Decrease In Stimulus Intensity Research Articles

Synchrony-Division Neural Multiplexing: An Encoding Model

Cortical neurons receive mixed information from the collective spiking activities of primary sensory neurons in response to a sensory stimulus. A recent study demonstrated an abrupt increase or decrease in stimulus intensity and the stimulus intensity itself can be respectively represented by the synchronous and asynchronous spikes of S1 neurons in rats. This evidence capitalized on the ability of an ensemble of homogeneous neurons to multiplex, a coding strategy that was referred to as synchrony-division multiplexing (SDM). Although neural multiplexing can be conceived by distinct functions of individual neurons in a heterogeneous neural ensemble, the extent to which nearly identical neurons in a homogeneous neural ensemble encode multiple features of a mixed stimulus remains unknown. Here, we present a computational framework to provide a system-level understanding on how an ensemble of homogeneous neurons enable SDM. First, we simulate SDM with an ensemble of homogeneous conductance-based model neurons receiving a mixed stimulus comprising slow and fast features. Using feature-estimation techniques, we show that both features of the stimulus can be inferred from the generated spikes. Second, we utilize linear nonlinear (LNL) cascade models and calculate temporal filters and static nonlinearities of differentially synchronized spikes. We demonstrate that these filters and nonlinearities are distinct for synchronous and asynchronous spikes. Finally, we develop an augmented LNL cascade model as an encoding model for the SDM by combining individual LNLs calculated for each type of spike. The augmented LNL model reveals that a homogeneous neural ensemble model can perform two different functions, namely, temporal- and rate-coding, simultaneously.

No temporal contrast enhancement of simple decreases in noxious heat.

Offset analgesia (OA) studies have found that small decreases in the intensity of a tonic noxious heat stimulus yield a disproportionately large amount of pain relief. In the classic OA paradigm, the decrease in stimulus intensity is preceded by an increase of equal size from an initial noxious level. Although the majority of researchers believe this temporal sequence of two changes is important for eliciting OA, it has also been suggested that the temporal contrast mechanism underlying OA may enhance detection of simple, isolated decreases in noxious heat. To test whether decreases in noxious heat intensity, by themselves, are perceived better than increases of comparable sizes, we used an adaptive two-interval alternative forced choice task to find perceptual thresholds for increases and decreases in radiant and contact heat. Decreases in noxious heat were more difficult to perceive than increases of comparable sizes from the same initial temperature of 45°C. In contrast, decreases and increases were perceived equally well within a common range of noxious temperatures (i.e., when increases started from 45°C and decreases started from 47°C). In another task, participants rated the pain intensity of heat stimuli that randomly and unpredictably increased, decreased, or remained constant. Ratings of unpredictable stimulus decreases also showed no evidence of perceptual enhancement. Our results demonstrate that there is no temporal contrast enhancement of simple, isolated decreases in noxious heat intensity. Combined with previous OA findings, they suggest that long-lasting noxious stimuli that follow an increase-decrease pattern may be important for eliciting the OA effect.NEW & NOTEWORTHY Previous research suggested that a small decrease in noxious heat intensity feels surprisingly large because of sensory enhancement of noxious stimulus offsets (a simplified form of “offset analgesia”). Using a two-alternative forced choice task where participants detected simple increases or decreases in noxious heat, we showed that decreases in noxious heat, by themselves, are no better perceived than increases of comparable sizes. This suggests that a decrease alone is not sufficient to elicit offset analgesia.

Offset analgesia is not affected by cold pressor induced analgesia.

Background and aims Offset analgesia (OA) is a pain modulating mechanism described as a disproportionately large decrease in pain intensity evoked by a minor decrease in stimulus intensity. Precise mechanisms of OA are still not elucidated and studies are needed to evaluate factors modulating OA. The aim of this study was to investigate OA before and during tonic cold pain (thought to induce descending inhibition), in a group of healthy volunteers. Methods A randomized, crossover study was performed in 17 healthy participants (8 males and 9 females). The OA paradigm lasted 35 s and was induced by the traditional method using thermal stimulation applied to the forearm. A constant control heat stimulus (CTL) paradigm was used as control to assess adaptation. Pain intensity was assessed continuously. For induction of tonic cold pain, the participants immersed their hand into 2°C water for 2 min. After 1 min and 25 s, the heat stimulation (OA or CTL paradigm) was repeated to assess the modulatory effect of the cold pressor test. Results It was possible to induce OA both before and during the cold pressor test. Tonic cold pain modulated the peak pain reported during both the OA (p=0.015) and CTL paradigms (p=0.001) reflecting endogenous pain modulation. However, the magnitude of OA was not modulated by tonic cold pain (p>0.05). Conclusions The offset analgesia magnitude was not modulated by simultaneously tonic cold pain, thought to reflect another endogenous pain modulation mechanism. Implications Neither offset analgesia magnitude nor adaptation were modulated by cold pressor induced endogenous analgesia. This could be explained by the fact, that offset analgesia was already at maximum in healthy participants. Hence, offset analgesia may not be a suitable assessment tool to investigate modulation induced by experimental methods or pharmacology in healthy participants.

Role of explicit verbal information in conditioned analgesia.

The exact role of expectation in conditioned analgesia is still elusive as it is not clear whether conditioning is an automatic process or rather it is cognitively mediated. This study is aimed at understanding the role of explicit verbal information in conditioned analgesia. Two groups of healthy subjects received a conditioning procedure whereby two visual cues were paired with increase and decrease in stimulus intensity. In the 'conditioning/verbal information' group (VER), subjects were informed about the meaning of the cues, whereas no information was given to the second group (noVER). After two conditioning blocks, an evocation session was run in which the stimulus intensity was the same, irrespective of the cues. Pain perception was assessed according to a numerical rating scale from 0 (no pain) to 10 (maximal pain). The N2-P2 component of laser-evoked potentials (LEP) was used as an index of index of brain responses to nociceptive stimuli. In the evocation session, only the VER group reported a decrease in pain rating and LEP amplitude when the cues were presented, suggesting that the visual-analgesic association does not occur without explicit verbal information. In line with the cognitive theory of conditioning, our results indicate that just pairing a cue with different pain stimulus intensities is not sufficient, per se, to produce a learning process. What matters is the informational cognitive content of the cue, i.e. the meaning assigned to the cue itself. These findings may help understand the mechanisms of conditioned analgesia and more in general of learning.

Influence of Different Stimulation Parameters on the Somatosensory Evoked Potentials of the Nervus Vagus—How Varied Stimulation Parameters Affect VSEP

Vagus somatosensory evoked potentials are a method for assessing the function of the vagus nerve, which were shown to be altered in neurodegenerative diseases like Alzheimer's and Parkinson's disease. Various parameters of the stimulation such as the electrode position and the stimulus intensity have already been investigated. In this study, the focus is on the systematic examination of the other parameters of the stimulation of the vagus somatosensory evoked potentials: stimulus duration, interstimulus interval, and, again, the stimulation intensity. Thirty young and healthy subjects were examined using five different sets of stimulation parameters, and 24 were included in the further analysis. The results show that a reduction of the stimulus duration and a decrease in stimulus intensity have a significant effect on the amplitudes. A shortening of the interstimulus interval does not seem to have such an effect, but this stimulation is rated more painful and unpleasant than the standard stimulation. Overall, the standard stimulation used so far seems to be the most preferable condition.

Frequency Transitions in Odor-Evoked Neural Oscillations

In many species, sensory stimuli elicit the oscillatory synchronization of groups of neurons. What determines the properties of these oscillations? In the olfactory system of the moth, we found that odors elicited oscillatory synchronization through a neural mechanism like that described in locust and Drosophila. During responses to long odor pulses, oscillations suddenly slowed as net olfactory receptor neuron (ORN) output decreased; thus, stimulus intensity appeared to determine oscillation frequency. However, changing the concentration of the odor had little effect upon oscillatory frequency. Our recordings in vivo and computational models based on these results suggested that the main effect of increasing odor concentration was to recruit additional, less well-tuned ORNs whose firing rates were tightly constrained by adaptation and saturation. Thus, in the periphery, concentration is encoded mainly by the size of the responsive ORN population, and oscillation frequency is set by the adaptation and saturation of this response.

Differential effect of near-threshold stimulus intensities on sound localization performance in azimuth and elevation in normal human subjects.

The ability of humans to localize sounds remains relatively constant across a range of intensities well above detection threshold, and increasing the spectral content of the stimulus results in an improvement in localization ability. For broadband stimuli, intensities near detection threshold result in fewer and weaker binaural cues used in azimuth localization because the stimulus energy at the high- and low-frequency ends of the audible spectrum fall below detection threshold. Thus, the ability to localize broadband sounds in azimuth is predicted to be degraded at audible but near threshold stimulus intensities. The spectral cues for elevation localization (spectral peaks and notches generated by the head-related transfer function) span a narrower frequency range than those for azimuth. As the stimulus intensity decreases, the ability to detect the stimulus frequencies corresponding to the spectral notches will be more strongly affected than the ability to detect frequencies outside the range where these spectral cues are useful. Consequently, decreasing the stimulus intensity should degrade localization in both azimuth and elevation and create a greater deficit in elevation localization due to the narrower band of audible frequencies containing elevation cues compared to azimuth cues. The present study measured the ability of 11 normal human subjects to localize broadband noise stimuli along the midsagittal plane and horizontal meridian at stimulus intensities of 14, 22, and 30 dB above the subject's detection threshold using a go/no-go behavioral paradigm. Localization ability decreased in both azimuth and elevation with decreasing stimulus intensity, and this effect was greater on localization in elevation than on localization in azimuth. The differential effects of stimulus intensity on sound localization in azimuth and elevation found in the present study may provide a valuable tool in investigating the neural correlates of sound location perception.

Attentional Bias toward Low-Intensity Stimuli: An Explanation for the Intensity Dissociation between Reaction Time and Temporal Order Judgment?

If two stimuli need different times to be processed, this difference should in principle be reflected both by response times (RT) and by judgments of their temporal order (TOJ). However, several dissociations have been reported between RT and TOJ, e.g., RT is more affected than TOJ when stimulus intensity decreases. One account for these dissociations is to assume differences in the allocation of attention induced by the two tasks. To test this hypothesis, different distributions of attention were induced in the present study between two stimulus positions (above and below fixation). Only bright stimuli appeared in one position and either bright or dim stimuli in the other. In the two RT experiments, participants had to respond to every stimulus appearing in one of the two positions. Reaction times to bright stimuli were faster when they appeared in the position where dim stimuli were likely to occur. This finding suggests that the allocation of attention was adapted to the asymmetrical arrangement of stimuli, not suggested by explicit instruction. In the two TOJ experiments, the temporal order of stimuli appearing in the two positions had to be judged. Although bright stimuli appearing at the bright-and-dim location were judged to be earlier, this effect was small and insignificant. Further, the intensity dissociation between RT and TOJ was insensitive to random vs blockwise presentations of intensities, therefore was not modified by attentional preferences. Thus, asymmetrical arrangement of stimuli has an impact on the allocation of attention, but only in the RT task. Therefore dissociations between TOJ and response times cannot be accounted for by an attentional bias in the TOJ task but probably by different use of temporal information in the two tasks.

Effect of pharmacologically induced mydriasis on the normal variability of retinal oscillatory potentials in man.

In this study we describe certain aspects of normal variability of oscillatory potentials, with particular reference to interaction between pupil size and stimulus intensity. The mean latencies of the earlier oscillatory potentials, O1 and O2, were significantly shorter when adopting the dilated pupil condition than when stimulating with a normal pupil. This occurred at various light intensities, although the difference was more significant at the highest intensity. O1 and O2 latencies became significantly longer as the stimulus intensity was progressively reduced. The latencies of the later components O3 and O4 were unaffected. The O1 and O2 amplitudes were not influenced by pupil size, but a significant reduction occurred with the progressive decrease in stimulus intensity. The later components O3 and O4 are greatly reduced in amplitude after pupil dilatation using higher stimulus intensities; the O4 potential may even disappear in 10% of the cases. These data provide further support for the existence of two distinct behavior patterns for earlier and later oscillatory potentials. Although light adaptation may affect oscillatory potentials, we suggest that in our experimental conditions, oscillatory potential changes may occur as a result of the activity of the neural modulating system, not only as a result of photoreceptor interaction.

ネコのＡＢＲにおける位相スペクトル解析の研究

In order to apply the phase spectral analysis of auditory brainstem response (ABR) to the clinical diagnostic test, the following studies in cats were performed. At first, the phase spectra of normal ABRs were investigated under adequate stimulus intensities. In addition the component synchrony measures (CSM) of ABRs were calculated at each stimulus level. Moreover the waveforms and phase spectra of ABRs were investigated before and after the destructions of cochlear nerve and brainstem auditory pathways in cats. The results were as follows; (1) In normal ABRs, the phase spectra were mainly composed of three frequency components at 0-300Hz (component A), 300-900Hz (component B) and 900-1500Hz (component C). The CSMs of component A represented the degree of synchrony for a slow component of the ABR and the CSMs of components B and C represented the degree of synchrony for a fast component. (2) A decrease in stimulus intensity resulted in a decrease in each average of the CSMs of the three components. (3) A greater decrease of the CSM occurred if ipsilateral destruction was performed in a peripheral lesion. Lesions of the auditory pathway were followed by a decrease of the CSM of component C to contralateral stimulation. These results suggest that the phase spectral analysis of ABRs has significant clinical value in the detection of brainstem lesions.

Normal variability of the brain-stem auditory evoked response in young and old adult subjects

The brain-stem auditory evoked response was studied under three combinations of intensity and rate of click stimulation in 25 young and 25 old adult subjects. The responses show a great intra and inter subject variability. Waves 1, 3 and 5 are constant and reproducible markers of the response while waves 2, 4, 6 and 7 are variable and frequently asymmetrical or absent. Wave peak latencies increase with an increase in stimulation rate, a decrease in stimulus intensity and an increase in age. Interpeak conduction times, except those involving wave 2, are unaffected by a change in stimulus intensity. Wave 1–3 time increases with an increase in stimulus rate and an increase in age while wave 3–5 time is not affected by any change in stimulus parameters or age. Wave amplitude alone is not a reliable measure of normality. Normal values for peak latencies, interpeak conduction times and interear symmetry are presented. One tailed 95 and 99% confidence intervals for normal interpeak conduction times are presented with appropriate corrections for stimulus rate used and patient age. No corrections need to be made for changes in stimulus intensity. This suggests that no correction is necessary when studying patients with conductive type hearing deficits. Laboratories using the same stimulation technique appear to produce equivalent normative conduction time values.

Intracellular study of synaptic events related to phase‐locking responses of cat cochlear nucleus cells to low‐frequency tones

Intracellular recording techniques were used to record from 55 cochlear nucleus cells in 46 pentobarbital anesthetized cats. Computer‐generated peristimulus‐time histograms, stimulus phase‐locked histograms, and interspike interval histograms were used to study each unit. For units which show phase locking of discharges an excitatory postsynaptic potential (EPSP) is generated in response to each period of the sine wave. With lower‐frequency stimulation, the probability of a unit discharge occurring with each EPSP was high but never exceeded 0.8. As the frequency of the stimulus increased, the probability of firing to a given period dropped, even though the period of the stimulus was much longer than the refractory period of the neuron. Units showed variable degrees of phase locking. Primary‐like units of the same low characteristic frequency might or might not phase lock to the characteristic frequency. Units which showed phase locking at lower frequencies but not at higher frequencies often also changed response configuration from primary to onset, built‐up or pause. A decrease in stimulus intensity caused a decrease in the amplitude of the EPSD and a lowered probability of firing. Both the EPSP and the phase‐locked unit discharge can contribute to the frequency‐following response recorded with macroelectrodes in the cochlear nucleus. [Work supported by Bank of America—Gianinni postdoctoral fellowship.]

Properties of mechanosensitive neurons within Auerbach's plexus of the small intestine of the cat.

Three different kinds of mechanosensitive neurons were detected by direct electrical recording from Auerbach's plexus. Neurons classified as fast-adapting mechanoreceptors discharged spikes at the onset of stimulation, and the discharge stopped during a sustained stimulus of constant intensity. Slowly-adapting mechanoreceptors maintained during sustained stimulation, a steady discharge at a frequency that was a direct function of the intensity of the stimulus. Tonic type neurons responded to mechanical stimulation with a prolonged train of spikes which had a consistent pattern from preparation to preparation. Once the tonic-type cells were triggered, the discharge followed a characteristic time course that was unchanged by further increase or decrease in stimulus intensity. Tonic-type neurons are probably not first order sensory neurons, but they may be activated by input derived from primary mechanoreceptors. Frequency of discharge of slowly-adapting mechanoreceptors was increased by histamine. This appeared to be secondary to histamine-induced contractile activity of the musculature.

Sensation and set in reaction time.

Simple auditory RT was measured in relation to 6 levels of stimulus intensity (55, 60, 65, 70, 75, and 85 db) and 2 variations of regular preparatory intervals (0.5 and 6.0 sec.). The results showed the louder the stimulus, the quicker the RT; more equivocally, and to a lesser extent, as stimulus intensity decreases, the role of preparatory interval increases. These results were discussed in terms of the factors set and sensation.

Effect of intensity and wavelength of photic stimulus on evoked responses of general cortex and optic tectum in turtles

1. In response to a photic stimulus the amplitude of the evoked response of the turtle general cortex is 400–600 μv, several times greater than the amplitude of the evoked response of the optic tectum. With a decrease in stimulus intensity, the response disappears sooner in the tectum than in the cortex. 2. Two areas can be distinguished in the general cortex in the region from which the visual evoked response is recorded: 1) a rostral area, where the relationship between amplitude and latent period of the visual evoked response to intensity of the photic stimulus is similar to those observed in the principal visual center of turtles, the tectum (amplitude directly proportional, latent period inversely proportional to logarithm of stimulus intensity) and 2) central and caudal areas, not showing this resemblance. 3. The changes in the principal parameters of the evoked response in the rostral portion of the general cortex to long versus short wavelengths resembles that in the tectum and retina (as shown by ERG data). Evoked responses of greatest amplitude and shortest latent period are recorded in response to a red stimulus (wavelength=648 nm).