Physical limits to spatial resolution of optical recording: Clarifying the spatial structure of cortical hypercolumns

Abstract

Neurons in macaque primary visual cortex are spatially arranged by their global topographic position and in at least three overlapping local modular systems: ocular dominance columns, orientation pinwheels, and cytochrome oxidase (CO) blobs. Individual neurons in the blobs are not tuned to orientation, and populations of neurons in the pinwheel center regions show weak orientation tuning, suggesting a close relation between pinwheel centers and CO blobs. However, this hypothesis has been challenged by a series of optical recording experiments. In this report, we show that the statistical error associated with photon scatter and absorption in brain tissue combined with the blurring introduced by the optics of the imaging system has typically been in the range of 250 m. These physical limitations cause a systematic error in the location of pinwheel centers because of the vectorial nature of these patterns, such that the apparent location of a pinwheel center measured by optical recording is never (on average) in the correct in vivo location. The systematic positional offset is Ϸ116 m, which is large enough to account for the claimed misalignment of CO blobs and pinwheel centers. Thus, optical recording, as it has been used to date, has insufficient spatial resolution to accurately locate pinwheel centers. The earlier hypothesis that CO blobs and pinwheel centers are coterminous remains the only hypothesis currently supported by reliable observation.hypercolumn model ͉ orientation maps ͉ visual cortex ͉ cytochrome oxidase blobs T he columnar organization of the neocortex is one of the seminal discoveries in neurobiology (1, 2). Neurons in vertical register within the cortex tend to have similar response properties, such as selectivity for oriented visual stimuli and ocular dominance (2). Perhaps even more important, however, was the discovery that columns were themselves organized into larger structures, termed hypercolumns, comprised of the full 180°range of orientation tuning for both eyes on the scale of Ϸ1 mm (3, 4). This idea has proven vital because it suggested a basic uniformity of cortical structure and a principle around which it might be organized. Because hypercolumn structure is critical to theories of both cortical function and its development, clarifying its details has been a central goal of neuroscience.Horton and Hubel (5, 6) showed that an additional periodic anatomical feature of primate visual cortex, the cytochrome oxidase (CO) blob system, consists of patches of cortical tissue most prominently appearing in layers II͞III but also appearing in layers I, IVb, V, and VI, with a size of Ϸ150 ϫ 250 m in primary visual cortex (V1) of macaque, and exhibiting higherthan-average metabolic activity, hence, staining darkly for CO activity. They observed the CO blob density to be approximately five per mm 2 , although Horton and Hocking (7) showed variation of a factor of two across subjects. The receptive field properties of neurons in the blobs is still an area of active research, but Livingstone and H...

Monte Carlo simulation of the spatial resolution and depth sensitivity of two-dimensional optical imaging of the brain

“…The light scattering and absorption in biological tissues make it difficult to do so; however, Polimeni et al have estimated the spatial resolution of IOI for a specific optical configuration using Monte Carlo simulation with tissue parameters describing the mammalian cortex. 9 Their result of 240 μm is consistent with the experimental result of Orbach and Cohen. 10 Other authors have mentioned a much smaller spatial resolution, though without offering evidence to support it.…”

High-field fMRI unveils orientation columns in humans

“…Subtle differences in eccentricity and/or inaccurate assignments of pinwheel centers might also contribute to the differences in pinwheel density (39). Assigning a ''pinwheel'' center (which is by definition a ''point'' in space) has limited accuracy because of the limited image resolution (40). Even in optical imaging, where the image resolution is much higher, the precise localization of pinwheel centers is limited.…”

Suppressed Neuronal Activity and Concurrent Arteriolar Vasoconstriction May Explain Negative Blood Oxygenation Level-Dependent Signal

“…Therefore, the observation that stimulus-evoked diameter change occurred on the arterial side suggests, but does not prove, the presence of active rather than passive processes. In the present study, we chose to examine surface vessels to facilitate the comparison with spectroscopic imaging that has high sensitivity to the cortical surface (Polimeni et al, 2005). The surface vessels have a lesser specificity to cortical columns and larger "upstream" dilation effects (Iadecola, 2004) than diving arterioles and their branches that feed well defined territories (Nishimura et al, 2007).…”