Attention spans multiple stimulus dimensions in macaque visual cortex

Jitendra Sharma*, James Schummers, Paymon Hosseini, Mriganka Sur

Department of Brain & Cognitive Sciences, The Picower Center for Learning and Memory, MIT, Cambridge, MA, USA

Visuo-spatial attention involves a two way process of enhancing perception of the attended stimulus while inhibiting non-attended stimuli. Attentional modulations have been likened to contrast gain mechanisms that serve to bias competitive interactions between multiple stimuli towards the attended stimulus. However, the locus and mechanism underlying such biased processing remains unresolved. We have investigated the effect of attention on responses of V1 neurons in monkeys trained to attend to a cue, by asking whether the same visual stimulus, placed on the receptive field of the neuron, leads to different responses when attention is directed towards the receptive field (attend-to) compared to when attention is directed away from the receptive field (attend-away). In 128 orientation tuned V1 neurons recorded from 2 monkeys, we find that in 65% of neurons there was significant increase in response modulation (p<0.01) in attend-to condition. We also find that attention differentially affects superficial and deep layer neurons. In superficial layers, significantly more neurons were modulated in attend-to condition whereas deeper layers, neurons showed preferential modulation in the attend-away condition. Next we studied temporal dynamics of attentional modulation in various stimulus dimensions in striate (V1) and extra-striate (V4) cortex. Analysis of the time course of attentional modulation shows significant differences in individual neurons for stimulus size, orientation and contrast. Our results suggest that visual spatial attention directed towards the receptive field of V1 and V4 neurons modulate responses in multiple stimulus dimensions. This argues against the notion of a “hard wired” substrate that enhances contrast of the attended stimulus, as a mechanism for attentional modulation. Further, in many neurons, this modulation is a non-linear enhancement in response to the preferred stimulus feature. These data implicate attention as a dynamic interaction between top-down and bottom-up processes.

Supported by NIH grant EY07023.