The effects of attentional field size on neuronal population responses in V1 of behaving monkeys

Ma’ayan Gadot, Itay Shamir and Hamutal Slovin

Spatial selective attention allocates the brain’s resources to process certain location (or locations) in the visual field, where stimuli can appear. Ample evidence demonstrated that visual attention facilitates perception and improves behavioral performance; however the neuronal mechanisms underlying spatial attention are not fully understood.

To investigate the neural correlates of spatial attention we used voltage sensitive dye (VSD) imaging and measured neural population responses from the primary visual cortex (V1) of behaving monkeys. We trained two macaque monkeys to perform a covert spatial attention task, where a briefly presented spatial cue (SC) indicated with high reliability to the location of a variable contrast target (focal attention). In addition, we manipulated the attentional field by presenting multiple SCs simultaneously (distributed attention).

Our behavioral results show improved performance under focal attention in respect to catch trials (invalid trials) and in the distributed condition, performance was better than invalid trials, but lower than the focal attention condition. In addition RTs were shorter in focal attention then on invalid cueing. We investigated the effect of spatial attention on the signal to noise ratio (SNR) of the neuronal responses. The VSD signal revealed that attention increased the population activity of V1 neurons. Neuronal responses in the imaged area were enhanced for the attend-in condition, relative to the attend-away condition. This attentional modulation persisted long after the SC was turned off. Furthermore, in the distributed attention condition, the attentional modulation was smaller compared to the attend-in condition. We also found that relative to the distributed condition, the activity in the attend-in condition was enhanced in regions retinotopicaly corresponding to the SC and its surrounding region but this enhancement declined at regions located more remotely from the SC. Finally, we tested the effect of attention on the neuronal variability. We calculated the trial-to-trial variability and noise correlation in the attend-in, attend-away and distributed conditions, but could not find a significant attentional effect.

To conclude, spatial attention induces increased responses of V1 neurons and modulates the attentional field size. Our results therefore, support the limited capacity hypothesis.