Spatial attention affects brain activity in human primary visual cortex

Abstract

Functional MRI was used to test whether instructing subjects to attend to one or another location in a visual scene would affect neural activity in human primary visual cortex. Stimuli were moving gratings restricted to a pair of peripheral, circular apertures, positioned to the right and to the left of a central fixation point. Subjects were trained to perform a motion discrimination task, attending (without moving their eyes) at any moment to one of the two stimulus apertures. Functional MRI responses were recorded while subjects were cued to alternate their attention between the two apertures. Primary visual cortex responses in each hemisphere modulated with the alternation of the cue; responses were greater when the subject attended to the stimuli in the contralateral hemifield. The attentional modulation of the brain activity was about 25% of that evoked by alternating the stimulus with a uniform field.

Figure 1

Experimental design and protocol. (a) Stimuli were moving sinusoidal gratings restricted to two peripheral, circular apertures (3° diameter, centered at 7° eccentricity). Fixation mark indicates “attend left.” (b) Main experiment: Subjects performed a speed discrimination task, attending alternately for a series of trials to the right, then for a series of trials to the left, and so on. (c) Baseline experiment: Subjects alternately performed the speed discrimination task for a series of trials (as in the main experiment) and then viewed a uniform gray field.

Figure 2

Attentional modulation in V1. Polar plots of fMRI responses while subjects alternated attention between the right and left stimulus apertures. (a) subject S.P.G., (b) subject G.M.B. The response amplitude (percentage magnetic resonance signal modulation) indicated by radial distance from the origin and response temporal phase indicated by the angle from the horizontal axis. Responses from left hemisphere V1 (open symbols) are near 0°, in phase with the cue to attend right. Responses from right hemisphere V1 (filled symbols) are near 180°, in phase with the cue to attend left. Small symbols plot results from each of eight repeated measurements (separate scans), for each of the two subjects. Large symbols represent the vector mean of the eight repeats.

Figure 3

Distributions of the V1 attentional modulation index for the 16 repeated measurements (eight scans, two hemispheres) in each subject. Each panel corresponds to one of the two subjects. The dotted vertical line indicates an attentional modulation index of 0.