The common view of surround suppression is that it is mostly due to intracortical inhibition (Haider et al., 2010). However, others think that it operates through withdrawal of intracortical excitation (Ozeki et al., 2009). Perhaps intracortical excitation amplifies maximally the responses to stimuli that are small and have low contrast, and surround suppression is a loss

in this amplification. The traveling waves may reflect this amplification, and their disappearance at high contrast would be synonymous Selleckchem PF 01367338 with the appearance of surround suppression. To summarize, perhaps traveling waves participate both in facilitation (through their presence) and in suppression (through their absence). Indeed, long-range stimulus interactions turn from overall facilitatory at low contrast to overall suppressive when there is high contrast in a large region of visual space (Cavanaugh et al., 2002a; Kapadia et al., 1999; Polat et al., 1998; Sceniak ISRIB cell line et al., 1999). This idea is in line with the normalization model, a quantitative framework that can describe both facilitatory and suppressive stimulus interactions. In the model, the responses of neurons result from a division: in the numerator, there are signals from a region of space that drive the neuron, and in the denominator, there is a constant plus the signals

from the normalization pool (Carandini and Heeger, 2012). If the regions of space driving the numerator and denominator are suitably wide, normalization accounts for crotamiton multiple aspects of long-range stimulus interactions (Bonin et al., 2005; Cavanaugh et al., 2002b; Chen et al., 2001; Schwartz and Simoncelli, 2001). When overall contrast is high, the signals in the denominator reduce gain and limit the extent of spatial integration. Conversely, when overall contrast is low, the signals from the normalization pool are small relative to the constant in the denominator and do little to reduce gain and limit spatial integration. Indeed, an imaging study

showed that the traveling waves are well described by a common implementation of the normalization model (Sit et al., 2009). This study used VSD imaging to measure V1 responses to a small, briefly flashed stimulus (Figure 7). The time to peak of these responses was progressively delayed at greater distances from the center of activation, consistent with a traveling wave (Figure 7C). These data were fit by a version of the normalization model in which the divisive interaction is mediated by a resistor-capacitor circuit (Figure 7A). Increasing the conductance of this circuit causes not only a divisive reduction of response gain but also a shortening of response latency (Carandini and Heeger, 2012). This effect is largest at the center of the stimulated region, where local contrast is highest. The responses at the center therefore rise at a faster rate than those at the periphery.