Neurophysiological effects of selective attention

Electroencephalographic data were collected to investigate the effects of selective attention on visual evoked potential (VEP) responses to contrast-modulated flicker. VEP amplitude was determined by Fourier analysis (FFT) of steady-state flicker epochs. Attention was controlled by a psychophysical task in which the subject was required to detect a faint isoluminant chromatic change in the attended target. Two types of visual target sets were employed. First, while the subject foveated on a central fixation-point, attention was directed either to an eccentric flickering wedge or to the fixation-point itself For every target set of this type, in the Attend-Fixation-Point condition, VEP amplitude was reduced to the level of noise in the spectral band surrounding the stimulus-driven frequency or was substantially attenuated. The second type of target set comprised a flickering wedge and an offset non-flickering wedge, both at an eccentric location in the visual field. For target sets of this type, comprising small flickering stimuli 0.36 x 0.36 degree of visual angle (v.a.) in size, separated by as little as 0.36 degree v.a. in the right visual hemifield, VEP amplitude was again reduced to the level of noise or substantially attenuated in the Attend-Nonflickering-Target condition. Small separated target sets in the left visual hemifield and small adjacent target sets in both visual hemifields showed less consistent attentional effects. Contrast response functions obtained from small separated target sets in the right visual hemifield evidenced a nearly linear increase of VEP amplitude for contrasts up to 25%, with saturation at higher contrasts. A variety of temporal flicker frequencies were tested, and all VEP responses obtained were in the beta bandwidth, whether at the fundamental flicker frequency, the second, or third harmonic. Attentional effects were replicated in both the contrast and frequency response experiments, and high resolution eye-tracking data demonstrated no differences in visual fixation between attentional conditions. Results demonstrated an absence of spatial gradients of attention above the threshold of perception. The slope and half-saturation values of the contrast response curves indicate that magnocellular pathways in V1 and V2 were the primary cortical areas involved in the VEP responses silenced by selective attention