Perceptual grouping boosts visual working memory capacity and reduces effort during retention.

Consistent, robust boosts to visual working memory capacity are observed when colour-location arrays contain duplicate colours. The prevailing explanation suggests that duplicated colours are encoded as one perceptual group. If so, then we should observe not only higher working memory capacity overall for displays containing duplicates, but specifically an improved ability to remember unique colours from displays including duplicates compared with displays comprising all uniquely coloured items. Furthermore, less effort should be required to retain displays as colour redundancy increases. I recorded gaze position and pupil sizes during a visual change detection task including displays of 4-6 items with either all unique colours, two items with a common colour, or three items with a common colour in samples of young and healthy elderly adults. Increased redundancy was indeed associated with higher estimated working memory capacity, both for tests of duplicates and uniquely coloured items. Redundancy was also associated with decreased pupil size during retention, especially in young adults. While elderly adults also benefited from colour redundancy, spillover to unique items was less obvious with low redundancy than in young adults. This experiment confirms previous findings and presents complementary novel evidence linking perceptual grouping via colour redundancy with decreased mental effort.