Cognitive neuroscience

Neural mechanisms for detecting and remembering novel events

Key Points

  • Novel or unexpected events or stimuli draw our attention and are more easily remembered than predictable or familiar ones. Evidence from a variety of sources is beginning to clarify the neural mechanisms that contribute to this response to novelty.

  • When a stimulus is repeated, the neuronal response to it (in many parts of the brain) often decreases rapidly. The response is highest when the stimulus is novel. Behavioural responses also depend on stimulus novelty: the initial presentation seems to produce 'priming' so that repeated stimuli are processed more efficiently. Repetition suppression of neuronal responses occurs over short timescales and is stimulus-specific. Stimulus repetition can also cause suppression of responses over longer time periods, including over weeks of training on a task.

  • This effect seems to rely on synaptic plasticity mediated by NMDA (N-methyl-D-aspartate) receptors. It has been proposed that the reduction in population activity represents a sparsening of the response to a stimulus that leaves a more robust response carried by fewer neurons. Reductions in responses in early visual areas might result from decreased feedback from higher visual areas of cortex.

  • Another type of novelty that has been studied is contextual novelty. This arises when an unexpected stimulus or event occurs, for example a dog's bark amidst a string of birdsong. Stimuli that are contextually novel produce characteristic event-related potentials in human electroencephalograms. These potentials seem to be generated by frontal and temporal areas of cortex. Evidence from functional imaging studies supports the idea that a network of brain areas responds to contextual novelty. This network probably produces the enhanced memory that is seen for contextually novel stimuli.

  • The neurotransmitter acetylcholine and the neuromodulator noradrenaline seem to be important for the generation of this response. An integrated view of novelty processing could see the release of acetylcholine and noradrenaline being increased in response to a novel stimulus, and modulating the activity of the cortical areas to which they project. This in turn would lead to the production of novelty-related potentials and the modulation of synaptic plasticity.


The ability to detect and respond to novel events is crucial for survival in a rapidly changing environment. Four decades of neuroscientific research has begun to delineate the neural mechanisms by which the brain detects and responds to novelty. Here, we review this research and suggest how changes in neural processing at the cellular, synaptic and network levels allow us to detect, attend to and subsequently remember the occurrence of a novel event.

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Figure 1: Stimulus novelty effects in humans and monkeys.
Figure 2: The novelty P3.
Figure 3: Effects of lateral prefrontal or posterior medial temporal lesions on the P3.


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We thank C. Clayworth, R. Knight, K. Lamberty, K. Paller and M. Soltani for their generous assistance in figure preparation, and C. Brozinsky, N. Kroll and M. Kishiyama for helpful comments on earlier versions of this article. G.R. was supported by the Austrian Academy of Sciences and the Max Planck Society.

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Recognition memory tasks in which presentation of a stimulus is followed by a delay, after which a choice is offered. In matching tasks, the originally presented stimulus must be chosen; in non-matching tasks, a new stimulus must be selected. With small stimulus sets, the stimuli are frequently repeated and therefore become highly familiar. So, typically, such tasks are most readily solved by short-term or working memory rather than by long-term memory mechanisms.


If you stare at a set of lines that are tilted in one direction from upright, upright lines will subsequently look as though they are tilted in the opposite direction.


Small eye movements made during free viewing of natural scenes tend to expose neurons to similar but not identical structure. Adaptation can reduce responses to structure that is similar across fixations, removing correlations.


An area that receives visual inputs and produces movements of the eye.


An association between a set of stimuli and a set of eye movements has to be learned by trial and error, where each stimulus is associated with a particular eye movement.


Electrical potentials that are generated in the brain as a consequence of the synchronized activation of neuronal networks by external stimuli. These evoked potentials are recorded at the scalp and consist of precisely timed sequences of waves or 'components'.


 A variant of functional magnetic resonance imaging (fMRI) methods that allows neural correlates of individual trials or classes of trials to be isolated and compared.

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Ranganath, C., Rainer, G. Neural mechanisms for detecting and remembering novel events. Nat Rev Neurosci 4, 193–202 (2003).

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