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Rodents use active whisker movements to explore their environment. The ‘slip hypothesis’ of whisker-related tactile perception entails that short-lived kinematic events (abrupt whisker movements, called ‘slips’, due to bioelastic whisker properties that occur during active touch of textures) carry the decisive texture information. Supporting this hypothesis, previous studies have shown that slip amplitude and frequency occur in a texture dependent way. Further, experiments employing passive pulsatile whisker deflections revealed that perceptual performance based on pulse kinematics (i.e. signatures that resemble slips) is far superior to the one based on time-integrated variables like frequency and intensity. So far, pulsatile stimuli were employed in a noise free environment. However, the realistic scenario involves background noise (e.g. evoked by rubbing across the texture). Therefore, if slips are used for tactile perception, the tactile neuronal system would need to differentiate slip-evoked spikes from those evoked by noise. To test the animals under these more realistic conditions, we presented passive whisker-deflections to head-fixed trained rats, consisting of 'slip-like' events (waveforms mimicking slips occurring with touch of real textures) embedded into background noise. Varying the i) shapes (ramp or pulse), ii) kinematics (amplitude, velocity, etc.), and iii) the probabilities of occurrence of slip-like events, we observed that rats could readily detect slip-like events of different shapes against noisy background. Psychophysical curves revealed that the difference of slip event and noise amplitude determined perception, while increased probability of occurrence (frequency) had barely any effect. These results strongly support the notion that encoding of kinematics dominantly determines whisker-related tactile perception while the computation of frequency or intensity plays a minor role.

The primary (S1) and secondary (S2) somatosensory cortices project to several trigeminal sensory nuclei. One putative function of these corticofugal projections is the gating of sensory transmission through the trigeminal principal nucleus (Pr5), and some have proposed that S1 and S2 project differentially to the spinal trigeminal subnuclei, which have inhibitory circuits that could inhibit or disinhibit the output projections of Pr5. Very little, however, is known about the origin of sensorimotor corticofugal projections and their patterns of termination in the various trigeminal nuclei. We addressed this issue by injecting anterograde tracers in S1, S2 and primary motor (M1) cortices, and quantitatively characterizing the distribution of labeled terminals within the entire rostro-caudal chain of trigeminal sub-nuclei. We confirmed our anterograde tracing results by injecting retrograde tracers at various rostro-caudal levels within the trigeminal sensory nuclei to determine the position of retrogradely labeled cortical cells with respect to S1 barrel cortex. Our results demonstrate that S1 and S2 projections terminate in largely overlapping regions but show some significant differences. Whereas S1 projection terminals tend to cluster within the principal trigeminal (Pr5), caudal spinal trigeminal interpolaris (Sp5ic), and the dorsal spinal trigeminal caudalis (Sp5c), S2 projection terminals are distributed in a continuum across all trigeminal nuclei. Contrary to the view that sensory gating could be mediated by differential activation of inhibitory interconnections between the spinal trigeminal subnuclei, , we observed that projections from S1 and S2 are largely overlapping in these subnuclei despite the differences noted earlier.

Rats are highly skilled in discriminating objects and textures by palpatory movements of their whiskers. If they used spatial frequency cues, they would be able to optimize performance in a stimulus dependent way - by moving their whisker faster or slower across the texture surface, thereby shifting the frequency content of the neuronal signal toward an optimal working range for perception. We tested this idea by measuring discrimination performance of head-fixed rats that were trained to actively sample from virtual grids. The virtual grid mimicked discrete and repetitive whisker deflections generated by real objects (e.g. grove patterns) with single electrical microstimulation pulses delivered directly to the barrel cortex, and provided the critical advantage that stimuli could be controlled at highest precision. Surprisingly, rats failed to use the spatial frequency cue for discrimination as a matter of course, and also failed to adapt whisking patterns in order to optimally exploit frequency differences. In striking contrast they could be easily trained to discriminate stimuli differing in electrical pulse amplitudes, a stimulus property that is not malleable by whisking. Intermingling these 'easy-to-discriminate' discriminanda with others that solely offered frequency/positional cues, rats could be guided to base discrimination on frequency and/or position, albeit on a lower level of performance. Following this training, abolishment of electrical amplitude cues and reducing positional cues led to initial good performance which, however, was unstable and ran down to very low levels over the course of hundreds of trials. These results clearly demonstrate that frequency cues, while definitely perceived by rats, are of minor importance and they are not able to support consistent modulation of whisking patterns to optimize performance.

Sensory receptors determine the type and the quantity of information available for perception. Here, we quantified and characterized the information transferred by primary afferents in the rat whisker system using neural system identification. Quantification of ‘how much’ information is conveyed by primary afferents, using the direct method, a classical information theoretic tool, revealed that primary afferents transfer huge amounts of information (up to 529 bits/s). Information theoretic analysis of instantaneous spike-triggered kinematic stimulus features was used to gain functional insight on ‘what’ is coded by primary afferents. Amongst the kinematic variables tested - position, velocity, and acceleration - primary afferent spikes encoded velocity best. The other two variables contribute to information transfer, but only if combined with velocity. We further revealed three additional characteristics that play a role in information transfer by primary afferents. Firstly, primary afferent spikes show preference for well separated multiple stimuli (i.e. well separated sets of combinations of the three instantaneous kinematic variables). Secondly, spikes are sensitive to short strips of the stimulus trajectory (up to 10 ms pre-spike time), and thirdly, they show spike patterns (precise doublet and triplet spiking). In order to deal with these complexities, we used a flexible probabilistic neuron model fitting mixtures of Gaussians to the spike triggered stimulus distributions, which quantitatively captured the contribution of the mentioned features and allowed us to achieve a full functional analysis of the total information rate indicated by the direct method. We found that instantaneous position, velocity, and acceleration explained about 50% of the total information rate. Adding a 10 ms pre-spike interval of stimulus trajectory achieved 80-90%. The final 10-20% were found to be due to non-linear coding by spike bursts.

Single-unit recordings conducted during perceptual decision-making tasks have yielded tremendous insights into the neural coding of sensory stimuli. In such experiments, detection or discrimination behavior (the psychometric data) is observed in parallel with spike trains in sensory neurons (the neurometric data). Frequently, candidate neural codes for information read-out are pitted against each other by transforming the neurometric data in some way and asking which code’s performance most closely approximates the psychometric performance. The code that matches the psychometric performance best is retained as a viable candidate and the others are rejected. In following this strategy, psychometric data is often considered to provide an unbiased measure of perceptual sensitivity. It is rarely acknowledged that psychometric data result from a complex interplay of sensory and non-sensory processes and that neglect of these processes may result in misestimating psychophysical sensitivity. This again may lead to erroneous conclusions regarding the adequacy of neural candidate codes. In this review, we first discuss requirements on the neural data for a subsequent neurometric-psychometric comparison. We then focus on different psychophysical tasks for the assessment of detection and discrimination performance and the cognitive processes that may underlie their execution. We discuss further factors that may compromise psychometric performance and how they can be detected or avoided. We believe that these considerations point to shortcomings in our understanding of the processes underlying perceptual decisions, and therefore offer potential for future research.

Background: Elderly residents of nursing homes often have multiple comorbidities and functional limitations. The status of 4,525 residents of complex and standard care units in 177 nursing homes where the nursing home was determined to provide adequate care, and from 14 specialized Alzheimer's units, was evaluated on 111 measures of medical condition, functional status, psychological well-being and cognitive performance in a demonstration study assessing quality of care in six states. Detailed measurements were also made of the types and amounts of services used (in minutes per day) by the residents., Methods: Given the number of health measures, and the possibility of assessment error, a multivariate analytic procedure called Grade of Membership (GoM) was used. This procedure identified profiles of health and functioning measures to identify the characteristics of clinically distinct groups of nursing home residents., Results: The analysis identified 11 profiles of health and functioning characteristics which described the 111 resident measurements. The 11 profiles predicted differentials in nursing home length of stay, and service use by various classes of caregivers. The GoM profiles described the data better than several other classification procedures applied to the same data., Conclusions: In nursing homes, elderly and oldest-old residents often have multiple comorbidities and disabilities. A multivariate procedure was able to identify the fundamental dimensions describing residents' variation on a number of health measures. These profiles predicted differences in service use so they had predictive validity. Thus, multivariate procedures may help identify clinically distinct groups in studies where complex measures are made.

Research has shown that the nursing home patient population is quite heterogeneous in terms of both individual patient characteristics and service needs. Furthermore, existing administrative classifications do a poor job of representing this heterogeneity. As a consequence we have conducted an analysis of the individual and service characteristics of two types of patients represented in the National Nursing Home Survey of 1977 (i.e., patients whose primary source of payment was Medicare and patients whose primary payment source was not Medicare). In this analysis we identified patterns of individual characteristics within the two patient groups and showed how these patterns related to their service needs. The logic of the model permitted us both to establish patterns of characteristics within the two payment types and to examine the implications of individual heterogeneity remaining in the classification. This makes the methodology useful both as a research tool for understanding the nature of the nursing home population and as a tool for studying the consequences of various classification schemes for questions of identifying service patterns and needs as well as the evaluation of policy options.