Single-unit chronic recordings from the sensorimotor cortex of unrestrained cats during locomotion

Contributions of the motor cortex to the control of the hindlimbs during locomotion in the cat

Although the corticospinal tract is not essential for the production of the basic locomotor rhythm in cats, it does contribute to the regulation of locomotion, particularly in situations in which there is a requirement for precise control over paw placement or limb trajectory. Lesions of the dorsolateral funiculi at the low thoracic level (T(13)) that completely interrupted both the cortico- and rubrospinal pathways produced long-term deficits in locomotion on a level surface. These deficits included a paw-drag that was probably caused both by a loss of cortico- and rubrospinal input to motoneurones controlling distal muscles as well as by a change in the relative timing of muscles acting around the hip and knee. Smaller lesions produced similar deficits from which the cats recovered relatively quickly. Cats with the largest lesions of the dorsolateral funiculi were unable to modify their gait sufficiently to step over obstacles attached to the treadmill belt even 3-5 months postlesion. These results imply that the medial pathways, the reticulo- and vestibulospinal pathways, are unable to fully compensate for damage to the lateral pathways. Single unit recordings from identified pyramidal tract neurones (PTNs) within the hindlimb representation of the primary motor cortex (area 4) showed that a substantial proportion of neurones (67%) significantly increased their discharge frequency when the cats modified their gait to step over obstacles attached to the treadmill belt. Of those PTNs that showed increased activity during the swing phase, populations of neurones were activated at different times. A large proportion of PTNS discharged early in swing, in phase with knee flexors such as the semitendinosus. Others discharged slightly later, in phase with the activity of ankle flexors, such as tibialis anterior, while still others discharged at the end of swing, in phase with digit dorsiflexors, such as the extensor digitorum brevis. We suggest that different populations of cortical neurones may specifically modify the activity of selected groups of close synergistic muscles during different parts of the swing phase. We further suggest that these modifications are mediated, in part, by groups of interneurones that are involved in determining the base locomotor rhythm. This provides a means by which the changes specified by the descending signal from the motor cortex may be smoothly, and appropriately, incorporated into the locomotor cycle.

Stability of the interface between neural tissue and chronically implanted intracortical microelectrodes

The stability of the interface between neural tissue and chronically implanted microelectrodes is very important for obtaining reliable control signals for neuroprosthetic devices. Stability is also crucial for chronic microstimulation of the cerebral cortex. However, changes of the electrode-tissue interface can be caused by a variety of mechanisms. In the present study, intracortical microelectrode arrays were implanted into the pericruciate gyrus of cats and neural activities were recorded on a regular basis for several months. An algorithm based on cluster analysis and interspike interval analysis was developed to sort the extracellular action potentials into single units. We tracked these units based on their waveform and their response to somatic stimulation or stereotypical movements by the cats. Our results indicate that, after implantation, the electrode-tissue interface may change from day-to-day over the first 1-2 weeks, week-to-week for 1-2 months, and become quite stable thereafter. A stability index is proposed to quantify the stability of the electrode-tissue interface. The reasons for the pattern of changes are discussed.

Long-term stability of the place-field activity of single units recorded from the dorsal hippocampus of freely behaving rats

Over 90% of all spontaneously active hippocampal pyramidal cells in freely moving rats signal the animal's spatial position by reliably changing their firing rate each time the animal enters a given place within an environment. This place-field activity exhibits plasticity when specific environmental variables are manipulated. Indeed, the hippocampus is perhaps best known as a system that serves as a model of neuronal plasticity. Although place-field activity has previously been examined only over relatively short experimental sessions, this behavioral correlate of hippocampal functional activity has been assumed to exhibit stability rather than plasticity in the absence of environmental changes. The present study shows that hippocampal neurons have stable place-field correlates that persist over very long periods of time. Single-unit activity was chronically recorded from the dorsal hippocampus of rats foraging repeatedly in a stable spatial environment. The location of the place fields of all units were stable over all time periods tested, for intervals up to 153 days in duration. The consistency of the information conveyed by this single-unit activity in a fixed spatial environment indicates that stability of neuronal activity may be as important as plasticity in the integrated processing of information that occurs in the hippocampus and throughout the nervous system.

Long-term recordings and receptive field measurements from single units of the visual cortex of awake unrestrained kittens

A method is described which allows simultaneous recording of the activity of several neurons in the visual cortex of awake unrestrained kittens for a period sufficiently long to follow experience-dependent changes in receptive field properties. The electrodes consisted of 25 microns thick Teflon-coated platinum-iridium wires whose tips were tapered and coated with platinum black. They were implanted individually into the striate cortex and fixed with tissue glue to the pia mater. The other end of the electrodes was connected to a 21-pin plug and fixed to the skull in a way which allowed the wires to follow freely any movements of the brain. With this arrangement single and multi unit activity could be recorded from up to 10 sites over several weeks. In fortuitous cases the activity of a single cell could be followed over several days. The receptive field properties of these neurons were determined in awake animals by computing response histograms to moving gratings that covered the whole visual field and whose orientation could be changed in small steps. Comparison with receptive field properties that were determined for the same neurons after the kittens had been lightly anesthetized, revealed that this method allowed reliable determination of the eye preference of the neurons as well as of their orientation and direction selectivity. Since this method allowed repeated non-invasive measurements of neuronal response properties in only lightly restrained kittens, it was possible to study, in individual units, the nature and time course of changes in receptive field properties such as result from monocular deprivation and reverse occlusion.

Motor cortical cell discharge during voluntary gait modification

Kinematic and electromyographic data were recorded together with motor cortical cell discharge during a task which required the cat to modify its gait in order to step over 3 different types of obstacles fixed to a moving treadmill belt. In order to negotiate the obstacles the cat made large adjustments in limb trajectory which were associated with equally large changes in forelimb flexor muscle activity. Sixteen of 57 identified pyramidal tract neurones recorded from area 4 of two cats increased their peak discharge rate during this gait adjustment. It is suggested that the motor cortex plays a role in adjusting the flexor muscle activity to the requirements of the locomotor task.

A multiple floating microelectrode for chronic implantation and longterm single unit recording in the cat

This paper describes a multiple electrode, suitable for chronic implantation and single unit recording in the superficial cerebral cortex of the cat. The electrodes are fine wires, 5-8 per implant, not kept together in a bundle; a segment of them floats freely in saline in a closed chamber over the place of penetration. The electrode tips are not sharpened electrolytically, but cut at a bevel during surgery. Longevity of the implants is improved by slowing down the occasional rapid head shaking characteristic of cats, which tends to tear out implanted electrodes, by means of a small weight attached by a bar to the head.