Data-robust tight lower bounds to the information carried by spike times of a neuronal population.

We develop new data-robust lower-bound methods to quantify the information carried by the timing of spikes emitted by neuronal populations. These methods have better sampling properties and are tighter than previous bounds based on neglecting correlation in the noise entropy. Our new lower bounds are precise also in the presence of strongly correlated firing. They are not precise only if correlations are strongly stimulus modulated over a long time range. Under conditions typical of many neurophysiological experiments, these techniques permit precise information estimates to be made even with data samples that are three orders of magnitude smaller than the size of the response space.

Population coding of stimulus location in rat somatosensory cortex.

This study explores the nature of population coding in sensory cortex by applying information theoretic analyses to neuron pairs recorded simultaneously from rat barrel cortex. We quantified the roles of individual spikes and spike patterns in encoding whisker stimulus location. 82%-85% of the total information was contained in the timing of individual spikes: first spike time was particularly crucial. Spike patterns within neurons accounted for the remaining 15%-18%. Neuron pairs located in the same barrel column coded redundantly, whereas pairs in neighboring barrel columns coded independently. The barrel cortical population code for stimulus location appears to be the time of single neurons' first poststimulus spikes-a fast, robust coding mechanism that does not rely on "synergy" in crossneuronal spike patterns.

The role of spike timing in the coding of stimulus location in rat somatosensory cortex.

Although the timing of single spikes is known to code for time-varying features of a sensory stimulus, it remains unclear whether time is also exploited in the neuronal coding of the spatial structure of the environment, where nontemporal stimulus features are fundamental. This report demonstrates that, in the whisker representation of rat cortex, precise spike timing of single neurons increases the information transmitted about stimulus location by 44%, compared to that transmitted only by the total number of spikes. Crucial to this code is the timing of the first spike after whisker movement. Complex, single neuron spike patterns play a smaller, synergistic role. Timing permits very few spikes to transmit high quantities of information about a behaviorally significant, spatial stimulus.

Practices in forensic neuropsychology: perspectives of neuropsychologists and trial attorneys.

Members of the National Academy of Neuropsychology and the Association of Trial Lawyers of America were surveyed concerning current practices in forensic neuropsychology. The majority of neuropsychologists and attorneys reported that attorneys never observe neuropsychological testing. Attorneys reported receiving raw data in almost all of their brain injury cases, but neuropsychologists reported that they produce raw data in only a minority of their forensic cases. Similarly, fewer neuropsychologists than attorneys acknowledged that they are asked to provide information to assist the lawyer in preparing for the cross-examination of the opposition's expert or to prepare the plaintiff for the opposition's evaluation. Lawyers typically spend up to an hour preparing their clients for neuropsychological evaluations and commonly cover test content, detection of malingering, and brain injury symptoms. Other topics addressed include attorney influence on findings, fees and billing, board certification, use of technicians, and methods used to generate referrals or locate experts. Areas of agreement and divergence between the groups were identified and ethical issues raised by identified practices were examined.

Spatial-temporal distribution of whisker-evoked activity in rat somatosensory cortex and the coding of stimulus location.

Rats use their facial vibrissae ("whiskers") to locate and identify objects. To learn about the neural coding of contact between whiskers and objects, we investigated the representation of single-vibrissa deflection by populations of cortical neurons. Microelectrode arrays, arranged in a geometric 10 x 10 grid, were inserted into the thalamo-recipient layers of "barrel cortex" (the vibrissal region of somatosensory cortex) in urethane-anesthetized rats, and neuronal activity across large sets of barrel-columns was measured. Typically, 5 msec after deflection of a whisker a 0.2 mm(2) focus of activity emerged. It rapidly expanded, doubling in size by 7 msec, before retracting and disappearing 28-59 msec after stimulus onset. The total territory engaged by the stimulus ranged from 0.5 to 2.9 mm(2) (2-11 barrels). Stimulus site dictated the domain of activity. To quantify the coding of whisker location, we applied the population d' measure of discriminability. Activity patterns elicited by two whiskers were highly discriminable at the initial cortical response; peak discriminability typically occurred within 16 msec of stimulus onset. To determine how widely information about stimulus location was distributed, we measured population d' while excluding response data from the on-center electrodes of the two tested whiskers. Response patterns remained discriminable, indicating that information about stimulus location was distributed across barrel cortex. Taken together, these results show that single-whisker deflections are represented in a multicolumn region constrained by barrel cortex map topography. The nature of this coding allows information about stimulus location to be coded extremely rapidly and unambiguously by one to two spikes per neuron.

Selection criteria for clinical neuropsychology internships.

Criteria used in the evaluation and selection of applicants for clinical neuropsychology internships were identified by a survey of the programs listed in The Clinical Neuropsychologist. Neuropsychologists from 40 programs replied to the survey, yielding an 80% response rate. Clinical experience in neuropsychological assessment, specialization in clinical neuropsychology during graduate training, personal interview, and letters of recommendation were reported to be the most salient selection criteria. Practica that provide experience with flexible batteries at university-affiliated medical centers and doctoral curricula that follow INS/APA Division 40 guidelines, with teaching and supervision provided by neuropsychologists were preferred prerequisites to internship. Selection criteria reflect a vertically integrated model of education and training in accordance with the Houston Conference model.

Examination of the spatial and temporal distribution of sensory cortical activity using a 100-electrode array.

This paper introduces improved techniques for multichannel extracellular electrophysiological recordings of neurons distributed across a single layer of topographically mapped cortex. We describe the electrode array, the surgical implant techniques, and the procedures for data collection and analysis. Neural events are acquired through an array of 25 or 100 microelectrodes with a 400-microm inter-electrode spacing. One advantage of the new methodology is that implantation is achieved through transdural penetration, thereby reducing the disruption of the cortical tissue. The overall cortical territory sampled by the 25-electrode array is 1.6 x 1.6 mm (2.56 mm2) and by the 100-electrode array 3.6 x 3.6 mm (12.96 mm2). Using a recording system with 100 channels available, neural activity is simultaneously acquired on all electrodes, amplified, digitized, and stored on computer. In our data, average peak-to-peak signal/noise ratio was 11.5 and off-line waveform analysis typically allowed the separation of at least one well-discriminated single-unit per channel. The reported technique permits analysis of cortical function with high temporal and spatial resolution. We use the technique to create an 'image' of neural activity distributed across the whisker representation of rat somatosensory (barrel) cortex.

Learning through maps: functional significance of topographic organization in primary sensory cortex.

The presence of "maps" in sensory cortex is a hallmark of the mammalian nervous system, but the functional significance of topographic organization has been called into question by physiological studies claiming that patterns of neural behavioral activity transcend topographic boundaries. This paper discusses recent behavioral and physiological studies suggesting that, when animals or human subjects learn perceptual tasks, the neural modifications associated with the learning are distributed according to the spatial arrangement of the primary sensory cortical map. Topographical cortical representations of sensory events, therefore, appear to constitute a true structural framework for information processing and plasticity.

Distribution of tactile learning and its neural basis.

The brain's sensory processing systems are modified during perceptual learning. To learn more about the spatial organization of learning-related modifications, we trained rats to utilize the sensory signal from a single intact whisker to carry out a behavioral task. Once a rat had mastered the task, we clipped its "trained" whisker and attached a "prosthetic" one to a different whisker stub. We then tested the rat to determine how quickly it could relearn the task by using the new whisker. We observed that rats were immediately able to use the prosthetic whisker if it were attached to the stub of the trained whisker but not if it were attached to a different stub. Indeed, the greater the distance between the trained and prosthetic whisker, the more trials were needed to relearn the task. We hypothesized that this "transfer" of learning between whiskers might depend on how much the representations of individual whiskers overlap in primary somatosensory cortex. Testing this hypothesis by using 100-electrode cortical recordings, we found that the overlap between the cortical response patterns of two whiskers accounted well for the transfer of learning between them: The correlation between the electrophysiological and behavioral data was very high (r = 0.98). These findings suggest that a topographically distributed memory trace for sensory-perceptual learning may reside in primary sensory cortex.

Rehabilitation of Danish dialysis and transplant patients.

An evaluation of the extent of rehabilitation of 512 Danish dialysis and transplant patients, based on a questionnarie, has been carried out. All transplanted patients and dialysis patients and dialysis patients with only slightly limited working capacity before treatment experienced an improvement, whereas disabled dialysis patients did not experience major changes. Sixty per cent of the patients receive disablement pension and the allotment of pension early in the course of treatment is suspected to be one of the main reasons for the low degree of vocational rehabilitation.

Rehabilitation of haemodialysis and transplant patients in denmark.

An evaluation of the extent of rehabilitation of 159 haemodialysis patients and 36 transplant patients from 6 of the 9 Danish nephrological centers was carried out. Half of the patients experienced their working capacity to be severely reduced prior to treatment and the percentage did not change during treatment. At the time of interview 88 percent of the patients received full disablement support and only ten per cent had retained their previous vocational position.