Thalamic deep brain stimulation as a paradigm to reduce consciousness: implications for cortico-striatal dynamics, absence epilepsy and consciousness studies

Abstract

Anesthetic manipulations provide much-needed causal evidence for neural correlates of consciousness, but non-specific drug effects complicate their interpretation. Evidence suggests that thalamic deep brain stimulation (DBS) can either increase or decrease consciousness, depending on the stimulation target and parameters. The putative role of the central lateral thalamus (CL) in consciousness makes it an ideal DBS target to manipulate circuit-level mechanisms in cortico-striato-thalamic (CST) systems, thereby influencing consciousness and related processes. We used multi-microelectrode DBS targeted to CL in macaques while recording from frontal, parietal, and striatal regions. DBS induced episodes reminiscent of absence epilepsy, here termed absence-like activity (ALA), with decreased behavior and vacant staring coinciding with low-frequency oscillations. DBS modulated ALA likelihood in a frequency-specific manner. ALA events corresponded to decreases in measures of neural complexity (entropy) and integration ({Phi}*), an index of consciousness, and substantial changes to communication in CST circuits. During ALA, power spectral density and coherence at low frequencies increased across CST circuits, especially in thalamo-parietal and cortico-striatal pathways. Decreased consciousness and neural integration corresponded to shifts in cortico-striatal network configurations that dissociated parietal and subcortical structures. Overall, the features of ALA and implicated networks were similar to those of absence epilepsy. As this same multi-microelectrode DBS method - but at different stimulation frequencies - can also increase consciousness in anesthetized macaques, it can be used to flexibly address questions of consciousness with limited confounds, as well as inform clinical investigations of absence epilepsy and other consciousness disorders. SIGNIFICANCEWe use tailored, multi-microelectrode thalamic deep brain stimulation to reversibly decrease consciousness for otherwise healthy, wakeful animals in a stimulation frequency-dependent manner. This represents a bidirectional mechanism for controlling consciousness, as the same method can increase consciousness under certain conditions. Theories of consciousness debate the relative contribution of parietal and frontal lobes, and largely ignore subcortical contributions. In this study, changes in consciousness predominantly involve changes in subcortical and parietal regions, implying that they contribute more to consciousness than frontal regions. Further, decreases in consciousness (indexed by {Phi}*) coincide with decreased movement, staring, and low-frequency activity in the EEG, similar to absence epilepsy. Thus, the systems-level mechanisms for decreased consciousness in this study have broader clinical implications for absence epilepsy.