Aging affects individuals of every species, with sometimes detrimental effects on memory and cognition. The simultaneous-chaining task, a sequential-learning task, requires subjects to select items in a predetermined sequence, putting demands on memory and cognitive processing capacity. It is thus a useful tool to investigate age-related differences in these domains. Pigeons of three age groups (young, adult and aged) completed a locomotors adaptation of the task, learning a list of four items. Training began by presenting only the first item; additional items were added, one at a time, once previous items were reliably selected in their correct order. Although memory capacity declined noticeably with age, not all aged pigeons showed impairments compared to younger pigeons, suggesting that inter-individual variability emerged with age. During a subsequent free-recall memory test in the absence of reinforcement, when all trained items were presented alongside novel distractor items, most pigeons did not reproduce the trained sequence. During a further forced-choice test, when pigeons were given a choice between only two of the trained items, all three age groups showed evidence of an understanding of the ordinal relationship between items by choosing the earlier item, indicating that complex cognitive processing, unlike memory capacity, remained unaffected by age. As we age, so does our brain, and with it many cognitive functions decline. Age-related degeneration of cognitive and memory capacities is well-documented for humans, and is evident even for healthy individuals. The impact of aging on nonhuman animal cognition is less well explored, but this knowledge is important for understanding common effects of non-pathological aging on cognition. The aging process affects every living being regardless of species, but the way in which it manifests in cognitive changes or impairments can sometimes differ greatly among individuals. Establishing how, and to what degree, aging can affect vital cognitive capacities is a first step in assessing the range of normal and abnormal impairments.

The simultaneous-chaining task is cognitively demanding as it requires subjects to reproduce a list of items in a specific sequence, with the only feedback provided regarding the correctness of a choice being the continuation of the trial. This task puts demands on an individual’s memory capacity, in terms of both reference memory to learn the sequence, and working memory to update the last choice made in order to determine the next required response. An individual’s memory capacity can be measured through the successive chaining of subsequent sequence items. Each time a subject learns to respond correctly to a sequence of n items, another item is added to the chain (n+1). Using this method, pigeons have successfully been trained to reproduce lists of four to five items.

In addition to the task’s adoption for evaluating memory capacity, it has also been selected as an assessment tool for cognitive capacity, and as such has been used to study a diverse range of species, from humans and apes to pigeons and chickens. As suggested by the task’s name, successful acquisition of the sequence can be achieved through simple associative chaining, with a response to the first item serving as a cue to respond to the second item, and so on. However, a more cognitively complex solution would be to form a mental representation of the item order, whereby each item in the sequence is acquired not only relative to the immediately preceding and following items, but also in terms of its unique ordinal position within the sequence as a whole (e.g., is the item the first, second, third, etc.). One way to assess a subject’s ability to form a representation of order is during subsequent pairwise-choice tests, for which only two items of the learned sequence are presented together, and the subject is allowed a single choice. If item order had been represented, subjects would be expected to recognise the ordinal relationship between the two presented items even in the absence of the complete sequence, and accordingly to choose the item that appeared earlier in the trained sequence. However, if learning occurred based on associative chaining alone, subjects would be expected to perform well only when the first item is present, but to be unable to distinguish between any later items, as the cue to respond to either—a successful response to the preceding item—would not have occurred.

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