Multiple Scales of Stimulus Representation in the Human Brain

National Institute Of Mental Health

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Abstract

During the past year, we have focused on 1) characterizing the representation of stimulus features in primary visual cortex and 2) studying how these stimulus representations interact with top-down cognitive signals, such as general arousal. Studies were carried out under clinical protocol NCT00001360. 1) Characterizing stimulus representations in human visual cortex. Stimulus orientation is one of the most basic stimulus features represented in primary visual cortex (V1). Yet, after more than 50 years of research, the representation of orientation is inadequately understood. Orientation-selective V1 neurons are organized at a fine spatial scale in a pseudo-periodic, columnar structure across the cortical surface. Using functional magnetic resonance imaging (fMRI), we have previously discovered an additional level of organization, a coarse-scale orientation bias in which each fMRI voxel in V1 exhibits an orientation preference that depends on the region of space that it represents. A central goal in the laboratory is to understand the neural computations that give rise to coarse-scale biases for stimulus features, and to learn how these biases interact with fine-scale patterns of stimulus selectivity. To accomplish this goal, we have developed a computational model of neural activity in V1. Using the model, we have demonstrated that three theoretically distinct neural mechanisms could give rise to the coarse-scale orientation bias: stimulus vignetting, neural gain fields, and asymmetric surround suppression. This modeling result is significant because it shows that these three distinct mechanisms can be confused with one another. For example, attempts to measure surround suppression, which is thought to be a marker for excitatory/inhibitory imbalance in cortex, and implicated in a wide range of neuropsychiatric disorders, could in fact be confounded with other neural mechanisms. Our empirical studies to date have focused on stimulus vignetting as a source of coarse-scale bias. However, our ongoing fMRI experiments are designed to isolate and separately measure all three neural mechanisms. Our work on understanding neural computations in the intact visual system is vital in determining how computations are altered in mental health and neurological disorders. 2) Endogenous brain states. A second theme in the laboratory is to test the influence of non-sensory cognitive signals in primary visual cortex. We have identified a type of brain activity that reflects a subject's general engagement in a task. This task-related activity contributes prominently to brain hemodynamic responses measured with fMRI, is independent of external visual stimuli, and instead reflects internal brain states. Task-related activity appears to be distinct from spatial attention in that it is global in cortex rather than retinotopically-specific. This activity may be related to general arousal. To test this hypothesis, in our ongoing experiments, we systematically varied 1) task difficulty, 2) the expected reward for correct performance, and 3) the temporal predictability of the task structure. We found that a global fMRI signal tracks arousal level in each of these task conditions, and shows a significance correlation with pupil size, which is a proxy for arousal. Understanding the neural mechanisms of global brain states has implications for the study of a number of neurological and psychiatric disorders, including schizophrenia, autism and Attention Deficit Hyperactivity Disorder (ADHD).