Increasing evidence suggests that sensory stimulation not only changes the level of cortical activity with respect to baseline but also its structure. Despite having been reported in a multitude of conditions and preparations (for instance, as a quenching of intertrial variability, Churchland et al., 2010), such changes remain relatively poorly characterized. Here, we used optical imaging of voltage-sensitive dyes to explore, in V4 of an awake macaque, the spatiotemporal characteristics of both visually evoked and spontaneously ongoing neuronal activity and their difference. With respect to the spontaneous case, we detected a reduction in large-scale activity (cortical extent>1 mm) in the alpha range (5 to 12.5 Hz) during sensory inflow accompanied by a decrease in pairwise correlations. Moreover, the spatial patterns of correlation obtained during the different visual stimuli were on the average more similar one to another than they were to that obtained in the absence of stimulation. Finally, these observed changes in activity dynamics approached saturation already at very low stimulus contrasts, unlike the progressive, near-linear increase of the mean raw evoked responses over a wide range of contrast values, which could indicate a specific switching in the presence of a sensory inflow.
Depth selectivity is crucial for accurate depth volume probing in vivo in a large
number of medical applications such as brain monitoring. Polarization gating has been widely
used to analyze biological tissues. It is shown that using polarized light allows probing tissues
on a specific depth depending on the polarization illumination type (linearly, circularly) and
the tissues properties. However, accurate depth investigation of the tissue requires a high
selectivity of the probed depth. We propose and simulate the use of different elliptically
polarized illuminations for continuous depth examination between linearly and circularly
polarized illumination. Monte Carlo simulations verify that circularly polarized illumination
penetrates deeper than linearly polarized illumination in biological scattering media.
Furthermore, we show that elliptically polarized light can be tuned in its penetration depth
continuously between the penetration depth of linearly polarized light and circularly polarized
light. Experimental results obtained on phantoms mimicking in vivo situations are presented.
The method proposed here allows to perform a selection of a well defined
subsurface volume in a turbid medium allowing SNR enhancement for functional imaging of
the cortex. The principle consists in sequentially probing the biological tissue with light
polarized linearly or circularly. The method and preliminary results obtained on phantoms are