  Classical center-surround antagonism in the early visual system is thought to
serve important functions such as enhancing edge detection and increasing
sparseness. The relative strength of the center and surround determine the
specific computation achieved. For example, weak surrounds achieve low-pass
spatial frequency filtering and are optimal for denoising when signal-to-noise
ratio (SNR) is low. Balanced surrounds achieve band-pass spatial frequency
filtering and are optimal for decorrelation of responses when SNR is high.
Surround strength has been measured in the retina and dorsal Lateral Geniculate
Nucleus (dLGN), primarily in anesthetized or ex vivo preparations. Here we
revisit the center-surround architecture of dLGN neurons in the un-anesthetized
rat. We report the spatial frequency tuning responses of N=47 neurons. We fit
these tuning curves to a difference-of-Gaussians (DOG) model of the spatial
receptive field. We find that some dLGN neurons in the awake rat (N=8/47) have
weak surrounds. The majority of cells in our sample (N=29/47), however, have
well-balanced center and surround strengths and band-pass tuning curves. We
also observed several neurons (N=10/47) with notched or dual-band-pass tuning
curves, a response class that has not been described previously. Within the
space of circularly concentric DOG models, strong surrounds were necessary and
sufficient to explain the dual-band-pass spatial frequency tuning of these
cells. It remains to be determined what advantage if any is conferred by this
novel response class, or by the heterogeneity of surround strength as such. We
conclude that surround antagonism can be strong in the dLGN of the awake rat.
