Hubel and Wiesel (
1965,
1968) found that the response of V1 neurons in cat and monkey was reduced when an otherwise optimal stimulus was extended beyond the neuron's CRF. Since then, this type of inhibition, now referred to as surround suppression (which we will abbreviate as SS), has been widely explored in physiological and psychophysical studies (Bair, Cavanaugh, & Movshon,
2003; Cannon & Fullenkamp,
1991; DeAngelis, Freeman, & Ohzawa,
1994; Petrov, Carandini, & McKee,
2005; Webb, Dhruv, Solomon, Tailby, & Lennie,
2005). Despite years of research, the mechanism underlying SS remains unclear. Thalamocortical feedforward (Ozeki et al.,
2004), intracortical horizontal (Stettler, Das, Bennett, & Gilbert,
2002) and extrastriate feedback (Bair et al.,
2003) connections to V1 have been proposed to serve as a possible source of SS. It is assumed that SS is not attributable to a single mechanism but rather a combination of multiple sources (Angelucci & Bressloff,
2006; Sèries, Lorenceau, & Fregnac,
2003). Webb et al.'s (
2005) recent physiological study supports this hypothesis. They suggested that SS in primate V1 was generated by two distinct mechanisms, one early and one late. The early SS is untuned and monocularly driven and may originate in the lateral geniculate nucleus (LGN) or input layers of V1. The late one, however, is stimulus tuned and binocularly driven and is a cortical mechanism.