Anthony Stigliani

Repetition of a stimulus typically leads to a reduction in neural response. This adaptation effect, sometimes known as repetition suppression or neural priming, can be observed both in individual neurons (as illustrated by a reduction in firing rate) and fMRI voxels containing hundreds of thousands of neurons (as illustrated by a reduction in BOLD response). When measured with fMRI, this repetition-related reduction in neural activity is known as fMRI-Adaptation (fMRI-A) and can be used to make inferences about the nature of neuron representations and their sensitivity to various stimulus transformations (e.g., Grill-Spector et al. 1999). While fMRI-A has proven to be a powerful and flexible tool for studying a variety of topics, some experimental designs are more optimal for studying particular phenomena than others. The purpose of this wiki page is to outline the critical components of fMRI-A methods and describe different types of experimental designs that may be used.

Background

Response of an example IT neuron to first the presentation of stimulus A (left), the first presentation of stimulus B (center), and repeated presentation of stimulus A (right) (courtesy of Grill-Spector et al. 2006).

Reductions in neural firing related to stimulus repetition were first observed in electrophysiological recordings of neurons in macaque inferior temporal (IT) cortex (e.g., Li et al., 1993). Instead of representing a general reduction in neural response when any two stimuli are shown in quick succession, response attenuation is only observed only when the same stimulus is repeated. This raises the questions of what constitutes a stimulus or item, but to some extent this depends on the particular question you want to address.

In studies of visual object recognition, items are typically construed as specific object exemplars or faces, but fMRI-A has been observed when different objects belonging to the same category are presented sequentially (Vuilleumier et al. 2002). However, the power of this approach lies in its ability examine how various transformations to a stimulus affects its neural representation. For example, one can probe whether the same populations of neurons in a particular brain region respond to the same face when viewed from different angles (e.g., front-facing vs. profile views).

Measuring fMRI-A

In order to use repetition-related response reductions to make inferences about the selectivity of a neuron or population of neurons involved in representing a stimulus, three measurements must be made:

1. Neural response when an identical stimulus is repeated
2. Neural response when different stimuli are presented
3. Neural response when a stimulus is repeated but varied along one dimension

Neural responses to the first two conditions must be measured in order to define a dynamic range within which to compare response to the third condition.

References

Grill-Spector, K., Henson, R., & Martin, A. (2006). Repetition and the brain: neural models of stimulus-specific effects. Trends in Cognitive Science, 10(1), 14-23.
Vuilleumier, P., Henson, R.N., Driver, J., & Dolan, R.J. (2002). Multiple levels of visual object constancy revealed by event-related fMRI of repetition priming. Nature Neuroscience, 5(2), 491-499.