title: Enhanced sensitivity for peripherally-presented collinearly-aligned stimulus elements: contour detection or spatial summation?
abstract: BACKGROUND: It has been shown that the detection threshold for a foveally presented Gabor stimulus (Gaussian-windowed sinusoidal grating) is reduced in the presence of two suprathreshold flanking Gabors and that such facilitation is orientation and configuration dependent, with maximal threshold reductions occurring with collinear flanking elements. METHODS: We assessed the effect of varying relative orientation on detection thresholds for chains of three Gabors, with 0.96(o) centre-to-centre spacing, presented at 4.8(o) in the periphery. The central Gabor element in each chain was either collinear with its flanking elements or oriented orthogonally to them. RESULTS: Thresholds for the configuration as a whole were found to be lower using the collinear configuration compared with the orthogonal configuration, consistent with the proposal that orientation-specific long-range intracortical interactions are involved. However, this configuration-dependent threshold difference disappeared when the spatial frequency of the Gabors was doubled from 3.33 cpd to 6.66 cpd. CONCLUSIONS: Configurational dependencies may disappear at higher spatial frequencies because, despite increased spatial summation at detection threshold contrast levels, the intrinsically smaller receptive field sizes of neurons tuned to high spatial frequencies render them incapable of encapsulating more than one of the spatially separated Gabors within their individual receptive fields.