Abstract
A recent hemodynamic model is extended and applied to simulate and explore the feasibility of detecting ocular dominance (OD) and orientation preference (OP) columns in primary visual cortex by means of functional magnetic resonance imaging (fMRI). The stimulation entails a short oriented bar stimulus being presented to one eye and mapped to cortical neurons with corresponding OD and OP selectivity. Activated neurons project via patchy connectivity to excite other neurons with similar OP in nearby visual fields located preferentially along the direction of stimulus orientation. The resulting blood oxygen level dependent (BOLD) response is estimated numerically via the model’s spatiotemporal hemodynamic response function. The results are then used to explore the feasibility of detecting spatial OD-OP modulation, either directly measuring BOLD or by using Wiener deconvolution to filter the image and estimate the underlying neural activity. The effect of noise is also considered and it is estimated that direct detection can be robust for fMRI resolution of around 0.5 mm, whereas detection with Wiener deconvolution is possible at a broader range from 0.125 mm to 1 mm resolution. The detection of OD-OP features is strongly dependent on hemodynamic parameters, such as low velocity and high damping reduce response spreads and result in less blurring. The short-bar stimulus that gives the most detectable response is found to occur when neural projections are at 45 relative to the edge of local OD boundaries, which provides a constraint on the OD-OP architecture even when it is not fully resolved.
Highlights
The overall field of view of the eyes is mapped in a topographic fashion to the primary visual cortex (V1) via a one-to-one retinotopic mapping [1,2,3,4]
A short oriented bar with ocular dominance (OD)-orientation preference (OP) features is mapped to the primary visual cortex and the blood oxygen level dependent (BOLD) response is numerically estimated via a spatiotemporal hemodynamic response
Our results show that direct OD-OP detection from BOLD is feasible for a resolution of 0.5 mm, while detection via Wiener deconvolution can be attained to resolutions from Δx 0.125 − 1.0 mm
Summary
The overall field of view of the eyes is mapped in a topographic fashion to the primary visual cortex (V1) via a one-to-one retinotopic mapping [1,2,3,4]. V1 cells are characterized by two response properties, ocular dominance (OD) and orientation preference (OP) [5,6,7,8,9]. OP is related to cells that respond best to a specific stimulus orientation, which changes linearly in patches of size 0.5 to 1 mm [10,11,12]. This demonstrates that all OD-OP features are mapped to within about 1 mm of any given point. A region that contains all feature preferences is termed a hypercolumn, and the part of the field of view mapped to a given hypercolumn is termed a visual field (VF). V1 contains patchy horizontal connections that preferentially connect neurons with similar OP in neighboring VFs, as seen in Fig 1(b); these projections extend furthest along an axis corresponding to the OP direction, with the overall zone of projections being elongated along the OP direction [13]
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