Tuned Normalization Explains the Size of Attention Modulations

The effect of attention on firing rates varies considerably within a single cortical area. The firing rate of some neurons is greatly modulated by attention while others are hardly affected. The reason for this variability across neurons is unknown. We found that the variability in attention modulation across neurons in area MT of macaques can be well explained by variability in the strength of tuned normalization across neurons. The presence of tuned normalization also explains a striking asymmetry in attention effects within neurons: when two stimuli are in a neuron's receptive field, directing attention to the preferred stimulus modulates firing rates more than directing attention to the nonpreferred stimulus. These findings show that much of the neuron-to-neuron variability in modulation of responses by attention depends on variability in the way the neurons process multiple stimuli, rather than differences in the influence of top-down signals related to attention. ► MT neurons show varying degrees of modulation by attention and normalization ► Attention and normalization modulation are correlated across the neuronal population ► An extension of normalization models, tuned normalization, explains this correlation ► The model also explains greater modulation from attending preferred versus null stimuli Attentional modulation of neuronal firing rates varies considerably within a cortical area. Ni et al. report that variability in the strength of tuned normalization can explain this effect, suggesting that variability in attentional response modulation depends on the way neurons process multiple stimuli.