Abstract
The way animals respond to a stimulus depends largely on an internal comparison between the current sensation and the memory of previous stimuli and outcomes. We know little about the accuracy with which the physical properties of the stimuli influence this type of memory-based discriminative decisions. Research has focused largely on discriminations between stimuli presented in quick succession, where animals can make relative inferences (same or different; higher or lower) from trial to trial. In the current study we used a memory-based task to explore how the stimulus’ physical properties, in this case tone frequency, affect auditory discrimination and generalization in mice. Mice performed ad libitum while living in groups in their home quarters. We found that the frequency distance between safe and conditioned sounds had a constraining effect on discrimination. As the safe-to-conditioned distance decreased across groups, performance deteriorated rapidly, even for frequency differences significantly larger than reported discrimination thresholds. Generalization width was influenced both by the physical distance and the previous experience of the mice, and was not accompanied by a decrease in sensory acuity. In conclusion, memory-based discriminations along a single stimulus dimension are inherently hard, reflecting a high overlap between the memory traces of the relevant stimuli. Memory-based discriminations rely therefore on wide sensory filters.
Highlights
An animal’s response to external stimuli depends largely on the animal’s capacity to identify the current stimulus as the same or similar to previously encountered stimuli
Neuroligin 2 knockout (Nlgn2 KO) mice [20,21] were used in one test
We investigated how (1) the frequency distance between trained stimuli, and (2) the mice past experience with these stimuli, affected learning speed, Memory-based discrimination and generalization discrimination performance and generalization gradients
Summary
An animal’s response to external stimuli depends largely on the animal’s capacity to identify the current stimulus as the same or similar to previously encountered stimuli. The study of absolute judgement [16], which tests the capacity of a subject to order a given stimulus among a group of stimuli varying along a single dimension, relies on experimental designs in which stimuli are presented successively. Animals often have to decide how to respond to a stimulus that is presented in spatial and temporal isolation from others that resemble it This type of memory-based judgement is reflected in some forms of stimulus categorization, which has been the subject of substantial research in several species, including humans, monkeys and pigeons [17,18], but not in simpler discriminations. Little is known about the role played by differences in a given physical dimension, such as sound frequency, in memory-based discriminations. Tone frequency judgements, in isolation, are generally difficult if the subject lacks absolute pitch but can be ameliorated with the use of reference frequencies [19]
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