Rapid prototyping for neuroscience and neural engineering.

Rapid prototyping (RP) is a useful method for designing and fabricating a wide variety of devices used for neuroscience research. The present study confirms the utility of using fused deposition modeling, a specific form of RP, to produce three devices commonly used for basic science experimentation. The accuracy and precision of the RP method varies according to the type and quality of the printer as well as the thermoplastic substrate. The printer was capable of creating device channels with a minimum diameter of 0.4 or 0.6mm depending on the orientation of fabrication. RP enabled the computer-aided design and fabrication of three custom devices including a cortical recording/stroke induction platform capable of monitoring electrophysiological function during ischemic challenge. In addition to the recording platform, two perfusion chambers and a cranial window device were replicated with sub-millimeter precision. The ability to repeatedly modify the design of each device with minimal effort and low turn-around time is helpful for oft-unpredictable experimental conditions. Results obtained from validation studies using both the cortical recording platform and perfusion chamber did not vary from previous results using traditional hand-fabricated or commercially available devices. Combined with computer-aided design, rapid prototyping is an excellent alternative for developing and fabricating custom devices for neuroscience research.

Fast wave propagation in auditory cortex of an awake cat using a chronic microelectrode array.

We investigated fast wave propagation in auditory cortex of an alert cat using a chronically implanted microelectrode array. A custom, real-time imaging template exhibited wave dynamics within the 33-microwire array (3 mm(2)) during ten recording sessions spanning 1 month post implant. Images were based on the spatial arrangement of peri-stimulus time histograms at each recording site in response to auditory stimuli consisting of tone pips between 1 and 10 kHz at 75 dB SPL. Functional images portray stimulus-locked spiking activity and exhibit waves of excitation and inhibition that evolve during the onset, sustained and offset period of the tones. In response to 5 kHz, for example, peak excitation occurred at 27 ms after onset and again at 15 ms following tone offset. Variability of the position of the centroid of excitation during ten recording sessions reached a minimum at 31 ms post onset (sigma = 125 microm) and 18 ms post offset (sigma = 145 microm), suggesting a fine place/time representation of the stimulus in the cortex. The dynamics of these fast waves also depended on stimulus frequency, likely reflecting the tonotopicity in auditory cortex projected from the cochlea. Peak wave velocities of 0.2 m s(-1) were also consistent with those purported across horizontal layers of cat visual cortex. The fine resolution offered by microimaging may be critical for delivering optimal coding strategies used with an auditory prosthesis. Based on the initial results, future studies seek to determine the relevance of these waves to sensory perception and behavior.

Electrophysiological response dynamics during focal cortical infarction.

While the intracellular processes of hypoxia-induced necrosis and the intercellular mechanisms of post-ischemic neurotoxicity associated with stroke are well documented, the dynamic electrophysiological (EP) response of neurons within the core or periinfarct zone remains unclear. The present study validates a method for continuous measurement of the local EP responses during focal cortical infarction induced via photothrombosis. Single microwire electrodes were acutely implanted into the primary auditory cortex of eight rats. Multi-unit neural activity, evoked via a continuous 2 Hz click stimulus, was recorded before, during and after infarction to assess neuronal function in response to local, permanent ischemia. During sham infarction, the average stimulus-evoked peak firing rate over 20 min remained stable at 495.5+/-14.5 spikes s-1, indicating temporal stability of neural function under normal conditions. Stimulus-evoked peak firing was reliably reduced to background levels (firing frequency in the absence of stimulus) following initiation of photothrombosis over a period of 439+/-92 s. The post-infarction firing patterns exhibited unique temporal degradation of the peak firing rate, suggesting a variable response to ischemic challenge. Despite the inherent complexity of cerebral ischemia secondary to microvascular occlusion, complete loss of EP function consistently occurred 300-600 s after photothrombosis. The results suggest that microwire recording during photothrombosis provides a simple and highly efficacious strategy for assessing the electrophysiological dynamics of cortical infarction.

On variability and use of rat primary motor cortex responses in behavioral task discrimination.

The success of a cortical motor neuroprosthetic system will rely on the system's ability to effectively execute complex motor tasks in a changing environment. Invasive, intra-cortical electrodes have been successfully used to predict joint movement and grip force of a robotic arm/hand with a non-human primate (Chapin J K, Moxon K A, Markowitz R S and Nicolelis M A L 1999 Real-time control of a robotic arm using simultaneously recorded neurons in the motor cortex Nat. Neurosci. 2 664-70). It is well known that cortical encoding occurs with a high degree of cortical plasticity and depends on both the functional and behavioral context. Questions on the expected robustness of future motor prosthesis systems therefore still remain. The objective of the present work was to study the effect of minor changes in functional movement strategies on the M1 encoding. We compared the M1 encoding in freely moving, non-constrained animals that performed two similar behavioral tasks with the same end-goal, and investigated if these behavioral tasks could be discriminated based on the M1 recordings. The rats depressed a response paddle either with a set of restrictive bars ('WB') or without the bars ('WOB') placed in front of the paddle. The WB task required changes in the motor strategy to complete the paddle press and resulted in highly stereotyped movements, whereas in the WOB task the movement strategy was not restricted. Neural population activity was recorded from 16-channel micro-wire arrays and data up to 200 ms before a paddle hit were analyzed off-line. The analysis showed a significant neural firing difference between the two similar WB and WOB tasks, and using principal component analysis it was possible to distinguish between the two tasks with a best classification at 76.6%. While the results are dependent upon a small, randomly sampled neural population, they indicate that information about similar behavioral tasks may be extracted from M1 based on relatively few channels of neural signal for possible use in a cortical neuroprosthetic system.

A method for monitoring intra-cortical motor cortex responses in an animal model of ischemic stroke.

Neuroplasticity is believed to play a key role in functional recovery after stroke. Neuroplastic effects can be monitored at the cellular level via e.g. neurotransmitter assessment, but these studies require sacrifice of the animal. FMRI can be used to assess functional neuronal performance, but the spatial and temporal resolution is far from the single cell level. The objective was to establish an effective method for short-term analysis of single and multi-unit electrophysiological function before, during and after stroke. We instrumented one rat with a 16-ch array in the primary motor cortex (100 microm wire diameter) to monitor cortical activity. A bipolar cuff electrode was implanted around the Ulnar nerve in the contralateral forelimb to provide a controlled electrical stimulus input to the sensory-motor system. A 3 mm diameter ischemic infarct was created immediately posterior to the electrode array by light activation of a photosensitive dye (Rose Bengal, 1.3 mg/100 mg body weight) at the cortical surface. M1 activity in response to the peripheral electrical stimulus was recorded before, during and after the cortical ischemic infarct. At 425 min following ischemic infarct the peak peri-stimulus time response had decreased to 30 +/- 11% (electrodes placed 1.5 mm from the infarct core) of the activity before the ischemic onset. The mean response latency increased from 30.1 +/- 4.5 ms (before infarct) to 40.6 +/- 8.5 ms (at 425 min). This dynamic view of neuroplasticity may eventually assist in optimizing acute stroke therapies and optimize functional recovery further.

Microstimulation in auditory cortex provides a substrate for detailed behaviors.

Sensory cortical prostheses have potential to aid people suffering from blindness, deafness and other sensory deficits. However, research to date has shown that sensation thresholds via epicortical stimulation are surprisingly large. These thresholds result in potentially deleterious electrical currents, as well as large activation volumes. Large activation volumes putatively limit the corresponding number of independent stimulation channels in a neural prosthesis. In this study, penetrating stimulation of the auditory cortex was tested for its ability to transmit salient information to behaving rat subjects. Here, we show that subjects that were previously trained to discriminate natural stimuli immediately discriminated different microstimulation cues more accurately and with shorter response latencies than the natural stimuli. Additionally, the cortical microstimulation resulted in a generalization gradient across locations within the cortex. The results demonstrate the efficacy of using closely spaced cortical microstimulation to efficiently transmit highly salient and discriminable information to a behaving subject.

Cortical microstimulation in auditory cortex of rat elicits best-frequency dependent behaviors.

Electrical activation of the auditory cortex has been shown to elicit an auditory sensation; however, the perceptual effects of auditory cortical microstimulation delivered through penetrating microelectrodes have not been clearly elucidated. This study examines the relationship between electrical microstimulus location within the adult rat auditory cortex and the subsequent behavior induced. Four rats were trained on an auditory frequency discrimination task and their lever-pressing behavior in response to stimuli of intermediate auditory frequencies was quantified. Each trained rat was then implanted with a microwire array in the auditory cortex of the left hemisphere. Best frequencies (BFs) of each electrode in the array were determined by both local field potential and multi-unit spike-rate activity evoked by pure tone stimuli. A cross-dimensional psychophysical generalization paradigm was used to evaluate cortical microstimulation-induced behavior. Using the BFs of each electrode, the microstimulation-induced behavior was evaluated relative to the auditory-induced behavior. Microstimulation resulted in behavior that was dependent on the BFs of the electrodes used for stimulation. These results are consistent with recent reports indicating that electrophysiological recordings of neural responses to sensory stimuli may provide insight into the sensation generated by electrical stimulation of the same sensory neural tissue.

Decoding of auditory cortex signals with a LAMSTAR neural network.

OBJECTIVES: Each neuron has a specific set of stimuli, which it preferentially responds to (the receptive field of the neuron). For implantable cortical prosthetic devices specific points of the cortex (or groups of neurons) have to be stimulated to create perceptions of sensory stimulus with specific attributes (such as frequency, temporal characteristics, etc). Such applications would need real time decoding of signals. Previously mathematical techniques, such as computing the receptive field (using electrophysiology data) and artificial neural networks (Kohonen network or SOM and back propagation network) have been used to decode neural signals. METHODS: A Large Adaptive Memory Storage and Retrieval (LAMSTAR) neural-network-based decoder was designed to decode responses recorded from neurons in the auditory cortex. It was designed to identify the frequency of the tonal stimuli that elicited a particular discharge rate pattern recorded on two channels of a tungsten wire electrode array. RESULTS: The network functioned efficiently as a decoder with 100% accuracy for the small sample of stimulus-response data used. DISCUSSION: The results show that the network is effective in studying the functional organization of the auditory cortex and other sensory systems. Depending on the input sub-word, information about the kind of stimuli that activates particular parts of the sensory cortex can be studied.

Preliminary study of neurite outgrowth within polyimide microtubes.

The cone electrode first developed by P.R. Kennedy paved headway in the area of cortical prostheses. While effective, to date, no optimization of the materials, length, diameter or controlling neurotrophic effects have been extensively quantified for such systems. This paper describes an in-vitro model system for the study of neurite outgrowth using PC-12 cells and an array of polyimide microtubes. Our aim is to obtain preliminary design specifications for the eventual optimization of in-vivo neurotrophic electrodes. We performed preliminary characterization of the number and average lengths of PC-12 neurites that penetrated into the tubes mounted within a standard Petri dish. To describe system performance, we observed an increase in the average number of neurites that grew into the tubes over a period of days. We also observed an increase in the average length of the neurites (with a 95% confidence) between day 3 and day 4 of between 14.97 microm and 62.27 microm. In addition, we measured a length change (with a 95% confidence) between day 4 and day 6 of 93.51 microm and 145.45 microm. These results will soon be augmented by quantification of neurites using a photo lithographically patterned glass microgroove system.

Modified spike-triggered averaging to assess rat auditory cortex time-frequency receptive fields.

Receptive fields have been used as a tool to study the functional organization of the auditory system in several animals. In this study, they have been used to characterize the primary auditory cortex of rats, specifically to address the differences in auditory processing at different depths of the cortex. The depths chosen; 500, 800 and 1300 microm correspond to layers IV, V and VI of the cortex. This study aims at quantifying the differences in the receptive field in terms of changes in latency, differences in tuning curves, spectral bandwidth and the complexity of the receptive fields. The following preliminary trends were observed: the mean peak latency changes from 10 +/- 4 ms at a depth of 500 microm to 46 +/- 13.08 ms at a depth of 1300 microm. Mean spectral bandwidth changes from 6.4 +/- 0.95 kHz at 500 microm to 8.9 +/- 1.73 KHz at 800 microm to 8 +/- 2.53 KHz at 1300 microm. The mean temporal width changes with increasing depth from 13.6 +/- 1.15 ms at 500 microm to 9.4 +/- 1.88 ms at 1300 microm. Quantitative characterization of the receptive field can be used to generate mathematical models of the auditory neurons, which could aid the computation of stimulation levels for implantable cortical prosthetics. Preliminary data from our experiment on three animals has been presented here.

Single electrode micro-stimulation of rat auditory cortex: an evaluation of behavioral performance.

A combination of electrophysiological mapping, behavioral analysis and cortical micro-stimulation was used to explore the interrelation between the auditory cortex and behavior in the adult rat. Auditory discriminations were evaluated in eight rats trained to discriminate the presence or absence of a 75 dB pure tone stimulus. A probe trial technique was used to obtain intensity generalization gradients that described response probabilities to mid-level tones between 0 and 75 dB. The same rats were then chronically implanted in the auditory cortex with a 16 or 32 channel tungsten microwire electrode array. Implanted animals were then trained to discriminate the presence of single electrode micro-stimulation of magnitude 90 microA (22.5 nC/phase). Intensity generalization gradients were created to obtain the response probabilities to mid-level current magnitudes ranging from 0 to 90 microA on 36 different electrodes in six of the eight rats. The 50% point (the current level resulting in 50% detections) varied from 16.7 to 69.2 microA, with an overall mean of 42.4 (+/-8.1) microA across all single electrodes. Cortical micro-stimulation induced sensory-evoked behavior with similar characteristics as normal auditory stimuli. The results highlight the importance of the auditory cortex in a discrimination task and suggest that micro-stimulation of the auditory cortex might be an effective means for a graded information transfer of auditory information directly to the brain as part of a cortical auditory prosthesis.