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K O Johnson

Publications and source records attributed to K O Johnson.

At least 55 records · Page 3Linked to original sources
PubMed1990

Tactile roughness: neural codes that account for psychophysical magnitude estimates.

Hypothetical neural codes underlying the sensation of tactile roughness were investigated in a combined psychophysical and neurophysiological study. The stimulus set consisted of plastic surfaces embossed with dot arrays of varying dot diameter and center-to-center spacing. Human subjects explored each surface with the pad of the index finger and reported their subjective sense of roughness magnitude. The same surfaces were scanned across the receptive fields of cutaneous mechanoreceptive afferents in monkeys while recording the evoked action potentials. Hypothetical neural codes for roughness magnitude were computed from the neural response patterns and tested for their ability to account for the psychophysical data. The psychophysical results showed that subjective roughness magnitude is an inverted U-shaped function of dot spacing that peaks near 3.0 mm spacing, and that increased dot diameter produces decreased roughness sensations at all dot spacings. Hypothetical neural codes that do not bear a consistent relationship to roughness magnitude across all of these stimulus conditions can be rejected as the code for roughness. Four types of neural codes were considered. They were based on (1) mean firing rate, (2) general variation in firing rate, (3) short-term temporal variation in firing rate, and (4) local spatial variation in firing rate. Mean firing rate failed to explain the psychophysical results: surfaces that evoked the same firing rate often evoked very different roughness judgments. In contrast, neural codes based on firing-rate variation, especially in slowly adapting afferents, account for the psychophysical results.

Action Potentials↗
PubMed1988

A rotating drum stimulator for scanning embossed patterns and textures across the skin.

A tactile stimulator is described that moves embossed or textured patterns tangentially across the skin. Patterns constructed by standard photoetching are mounted on the outer surface of a cylinder that rotates at a selected speed and is held in contact with the skin at a selected force. The stimulator operates in several modes to meet the different requirements of psychophysical and neurophysiological experiments. The features of note are (i) relatively small size and weight; (ii) flexible automated control of drum contact with the skin, angular velocity, axial position, and contact force; (iii) monitoring of drum angular and axial location to better than 10 micron accuracy; (iv) construction with commercially available devices; (v) electronic monitoring of skin contact; and (vi) rapid drum changes (2 seconds) during psychophysical or neurophysiological experiments.

Electronics, Medical↗
PubMed1988

Spatial pattern representation and transformation in monkey somatosensory cortex.

Embossed letters, used previously in pattern recognition experiments in humans, were used to study the spatial patterns of neural activity evoked in peripheral fibers and cortical neurons in areas 3b and 1 of the primary somatosensory cortex of alert rhesus (Macaca mulatta) monkeys. The object was to investigate the representation and transformation of spatial information during the early stages of peripheral and cortical neural processing. Our method consisted of sweeping each letter of the alphabet across the skin repeatedly and constructing a two-dimensional plot (called a spatial event plot) of the action potentials evoked in afferent fibers and cortical neurons. By using this method, slowly and rapidly adapting primary afferents were shown to transmit isomorphic neural images of the letters. Although the slowly adapting images were more spatially acute, both populations conveyed images of sufficient quality to account for human psychophysical performance. In the cortical areas studied, the slowly adapting neurons of area 3b stood out for the acuity, complexity, and variety of their responses. Some of the spatial event plots for these neurons were isomorphic and at least as acute as those obtained from any primary afferent. Others were highly structured but nonisomorphic. The quality and variety of responses in area 3b slowly adapting neurons suggest that they play an important role in the processing of information underlying tactual pattern recognition. The rapidly adapting neurons of area 3b and all types of neurons in area 1 yield much less structured and differentiated responses.

Animals↗
PubMed1985

Neural mechanisms of scanned and stationary touch.

The neural mechanisms subserving the sense of touch set the limits for the acquisition of information regarding the spatial and temporal characteristics of stimuli impinging on the skin surface. The results of three different psychophysical experiments imply that the skin of the finger pad can resolve the elements of a stimulus separated by 0.9 mm when the stimulus is applied to the skin and held stationary. This resolution limit is only slightly improved (to about 0.7 mm) when movement between the stimulus and skin is allowed. Single-unit recordings from three classes of primary mechanoreceptive afferents in anesthetized monkeys shows that only one class, the slowly adapting afferents, resolve spatial detail of stationary stimuli near the resolution limit. In addition, slow adaptors appear to resolve moving stimuli (e.g., Braille-dot patterns) more effectively than do the other two classes. However, these observations do not explain the extraordinary capacity of the finger-pad skin for discriminating between fine textures. Neurophysiological evidence suggests that information about such textures (i.e., surfaces with spatial details below the resolution limit) may be conveyed by a code based on the relative engagement of the three receptor populations.

Adaptation, Physiological↗
PubMed1983

Neural mechanisms of tactual form and texture discrimination.

The mechanisms of fine form discrimination and texture discrimination are discussed in terms of a set of working hypotheses. 1) The limits of fine form discrimination at the fingertips are set by peripheral, not central, factors. 2) The critical information is relayed by slowly adapting (SA) afferents. 3) Texture is coded by the relative discharge rates in the quickly adapting, Pacinian, and SA populations. 4) SA transduction and transmission properties are particularly adapted for signaling spatial information. 5) The skin of the finger pad behaves like an ideal continuous medium.

Acclimatization↗
PubMed1982

Cutaneous mechanoreceptors in macaque monkey: temporal discharge patterns evoked by vibration, and a receptor model.

1. Vibratory stimuli applied to the hand of a monkey evoke phase locked impulse trains in the three classes of low threshold mechanoreceptive afferents which innervate the area. The responses of each class of afferent (slowly adapting (SA), rapidly adapting (RA), and Pacinian (PC) vary in a systematic but complex way across the range of frequencies and intensities to which they are sensitive. The receptors are not accessible for electrophysiological recording. The aim in this study was to infer the mechanisms underlying their responses from detailed examination of the statistical properties of the impulse trains.2. A very simple receptor model with four degrees of freedom was chosen as a starting point. The independent variables consisted of the resting membrane time constant, tau, a variable membrane conductance, G(r), the fraction of each sinusoidal stimulus cycle producing depolarization, p(r), and the noise level, sigma, which was assigned to the impulse threshold. The aim was to use the deviations between observed data and predictions from the basic model to construct a more effective model. In fact, the deviations were minor and were mostly explained by periods of increased excitability in the wake of each action potential. Almost all of the differences between the responses of the three mechanoreceptive classes examined in this paper were accounted for by differences in time constants.3. The temporal structure of the responses from each mechanoreceptive class was examined at two levels of resolution, a coarse level where the resolution unit was a full cycle, and a fine level where the unit was 0.1 ms.4. The coarse structure of each response was represented by the presence or absence of an impulse on each stimulus cycle. In each mechanoreceptive class, the impulse sequences were random at low stimulus frequencies and regular at high frequencies. The transition frequencies were roughly 5 Hz for the slowly adapting afferents, 7 Hz for the rapidly adapting afferents, and 110 Hz for the Pacinian afferents. The model matched these data closely when the time constants were set at 80, 60 and 3.4 ms for SAs, RAs and PCs, respectively.5. The fine structure of the responses of each mechanoreceptive class exhibited impulse phase locking, phase advance with increasing intensity, and bimodal phase distributions at higher frequencies. Impulses contributing to the first mode of bimodal distributions always occurred in cycles following cycles in which no impulse occurred. Impulses contributing to the retarded mode always occurred in cycles following filled cycles. The mean phase differences between the two modes was called phase retardation. Phase retardation grew with stimulus frequency for both the receptors and the model; the time constants required to match the model against neural phase retardation curves were 123, 64 and 4.8 ms for SAs, RAs and PCs, respectively.

Animals↗
PubMed1982

A model accounting for effects of vibratory amplitude on responses of cutaneous mechanoreceptors in macaque monkey.

1. A mechanoreceptor model, developed in the preceding paper (Freeman & Johnson, 1982), was used to study the effects of vibratory intensity and frequency on the responses of slowly adapting, rapidly adapting and Pacinian afferents in monkey hairless skin. As in the previous paper almost all of the response properties studied here were accounted for by the equivalent circuit model; changes in membrane time constant and amplitude sensitivity accounted for the differences between the three mechanoreceptive fibre types.2. The stimulus-response function of primary concern was the relationship between impulse rate and vibratory amplitude. This relationship had the same general form in each of the three fibre types. Amplitudes, I, less than I(0) produced no impulse on any stimulus cycles. Amplitudes greater than I(1) produced one impulse on every cycle. As I rose from I(0) to I(1) the impulse rate rose monotonically from 0 to 1 impulse/cycle. For each fibre type the form of this ramp depended on the stimulus frequency.3. At stimulus frequencies low in the frequency range of each fibre type the (I(0), I(1)) ramp tended to be steep and sigmoidal in shape. Two or more impulses occurred on some cycles and none on others.4. At intermediate frequencies the (I(0), I(1)) ramps became linear with at most one impulse on each cycle. A short plateau appeared at 0.5 impulses/cycle (i.e. there was a range of intensities yielding one impulse on alternate cycles). All of these response properties at low and intermediate frequencies were explained by the model.5. At higher frequencies the (I(0), I(1)) ramps became shallower and developed discontinuities in slope at impulse rates of 0.5 impulses/cycle. At stimulus frequencies greater than 20 Hz for SAs and RAs, the upper segment of the (I(0), I(1)) slope became steeper. For frequencies greater than 80 Hz, the upper segments of the Pacinian (I(0), I(1)) slopes were shallower than the lower segments. These effects suggested transient periods of hyperexcitability following each action potential, and reductions in sensitivity due to high impulse rates, respectively.6. The model's membrane time constant was adjusted to match the observed reduction in the (I(0), I(1)) slope with increasing stimulus frequency. The time constants required for least-squares fitting were 58, 29 and 4.2 msec for slowly adapting, rapidly adapting and Pacinian afferents, respectively; these values are of the same order as those obtained in the preceding paper.7. Receptor sensitivity varied across the frequency spectrum, slow adaptors being most sensitive at low frequencies, rapidly adapting units at mid-range, and Pacinians at the high frequencies. According to the model, the high frequency roll-off in a receptor's tuning curve is due to the current integrating properties of receptor membrane, and the low frequency roll-off is due to a high pass filter, presumably mechanical, situated in the tissues between the stimulus probe and receptor membrane.8. Impulse phase advances with increasing stimulus intensity in both receptor and model. The ability of the model to fit both the rate-intensity function and phase advance functions in individual receptors is demonstrated.

Animals↗
PubMed1981

The utilization of trifluorothymidine (TFT) to select for thymidine kinase-deficient (TK-/-) mutants from L5178Y/TK+/- mouse lymphoma cells.

Trifluorothymidine (TFT), a thymidine analog, was analyzed for its ability to select for thymidine kinase-deficient (TK-/-) mutants. In comparison with BUdR, the traditional selective agent for TK-/- cells, it was determined that TFT at 1/50th the dose (1 microgram/ml vs. 50 microgram/ml) is a more effective and versatile selective agent for TK-/- mutants arising from the TK+/- -3.7.2C heterozygote of L5178Y mouse lymphoma cells. Since TFT acts more rapidly than BUdR, it can be utilized in procedures (such as the analysis of the phenotypic lag) requiring the fast arrest of cell division. Reconstruction analyses of effective TK-/- mutant recovery indicate that TFT can be used to recover mutants from significantly higher densities of TK+/- cells than can BUdR. In addition, TK-/- mutants can attain larger colony size in TFT than in BudR where severe stunting of growth occurs at high TK-/- cell densities. 190 of 194 isolated TFT-resistant large and small colony mutants (both spontaneous and induced).

Animals↗
PubMed1981

Neural mechanisms of spatial tactile discrimination: neural patterns evoked by braille-like dot patterns in the monkey.

1. The experiments reported here were designed to investigate the responses of cutaneous mechanoreceptive afferents to spatially configured dot patterns scanned across the skin. Braille-like patterns were selected because the discrimination of Braille characters must depend on spatial patterning rather than some other facet of the afferent discharge. 2. A multifactorial experimental design was used in which each afferent fibre was studied using every combination of six dot patterns, two dot sizes, two dot spacings, two contact forces and two scanning velocities. Two other factors, scanning direction relative to the skin ridges and intermittent versus continuous scanning, were studied. 3. Beside the general question concerning the response properties of the mechanoreceptive afferents, three major questions were addressed here. (i) What is the critical spatial dimension at which neural spatial patterning breaks down and below which tactual discrimination must depend on facets of the afferent discharge other than spatial neural patterning? (ii) Which mechanoreceptive population sets this critical dimension? (iii) Why is tactual discrimination enhanced by lateral scanning? 4. The results presented here suggest that the critical dimension, below which spatial neural patterning breaks down, is of the order of 1.0 mm and that the slowly adapting (SA) afferent fibres are responsible for this limit. 5. At dimensions above approximately 1.0 mm the spatial contrast between peaks and troughs in the SA discharge is markedly enhanced during scanning. When the skin is stationary the discharge rates in the SA population drop rapidly to low levels. A second possible reason for enhanced tactual discrimination during scanning is related to the increased spatiotemporal information in a coherent pattern of neural activity moving across a discrete population of afferent fibres. 6. The effects of variations in conduction velocity are analysed and it is shown that they place serious constraints on the transmission of spatiotemporal information.

Animals↗
PubMed1980

Peripheral neural representation of spatial dimensions of a textured surface moving across the monkey's finger pad.

1. The responses of single cutaneous mechanoreceptive fibres evoked by a geometrically patterned surface sweeping across the skin were examined. The surfaces used were each a fine diamond-shaped array of "dots' in relief against a flat background. The fibres examined were rapidly adapting, slowly adapting and Pacinian fibres innervating the monkey's finger pad skin. 2. Each fibre's response to the two-dimensioned surface was assessed using a procedure in which the surface was swept across the fibre's receptive field many times, the position of the surface relative to the underlying receptive field being precisely known at any instant in time. Between each successive sweep the stimulus surface was shifted lateral to the direction of movement by a small increment. The response pattern generated by this scanning procedure specified the extent to which a single response of the fibre was dependent on the spatial and temporal dimensions of the stimulus. 3. The invariant spatial features of the moving surface were represented only in the responses of populations of fibres; responses of the constituent fibres always confounded the information relayed about the surface pattern and its rate of movement across the skin. 4. Factors determining the representation of the spatial and temporal characteristics of the moving surface in the responding mechanoreceptive fibre population were examined. These included the response characteristics of the constituent fibres, the innervation density, and the total number of fibres engaged by the moving surface.

Action Potentials↗
PubMed1980

Sensory discrimination: decision process.

1. This paper and a following paper deal with problems, such as the following, that arise in experimental studies of the neural mechanisms underlying sensory discrimination: What measures of neural activity are relevant in such a study? How can sample data from the responses of single neurons be combined to represent the information relayed by a population of neurons? How can neural data be compared with results from psychophysical studies? What assumptions are implicit in any such comparison? What are the implications of assumptions that neurons respond independently or that they have homogeneous response properties? How can neural codes be assessed in a systematic way? Can psychophysical and neurophysiological observations be combined to infer mechanisms or relationships in the processes underlying discrimination? All of these questions require some theoretical framework before they can be answered. These papers set out such a framework, they deal with most of those questions, and they provide practicable formulas for relating sample data from neurophysiological experiments to behavioral measures derived from psychophysical experiments. 2. The processes that intervene between a relatively peripheral array of neural activity and a subject's decision in a discrimination task are split into two sections: a) the ascending sensory processes that provide the final patterns of neural activity on which discrimination is based, and b) a process that yields decisions of the type required by the experimental design used in the psychophysical study. The approach is to develop a theory of the decision process in this paper, and then to expand it to incorporate the ascending processes in the following paper. 3. The decision theory deals with a class of experimental designs in which a subject is required to make a decision about two stimuli S1 and S2 (e.g., S1 is larger than S2, S2 is the same as S1, S2 was the modified stimulus, and so on). A mathematical representation for experimental designs of this type is developed. 4. The decision process is analyzed in two forms: a) a multivariate form in which the discrimination decision results directly from multidimensional neural representation of the two stimuli, and b) a vivariate form in which the final representation of each stimulus is a unidimensional variable. Conditions required for equivalence of these formulations are examined. 5. The theory includes as explicit variables a) the experimental design, b) the subject's discrimination strategy, c) bias, d) memory variance, e) bias variance, f) variance in the final neural representations of the stimuli, and g) their functional dependence on the stimuli that they represent. 6. Formulas are developed for the expected values of commonly used psychophysical measures such as the classical psychometric function, receiver operating characteristic (ROC) functions, discriminatory separation index (d'), and the difference limen. 7. Optimum discrimination behavior is analyzed.

Afferent Pathways↗