It is also important to note here the difference between a network tuning curve and a single-neuron tuning curve. The former is a coarse-grained representation of the CO blob response,with the horizontal axis representing the gamut of hue preferences within a single network. A relatively large tuning width would therefore indicate considerable responses from a wide range of hue tuning cells and poor network selectivity. The single-neuron tuning curve, onthe other hand, is an electrophysiological recording of an individual cell 's response to a set of hue stimuli, with the horizontal axis representing the range of stimulus hue angles used in the experiment. The peak location of the single-neuron tuning curve would therefore indicate the hue preference of the individual neuron, while the width would represent its selectivity forthat specific hue. Thus, though the two types of tuning curves are labeled and shaped similarly, the latter is only useful to characterize our network 's constituent neurons and notthe emergent properties of the population as a whole ( Bressloff & Cowan, 2003b ). Roles of the stimulus strength and cortical threshold. We begin by considering the role of the stimulus signal strength con the hue tuning width and peak height. Figure 6 shows typical tuning curves for two values of J1. We find that the stimulus strength has a quickly saturating effect on Δcfor all J1> 0, which is more pronounced at lower values of cas J10. Above saturation, the main contribution of the chromatic signal is to increase the network response, that is, to increase a∞(θ/C22). We also note that at T= 0, the trend reverses, such that increasing chas no effect on the tuning width at T= 0 and a widening effect for T>0. Figure 7 illustrates this reversal with four tuning curves of matched parameters and varying values of T. The coupling of cand Tmust be considered because some neural field models (see Amari, 1977 ;Carandini & Ringach, 1997 ; Dayan et al., 2001 ) take T= 0 for mathematical simplicity. Indeed, we might expect that there is no more physiological significance to choosing a threshold potential of T= 0 m V than any other value, beyond their relative magnitudes to h(θ,t). However, the independence of cand Δcat T= 0 and the significance of the relative signs of cand Telsewhere suggest quite the opposite. The effect of the chromatic input on tuning the network hue selectivity weakens notonly once the anisotropic strength parameter, J 1, is large enough to predominate, but also as T0. The coupling of cand Tis equally significant to the effects of Ton the tuning curve prop-erties. Figure 8 shows that below a certain value, Tprimarily modulates a∞(θ/C22). However, for Figure 6. Effect of con the tuning curve properties. The tuning role of cquickly saturates, while its effect on the network response rate grows without bound. For θ/C22=0, β=1, T=-1, and J0=-1. (A) J1=0. 2. ( B ) J1= 0. 7. Network Neuroscience 691Pattern forming mechanisms of color vision Downloaded from http://direct. mit. edu/netn/article-pdf/7/2/679/2118501/netn_a_00294. pdf by guest on 09 August 2024 