Intralaminar and medial thalamic influence on cortical synchrony, information transmission and cognition

The intralaminar and medial thalamic nuclei are part of the higher-order thalamus, which receives little sensory input, and instead forms extensive cortico-thalamo-cortical pathways. The large mediodorsal thalamic nucleus predominantly connects with the prefrontal cortex, the adjacent intralaminar nuclei connect with fronto-parietal cortex, and the midline thalamic nuclei connect with medial prefrontal cortex and medial temporal lobe. Taking into account this connectivity pattern, it is not surprising that the intralaminar and medial thalamus has been implicated in a variety of cognitive functions, including memory processing, attention and orienting, as well as reward-based behavior. This review addresses how the intralaminar and medial thalamus may regulate information transmission in cortical circuits. A key neural mechanism may involve intralaminar and medial thalamic neurons modulating the degree of synchrony between different groups of cortical neurons according to behavioral demands. Such a thalamic-mediated synchronization mechanism may give rise to large-scale integration of information across multiple cortical circuits, consequently influencing the level of arousal and consciousness. Overall, the growing evidence supports a general role for the higher-order thalamus in the control of cortical information transmission and cognitive processing

Cognitive and Perceptual Functions of the Visual Thalamus

The thalamus is classically viewed as passively relaying information to the cortex. However, there is growing evidence that the thalamus actively regulates information transmission to the cortex and between cortical areas using a variety of mechanisms, including the modulation of response magnitude, firing mode, and synchrony of neurons according to behavioral demands. We discuss how the visual thalamus contributes to attention, awareness, and visually guided actions, to present a general role for the thalamus in perception and cognition

Electrophysiological Low-Frequency Coherence and Cross-Frequency Coupling Contribute to BOLD Connectivity

SummaryBrain networks are commonly defined using correlations between blood oxygen level-dependent (BOLD) signals in different brain areas. Although evidence suggests that gamma-band (30–100 Hz) neural activity contributes to local BOLD signals, the neural basis of interareal BOLD correlations is unclear. We first defined a visual network in monkeys based on converging evidence from interareal BOLD correlations during a fixation task, task-free state, and anesthesia, and then simultaneously recorded local field potentials (LFPs) from the same four network areas in the task-free state. Low-frequency oscillations (<20 Hz), and not gamma activity, predominantly contributed to interareal BOLD correlations. The low-frequency oscillations also influenced local processing by modulating gamma activity within individual areas. We suggest that such cross-frequency coupling links local BOLD signals to BOLD correlations across distributed networks

Inhibition and inhibitory plasticity in the mammalian auditory midbrain

Neuronal plasticity that involves large scale anatomical changes is restricted to the early period of development. Beyond this there is a critical period during which processes of formation of arborization patterns of afferent axons and of synaptic contacts form a basis for plasticity (for a review see Rauschecker, 1991). Although these findings set limits on the prospects for plasticity in the mature nervous system, a number of experimental paradigms have demonstrated considerable functional plasticity in the adult brain, These involve study of the topographic representations of the somatosensory and motor cortices (for reviews see Dykes, 1990; Kaas, 1991; Colford, 1995), the auditory cortex (Robertson and Irvine, 1989; Rajan et aI., 1993) and primary visual cortex (Kaas et aI., 1990; Gilbert and Wiesel, 1992) following a restricted nerve injury or a behavioural manipulation (Recanzone et aI., 1992a,b, 1993). These topographic representations have been chosen as appropriate models because they serve as scales against which any induced changes can be measured. Work in this laboratory over the past nine years has given emphasis to the role of inhibition in the early events following the loss (or inactivation) of a subset of the inputs to a brain area. The initial event is a disinhibition which allows expression (unmasking) of otherwise ineffective inputs

The Cognitive Thalamus

Cognitive processing is commonly conceptualized as being restricted to the cerebral cortex. Accordingly, electrophysiology, neuroimaging and lesion studies involving human and animal subjects have almost exclusively focused on defining roles for cerebral cortical areas in cognition. Roles for the thalamus in cognition have been largely ignored despite the fact that the extensive connectivity between the thalamus and cerebral cortex gives rise to a closely coupled thalamo-cortical system. However, in recent years, growing interest in the thalamus as much more than a passive sensory structure, as well as methodological advances such as high-resolution functional magnetic resonance imaging of the thalamus and improved electrode targeting to subregions of thalamic nuclei using electrical stimulation and diffusion tensor imaging, have fostered research into thalamic contributions to cognition. Evidence suggests that behavioral context modulates processing in primary sensory, or first-order, thalamic nuclei (for example, the lateral geniculate and ventral posterior nuclei), allowing attentional filtering of incoming sensory information at an early stage of brain processing. Behavioral context appears to more strongly influence higher-order thalamic nuclei (for example, the pulvinar and mediodorsal nucleus), which receive major input from the cortex rather than the sensory periphery. Such higher-order thalamic nuclei have been shown to regulate information transmission in frontal and higher-order sensory cortex according to cognitive demands. This Research Topic aims to bring together neuroscientists who study different parts of the thalamus, particularly thalamic nuclei other than the primary sensory relays, and highlight the thalamic contributions to attention, memory, reward processing, decision-making, and language. By doing so, an emphasis is also placed on neural mechanisms common to many, if not all, of these cognitive operations, such as thalamo-cortical interactions and modulatory influences from sources in the brainstem and basal ganglia. The overall view that emerges is that the thalamus is a vital node in brain networks supporting cognition

Pregnane steroids and short-term neural plasticity

Gamma-aminobutyric acid (GABA) is the major inhibitory transmitter in the brain, and its fast effects are mediated by the GABA-A receptor. It is well known, from pharmacological manipulations, that many exogenous agents alter the efficacy of GABA-A receptors. For example, benzodiazepines increase the effect of GABA and some β-carbolines reduce the effect of GABA at these receptors. Increasing the strength of neuronal inhibition can prevent seizures, reduce anxiety and be neuroprotective. There are also endogenous mechanisms that increase efficacy. For example, more GABA-A receptors can be synthesized and inserted into synapses, but this requires up to 1 h or more. On a shorter timescale, GABAergic inhibition can be potentiated by steroids, e.g., allopregnanolone, synthesized de novo in neural tissue or derived from peripheral endocrine organs. The widespread distribution of these neuroactive steroids across the brain suggests an extensive role in short-term neural plasticity

A minimally invasive and reversible system for chronic recordings from multiple brain sites in macaque monkeys

We have developed a reversible system for performing simultaneous recordings from multiple brain areas of trained macaque monkeys. It consists of a near-circular halo fitted around the head of the monkey with 5-10 thin plastic or stainless steel posts that either jut against or are screwed into the skull, respectively. Both methods of implantation of the posts are easily reversible, enabling protracted recordings over many years and training the monkeys in more complex tasks. The former is more useful for shorter periods of recordings (2-4 months) separated by long intervals and the latter for longer periods of recordings at a time (6-12 months). With both systems, essentially the entire scalp is intact, allowing multi-site recordings from a number of dorsal cortical areas, as well as other areas, simultaneously. These recordings are performed through tiny craniotomies of usually less than 2 mm diameter, which are fitted with small plastic cones that serve as guide tubes for the microelectrodes. The surgery involved in these procedures is relatively minor compared to classical methods and the implants are also usually free of infections, thus requiring little maintenance of recording chambers

Rhythmic Sampling within and between Objects despite Sustained Attention at a Cued Location

The brain directs its limited processing resources through various selection mechanisms, broadly referred to as attention. The present study investigated the temporal dynamics of two such selection mechanisms: space- and object-based selection. Previous evidence has demonstrated that preferential processing resulting from a spatial cue (i.e., space-based selection) spreads to uncued locations, if those locations are part of the same object (i.e., resulting in object-based selection). But little is known about the relationship between these fundamental selection mechanisms. Here, we used human behavioral data to determine how space- and object-based selection simultaneously evolve under conditions that promote sustained attention at a cued location, varying the cue-to-target interval from 300—1100 ms. We tracked visual-target detection at a cued location (i.e., space-based selection), at an uncued location that was part of the same object (i.e., object-based selection), and at an uncued location that was part of a different object (i.e., in the absence of space- and object-based selection). The data demonstrate that even under static conditions, there is a moment-to-moment reweighting of attentional priorities based on object properties. This reweighting is revealed through rhythmic patterns of visual-target detection both within (at 8 Hz) and between (at 4 Hz) objects

Neurosteroids involved in regulating inhibition in the inferior colliculus.

Fast inhibitory neurotransmission in the brain is largely mediated by the gamma-aminobutyric acid-type A (GABA(A)) receptor. The 3alpha,5alpha-reduced neurosteroids (e.g., allopregnanolone) are the most potent endogenous modulators of the GABA(A) receptor. Although it is known that 3alpha,5alpha-reduced neurosteroid levels change during stress or depression and over the estrus cycle, a basic physiological role consistent with their pharmacological action remains elusive. We used the unique architecture of the auditory midbrain to reveal a role for 3alpha,5alpha-reduced neurosteroids in regulating inhibitory efficacy. After blocking the massive GABAergic projection from the dorsal nucleus of the lateral lemniscus (DNLL) to the contralateral central nucleus of the inferior colliculus (ICC) in anesthetized rats, a reactive increase in the efficacy of other inhibitory circuits in the ICC (separable because of the dominant ear that drives each circuit) was demonstrated with physiological measures-single-neuron activity and a neural-population-evoked response. This effect was prevented by blocking 3alpha,5alpha-reduced neurosteroid synthesis with a 5alpha-reductase inhibitor: finasteride. Immunohistochemistry confirmed that the DNLL blockade induced an increase in 3alpha,5alpha-reduced neurosteroids in the contralateral ICC. This study shows that when GABAergic inhibition is reduced, the brain compensates within minutes by locally increasing synthesis of neurosteroids, thereby balancing excitatory and inhibitory inputs in complex neural circuits