The spatial summation properties of visual signals were analyzed for geniculocortical afferents in the primary visual cortex (V1) of anaesthetized paralyzed macaque monkeys. Afferent input responses were recorded extracellularly during cortical inactivation through superfusion of the cortex with muscimol, allowing investigation of LGN cell properties in the absence of cortical feedback. Responses from afferent inputs were classified as magno-, parvo- or konio-cellular based on anatomical organization within the cortex, established through histological reconstructions, and visual response wavelength sensitivity. Greater than 80% of afferents showed strong surround suppression (SI > 0.5) and 14% showed negligible surround suppression (SI < 0.2). Afferent responses with weak and strong surround suppression were found throughout cortical input layers 4C and 4A. High contrast estimates of the spatial extent of the classical surround were similar to the non-classical surround. The classical and non-classical surrounds were, on average, 1.5 times larger than the excitatory center. Unlike neurons within V1, the spatial extent of excitatory summation for geniculocortical afferents was contrast invariant. Non-classical surround suppression showed slight contrast dependence with estimates larger (20%) at lower contrasts and stronger at higher contrasts (13%). Surround suppression is inherent in cortical input responses and likely derives from lateral inhibition in either the LGN or retina. Although surround suppression within afferent responses increases slightly with contrast, the spatial spread of excitation remains fixed with contrast. This argues for distinct mechanisms of action for contrast-dependent modulation in cortical and subcortical responses.