Magnitude-matching: the measurement of taste and smell

In the method of magnitude-matching, subjects try to judge intensitiesof sensations from two or more modalities on a single, commonscale. Using responses to one modality as a standard makes itpossible to compare subjects' suprathreshold perceptions onthe other, test modality. A series of ten experiments revealedthe following: (i) magnitude-matching ‘works’: withboth loudness of tones and lightness of grays as standards,tasters versus nontasters of 6-n-prophylthiouracil (PROP) (asdefined by a threshold criterion) show much greater responceto suprathreshold PROP and slightly greater response to surcose;(ii) though superior to rating-scale judgements of sensory intensitymade without reference to a second modality, magnitude-matchingis not, however, flawless: the cross-modality matching relationproduced by a set of magnitude-matches depends systematicallyon the contextual sets of stimulus levels presented for judgement;(iii) with taste as the standard, old versus young subjectsshowed only a 25% decrement in responce to the odor intensityof butanol when both groups recieved the same physical (concentration)levels, but a >50% decrement in responce when both groupsrecieved about the same perceptual levels; (iv) magnitude-matchesare much the same whether subjets make their judgements on abounded rating-scale or an open-ended magnitude-estimation scale:and (v) loudness, lightness and odor intensity serve about equallyin magnitude-matching with taste intensity.     

Determination of saltiness from the laws of thermodynamics--estimating the gas constant from psychophysical experiments

One can relate the saltiness of a solution of a given substance to the concentration of the solution by means of one of the well-known psychophysical laws. One can also compare the saltiness of solutions of different solutes which have the same concentration, since different substances are intrinsically more salty or less salty. We develop here an equation that relates saltiness both to the concentration of the substance (psychophysical) and to a distinguishing physical property of the salt (intrinsic). For a fixed standard molar entropy of the salt being tasted, the equation simplifies to Fechner's law. When one allows for the intrinsic 'noise' in the chemoreceptor, the equation generalizes to include Stevens's law, with corresponding decrease in the threshold for taste. This threshold reduction exemplifies the principle of stochastic resonance. The theory is validated with reference to experimental data.     

An informational approach to reaction times

Simple reaction time is the minimum time required to respond to a signal such as a steady light or tone. Such a reaction time is taken to be the time required for transmission of a fixed quantity of information, ΔH, from stimulus to subject. That is, information summation replaces energy summation. This information is calculated from consideration of the quantum nature of the stimulus. The theoretically derived equation for reaction time is fitted to experimental data. Piéron's empirical law for reaction time is obtained as an approximation from a proposed informational equation. The exponent in Piéron's law is found to be the same as the exponent in the power law of sensation. Threshold appears to be the smallest stimulus capable of transmitting the quantity of information ΔH.     

On the information received by sensory receptors

It is hypothesized that a sensory neuron, a neuron issuing from a sensory receptor, encodes the rate at which entropy or uncertainty is removed at the receptor level. This hypothesis is tested for the case of the entropy associated with the magnitude of a signal (stimulus) applied at the sensory receptor. A simple mathematical model of the process is presented and a number of well-known stimulus-response relationships are seen to emerge. For example, the adaptation of a receptor may be seen to occur as a consequence of reduced uncertainty regarding stimulus intensity. A general equation relating stimulus and response is developed, and this equation will simplify, depending upon the ratio of signal power to noise power, to either a logarithmic or a power law.     

A theoretical approach to G-protein modulation of cellular responsiveness

 Structure and function of cells often depend critically on molecular signals arriving at their surface. There are universal mechanisms of signal transduction and signal processing across cell membranes. In this paper the mechanisms involving guanine-nucleotide regulatory proteins (“G-proteins”) and certain receptor-kinases are considered. On the basis of recent findings in molecular biology a mathematical model is developed taking into account all essential components in the biochemical network between first and second messenger. There are two coupled feedback loops inherent in this process. The model finally consists of three nonlinear equations, which are obtained from a system of originally ten equations by using conservation laws and quasi-steady state conditions. The second part of the paper contains a mathematical analysis of the model. Solutions describing the temporal development of the involved biochemical species are shown to be bounded, more specifically to remain, independent of the size of the input signal, in a bounded domain of the state space. For the situation of stationary input signals existence, uniqueness and asymptotic stability of steady states are derived. We also demonstrate biologically relevant stimulus-response properties like monotonicity and saturation effects. For temporally non-constant input signals we show numerically that the model is able to produce phenomena of hypersensitivity and desensitization which are important characteristics of cellular responsiveness. Received 18 March 1996; received in revised form 15 April 1996     

Salt as an oral irritant

The temporal characteristics of oral irritation produced byNaCl and KCl were investigated in two experiments. Subjectswere exposed to 0.4 and 0.8 M solutions of both salts at therate of once per min for 15 min. No water rinses were allowedbetween sips. The results showed that the irritation producedby NaCl increased in intensity as a function of number of exposures.In contrast, the intensity of the salt taste remained approximatelyconstant over the 15 min period. The relative contribution ofirritation to the sensation of saltiness therefore increasedover time. The same pattern of results was found for 0.8 M butnot for 0.4 M KCl. The growth in perceived intensity of saltirritation with repeated exposures may be attributable to thephenomenon of ‘sensitization’ that is a characteristicof some nociceptive afferents. Overall, the results demonstratethat salts produce perceptible sensations of irritation at concentrationstypically used in psychophysical studies of salt taste, andthat this apparently non-gustatory component of saltiness vanesdirectly with both concentration and duration of exposure.     

Interactions between oral chemical irritation, taste and temperature

The oral chemical irritant, capsaicin, at 2, 4 and 8 p.p.m.,was combined in mixtures with sucrose (Experiment 1), sodiumchloride (Experiment 2) and soup (Experiment 3), each evaluatedat two temperatures. These mixtures were rated for their sweetnessand/or saltiness, intensity of burning sensation and total mixtureintensity. In both solution and soup, sweetness was suppressed,whereas saltiness showed only minor suppression in low NaCl,high capsaicin mixtures. The burning sensation produced by capsaicinwas uninfluenced by sucrose, while NaCl increased the burningsensation. Total mixture intensity was entirely determined bycapsaicin concentration in mixtures with sucrose, although NaClcontributed in NaCl/capsaicin mixtures. Varying temperatureinfluenced the burning sensation and total intensity of sucrose/capsaicinmixtures, but did not modulate the effects of capsaicin on taste.Explanations of taste suppression in terms of cognitive andstructural models are examined. The differential effect of capsaicinon sweetness and saltiness is also considered in terms of theirritant properties of NaCl.     

The Dynamic Range Paradox: A Central Auditory Model of Intensity Change Detection

In this paper we use empirical loudness modeling to explore a perceptual sub-category of the dynamic range problem of auditory neuroscience. Humans are able to reliably report perceived intensity (loudness), and discriminate fine intensity differences, over a very large dynamic range. It is usually assumed that loudness and intensity change detection operate upon the same neural signal, and that intensity change detection may be predicted from loudness data and vice versa. However, while loudness grows as intensity is increased, improvement in intensity discrimination performance does not follow the same trend and so dynamic range estimations of the underlying neural signal from loudness data contradict estimations based on intensity just-noticeable difference (JND) data. In order to account for this apparent paradox we draw on recent advances in auditory neuroscience. We test the hypothesis that a central model, featuring central adaptation to the mean loudness level and operating on the detection of maximum central-loudness rate of change, can account for the paradoxical data. We use numerical optimization to find adaptation parameters that fit data for continuous-pedestal intensity change detection over a wide dynamic range. The optimized model is tested on a selection of equivalent pseudo-continuous intensity change detection data. We also report a supplementary experiment which confirms the modeling assumption that the detection process may be modeled as rate-of-change. Data are obtained from a listening test (N = 10) using linearly ramped increment-decrement envelopes applied to pseudo-continuous noise with an overall level of 33 dB SPL. Increments with half-ramp durations between 5 and 50,000 ms are used. The intensity JND is shown to increase towards long duration ramps (p<10⁻⁶). From the modeling, the following central adaptation parameters are derived; central dynamic range of 0.215 sones, 95% central normalization, and a central loudness JND constant of 5.5×10⁻⁵ sones per ms. Through our findings, we argue that loudness reflects peripheral neural coding, and the intensity JND reflects central neural coding.     

A computational framework for topographies of cortical areas

Self-organizing feature maps (SOFMs) represent a dimensionality-reduction algorithm that has been used to replicate feature topographies observed experimentally in primary visual cortex (V1). We used the SOFM algorithm to model possible topographies of generic sensory cortical areas containing up to five arbitrary physiological features. This study explored the conditions under which these multi-feature SOFMs contained two features that were mapped monotonically and aligned orthogonally with one another (i.e., “globally orthogonal”), as well as the conditions under which the map of one feature aligned with the longest anatomical dimension of the modeled cortical area (i.e., “dominant”). In a single SOFM with more than two features, we never observed more than one dominant feature, nor did we observe two globally orthogonal features in the same map in which a dominant feature occurred. Whether dominance or global orthogonality occurred depended upon how heavily weighted the features were relative to one another. The most heavily weighted features are likely to correspond to those physical stimulus properties transduced directly by the sensory epithelium of a particular sensory modality. Our results imply, therefore, that in the primary cortical area of sensory modalities with a two-dimensional sensory epithelium, these two features are likely to be organized globally orthogonally to one another, and neither feature is likely to be dominant. In the primary cortical area of sensory modalities with a one-dimensional sensory epithelium, however, this feature is likely to be dominant, and no two features are likely to be organized globally orthogonally to one another. Because the auditory system transduces a single stimulus feature (i.e., frequency) along the entire length of the cochlea, these findings may have particular relevance for topographic maps of primary auditory cortex. This research was supported by The McDonnell Center for Higher Brain Function, The Wallace H. Coulter Foundation and NIH grant DC008880.     

“Ignition” phenomena in random nets

The spread of excitation in a “random net” is investigated. It is shown that if the thresholds of individual neurons in the net are equal to unity, a positive steady state of excitation will be reached equal to γ, which previously had been computed as the weak connectivity of the net. If, however, the individual thresholds are greater than unity, either no positive steady state exists, or two such states depending on the magnitude of the axone density. In the latter case the smaller of the two steady states is unstable and hence resembles an “ignition point” of the net. If the initial stimulation (assumed instantaneous) exceeds the “ignition point,” the excitation of the net eventually assumes the greater steady state. Possible connections between this model and the phenomenon of the “preset” response are discussed.     

A note on the thermodynamics and kinetics of open and steady state systems

Some conclusions of irreversible thermodynamics are summarized. It is shown that θ, the rate of irreversible entropy production, is not minimized in the steady state. It is also postulated that multiple steady states are possible in nonlinear kinetic systems, giving rise to situations of possible biological interest. The necessity of examining particular kinetic models is mentioned.     

Entropy budget of conifer branches

From energy budget data for a branch of ponderosa pine given by Gates, Tibbals and Kreith, entropy fluxes into or out of the branch due to solar radiation, infrared radiation, transpiration and convection are calculated. Net entropy flow into the branch is negative. Assuming that the entropy in the branch is at steady state, the entropy production in the branch of ponderosa pine is calculated and shown to be positive. A positive entropy production indicates that the Second Law of Thermodynamics is certainly valid in the branch. Entropy productions in other conifers, blue spruce and white fir, and in a single pine needle in a horizontal position are also calculated. The entropy production (S_prod) increases linearly with the solar energy absorbed by the plant surface (E_solar); S_prod≈(30.6 E_solar)×10⁻⁴. The ratio (S_prod/E_solar) does not differ between deciduous leaves reported earlier and conifer branches. The theorem of oscillating entropy production proposed earlier holds also for conifer branches and will be of universal nature, applicable to all plant leaves.     

Successive contrast cannot explain suppression release after repetitious exposure to one of the components of a taste mixture

Repetitious exposure to one of the components of a NcCl-sucrosemixture causes the other component to recover from suppression.This so-called‘suppression release’ might be explainedby assuming that the subjects habituate to the repeated component,which is thereby disabled as a suppressor in the mixture. Twoexperiments are reported that test successive contrast as analternative explanation of this type of suppression release.This alternative was investigated by substituting an unmixedtest stimulus of about equal subjective sweetness (or saltiness)for the mixture. The results indicate that successive contrastcontributes only partially to the suppression-release effect.When the contrast contribution is subtracted, a significantsuppression-release effect remains.     

Modality interactions alter the shape of acoustic mate preference functions in gray treefrogs

Sexual selection takes place in complex environments where females evaluating male mating signals are confronted with stimuli from multiple sources and modalities. The pattern of expression of female preferences may be influenced by interactions between modalities, changing the shape of female preference functions, and thus ultimately altering the selective landscape acting on male signal evolution. We tested the hypothesis that the responses of female gray treefrogs, Hyla versicolor, to acoustic male advertisement calls are affected by interactions with visual stimuli. We measured preference functions for several call traits under two experimental conditions: unimodal (only acoustic signals presented), and multimodal (acoustic signals presented along with a video‐animated calling male). We found that females were more responsive to multimodal stimulus presentations and, compared to unimodal playbacks, had weaker preferences for temporal call characteristics. We compared the preference functions obtained in these two treatments to the distribution of male call characteristics to make inferences on the strength and direction of selection expected to act on male calls. Modality interactions have the potential to influence the course of signal evolution and thus are an important consideration in sexual selection studies.     

Density-dependent migration and stability in a system of linked populations

The effect of adding density-dependent migration between nearest neighbour populations of a single discrete-generation species in a chain of habitat fragments is investigated. The larger the population on a particular habitat fragment, the greater the fraction of inhabitants who migrate before reproducing. It has previously been shown for similar models with density-independent migration that coupling populations in this way has no effect on the stability of these populations. Here, it is demonstrated that this effect is also generally true if migration is density-dependent. However, if the migration rate is large enough and has density dependence of the correct form, then the steady state (with all the populations remaining at the same constant value through time) can be destabilised. The conditions for this to occur are obtained analytically. When this “destabilisation” occurs, the system settles down to an alternative steady state where half of the populations take one constant value which is below that of an equivalent isolated system, and the other populations all share a population value which is greater than the steady state of the isolated populations. Once this configuration is reached, the population size on each patch remains constant over time. hence the change might more properly be described as a decrease in homogeneity rather than in stability.     

After repetitious sucrose stimulation saltiness suppression in NaClsucrose mixtures is diminished: implications for a central mixture suppression mechanism

Human subjects estimated the saltiness of sequences of tastestimuli by a modified magnitude-estimation procedure. The laststimulus in a sequence was always a NaCl/sucrose mixture, whereasthe preceding stimuli were a number of sucrose stimuli. Sensoryadaptation was prevented by choosing short stimulus durationsand long inter-stimulus intervals and by mouth-rinsing betweenstimuli. Saltiness suppression in the mixture was diminishedin a logarithmic-like way by habituation to NaCl and this effectbecame more pronounced with increasing numbers of precedingsucrose stimuli. The steepness of the saltiness-recovery functiondepends upon the amount of mixture suppression and all functionsconverge at a point where mixture suppression is about zero.On the basis of these and previous results it is hypothesizedthat NaCl/sucrose mixtures excite two independent neuronal tastecenters which have inhibitory projections to each other. Furthermore,the results suggest a functional distinction between neuralactivity leading to intensity sensation and neural activityassociated with mixture suppression.     

Wing-shaking and wing-patch as nestling begging strategies: their importance and evolutionary origins

Avian chicks use different begging strategies when soliciting parental care. A novel begging strategy was recently observed in Horsfield’s hawk-cuckoo Hierococcyx hyperythrus (=Cuculus fugax). Chicks of this brood-parasitic species raise and shake their wings and display to fosterers a gape-coloured patch on the undersides of their wings. Although the gape-coloured wing-patch may be a unique trait of Horsfield’s hawk-cuckoo, wing-shaking in the context of begging is virtually universal in both brood parasites and their hosts. A simple qualitative comparison across different avian taxa suggests that wing-shake begging is most probably an ancestral feature of cuckoos and perhaps all altricial birds. The wing-shaking may be an honest signal of chick quality. It could also reduce the risk of predation if wing-shaking was coupled with reduced loudness of begging. Horsfield’s hawk-cuckoo chicks could have exploited the universal pre-existing host responsiveness to wing-shake begging. Evolution could have then further proceeded by making the wing-shaking more conspicuous by addition of another stimulus—the unique colourful wing-patch. I also hypothesize that wing-shake begging may have evolved from pre-fledging restlessness and is used secondarily in courtship displays, threatening postures, and distraction displays by adults. Further discussions and tests of these hypotheses may facilitate research into the so far unstudied phylogenetic history of avian chick-begging strategies. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Comfortable Loudness Level and Phrase Intelligibility of Spectrally Deprived Speech

It is known from the literature that (1) sounds with complex spectral composition are assessed by summing the partial outputs of the spectral channels; (2) electrical stimuli used in cochlear implant systems bring about the perception of a frequency band; and (3) removal of different parts of the auditory spectrum significantly affects phrase intelligibility. The level of acoustic pressure (AP) at a comfortable loudness level and the phrase intelligibility after comb filtering of a speech signal were measured in normally hearing subjects. Using a software program for spectral transformation of the speech signal, the phrase spectrum was divided into frequency bands of various width and only the bands with odd numbers were summed. In three series, the width of odd bands was 50, 100, or 150 Hz and the width of even bands was varied. The filter period was equal to the sum of the even and odd bands. With the same period, the acoustic pressure of the output signal should be increased to reach the comfortable loudness level of a speech signal passed via the comb filter; the narrower the width of the test bands, the higher the AP increase. With the same width of the test band, the acoustic pressure of the output signal should be increased to reach the comfortable loudness level; the greater the filter period, the higher the increase should be. The speech signal redundancy with respect to its spectral content can be equal to or even exceed 97.5%.     

Specific features in realization of the principle of minimum energy dissipation during individual development

Realization of the principle of minimum energy dissipation (Prigogine??s theorem) during individual development has been analyzed. This analysis has suggested the following reformulation of this principle for living objects: when environmental conditions are constant, the living system evolves to a current steady state in such a way that the difference between entropy production and entropy flow (?? _u function) is positive and constantly decreases near the steady state, approaching zero. In turn, the current steady state tends to a final steady state in such a way that the difference between the specific entropy productions in an organism and its environment tends to be minimal. In general, individual development completely agrees with the law of entropy increase (second law of thermodynamics).