Mechanisms and Meaning of Neocortical Dynamics

The neocortex evolved after vertebrates were able to perform complex perceptual tasks. Its added value is almost certainly in providing mammals the capability to optimally function beyond genetic programming, optimization that emerges from rapid and long-term time scale dynamics in this structure and with its interconnected networks. A canonical example of rapid time-scale neocortical dynamics occurs with behavioral allocation of attention, following which the sensitivity and specificity of neocortical neurons changes substantially, in ways that predict enhanced information processing.

We study neocortical dynamics, with the goal of understanding their meaning for information processing. Crucial to understanding the meaning of dynamics, for example the expression of a neural oscillation during attention, is to understand underlying mechanisms, as these provide the real world 'embodied' constraints as to the potential functionality of the system.

CASE STUDY: NEOCORTICAL GAMMA OSCILLATIONS CAN ENHANCE PERCEPTION

An example of the progression between mechanism and meaning in our laboratory are recent studies of neocortical 'gamma' oscillations. Gamma oscillations are rhythmic activity patterns in the range from ~30-80 Hz that are increased in many neocortical areas during active processing, for example with the allocation of attention. The importance of this dynamics is a much fought over topic, with views ranging from the belief that these signals are key to consciousness perception, to the view that they are an ‘exhaust fume’ of computation, an epiphenomenal accident with no link to optimal sensory processing.

In initial mechanistic studies (Cardin et al., 2009 Nature; Cardin et al., 2010; Carlen et al., 2011), we used optogenetics to causally test in vivo the hypothesis that synchronization of a specific type of interneuron, fast-spiking cells, was key to the expression of gamma. We found that a highly naturalistic and specific expression of gamma was generated by selective drive of this ‘FS’ cell class, in agreement with a wealth of prior computational and correlative studies. We are now using this highly specific form of control to test the hypothesis that induced FS-gamma can enhance detection performance. We have found that endogenous expression of FS-gamma and specific emulation of FS-gamma by optogenetic drive both predict increased likelihood of correctly detecting a sensory input. These effects are interestingly unique to less salient stimuli—as with many effects in attention, there is no added benefit for the processing of innately salient, easier to perceive inputs.