Attentional modulation in the human visual cortex: the time-course of the BOLD response and its implications.

Throughout the visual areas of the brain, the sensory response to a stimulus is enhanced by attending to the stimulus. Neurophysiological studies in primates show that such enhancement is marked in posterior parietal cortex and some anterior occipital areas, but much more modest in the earliest processing stages, such as the primary visual cortex (V1). In contrast, human fMRI studies show large and robust attentional modulation in all visual areas, including V1. We investigate the possibility that, in the case of fMRI, the BOLD (blood oxygen level dependent) response may be increased not only by local attention-related increases in neural activity, but also by local blood-flow increases caused by remote control systems that anticipate an impending need for oxygen at the attended location. Such changes could be much more rapid than the rather slow response to oxygenation change that typifies the BOLD response. We have employed a paradigm that isolates the component of the BOLD response due to attentional modulation and the component due to the mere presence of a visual stimulus. The results show that the temporal profiles of the BOLD responses in human V1 to the onset of a stimulus and to the onset of attention are extremely similar. The time-course of the attention-related BOLD response is not consistent with the action of remote, anticipatory control mechanisms and suggests that the modulatory effect of attention seen in human V1 with fMRI probably reflects genuine changes in local neural activity that are considerably larger than in non-human primates.