The balance between transient and sustained temporal response varies across the V1 visual field map

Abstract

Purpose: The temporal impulse response in V1 reflects multiple neural and hemodynamic components. We analyze these components by modeling the temporal response as a mixture of several terms and comparing the relative contributions of these terms across the cortical surface.

Methods: Functional MRI measurements were made using a 3T scanner (Siemens, Trio, Erlangen, Germany). We dilated subjects' pupils with mydriatics and placed semi-transparent, hemisphere-shaped diffusers on their eyes to eliminate spatial contrast (Ganzfeld). Subjects viewed spatially uniform brightness that alternated between a low and high luminance every 24 seconds (square wave). We modeled the temporal response as the sum of two terms. One corresponds to a signal at the luminance step (transient) and a second that corresponds to the mean illumination (sustained). Both regressors were convolved with the hemodynamic response function. We also measured the visual field eccentricity maps.

Results: The temporal responses varied as we measured from posterior to anterior calcarine sulcus. The responses spread far above 14 degrees isoeccentricity line which we could precisely determine from the eccentricity map. The sustained response decreased with visual field eccentricity, while the transient responses increased with increasing eccentricity. In the central representation, the ratio of the transient reponse to the sustained was 0.52 ± 0.18; in the periphery the ratio was 2.39 ± 0.26.

Discussion: The responses in central visual field representation depend more on the absolute luminance level (sustained) while in the periphery responses are influenced more by instantaneous luminance changes. This difference might reflect the differential weighting of cell types in these regions of V1.