Inhibitory processing in the aging brain

Inhibition prevents irrelevant information from intruding on ongoing goal-directed processes and is essential for successful completion of many daily tasks. Normal aging allegedly induces a loss of its efficiency. However, four factors crucial for an appropriately disentangled and fine-grained account of this effect tend to be neglected: (1) the degree of executive control required to deploy the inhibitory process; (2) the degree of match between the individual’s morning/evening optimal-functioning type (Chronotype, CT) and the time of day (ToD) when the task is executed, yielding the so-called “synchrony effect”; (3) the interaction (1) x (2), i.e. that between the degree of executive control required by the task and the synchrony effect; (4) in association with designs that consider any of the previous factors, the use of measurement techniques suitable for probing the relation between performance parameters (reaction time (RTs), correct/incorrect responses) and the brain processes underlying the observed variance in those parameters (e.g. Event Related Brain Potentials (ERPs), functional Magnetic Resonance Imaging).

Studies conducted by members of our team [2] have already explored the moderating impact of (1) on age-related inhibition changes. Controlled inhibition was shown to be hindered by aging, while automatic inhibition, when probed by means of negative priming (NP), seems impervious to this effect. This line of research poses the question of determining whether the effect of aging upon controlled inhibition is direct or mediated. A strong candidate for such a mediating factor is (2), i.e., an increased age-associated sensitivity of frontal processes, such as executive control, to the synchrony effect. In spite of this, studies of aging effects upon inhibition that appropriately include (2) in their design are rare. Another question raised by this previous research from our team pertains to the use of NP (a residual after effect of previous inhibition of task-irrelevant information) as a means to probe automatic inhibition: As an after effect, NP may reflect features of the original inhibitory event, which may itself have been automatic or controlled, and, in this latter case, could result in augmented NP when the required executive control is more effortful (e.g. when performed by older participants under CT/ToD asynchrony). Tackling these questions would require a design appropriately incorporating (3). Aging studies designed to investigate this higher level interaction, (3), or that incorporate (4), are, to the best of our knowledge, inexistent. We have therefore designed a study of aging effects tailored to probe (3), comparing the mediating effect of CT/ToD (as)synchrony upon age-related differences in negative priming (NP) resulting from automatic or controlled inhibition.

All participants will perform two ERP eliciting tasks, one probing NP resulting from automatic inhibition (a yes/no word-picture matching task, containing yes-yes trial-sequences featuring the subdominant meaning of an ambiguous word followed by its

dominant meaning, previously inhibited), the other probing NP resulting from controlled inhibition (a spatial stroop task, in which the left/right location of an arrow has to be ignored and its pointing direction determines which hand-held switch should be pressed, featuring trials that require the previously inhibited right/left switch press to be effected). Young and older participants will be selected in order to obtain an identical number of morning and evening chronotypes (30 of each chronotype in each age group, for a total of 120 participants). Each age/chronotype subgroup will be split in two 15-participants conditions by randomly assigning a time of participation that either matches or mismatches the participants’ chronotype.

This design will probe the overall hypothesis that aging impacts frontal inhibition-control processes differently from non-frontal inhibitory circuitry. Specifically, we hypothesize that (I) the impact of aging upon controlled inhibition mostly reflects increased sensitivity of executive control to circadian rhythms, in particular to the synchrony effect; (II) the impact of aging upon the functional integrity of non-frontal inhibitory circuitry, as reflected in automatic inhibition, results in intrinsic and irretrievable changes in this circuitry, and therefore should not show modulations resulting from the synchrony effect.

Both performance (RTs and proportion of correct responses) and ERP data will be analyzed. With respect to the latter, N200 and P300 components are expected to reflect NP modulations, respectively indexing conflict detection and conflict resolution caused by residual inhibition in NP trials. The use of dense array electroencephalography (64 electrodes), will further allow us to explore the brain locations associated with the detected ERP effects. To this end we will use the sLORETA source localization algorithm.