Academic journal article Psychonomic Bulletin & Review

Capacity and Limits of Associative Memory in Pigeons

Academic journal article Psychonomic Bulletin & Review

Capacity and Limits of Associative Memory in Pigeons

Article excerpt

How much information can a brain store over a lifetime's experience? The answer to this important, but little researched, question was investigated by looking at the long-term visual memory capacity of 2 pigeons. Over 700 sessions, the pigeons were tested with an increasingly larger pool of pictorial stimuli in a two-alternative discrimination task (incremented in sets of 20 or 30 pictures). Each picture was randomly assigned to either a right or a left choice response, forcing the pigeons to memorize each picture and its associated response. At the end of testing, 1 pigeon was performing at 73% accuracy with a memory set of over 1,800 pictures, and the 2nd was at 76% accuracy with a memory set of over 1,600 pictures. Adjusted for guessing, models of the birds' performance suggested that the birds had access, on average, to approximately 830 memorized picture-response associations and that these were retained for months at a time. Reaction time analyses suggested that access to these memories was parallel in nature. Over the last 6 months of testing, this capacity estimate was stable for both birds, despite their learning increasingly more items, suggesting some limit on the number of picture-response associations that could be discriminated and retained in the long-term memory portion of this task. This represents the first empirically established limit on long-term memory use for any vertebrate species. The existence of this large exemplar-specific memory capacity has important implications for the evolution of stimulus control and for current theories of avian and human cognition.

How does the brain encode, store, and retrieve an entire lifetime of experiences? Biological memory capacity has been extensively studied with regard to the short-term retention of recent information (Miller, 1956; Nelson, 2001), and a system with a limited capacity in the single digits has consistently been suggested. Such findings suggest that the brain may have multiple memory systems, and numerous longer term stores (e.g., procedural, episodic, long-term, semantic, associative, and reference memory) have been proposed to capture the extensive knowledge and memories stored beyond those of the recent past. One challenging issue concerns the storage capacity and information retrieval of these longer term memory mechanisms. Attempts to examine the capacity of such longer term memory stores have been limited and mainly computational (Dudai, 1997; Landauer, 1986; Standing, 1973). Although there is general agreement that the human capacity for recognizing pictorial information is considerable (Shepard, 1967; Standing, 1973) and the storage needs for language use substantial, the empirical study of long-term memory capacity has been virtually absent because of the impractical requirements of testing humans over extended periods.

Nonhuman animals are similarly guided by a combination of acquired knowledge and recent experiences, but here as well, little effort has been directed at understanding the capacity of their longer term memory systems. Vaughan and Greene ( 1984) conducted the most extensive study of animal memory capacity. They showed that pigeons could discriminate among 320 randomly assigned pictorial stimuli ( 160 positive and 160 negative) in a successive go/no-go discrimination. On the basis of a smaller stimulus set (160), they further demonstrated that some of these memories endure for at least 2 years. There are some important limitations, however, to these findings. Their rate-based discrimination procedure and rankorder measure of performance were not designed to yield a measure of how many stimuli were retained, for example. Thus, their pigeons' above-chance discrimination could have been accomplished with considerably fewer than the 320 pictures being memorized. Furthermore, the testing of all 320 stimuli was quite brief, making it impossible to judge the long-term effects of high memory load. Since then, the capacity of pigeons to remember large numbers of pictures has been confirmed. …

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