Closed-loop optical neural stimulation based on a 32-channel low-noise recording system with online spike sorting

OBJECTIVE

Closed-loop operation of neuro-electronic systems is desirable for both scientific and clinical (neuroprosthesis) applications. Integrating optical stimulation with recording capability further enhances the selectivity of neural stimulation. We have developed a system enabling the local delivery of optical stimuli and the simultaneous electrical measuring of the neural activities in a closed-loop approach.

APPROACH

The signal analysis is performed online through the implementation of a template matching algorithm. The system performance is demonstrated with the recorded data and in awake rats.

MAIN RESULTS

Specifically, the neural activities are simultaneously recorded, detected, classified online (through spike sorting) from 32 channels, and used to trigger a light emitting diode light source using generated TTL signals.

SIGNIFICANCE

A total processing time of 8 ms is achieved, suitable for optogenetic studies of brain mechanisms online.

Neural responses to facial expression and face identity in the monkey amygdala

The amygdala is purported to play an important role in face processing, yet the specificity of its activation to face stimuli and the relative contribution of identity and expression to its activation are unknown. In the current study, neural activity in the amygdala was recorded as monkeys passively viewed images of monkey faces, human faces, and objects on a computer monitor. Comparable proportions of neurons responded selectively to images from each category. Neural responses to monkey faces were further examined to determine whether face identity or facial expression drove the face-selective responses. The majority of these neurons (64%) responded both to identity and facial expression, suggesting that these parameters are processed jointly in the amygdala. Large fractions of neurons, however, showed pure identity-selective or expression-selective responses. Neurons were selective for a particular facial expression by either increasing or decreasing their firing rate compared with the firing rates elicited by the other expressions. Responses to appeasing faces were often marked by significant decreases of firing rates, whereas responses to threatening faces were strongly associated with increased firing rate. Thus global activation in the amygdala might be larger to threatening faces than to neutral or appeasing faces.

Dentate gyrus and ca1 ensemble activity during spatial reference frame shifts in the presence and absence of visual input

In rats shuttling between a variably placed landmark of origin and a fixed goal, place fields of hippocampal CA1 cells encode location in two spatial reference frames. On the initial part of the outbound journey, place fields encode location with respect to the origin while on the final segment, place fields are aligned with the goal (Gothard et al., 1996b). An abrupt switch of reference frame can be induced experimentally by shortening the distance between the origin and the goal. Two linked hypotheses concerning this effect were addressed: (1) that the persistent, landmark-referenced firing results from some internal dynamic process (e.g., path integration or "momentum") and is not a result of maintained sensory input from the landmark of origin; and (2) that this hypothetical process is generated by connections either within CA3 or between CA3 and CA1, in which case the effect might be absent from the dentate gyrus. Neuronal ensemble recordings were made simultaneously from CA1 and the dentate gyrus as rats shuttled on a linear track between a variably located box and a goal, under light or dark conditions. The box-referenced firing persisted significantly longer in the dark in both hippocampal subfields, suggesting a competitive interaction between an internal dynamic process and external sensory cues. The similarity between reference frame transitions in the dentate gyrus and the CA1 region suggests that this process probably occurs before CA3, possibly in the entorhinal cortex or subiculum.

Independence of firing correlates of anatomically proximate hippocampal pyramidal cells

In neocortex, neighboring neurons frequently exhibit correlated encoding properties. There is conflicting evidence whether a similar phenomenon occurs in hippocampus. To assess this quantitatively, a comparison was made of the spatial and temporal firing correlations within and between local groups of hippocampal cells, spaced 350-1400 microm apart. No evidence of clustering was found in a sample of >3000 neurons. Moreover, cells active in two environments were uniformly interspersed at a scale of <100 microm, as assessed by the activity-induced gene Arc. Independence of encoding characteristics implies uncorrelated inputs, which could enhance the capacity of the hippocampus to store arbitrary associations.

Stable and rapid recurrent processing in realistic autoassociative memories

It is shown that in those autoassociative memories that learn by storing multiple patterns of activity on their recurrent collateral connections, there is a fundamental conflict between dynamical stability and storage capacity. It is then found that the network can nevertheless retrieve many different memory patterns, as predicted by nondynamical analyses, if its firing is regulated by inhibition that is sufficiently multiplicative in nature. Simulations of a model network with integrate-and-fire units confirm that this is a realistic solution to the conflict. The simulations also confirm the earlier analytical result that cued-elicited memory retrieval, which follows an exponential time course, occurs in a time linearly related to the time constant for synaptic conductance inactivation and relatively independent of neuronal time constants and firing levels.