EAGER: Separating BOLD Nonlinearity from Neuronal Nonlinearity in Human with Achiasma

University Of Southern California, Los Angeles CA

Investigators

Abstract

Functional magnetic resonance imaging (fMRI) is a noninvasive technique for observing brain activity. It is difficult to overestimate the role fMRI plays in the modern study of neuroscience. While fMRI is an indispensible tool for studying the neural system, the specific relationship between neural activity and the most commonly used fMRI signal, called the blood oxygenation level dependent (BOLD) signal, is not well understood. With funding from the National Science Foundation, Bosco Tjan, Ph.D., of the University of Southern California, is determining the precise linkage between neural activity and the BOLD signal by studying the visually-evoked BOLD signal in the brain of an achiasmatic person. Achiasma is a rare congenital condition; a person with achiasma is born without an optic chiasm, which is the part of the brain where half of the nerve fibers from each eye cross over to reach the opposite side of the brain. Achiasma's unusual development of the visual system has striking effects upon the mapping of the visual world onto the visual cortex. Normally, there is a one-to-one topological relationship between a point in the visual world and a location on an early visual cortical area. For example, when looking at the center of a clock face, each of the numbers on the clock face is projected to one distinct location on the primary visual cortex. In contrast, when an achiasmatic person looks at the same clock face, the symmetric locations about the vertical midline are projected to the same cortical locations in the early visual areas. For example, the numbers 4 and 8 on the clock face are both projected to the same location in the primary visual cortex, while the numbers 5 and 7 are both projected to a nearby location. Fortunately, an achiasmatic person does not confuse the left from the right half of a clock face, or the visual world for that matter. As it turns out, there are two comparable groups of neurons packed in the same cortical location in the visual cortex of an achiasmatic person, with one responsible for a left location of the visual world, and the other for the symmetric right location. These two groups of neurons appear to function independently of each other, but nevertheless share part of the same blood supply. By presenting identical stimuli to one or both sides of a pair of symmetric locations about the vertical midline, the investigator can effectively half or double the underlying neural activity that drives the blood supply at that location, without knowing the exact relationship between the stimulus and the absolute amount of the evoked neural activity. He can then measure the resulting fMRI BOLD signal, which is a signal related to the local blood flow, blood volume and blood oxygen concentration, and thereby characterize the mathematical relationship between neural activity and the BOLD signal. Knowing the quantitative relationship between the BOLD signal and the underlying neural activity is of the utmost importance. It will allow researchers to use noninvasive fMRI measurement to quantify the underlying neural activity everywhere in the brain. In cognitive neuroscience, we may one day be able to use fMRI to quantitatively measure certain psychological constructs and provide deeper insight into the mechanics of the brain. For example, if the level of neural activity in a brain area is believed to be proportional to the subjective value of a reward a study participant has received, then observing noninvasively an x% change in the fMRI BOLD signal will allow a researcher to infer that the subjective value of the reward has changed by y%. The current investigation, although focused on the arguably aberrant visual system of an achiasmatic subject, can have a broad and sweeping impact on many subfields of cognitive neuroscience by providing a unique and decisive data set relating neural activity to the fMRI BOLD signal.