Neuronal Effects of Spatial and Feature Attention Differ Due to Normalization

Although spatial and feature attention have differing effects on neuronal responses in visual cortex, it remains unclear why. Response normalization has been implicated in both types of attention (Carandini and Heeger, 2011), and single-unit studies have demonstrated that the magnitude of spatial attention effects on neuronal responses covaries with the magnitude of normalization effects. However, the relationship between feature attention and normalization remains largely unexplored. We recorded from individual neurons in the middle temporal area of rhesus monkeys using a task that allowed us to isolate the effects of feature attention, spatial attention, and normalization on the responses of each neuron. We found that the magnitudes of neuronal response modulations due to spatial attention and feature attention are correlated; however, whereas modulations due to spatial attention are correlated with normalization strength, those due to feature attention are not. Additionally, spatial attention modulations are stronger with multiple stimuli in the receptive field, whereas feature attention modulations are not. These findings are captured by a model in which spatial and feature attention share common top-down attention signals that nonetheless result in differing sensory neuron response modulations because of a spatially tuned sensory normalization mechanism. This model explains previously reported commonalities and differences between these two types of attention by clarifying the relationship between top-down attention signals and sensory normalization. We conclude that similar top-down signals to visual cortex can have distinct effects on neuronal responses due to distinct interactions with sensory mechanisms.SIGNIFICANCE STATEMENT Subjects use attention to improve their visual perception in several ways, including by attending to a location in space or to a visual feature. Prior studies have found both commonalities and differences between the effects of spatial and feature attention on neuronal responses in visual cortex, although it is unclear what mechanisms could explain this range of effects. Normalization, a computation by which neuronal responses are modified by stimulus context, has been implicated in many neuronal mechanisms throughout the brain. Here we propose that normalization provides a simple explanation for how spatial and feature attention could share common top-down attention signals that still affect sensory neuron responses differently.