general declines but can still integrate auditory warning signals on a perceptual level.

Cognitive processes in aging effects on attentional alerting

Alerting, the process of achieving and maintaining a state of optimal vigilance, is crucial for detecting relevant stimuli and task performance. Age-related decline in the ability to use alerting cues is widely reported and attributed to changes in noradrenergic signaling. However, it remains to be determined whether aging affects all forms of alerting cues equally and whether older adults differently modulate their alerting sensitivity based on differences in cue predictivity relevant to the target task. We examined the performance of 135 younger adults and 103 older adults on three versions of the Attention Networks Test, using locational but spatially nonpredictive visual cues, locational spatially predictive visual cues, and spatially predictive auditory cues. Analysis of alerting effects indicated that while older adults derived less benefit from visual alerting cues than younger adults, they used auditory alerting cues equally well. Furthermore, cue spatial predictivity did not impact on aging effects on alerting. This heterogeneity in aging effects on alerting may indicate that they result primarily from cognitive rather than neuromodulatory changes.

Age-related changes in the functional integrity of the phasic alerting system: a pupillometric investigation

Enhanced processing following a warning cue is thought to be mediated by a phasic alerting response involving the locus coeruleus-noradrenergic (LC-NA) system. We examined the effect of aging on phasic alerting using pupil dilation as a marker of LC-NA activity in conjunction with a novel assessment of task-evoked pupil dilation. While both young and older adults displayed behavioral and pupillary alerting effects, reflected in decreased RT and increased pupillary response under high (tone) versus low (no tone) alerting conditions, older adults displayed a weaker pupillary response that benefited more from the alerting tone. The strong association between dilation and speed displayed by older adults in both alerting conditions was reduced in young adults in the high alerting condition, suggesting that in young (but not older) adults the tone conferred relatively little behavioral benefit beyond that provided by the alerting effect elicited by the target. These findings suggest a functioning but deficient LC-NA alerting system in older adults, and help reconcile previous results concerning the effects of aging on phasic alerting.

Phasic alerting effects on visual processing speed are associated with intrinsic functional connectivity in the cingulo-opercular network

Phasic alertness refers to short-lived increases in the brain's "state of readiness", and thus to optimized performance following warning cues. Parametric modelling of whole report task performance based on the computational theory of visual attention (TVA) has demonstrated that visual processing speed is increased in such cue compared to no-cue conditions. Furthermore, with respect to the underlying neural mechanisms, individual visual processing speed has been related to intrinsic functional connectivity (iFC) within the cingulo-opercular network, suggesting that this network's iFC is relevant for the tonic maintenance of an appropriate readiness or alertness state. In the present study, we asked whether iFC in the cingulo-opercular network is also related to the individual ability to actively profit from warning cues, i.e. to the degree of phasic alerting. We obtained resting-state functional magnetic resonance imaging (rs-fMRI) data from 32 healthy young participants and combined an independent component analysis of rs-fMRI time courses and dual regression approach to determine iFC in the cingulo-opercular network. In a separate behavioural testing session, we parametrically assessed the effects of auditory phasic alerting cues on visual processing speed in a TVA-based whole report paradigm. A voxel-wise multiple regression revealed that higher individual phasic alerting effects on visual processing speed were significantly associated with lower iFC in the cingulo-opercular network, with a peak in the left superior orbital gyrus. As phasic alertness was neither related to iFC in other attention-relevant, auditory, or visual networks nor associated with any inter-network connectivity pattern, the results suggest that the individual profit in visual processing speed gained from phasic alerting is primarily associated with iFC in the cingulo-opercular network.

Event-related Electroencephalographic Lateralizations Mark Individual Differences in Spatial and Nonspatial Visual Selection

Selective attention controls the distribution of our visual system's limited processing resources to stimuli in the visual field. Two independent parameters of visual selection can be quantified by modeling an individual's performance in a partial-report task based on the computational theory of visual attention (TVA): (i) top-down control α, the relative attentional weighting of relevant over irrelevant stimuli, and (ii) spatial bias w_λ, the relative attentional weighting of stimuli in the left versus right hemifield. In this study, we found that visual event-related electroencephalographic lateralizations marked interindividual differences in these two functions. First, individuals with better top-down control showed higher amplitudes of the posterior contralateral negativity than individuals with poorer top-down control. Second, differences in spatial bias were reflected in asymmetries in earlier visual event-related lateralizations depending on the hemifield position of targets; specifically, individuals showed a positivity contralateral to targets presented in their prioritized hemifield and a negativity contralateral to targets presented in their nonprioritized hemifield. Thus, our findings demonstrate that two functionally different aspects of attentional weighting quantified in the respective TVA parameters are reflected in two different neurophysiological measures: The observer-dependent spatial bias influences selection by a bottom-up processing advantage of stimuli appearing in the prioritized hemifield. By contrast, task-related target selection governed by top-down control involves active enhancement of target, and/or suppression of distractor, processing. These results confirm basic assumptions of the TVA framework, complement the functional interpretation of event-related lateralization components in selective attention studies, and are of relevance for the development of neurocognitive attentional assessment procedures.