Electrophysiological Advances on Multiple Object Processing in Aging

Abstract

EEG research conducted in the past 5 years on multiple object processing has begun to define how the aging brain tracks the numerosity of the objects presented in the visual field for different goals. We review the recent EEG findings in healthy older individuals (age range: 65-75 years approximately) on perceptual, attentional and memory mechanisms-reflected in the N1, N2pc and contralateral delayed activity (CDA) components of the EEG, respectively-during the execution of a variety of cognitive tasks requiring simultaneous processing of multiple elements. The findings point to multiple loci of neural changes in multi-object analysis, and suggest the involvement of early perceptual mechanisms, attentive individuation and working memory (WM) operations in the neural and cognitive modification due to aging. However, the findings do not simply reflect early impairments with a cascade effect over subsequent stages of stimulus processing, but in fact highlight interesting dissociations between the effects occurring at the various stages of stimulus processing. Finally, the results on older adults indicate the occurrence of neural overactivation in association to good levels of performance in easy perceptual contexts, thus providing some hints on the existence of compensatory phenomena that are associated with the functioning of early perceptual mechanisms.

Keywords: EEG; N2pc; aging; attention; contralateral delayed activity (CDA); enumeration; multiple object processing; working memory.

Figure 1

Examples of the paradigms used to explore multiple object processing. Left: in multiple object tracking tasks participants are typically presented with a varying number of elements and a cue indicating the targets (here the red circles; see cue array), before all elements start moving for a variable amount of time (tracking array). The movement trajectory (here indicated by the arrows) was linear and with a constant speed of 1°/s in Störmer et al. (2013a). A test array prompts participants to report if the probe element (the red circle in the example) was a target element or not. Center: in delayed match-to-sample judgments a set of elements is identified as the target by a cue (the arrow in the example), and participants need to retain the set of target elements for a certain amount of time (retention period, typically 1 s see Sander et al., 2011) for subsequent old/new recognition on a test array. Right: in enumeration tasks, a varying number of targets (here the red elements, as indicated at the beginning of the experimental session) is presented among distracters; participants report the number of targets either immediately or after a short delay (200 ms in Pagano et al., , see example).

Figure 2

N2pc and contralateral delayed activity (CDA) results on enumeration in aging (figure modified from Pagano et al., 2015). (A) Grand-average ERP waveforms for old and young participants as a function of target numerosity. A reduction of both N2pc (see gray area) and (partially) of CDA is visible for older adults. (B) The analysis on mean amplitude values for N2pc (180–300 ms) and CDA (400–600 ms) highlights that the age-related difference in N2pc amplitude is present for all target numerosities; in contrast, the CDA is equivalent in the two groups for up to three targets, and differentiates young and older participants only from approximately four elements.