Long-term episodic memory depends on the retention of associative information, such as the relationship between a friend's face and his name, a home and its neighborhood, and a mint and its odor. How many of these associative links can be stored and recalled? Many have considered memory's capacity, but relevant data are scant. Some previous experiments on humans have assessed the retention of thousands of visual impressions using forced-choice picture recognition and have concluded that there are virtually no constraints on how much information can be successfully retained. However, no previous experiments on humans have investigated the capacity of associative memory. I describe the first relevant data, which I obtained by systematically probing my own capacity during 58,560 memory trials for picture-response associations (approximately 1 year of testing). Estimated capacity was on the order of several thousand associations, and this and other indicators of memory function were remarkably similar to those obtained for baboons (Papio papio) under comparable circumstances. These findings, along with other data, suggest conservation of long-term memory mechanisms and effectiveness in humans relative to nonhuman primates, despite at least 20 million years of divergent evolution and vastly different behavioral and cognitive repertoires. The present findings also indicate that the associative processes that support our ability to remember episodes are limited in capacity relative to processes that support picture recognition.
How much information can be stored in memory? This question has daunted philosophers and scientists for over a thousand years (Dudai, 1997). No answer is currently available, partly because there are scant data on which to base capacity estimates or to constrain models of capacity (Landauer, 1986; Treves & Rolls, 1994).
Valuable evidence has been obtained by examining memory for large sets of pictures studied en masse (Brady, Konkle, Alvarez, & Oliva, 2008; Standing, 1973; Standing, Conezio, & Haber, 1970). In these studies, individuals attempted to memorize many briefly presented images (e.g., up to 10,000 photographs in Standing, 1973), and were later tested on their ability to discriminate studied images from novel images when they saw pairs comprising one of each type of image presented side by side in a forced-choice-format test. Discrimination was remarkably accurate in these studies, indicating that the brain's ability to serve as a storehouse for item-specific visual information is virtually limitless. However, long-term episodic memory also involves associative traces (Aggleton & Brown, 2006; Eichenbaum & Cohen, 2001; Lavenex & Amaral, 2000; Norman & O'Reilly, 2003; Tulving, 1983), and other approaches are necessary for determining capacity for associative information.
Data relevant to associative capacity in humans have been unavailable due to practical constraints on their collection. I therefore systematically probed my own capacity during 58,560 memory trials for picture-response associations distributed over approximately 1 year of testing. I used a modification of a paradigm originally designed to tax long-term memory in baboons (Papio papio) and pigeons (Columba livia) (Cook, Levison, Gillett, & Blaisdell, 2005; Fagot & Cook, 2006), so that information regarding the evolutionary trajectory of memory capacity could also be obtained. This paradigm is advantageous for making comparisons across species because associative traces comprise arbitrary pairings of visual stimuli with one of two behavioral response options, thus minimizing confounding influences of factors, such as conceptual/ semantic learning strategies, which have previously undermined comparisons of humans and nonhuman animals (Ekstrand, Wallace, & Underwood, 1966).
Visual stimuli comprised 4,980 unique color digital photographs of common matter (objects, people, buildings, animals, etc. …