Effect of attentive fixation in macaque thalamus and cortex

Attentional modulation of neuronal responsiveness is common in many areas o f visual cortex. We examined whether attentional modulation in the visual t halamus was quantitatively similar to that in cortex. Identical procedures and apparatus were used to compare attentional modulation of single neurons in seven different areas of the visual system: the lateral geniculate, thr ee visual subdivisions of the pulvinar [inferior, lateral, dorsomedial part of lateral pulvinar (Pdm)], and three areas of extrastriate cortex represe nting early, intermediate, and late stages of cortical processing (V2, V4/P M, area 7a). A simple fixation task controlled transitions among three atte ntive states. The animal waited for a fixation point to appear (ready state ), fixated the point until it dimmed (fixation state), and then waited idly to begin the next trial (idle state). Attentional modulation was estimated by flashing an identical, irrelevant stimulus in a neuron's receptive fiel d during each of the three states; the three responses defined a "response vector" whose deviation from the line of equal response in all three states (the main diagonal) indicated the character and magnitude of attentional m odulation. Attentional modulation was present in all visual areas except th e lateral geniculate, indicating that modulation was of central origin. Pre valence of modulation was modest (26%) in pulvinar, and increased from 21% in V2 to 43% in 7a. Modulation had a push-pull character (as many cells fac ilitated as suppressed) with respect to the fixation state in all areas exc ept Pdm where all cells were suppressed during fixation. The absolute magni tude of attentional modulation, measured by the angle between response vect or and main diagonal expressed as a percent of the maximum possible angle, differed among brain areas. Magnitude of modulation was modest in the pulvi nar (19-26%), and increased from 22% in V2 to 41% in 7a. However, average t rial-to-trial variability of response, measured by the coefficient of varia tion, also increased across brain areas so that its difference among areas accounted for more than 90% of the difference in modulation magnitude among areas. We also measured attentional modulation by the ratio of cell discha rge due to attention divided by discharge variability. The resulting signal -to-noise ratio of attention was small and constant, 1.3 +/- 10%, across al l areas of pulvinar and cortex. We conclude that the pulvinar, but not the lateral geniculate, is as strongly affected by attentional state as any are a of visual cortex we studied and that attentional modulation amplitude is closely tied to intrinsic variability of response.