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 well as fMRI voxels containing hundreds of thousands of neurons. 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 neuronal representations and their sensitivity to various stimulus transformations (e.g., Grill-Spector et al. 1999).

fMRI-A is a flexible method that has been applied to wide variety of topics, but some experimental designs are better suited for studying particular questions than others. The purpose of this wiki page is to describe the necessary components of fMRI-A designs as well as the advantages of a few different experimental designs.

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 object perception, items are typically construed as specific object exemplars or faces (e.g., Grill-Spector et al., 1999), but fMRI-A is also observed in left inferior frontal cortex when object category is repeated (Vuilleumier et al. 2002). fMRI-A has also been used to address a range of topics outside of this domain from the orientation tuning of neurons in visual cortex (Fang et al., 2005) to the representation of numerosity in parietal cortex (Piazza et al., 2004). 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

Fang, F., Murray, S.O., Kersten, D., & He, S. (2005). Orientation-tuned fMRI adaptation in human visual cortex. Journal of Neurophysiology, 94(6), 4188-4195.
Grill-Spector, K., Kushnir, T., Edelman, S., Avidan, G., Itzchak, Y., & Malach, R. (1999). Differential processing of objects under various viewing conditions in the human lateral occipital complex. Neuron, 24(1), 187-203. 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.
Li, L., Miller, E.K., & Desimone, R. (1993). The representation of stimulus familiarity in anterior inferior temporal cortex. Journal of Neurophysiology, 69(9), 1918-1929.
Piazza, M., Izard, V., Pinel, P., Le Bihan, D., & Dehaene, S. (2004). Tuning curves for approximate numerosity in the human intraparietal sulcus. Neuron, 44(3), 547-555.
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.