Neural mechanisms of transient neocortical beta rhythms: Converging evidence from humans, computational modeling, monkeys, and mice

Alpha-band oscillations are the dominant oscillations in the human brain and recent evidence suggests that they have an inhibitory function. Nonetheless, there is little doubt that alpha-band oscillations also play an active role in information processing. In this article, I suggest that alpha-band oscillations have two roles (inhibition and timing) that are closely linked to two fundamental functions of attention (suppression and selection), which enable controlled knowledge access and semantic orientation (the ability to be consciously oriented in time, space, and context). As such, alpha-band oscillations reflect one of the most basic cognitive processes and can also be shown to play a key role in the coalescence of brain activity in different frequencies.

Cortical connections within the occipital lobe (areas 17, 18 and 19) of the rhesus monkey are investigated with the autoradiographic and horseradish peroxidase procedures. Two efferent systems, each with a specific laminar organization, are observed. (1) Rostrally directed connections, from area 17 to 18, area 18 to 19 and area 19 to the inferotemporal region (area TE), originate from neurons in layer IIIc (and, in area 19, from a small complement of neurons in layer Va), and terminate in and around layer IV. (2) In contrast, connections in the reverse direction ('caudally directed' connections), from area TE to 19, area 19 to 18, and area 18 to 17, originate from neurons in layers Vb, VI and, to a lesser extent, IIIa, and terminate mainly in layer I. In addition, the laminar organization of several intrinsic and callosal connections are observed. In trinsic connections within areas 18 and 19 originate from neurons in layers IIIc and, to a lesser extent, Va, and terminate in vertical bands in layers I to IV. Callosal connections from areas 18, 19, and the caudal inferotemporal region originate from neurons mainly in layer IIIc. From areas 18 and 19, these callosal connections terminate in vertical bands in layers I through IV. Thus, different cortical projection systems are characterized by specific laminar distributions of efferent terminations as well as of their neurons of origin.