Animal memory: A review of delayed matching-to-sample data

Authors: Johan Lind, Magnus Enquist, Stefano Ghirlanda
Published: 2015 (available online 2014)
Journal: Behavioural Processes, 117, 90–96
DOI: 10.1016/j.beproc.2014.11.019

Abstract

We performed a meta-analysis of over 90 data sets from delayed matching-to-sample (DMTS) studies with 25 species (birds, mammals, and bees). In DMTS, a sample stimulus is first presented and then removed. After a delay, two (or more) comparison stimuli are presented, and the subject is rewarded for choosing the one matching the sample. We used data on performance vs. delay length to estimate two parameters informative of working memory abilities: the maximum performance possible with no delay (comparison stimuli presented as soon as the sample is removed), and the rate of performance decay as the delay is lengthened (related to memory span). We conclude that there is little evidence that zero-delay performance varies between these species. There is evidence that pigeons do not perform as well as mammals at longer delay intervals. Pigeons, however, are the only extensively studied bird, and we cannot exclude that other birds may be able to bridge as long a delay as mammals. Extensive training may improve memory, although the data are open to other interpretations. Overall, DMTS studies suggest memory spans ranging from a few seconds to several minutes. We suggest that observations of animals exhibiting much longer memory spans (days to months) can be explained in terms of specialized memory systems that deal with specific, biologically significant information, such as food caches. Events that do not trigger these systems, on the other hand, appear to be remembered for only a short time.

Keywords

Memory; Delayed matching-to-sample; Animal intelligence; Comparative psychology; Animal cognition

Key Findings

• Meta-analysis of 90+ DMTS data sets across 25 species (birds, mammals, bees)
• Two parameters estimated: zero-delay performance (intercept) and performance half-life (decay rate)
• Little evidence of species differences in zero-delay performance
• Pigeons perform worse than mammals at longer delays (but are the only extensively studied bird)
• Performance half-lives range from seconds to minutes; >1 min only in mammals
• Primates do not stand out from other mammals
• Training may improve memory span (titration yielded ~7 min half-life in macaques)
• Humans achieve errorless performance after 48 h delays (far exceeding other species)
• Specialized vs. generalist memory distinction: long-term memories reflect ecologically specific adaptations (food caching, taste learning), not general-purpose capacity

Species Studied

Includes pigeons, various primates (rhesus macaques, capuchins, baboons, chimpanzees), dolphins, rats, mice, dogs, parrots, corvids, and bees.