, 2011). To incorporate competitor strength-dependent

inhibition, we took the suppression factors sin and sout to be proportional to the activity I of the inhibitory units driven by stimuli located outside the Cilengitide in vitro RF: equation(4) sin=din·I,sout=dout·Isin=din·I,sout=dout·Iwhere din and dout were proportionality constants, and I was the inhibitory activity driven by the competitor. Recordings of Imc responses to single looming stimuli have shown that they are well fit by sigmoidal functions (S.M., unpublished data). Consequently, inhibitory activity as a function of the loom speed of the competitor stimulus was modeled as having the same form as Equation 1: equation(5) I=m+h(lklk+s50k) The free parameters were m, the minimum response; h, the maximum change in response; S50, the loom speed that yields a half-maximum response; and k, a factor that controls response saturation. The effect

of changing the values of each of these parameters on I is illustrated in Figure S1. A linear dependence between the input and output divisive factors (sin and sout) and the inhibitory activity (I) was assumed in Equation 4 for simplicity. This formulation minimized the number of free parameters in the model, while still allowing for nonlinear competitor strength-dependent response suppression, due to the FRAX597 datasheet nonlinearity of I. We now describe the special response properties underlying strongest versus other categorization that need to be accounted for by the model. These were revealed in experiments in which a looming stimulus of fixed speed was presented inside the RF, while a second competing stimulus of variable speed was presented far outside the RF, about 30° away. The resulting responses are referred to as the competitor strength-response profile, or CRP (Mysore et al., 2011). Essential to the explicit representation of categories in the OTid is the abrupt, switch-like increase in response suppression, observed in about 30% of OTid neurons, Carnitine dehydrogenase as the strength of a competing stimulus is increased (Figure 2D, right). The abruptness of the transition is quantified as the range of competitor strengths

over which CRP responses drop from 10% to 90% of the maximum change in response and is referred to as the transition range. Switch-like CRPs were defined as those for which the transition range was very narrow: ≤4°/s, equivalent to ≤1/5 of the full range of loom speeds tested. Population activity patterns that include switch-like responses (along with non-switch-like responses) explicitly categorize stimuli into two categories, strongest and others, as determined by crosscorrelational analysis (Mysore and Knudsen, 2011a). Conversely, excluding the top 20% of the neurons with the most abrupt response transitions (switch-like responses) from the population analysis eliminates categorization by the population activity.