Signal-to-noise ratio improvement in multiple electrode recording.

Recordings of spike trains made with microwires or silicon electrodes include more noise from various sources that contaminate the observed spike shapes compared with recordings using sharp microelectrodes. This is a particularly serious problem if spike shape sorting is required to separate the several trains that might be observed on a particular electrode. However, if recordings are made with an array of such electrodes, there are several mathematical methods to improve the effective signal (spikes) to noise ratio, thus considerably reducing inaccuracy in spike detection and shape sorting. We compare the theoretical basis of three such methods and evaluate their performance with simulated and real data.

Role of mammalian auditory cortex in the perception of elementary sound properties.

Studies in several mammalian species have demonstrated that bilateral ablations of the auditory cortex have little effect on simple sound intensity and frequency-based behaviors. In the rat, for example, early experiments have shown that auditory ablations result in virtually no effect on the rat's ability to either detect tones or discriminate frequencies. Such lesion experiments, however, typically examine an animal's performance some time after recovery from ablation surgery. As such, they demonstrate that the cortex is not essential for simple auditory behaviors in the long run. Our study further explores the role of cortex in basic auditory perception by examining whether the cortex is normally involved in these behaviors. In these experiments we reversibly inactivated the rat primary auditory cortex (AI) using the GABA agonist muscimol, while the animals performed a simple auditory task. At the same time we monitored the rat's auditory activity by recording auditory evoked potentials (AEP) from the cortical surface. In contrast to lesion studies, the rapid time course of these experimental conditions preclude reorganization of the auditory system that might otherwise compensate for the loss of cortical processing. Soon after bilateral muscimol application to their AI region, our rats exhibited an acute and profound inability to detect tones. After a few hours this state was followed by a gradual recovery of normal hearing, first of tone detection and, much later, of the ability to discriminate frequencies. Surface muscimol application, at the same time, drastically altered the normal rat AEP. Some of the normal AEP components vanished nearly instantaneously to unveil an underlying waveform, whose size was related to the severity of accompanying behavioral deficits. These results strongly suggest that the cortex is directly involved in basic acoustic processing. Along with observations from accompanying multiunit experiments that related the AEP to AI neuronal activity, our results suggest that a critical amount of activity in the auditory cortex is necessary for normal hearing. It is likely that the involvement of the cortex in simple auditory perceptions has hitherto not been clearly understood because of underlying recovery processes that, in the long-term, safeguard fundamental auditory abilities after cortical injury.

Recordings, behaviour and models related to corticothalamic feedback.

In this paper, we review recent work on aspects of corticothalamic interactions in the auditory and in the visual systems. There are gross similarities in the arrangements of these systems, but considerable contrasts in the processing computations and in the effects of corticothalamic feedback.