The representation of any stimulus can be seen as a unique and distributed pattern of activity in a large population of neurons. These neural representations are thought to undergo a series of transformations across processing stages in visual cortex, and to depend on behavioral demands. The nature of these transformations reveals important insights into the computational mechanisms underlying the formation of behaviorally meaningful neural representations from incoming sensory signals. In this proposal, the focus is on measuring the neural representations and characterizing the transformations for a specific visual modality: color. The representation of color takes on many different forms. For example, humans discriminate between many thousands of hues but use only a handful of discrete color categories. This makes color an ideal candidate to investigate distributed neural representations. The novel empirical and theoretical approaches in the current proposal aim to significantly advance understanding of 1) how the human visual system represents color, 2) how this distributed neural representation is transformed across the hierarchy of visual cortical areas 3) the dependence of these representations on behavioral demands, and 4) the dependence on context.
Aim 1 experiments will test the hypothesis that the neural representation of color is transformed as chromatic signals ascend the visual system. Neural color spaces will be derived from functional magnetic resonance imaging (fMRI) measurements, using novel experimental protocols and multivariate data analysis techniques. These neural color spaces will be compared with perceptual color spaces derived from psychophysical measurements of color discrimination and categorization.
Aim 1 experiments will also test the hypothesis that neural representations of color depend on behavioral demands. The proposed experiments will distinguish between two specific computational hypotheses: 1) that neural color spaces change for different behavioral tasks, indicating a change in the underlying selectivity and tuning of the neurons, versus 2) a (possibly selective) increase in response gain, with no evidence for a change in the color space.
Aim 2 experiments will test the hypothesize that changes in color perception, due to a dramatic visual illusion, are correlated with corresponding shifts in the underlying neural representation, and that the extent of the shift in the neural representation varies between visual areas, depending on the neural color space in each visual area. Ultimately, color provides a model for more complex neural representations (e.g., those underlying face and object recognition, control of movement, etc.) and the findings will provide general insights about distributed neural processes and representations. Consequently, the proposed research will provide information about how the brain transforms an incoming set of signals (of any modality) into a set of meaningful representations that subserve a multitude of tasks.
The current proposal builds on previous work to study how the brain represents visual information, the computations the brain performs to transform representations of visual information, and the relationship between neural activity and visual perception. Consequently, the outlined approach will be of great benefit in understanding amblyopia, low vision, and aperceptive visual agnosias, as the methods developed in the proposal can be used to characterize distributed neural representations of other stimulus features besides color including, for example, contrast, orientation, depth, motion, etc. In addition, these methods for characterizing large-scale, distributed neural representations, non-invasively in the human brain, will be helpful for evaluating the efficacy of any therapy for visio restoration, whether it be a therapy for cortical visual deficits in amblyopia or a retinal prostheis for degenerative retinal diseases.
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| Wang, Helena X; Merriam, Elisha P; Freeman, Jeremy et al. (2014) Motion direction biases and decoding in human visual cortex. J Neurosci 34:12601-15 |
| Brouwer, Gijs Joost; Heeger, David J (2013) Categorical clustering of the neural representation of color. J Neurosci 33:15454-65 |
| Freeman, Jeremy; Heeger, David J; Merriam, Elisha P (2013) Coarse-scale biases for spirals and orientation in human visual cortex. J Neurosci 33:19695-703 |
