Electrophysiological Correlates of Subitizing in Healthy Aging

Abstract

To understand the nature of age-related changes in enumeration abilities we measured two ERP responses -N2pc and CDA, associated respectively to attentive individuation and VWM- and posterior alpha band (8-15 Hz) event-related desynchronization (ERD), traditionally linked to enhanced target processing. Two groups of old and young participants enumerated a variable number (1-6) of targets presented among distractors. Older participants were less accurate in enumerating targets. ERP results in old participants showed a suppression of N2pc amplitudes for all numerosities, and a decrease in CDA only for the largest set (4-6 targets). In contrast with the pattern for young adults, time/frequency results on older adults revealed neither a modulation of alpha oscillations as a function of target numerosity, nor an effect of ERD lateralization. These patterns indicate that both attention and working memory contribute to the age-related decline in enumeration, and point to an overall decrease in the activity of the visual areas responsible for the processing of the hemifield where the relevant objects are presented.

Fig 1. Stimulus example and accuracy results.

(A) Example of a trial with four targets Participants had to indicate the number of targets by clicking on one of the six squares in the response display. (B) Accuracy results (error rates) as a function of age group and numerosity.

Fig 2. Event related potential results.

(A) ERP difference waveforms as a function of age group and numerosity. Panels represent respectively numerosities 1 to 6 for the old group group (left) and for the young group (right). (B) Mean amplitude values for the N2pc (200–300 ms) and CDA (400–600 ms) as a function of age group and numerosity.

Fig 3. Time/frequency analysis results.

(A) Left: Time-frequency plots for the old and young group averaged across numerosities and side at PO7/8. The white dotted squares indicate the time-frequency bins used to analyze ERS/ERD in the alpha band. Right: Topographical maps of the alpha band in the 300–600 ms post-stimulus window plotted separately for the old and young group and collapsed across numerosities. Contralateral side is plotted on the left. (B) Posterior (PO7/8) ipsilateral and contralateral ERS/ERD time course averaged across numerosities in the old and young group in the alpha band. The young group shows a significant alpha lateralization effect starting around 300 ms post stimulus. (C) Mean alpha ERD as a function of numerosity, laterality and age in the 300–600 time window.