Fourier Optical Imaging of Discrete Stimuli in Primate Somatosensory Cortex

Abstract

Optical imaging of intrinsic signals (OIS) has improved our understanding of the functional organization of the cerebral cortex. However, the technique places time constraints on experiments due to the averaging needed to overcome cardiovascular noise. A novel OIS method, using continuous image acquisition of a periodically presented stimulus, followed by a Fourier decomposition, promises to reduce experiment time while separating the stimulated response from the cardiovascular noise. Discrete, periodic finger stimulation and Fourier OIS were used to generate a functional map of finger somatotopy in squirrel monkey SI cortex. Phase delay correction was successfully performed through both stimulus reversal/subtraction and subtraction of a single, discrete stimulus. Magnitude borders between digits were described, and a harmonic analysis showed that they are caused by overlapping response from the adjacent finger representation. Digit selectivity analysis demonstrated a selectivity gradient within digit representations in both cortical areas 3b and 1. Significance maps were generated by calculating the signal-to-noise ratio (SNR) for each pixel. SNR maps required more stimulation repetitions than magnitude maps, but demonstrated higher sensitivity for regions with low activity and less overall blood vessel artifact. Finally, a comparison to episodic OIS and electrophysiological receptive field mapping showed that Fourier OIS maps are comparable to those standards.