Seminar | April 2 | 12-1:30 p.m. | 560 Evans Hall
Pascal Fries, Ernst Strüngmann Institute (ESI) for Neuroscience
Free viewing of natural images induces gamma-band oscillations in early visual cortex. If the gamma rhythm in a lower visual area entrains a gamma rhythm in a higher visual area, this might establish an effective communication protocol: The lower area sends a representation of the visual stimulus rhythmically, and the higher area is most excitable precisely when this representation arrives. At other times, the higher area is inhibited, which excludes competing stimuli. I refer to this scenario as the Communication-through-Coherence (CTC) hypothesis. Consistent with CTC, the gamma rhythm in awake macaque V4 modulates the gain of synaptic inputs. Constant optogenetic stimulation in anesthetized cat area 21a (homologue to V4) induces a local gamma rhythm, and this isolated gamma is sufficient to produce similar gain modulation. These gain modulation effects would be ideal to lend enhanced effective connectivity to attended stimuli. This is indeed the case between macaque V1 and V4. When two visual stimuli induce two local gamma rhythms in V1, only the one induced by the attended stimulus entrains V4. The gamma synchronization between V1 and V4 occurs at the phase relation that is optimal for stimulus transmission. I will then investigate how these changes in gamma synchronization between visual areas are controlled by influences from parietal cortex. Posterior parietal cortex influences visual areas primarily via beta-band synchronization. Generally, beta-band influences are stronger in the top-down direction, while gamma-band influences are stronger in the bottom-up direction. This holds across macaques and human subjects, and in both species it allows building a hierarchy of visual areas based on the directed influences. Finally, attentional selection occurs at a theta rhythm. When two objects are monitored simultaneously, attentional benefits alternate at 4 Hz, consistent with an 8 Hz sampling rhythm, sampling them in alternation. This theta rhythm can be seen in V1 and V4. The attended stimulus is monitored more continuously, whereas the unattended stimulus is sampled intermittently.