The extraction and detection of specific responses from a large amount of background noise has been the subject of a considerable body of research in brain functional imaging, and more specifically in optical intrinsic signal imaging. Recent work by Kalatsky and Stryker (2003) showed that by combining different conditions and using periodic stimuli, recording times can be reduced. Spectral decomposition is then used to provide amplitude and phase information locked to the stimulus. A drawback of the above method is that by focusing only on a single harmonic, response information is limited. The shape of the hemodynamic response function (HRF) and the temporal variations in the neural responses cannot be assessed. In this work it is argued that additional information about neural responses can be gathered by using higher harmonics. Moving bars were used to generate visuotopic maps on large portions of the cat visual cortex. Up to four simultaneously bars moving repetitively across the visual field at different frequencies were used to sample the HRF in the Fourier domain. The HRF profile obtained with continuous stimulation was spatially homogeneous throughout the cortex and similar to the HRF profile obtained using episodic stimulation. Furthermore, by modeling the optical response as a convolution between HRF and neuronal responses, the ratio of the second harmonic to the first provided an estimation of the receptive field size. This was further validated by measuring spatial frequency selectivity. Therefore, the use of higher harmonics opens new avenues to estimate receptive field size from temporal signals.
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