Changes In Stimulus Intensity Research Articles

Repetitive stimulation of olfactory receptor cells in female silkmoths Bombyx mori L.

The pheromone-sensitive receptor cells of male moth antennae are capable of detecting the rapid changes in stimulus intensity encountered in natural pheromone odour plumes. We investigated temporal response characteristics of the two receptor cell types of the sensillum trichodeum of female Bombyx mori, which are most sensitive to benzoic acid and 2,6-dimethyl-5-heptene-2-ol (DMH), respectively. The cells were repetitively stimulated with 50-ms pulses of benzoic acid and (±)-linalool, an effective mimic of DMH, at various pulse rates and different stimulus intensities. By recording receptor potentials and nerve impulses we demonstrated that both receptor cell types were able to follow stimulus pulses at least up to eight pulses per sec, with a more pronounced modulation of the responses in the DMH cell. The resolution capability of the two cell types showed little dependence on stimulus intensity. In their ability to resolve pulsed odour stimuli, the receptor cells for benzoic acid and DMH were as good as pheromone receptor cells.

Changes in the Second-order Kernel Component Obtained by the Techniques of the Multifocal Electroretinogram in Early Stages of Diabetes Mellitus

To detect changes in retinal function in diabetic eyes without ophthalmoscopic retinopathy and to find a clue for differential origins of the second-order kernel of the multifocal electroretinogram(m-ERG). m-ERGs were recorded from eyes of 22 healthy control subjects and 21 patients with diabetes mellitus(DM) by 19-hexagonal-element stimulus at stimulus intensities of 20, 63, 100, 200 and 331 cd/m2(VERIS III, Tomey). The response densities and the peak latencies of the first-order kernel(P 1) and the second-order kernels(P 2, P 3) were compared between the DM and the control groups. Full-field ERGs were also recorded from the same patients and compared with the results of m-ERGs. The response densities of P 1 and P 2 in the DM group were significantly(p < 0.05) larger than those in the control group. The response of the full-field ERG in the DM group did not significantly differ from that in the control group. The components of the m-ERG were divided into two groups based on their behavior to stimulus intensity changes: P 1 together with P 2 and P 3. With increase in the stimulus intensity, the response densities of P 1 and P 2 increased significantly(p < 0.05), while the response densities of P 3 did not change significantly. The fact that the response densities in the DM group were greater than those in the control group supports the hypothesis that the retinal blood flow increases in the early stage of diabetes. m-ERG is therefore beneficial to ascertain subclinical retinal changes and to evaluate the retinal function of early diabetic patients. Since the P 2 and P 3 differed in behavior according to the stimulus intensities, it can be inferred that origin of the P 2 of the second order kernel is different from that of P 3.

Effects of Stimulus Intensity on Multifocal Electroretinograms

To investigate origins of responses of the second order kernel components of multifocal electroretinograms (m-ERGs), the influence of stimulus intensities on the waveform were studied. M-ERGs were recorded from 20 normal eyes of 14 normal adults using a Visual Evoked Response Imaging System with 19 hexagonal stimulus elements with light intensities of 6.3, 20, 63, 200, or 331 cd/m2. The response densities and implicit times of the first (N1, P1, N2) and the second order kernel components (P1, N1, P2, N2, P3, N3) of the m-ERGs were measured. The components were divided into two groups based on the behavior of the each component to the stimulus intensity change. The first group consisted of all the first order kernel components and P1 and N1 of the second order kernel components whose response densities were significantly larger (p < 0.05) than those of components elicited by stimulus of one grade lower intensity. The second group consisted of N2, P3, and N3 of the second order kernel components whose response densities did not increase when the stimulus intensity was increased from 200 to 331 cd/m2. It is probable that in the second order kernel components, the origin of P1, N1, and P2 is different from N2, P3, and N3 because the response to stimulus intensity of the two groups of components was different.

Neurovascular relationships in human sensorimotor cortex can be modulated by movement

Introduction: The interpretation of fMRl is critically dependent on understanding the relationship between observed blood flow responses and the underlying neuronal changes. We have established a linear nemovascular coupling relationship between fMRl BOLD and somatosensory evoked potential (SEP) amplitude in human sensorimotor cortex using changes in stimulus intensity. Finger movement during a stimulus is known to attenuate electrical SEP responses in somatosensory cortex (movement gating), however it is also a common method of inducing fMRl BOLD activity in sensorimotor cortex, suggesting that electrical and haemodynamic cortical responses diverge. Here we examined the effects of finger movement during sensory stimulation on fMIU BOLD and SEP amplitude in sensorimotor cortex, in order to establish whether movement can modulate this relationship. Methods: Normal volunteers were subject to 0.2 millisecond square-wave electrical pulses delivered to the median nerve at the wrist, at 50 - 175 % of predetermined motor threshold. SEPs were recorded from contra-lateral parietal cortex, Cp3/4, referenced to Fz, over 450 averages. Gradient-echo EPI BOLD imaging was performed on a 3.0 Tesla Bruker Medical S300 using a blocked design with TB 4 sec. Imaging was acquired for all intensities, which were pseudo-randomised, then the experiment was repeated with subjects performing a self-paced thumb twitch during the stimulus. Images were analyzed using SPM99. Results: Without movement, fMBI BOLD responses paralleled changes in SEP N20-P27 amplitude (pcO.05) as both increased with increasing intensity (pcO.05; pcO.05 respectively). However, thumb movement increased fMRl BOLD signal intensity change (pcO.05) but caused a significant decrease in SEP N20-P27 amplitude across all intensities in all subjects (pcO.05). Conclusion: SEP amplitudes increased with increasing stimulus intensity, as did fMFU BOLD signal intensity changes, consistent with a close neurovascular coupling relationship. However the effect of simultaneous movement is to reset this relationship by increasing the cerebral blood flow response and suppressing the electrical response. The apparent coupling relationship can therefore be modulated by movement.

Odour-plume dynamics influence the brain's olfactory code.

The neural computations used to represent olfactory information in the brain have long been investigated. Recent studies in the insect antennal lobe suggest that precise temporal and/or spatial patterns of activity underlie the recognition and discrimination of different odours, and that these patterns may be strengthened by associative learning. It remains unknown, however, whether these activity patterns persist when odour intensity varies rapidly and unpredictably, as often occurs in nature. Here we show that with naturally intermittent odour stimulation, spike patterns recorded from moth antennal-lobe output neurons varied predictably with the fine-scale temporal dynamics and intensity of the odour. These data support the hypothesis that olfactory circuits compensate for contextual variations in the stimulus pattern with high temporal precision. The timing of output neuron activity is constantly modulated to reflect ongoing changes in stimulus intensity and dynamics that occur on a millisecond timescale.

Sex differences in response to red and blue light in human primary visual cortex: a bold fMRI study

Studies using a variety of investigative methods, including functional brain imaging and electroencephalography (EEG), have suggested that changes in central nervous system (CNS) dopamine function result in altered visual system processing. The discovery of abnormal retinal blue cone, but not red cone, electroretinogram in association with cocaine withdrawal and Parkinson's disease suggests that visual system response to blue light might be a marker for CNS dopamine tone. As there are numerous sex-related differences in central nervous system dopamine function, we predicted that blue and red light stimulation would produce sex-specific patterns of response in primary visual cortex when studied using the blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) technique. We analyzed the BOLD response to red and blue light in male and female human volunteers ( N=20). Red and blue light responses in primary visual cortex (V1) to stepped intensities of red and blue light were compared by sex for threshold to detectable BOLD signal increase and for stimulus intensity vs. BOLD signal response. Near threshold, males and females showed similar BOLD signal change to red light, but males showed a threefold greater increase (0.52%) to blue light stimulation when compared to females (0.14%). Log-linear regression modeling revealed that the slope coefficients for the red light stimulus intensity vs. signal change curve were not significantly different for males and females ( z=0.995, P=0.320), whereas the slope coefficients for the blue light stimulus intensity vs. signal change curve were significantly larger in males ( z=2.251, P=0.024). These findings support a sex and color-dependent differential pattern of primary visual cortical response to photic stimulation and suggest a method for assessing the influence of specific dopamine agonist/antagonist medications on visual function.

On the Reliability of Augmenting/Reducing

Abstract To investigate the impact of several methodological variations in the assessment of augmenting/reducing, auditory evoked potentials to 1000 Hz tones with varying stimulus intensity (59, 71, 79, 88, 92, 96 dB (SPL)) were recorded at 19 EEG sites in 24 participants during two separate recording sessions. The internal consistency analysis revealed only weak correlations for linear regression slopes based on high intensity levels when compared to slopes based on low intensity levels. For base-to-peak and peak-to-peak ERP component measurements, acceptable internal consistency and temporal stability were confirmed for the N1/P2-based slope, and partly for the P2 slope, whereas P1, N1, and P1/N1 slopes were not reliable. After submitting auditory evoked potentials to a covariance-based principal component analysis (PCA), followed by unscaled varimax rotation, temporal stability of slope measures for the corresponding factor scores substantially increased. The findings suggest that at least five to six intensity-levels are required over a relatively broad range to yield a reliable measure of auditory evoked augmenting/reducing. If measured reliably, P2 slopes may reflect stimulus intensity changes more precisely than N1/P2 slopes, and should therefore be evaluated in future studies of individual differences in augmenting/reducing.

Proceedings of the symposium on adaptation in vision and olfaction held at the XXI meeting of AChemS on april 15, 1999.

Adaptation is the ability of a neural system to adjust its sensitivity and dynamic range as the background level of stimulation changes. In sensory systems, adaptation increases threshold (decreases sensitivity) and extends the dynamic range of the behavioral stimulus–response function thereby allowing the system to respond to changes in stimulus intensity under different levels of background stimulation. In addition, adaptation properties of sensory receptor cells determine the magnitude of the response to the different frequency components present in a timevarying stimulus such as the odor plume sensed by an animal as it moves towards the source of the stimulus. During the XXI meeting of AChemS, a symposium was held where four speakers discussed different aspects of the characteristics and mechanisms underlying adaptation in the visual and olfactory systems. Dr Edward Pugh Jr presented data describing calcium-dependent and independent mechanisms underlying adaptation in rod photoreceptors. His talk indicated that multiple processes underlie adaptation in rod photoreceptors. These processes can be divided conceptually into those that result in extension of the dynamic range and those that result in diminished flash sensitivity. The calcium-dependent processes best understood at the molecular level in rod photoreceptors—the activation of guanilyl cylase by calcium and the calmodulin-dependent shift in the K1/2 of the cGMP-gated channel—contribute to extension of the dynamic range, but paradoxically elicit an increase in sensitivity, rather than a decrease. Thus, other processes must underlie the decrease in sensitivity to light flashes. Two such processes were discussed: an increase in steady cGMP hydrolysis rate and a decrease in the lifetime of activated rhodopsin. Therefore, as in the olfactory system (see below), adaptation in the visual system is a complex process with multiple underlying molecular mechanisms. Pugh and coworkers recently published an excellent review of adaptation in vertebrate photoreceptors (Pugh et al., 1999). The talk by Pugh was followed by three presentations discussing adaptation in the olfactory system from points of view ranging from molecular to behavioral. The present proceedings encompass three manuscripts covering the presentations on olfactory adaptation. The three manuscripts cover peripheral and central aspects of olfactory adaptation. The manuscript by Zufall and Leinders-Zufall presents a comprehensive discussion of the multiple mechanisms underlying adaptation in olfactory receptor cilia. Even when considering only the initial stages of olfactory transduction in the olfactory cilia, adaptation is a complex process, whose mechanisms vary depending on duration of exposure and intensity of the adapting stimulus. The mechanisms discussed in the manuscript by Zufall and Leinders-Zufall alter the stimulus–response function that the generator potential displays as a function of odor concentration. How this change in stimulus–response function alters sensitivity of the response of mitral/tufted cells in the olfactory bulb depends on several parameters, one of which is the dependence of action potential firing frequency on the magnitude of the generator potential. Whereas the Zufall and Leinders-Zufall manuscript describes mechanisms that cause adaptation of the generator potential of receptor cells, the second paper by Reisert and Matthews discusses adaptation measured as a decrease in the action potential firing frequency. Finally, in the third manuscript, Dalton discusses psychophysical studies of olfactory adaptation in adult humans and presents evidence for the involvement of central processes in adaptation. This series of manuscripts present a comprehensive review of recent advances in the understanding of the characteristics and mechanisms of olfactory adaptation. They indicate that adaptation is a complex process involving multiple mechanisms at peripheral and central levels of processing. This symposium was sponsored by a grant from the National Institute of Deafness and Communication Disorders.

Medullary Dorsal Horn Neuronal Activity in Rats with Persistent Temporomandibular Joint and Perioral Inflammation

Studies at spinal levels indicate that peripheral tissue or nerve injury induces a state of hyperexcitability of spinal dorsal horn neurons that participates in the development of persistent pain and hyperalgesia. It has not been demonstrated that persistent injury in the orofacial region leads to a similar state of central hyperexcitability in the trigeminal system. The purpose of the present study was to conduct a parametric analysis of the response properties of nociceptive and nonnociceptive neurons in trigeminal nucleus caudalis (medullary dorsal horn, MDH) in a rat model of persistent orofacial inflammation. Neurons were recorded extracellularly and classified as low-threshold mechanoreceptive (LTM, n = 49), wide dynamic range (WDR, n = 82), and nociceptive-specific (NS, n = 11) neurons according to their response properties to mechanical stimuli applied to their cutaneous receptive fields (RFs). The inflammation was induced 24 h before the recordings by injecting complete Freund's adjuvant (CFA) into the temporomandibular joint (TMJ) capsule or the perioral (PO) skin. The mean areas of the high-threshold RFs of WDR neurons in TMJ (8.66 +/- 0.61 cm(2), n = 25) and PO (5.61 +/- 2.07 cm(2), n = 25) inflamed rats were significantly larger than those in naive rats (1.10 +/- 0. 16 cm(2), n = 32). The mean RF size in TMJ-inflamed rats also was significantly larger than that in PO-inflamed rats (P < 0.01). Furthermore the mean area of the RFs of NS neurons (3.74 +/- 1.44 cm(2), n = 5) was significantly larger in TMJ inflamed rats as compared with naive rats (0.4 +/- 0.09 cm(2), n = 3) (P < 0.05). The background activity in the TMJ- and PO-inflamed rats was generally greater in WDR and NS neurons, but less in LTM neurons, when compared with naive rats. The responses of WDR neurons to noxious mechanical stimuli were increased significantly in TMJ-inflamed rats (P < 0.05) as compared with naive rats. WDR neuronal responses to mechanical stimulation also were increased in PO-inflamed rats but to a lesser extent than in TMJ-inflamed rats. The injection of CFA into the TMJ or PO skin resulted in reduced responses of LTM neurons to mechanical stimuli. The responses of MDH nociceptive neurons to 48-55 degrees C heating were greater in inflamed rats as compared with naive rats. A subpopulation of WDR neurons recorded from TMJ (n = 4 of 10)- or PO (n = 3 of 13)-injected rats responded to cooling in addition to heating of the RFs but did not grade their responses with changes in stimulus intensity. These results indicate that persistent orofacial inflammation produced hyperexcitability of MDH nociceptive neurons. TMJ inflammation resulted in more robust changes in MDH nociceptive neurons as compared with PO inflammation, consistent with previous studies of increased inflammation, increased MDH Fos-protein expression, and increased MDH preprodynorphin mRNA expression in this deep tissue orofacial model of pain and hyperalgesia. The inflammation-induced MDH hyperexcitability may contribute to mechanisms of persistent pain associated with orofacial deep tissue painful conditions.

Stimulation of a restricted region in the midline cerebellar white matter evokes coordinated quadrupedal locomotion in the decerebrate cat.

In the reflexively standing acute decerebrate cat, we have previously shown that pulse train microstimulation of the hook bundle of Russel in the midline of the cerebellar white matter, through which crossed fastigiofugal fibers decussate, augments the postural tone of neck, trunk, fore-, and hindlimb extensor muscles. In the present study we examined the possible role of such stimulation in evoking locomotion as the animal is supported by a rubber hammock with its feet contacting the moving surface of a treadmill. We were able to provoke well-coordinated, bilaterally symmetrical, fore- and hindlimb movements, whose cycle time and pattern were controlled by appropriate changes in stimulus intensity and treadmill speed. We carefully and systematically mapped this cerebellar locomotor region (CLR) through repeated dorsoventral penetrations with a glass-coated tungsten microelectrode in a single animal and between animals. We found that the optimal locus for evoking locomotion was centered on the midline, at Horsley-Clarke coordinates H0 and P7.0, and extended over a rostrocaudal and dorsolateral range of approximately 0.5 mm. The lowest effective stimulus intensity at the optimal site was in the range of 5-8 microA. Along penetration tracks to left or right of the midline, effective stimulus intensity increased and evoked locomotor patterns were no longer symmetrical, but rather shifted toward the contralateral limbs. In the same animals, controlled locomotion was evoked by stimulating the mesencephalic locomotor region (MLR). With concomitant stimulation of the optimal sites in the CLR and the MLR, each at subthreshold strength, locomotor movements identical to those seen with suprathreshold stimulation of each site alone were evoked. With concomitant stimulation at suprathreshold strength for each site, locomotion became vigorous, with a shortened cycle time. After making ablative lesions at either the CLR or MLR (unilateral or bilateral), controlled locomotion was still evoked at the prior stimulus strength by stimulating the remaining site. Together, these results demonstrate that selective stimulation of the hook bundle of Russel in the midsagittal plane of the cerebellar white matter evokes "controlled" locomotion identical to that evoked by stimulating the MLR. We have shown that the fastigial nucleus is one of the supraspinal locomotion inducing sites and that it can independently and simultaneously trigger brain stem and spinal locomotor subprograms formerly believed to be the domain of various brain stem regions including the MLR and the subthalamic locomotor region.

Some new insights into the effects of opioids in phasic and tonic nociceptive tests

In this review we critically appraise the value of some phasic and tonic nociceptive tests as models for differentiating the antinociceptive effects of opioid agonists. Using heat-evoked withdrawal of the hind paw or tail of a rodent, several early studies have assessed the effects of stimulus intensity upon antinociceptive potency of opioid agonists. After intrathecal (i.t.) administration of either morphine or sufentanil, for example, for any incremental change in stimulus intensity, the degree of right shift in the dose-response relationship was greater for morphine than for sufentanil. At first glance, such data appear to provide robust support for the pharmacological model of fractional receptor occupancy (FRO), which, according to the historical tenets of classical receptor theory, describes the relationship between intrinsic efficacy and the total receptor concentration. However, new data which elegantly characterize the relative contribution of small calibre unmyelinated and myelinated nociceptive afferents in mediating thermal-evoked responses challenge the exclusivity of such explanations with origins in classical theory. Within our review we report the results of experiments which provide direct electrophysiological evidence that noxious skin heating at a low rate activates C-polymodal nociceptors, but does not effectively activate A-delta mechanothermal nociceptors. In contrast, a high rate of skin heating activates both nociceptor classes, but produces a more intense activation of A-delta nociceptors that occurs after a shorter onset latency compared with the activation of C-fibre nociceptors. Thus, in direct challenge to the traditional model of FRO, a shift in the dose-response relationship of morphine to the right with a reduction in efficacy, may reflect the limited effectiveness of morphine to attenuate the A-delta-mediated component which assumes increasing dominance at high intensity heating. In our appraisal of other nociceptive models we provide an in-depth characterization of afferent processing in the early neonate rat, in which opioids have been tested in both phasic (tail flick and hot-plate) and tonic (formalin) tests. Afferent processing in this model is typified by several behavioural, anatomical and functional features which, although not pathological, are characteristic of those observed in models of nerve injury using the adult rat. Notably, these features include a lack of segmental inhibition (`disinhibition') and afferent input in large diameter myelinated fibres which make synaptic contacts within superficial laminae of the dorsal horn that in the adult are predominantly nociceptive. Paradoxically, because this paradigm demonstrates increased sensitivity to the antinociceptive effects of opioids it may have special merit as a model of tonic pain. It was recently announced that the i.t. administration of pertussis toxin (PTX) caused hyperalgesia and allodynia that appears similar to the symptoms reported by patients suffering from neuropathic pain. Unlike the effects of other opioids so far tested, buprenorphine-induced antinociception is not blocked in this model. This is an exciting finding and provides new optimism that some opioids, notably buprenorphine, may have a special role in managing some types of neuropathic pain.

Relation of olfactory event-related potentials to changes in stimulus concentration

Objective: The aim of this study was to investigate the influence of odorant concentration on the olfactory event-related potential (OERP). Design and Methods: OERP were evaluated in 8 men and 8 women (17–34 years of age) in response to 4 concentrations of vanillin (7, 28, 56 and 84% v/v). Sixteen presentations of each concentration (stimulus duration 200 ms, interval 40 s, flow 8 l/min) were applied in a randomized order. EEG recordings were made at 3 midline sites (pos. Fz, Cz, Pz). Amplitudes and latencies of four peaks were measured (latencies in ms at Pz after stimulation with 84% v/v vanillin): P1 (277), N1 (348), P2 (412) and P3 (496). Statistical analysis was performed with MANOVAs (`concentration', `recording site'=within-subject-factors; `age' as covariate). Results: With increasing stimulus concentration amplitudes became significantly larger; this was most pronounced for P3 (P1N1: F=2.90, P<0.05; N1P2: F=5.15, P<0.01; N1P3: F=35.7, P<0.001; P3: F=38.6; P<0.001). Correspondingly, latencies shortened with increasing concentrations (P1: F=25.2; N1: 17.51; P2: 14.8; P3: 13.4; all P<0.001). While there was no correlation between OERP amplitudes and butanol odor detection thresholds, latencies were the shorter the lower the subjects' thresholds (coefficients of correlations for peak latencies at Cz for 84% v/v: P1 r 15=minus0.59; N1 r 15=minus0.58; P2 r 15=minus0.55; P3 r 15=minus0.45). Conclusions: The results indicated that both OERP amplitudes and latencies are related to the concentration of olfactory stimuli. They also suggested that latencies exhibit a stronger relation to changes in stimulus intensity compared to OERP amplitudes.

Changes in seizure threshold over the course of electroconvulsive therapy affect therapeutic response and are detected by ictal EEG ratings.

Therapeutic effectiveness of electroconvulsive therapy is influenced by the degree to which the stimulus intensity exceeds the seizure threshold. However, the threshold rises variably over the treatment course, confounding maintenance of desired relative stimulus intensity. In 47 depressed patients, decreases in relative stimulus intensity between treatments 1 and 6 were associated with diminished therapeutic response at treatment 6 for unilateral (UL) ECT. A multivariate model including manual ratings of ictal EEG data predicted whether seizure threshold rose with 82% accuracy. The same EEG variables were also significantly related to therapeutic response. Thus, decreases in relative stimulus intensity over the ECT course affect the therapeutic potency of UL ECT. Further, ictal EEG indices have considerable potential for predicting such stimulus intensity changes and their effect on therapeutic outcome.

Primary somatosensory cortical neuronal activity during monkey's detection of perceived change in tooth-pulp stimulus intensity.

To elucidate the functional properties of primary somatosensory cortical neurons for the perception of tooth-pulp sensation, neuronal activity was recorded from the primary somatosensory cortex (SI) in awake behaving monkeys. Monkeys were trained to detect changes in tooth-pulp stimulus intensity applied to the upper canine or incisor tooth pulp. Stimulus intensities applied to the tooth pulp were multiples of the threshold intensity for the jaw opening reflex (1.0 T) elicited by tooth-pulp stimulation. When monkeys pressed a button, baseline electrical pulses (V1: 0.5 T, 1.0 T, 2.0 T, or 3.0 T) were applied to the tooth pulp. After 4-8 s, a V2 stimulus (0.3 T, 0.5 T, 1.0 T, or 2.0 T) was added to V1. Percent escapes at V1 stimulus intensity of 0.5 T and 1.0 T were approximately 10%, 22% at 2.0 T, and 40% at 3.0 T (total of 1,997 trials). A total of 862 single units were recorded from the SI. Thirty-seven SI neurons responded to electrical stimulation of the tooth pulp (tooth-pulp-driven neurons; TPNs), 139 SI neurons responded to tactile stimulation of the lateral face area, 90 to upper lip and 99 to lower lip, 44 to tongue and 102 to periodontal membrane, whereas 351 SI neurons were not responsive to tactile stimulation of the orofacial regions. Thirty of 37 TPNs were recorded long enough to test with V1 stimuli ranging from 0.5 T to 3.0 T. Eleven of 30 TPNs linearly increased their firing frequency following increases in stimulus intensity (encoding TPNs), whereas 19 did not (nonencoding TPNs). Mean first spike latency of encoding TPNs was 24.8 +/- 1.7 ms (n = 11), that of nonencoding TPNs was 23.6 +/- 1.5 ms (n = 19), and that of unclassified TPNs was 24.7 +/- 3.7 ms (n = 7). TPNs were distributed in the areas 1-2, 3a, and 3b within the oral projection area and the transition zone between the face and oral projection areas of the SI. All of them received inputs from the intraoral structures, facial skin, or both. The firing frequency of eight encoding and nonencoding TPNs was correlated with detection latency at stimulus intensities of 0.5 and 1.0 T. On the other hand, when the baseline stimulus was increased to 2.0 T and 3.0 T, the discharge of most TPNs did not increase in firing frequency with the reduction in detection latency. These results indicate that the discharge rates of some SI TPNs are correlated with detection latency at near-noxious threshold and noxious stimulus intensities. These findings suggest that some TPNs are involved in the sensory-discriminative aspect of tooth-pulp sensation in the near-pain threshold and pain ranges.

Combining experiences over time: the effects of duration, intensity changes and on-line measurements on retrospective pain evaluations

Two experiments were conducted to examine the effects of various factors on retrospective pain evaluation. The factors examined in Experiment 1 were the rate and pattern of change, the intensity (particularly the final intensity), and the duration of the painful experience. Experiment 2 manipulated these factors and, in addition, examined the effect of continuous (on-line) ratings on the overall retrospective evaluation. The two experiments utilized different pain modalities, heat in the first and mechanical pressure in the second. In addition, all subjects in Experiment 1 experienced stimuli with the same physical magnitude, while in Experiment 2 stimuli were individually tailored to make them subjectively equivalent. In both experiments, subjects were presented with a series of painful stimuli and evaluated the intensity of each stimulus immediately upon its termination. The stimuli themselves were composed of multiple intensity levels that differentially changed over time (Intensity-Patterns). Subjects' on-line ratings in Experiment 2 closely mirrored the physical patterns of the intensities. The main conclusion from both experiments is that the retrospective evaluations of painful experiences are influenced primarily by a combination of the final pain intensity and the intensity trend during the latter half of the experience. In addition, results indicated that duration has little impact on retrospective evaluations for stimuli of relatively constant intensity. However, when the stimulus intensity changes over time, duration does play a role. Finally, the task of continuously reporting the stimulus intensity had a moderating impact on the retrospective evaluations. © 1998 John Wiley & Sons, Ltd.

Neural contributions to the perstimulus compound action potential: implications for measuring the growth of the auditory nerve spike count as a function of stimulus intensity.

The perstimulus compound action potential (PCAP), unlike the more familiar compound action potential (CAP), can be recorded in response to asynchronous as well as synchronous auditory nerve activity. When all neurons contribute equally to the PCAP, the area under the PCAP (the PCAP area) is proportional to the number of action potentials fired by auditory nerve neurons (the auditory nerve spike count). The auditory nerve spike count is one proposed code for stimulus intensity, and our goal is to use the PCAP to test this hypothesis. In this study, two independent tests were developed to measure the contributions of neurons to the PCAP as a function of their characteristic frequency (CF). The test results were verified using a model of the auditory periphery designed to calculate the auditory nerve spike count as a function of pure tone intensity and frequency. In nearly all experiments, neurons having CFs that span contiguous three or four octave bands contribute equally to the PCAP. For pure tones that stimulate only those neurons contributing equally to the PCAP, the PCAP area grows over intensity ranges frequently exceeding 80 dB, and in one case equaling 108 dB. These results demonstrate that the auditory nerve spike count, at least for pure tones, is capable of encoding changes in stimulus intensity over the entire dynamic range of the auditory system.

Directionality of auditory nerve fiber responses to pure tone stimuli in the grassfrog, Rana temporaria . II. Spike timing

We studied the directionality of spike timing in the responses of single auditory nerve fibers of the grass frog, Rana temporaria, to tone burst stimulation. Both the latency of the first spike after stimulus onset and the preferred firing phase during the stimulus were studied. In addition, the directionality of the phase of eardrum vibrations was measured. The response latency showed systematic and statistically significant changes with sound direction at both low and high frequencies. The latency changes were correlated with response strength (spike rate) changes and were probably the result of directional changes in effective stimulus intensity. Systematic changes in the preferred firing phase were seen in all fibers that showed phaselocking (i.e., at frequencies below 500-700 Hz). The mean phase lead for stimulation from the contralateral side was approximately 140 degrees at 200 Hz and decreased to approximately 100 degrees at 700 Hz. These phaseshifts correspond to differences in spike timing of approximately 2 ms and 0.4 ms respectively. The phaseshifts were nearly independent of stimulus intensity. The phase directionality of eardrum vibrations was smaller than that of the nerve fibers. Hence, the strong directional phaseshifts shown by the nerve fibers probably reflect the directional characteristics of extratympanic pathways.

Inhibition of the human startle response is affected by both prepulse intensity and eliciting stimulus intensity

The present study evaluated the effects of the intensity of prepulses and eliciting stimuli on the modification of the adult human acoustic startle eyeblink. Eyeblinks were elicited by 85, 95, and 105 dB(A) noise bursts, preceded on some trials by 60 or 70 dB(A) tones at a 120 ms stimulus onset asynchrony. Prepulse intensity was a within-subject variable in Experiment 1 ( N = 19) and a between-groups variable in Experiment 2 ( N = 38). For no-prepulse trials, as startle stimulus intensity decreased, startle amplitude, probability, and magnitude decreased, and startle latency increased. As startle stimulus intensity decreased from 105 to 95 dB, the amount of inhibition of response amplitude and magnitude remained stable for 70 dB prepulses in Experiment 1 and for both 60 and 70 dB prepulses in Experiment 2, whereas inhibition of response probability became more pronounced. As startle stimulus intensity decreased from 95 to 85 dB, prepulse inhibition of response amplitude and magnitude lessened and inhibition of response probability became still more pronounced in both experiments. These data show that the inhibition of startle can be affected by eliciting stimulus intensity, and that startle response amplitude and probability are affected by stimulus intensity changes in different ways.

Magnetic resonance imaging (MRI) detection of the murine brain response to light: temporal differentiation and negative functional MRI changes.

Using a 9.4 T MRI instrument, we have obtained images of the mouse brain response to photic stimulation during a period between deep anesthesia and the early stages of arousal. The large image enhancements we observe (often >30%) are consistent with literature results extrapolated to 9.4 T. However, there are also two unusual aspects to our findings. (i) The visual area of the brain responds only to changes in stimulus intensity, suggesting that we directly detect operations of the M visual system pathway. Such a channel has been observed in mice by invasive electrophysiology, and described in detail for primates. (ii) Along with the typical positive response in the area of the occipital portion of the brain containing the visual cortex, another area displays decreased signal intensity upon stimulation.

Sensitivity to interaural intensity differences of neurons in primary auditory cortex of the cat. I. types of sensitivity and effects of variations in sound pressure level.

1. Interaural intensity differences (IIDs) provide the major cue to the azimuthal location of high-frequency narrowband sounds. In recent studies of the azimuthal sensitivity of high-frequency neurons in the primary auditory cortex (field AI) of the cat, a number of different types of azimuthal sensitivity have been described and the azimuthal sensitivity of many neurons was found to vary as a function of changes in stimulus intensity. The extent to which the shape and the intensity dependence of the azimuthal sensitivity of AI neurons reflects features of their IID sensitivity was investigated by obtaining data on IID sensitivity from a large sample of neurons with a characteristic frequency (CF) > 5.5 kHz in AI of anesthetized cats. IID sensitivity functions were classified in a manner that facilitated comparison with previously obtained data on azimuthal sensitivity, and the effects of changes in the base intensity at which IIDs were introduced were examined. 2. IID sensitivity functions for CF tonal stimuli were obtained at one or more intensities for a total of 294 neurons, in most cases by a method of generating IIDs that kept the average binaural intensity (ABI) of the stimuli at the two ears constant. In the standard ABI range at which a function was obtained for each unit, five types of IID sensitivity were distinguished. Contra-max neurons (50% of the sample) had maximum response (a peak or a plateau) at IIDs corresponding to contralateral azimuths, whereas ipsi-max neurons (17%) had the mirror-image form of sensitivity. Near-zero-max neurons (18%) had a clearly defined maximum response (peak) in the range of +/- 10 dB IID, whereas a small group of tough neurons (2%) had a restricted range of minimal responsiveness with near-maximal responses at IIDs on either side. A final 18% of AI neurons were classified as insensitive to IIDs. The proportions of neurons exhibiting the various types of sensitivity corresponded closely to the proportions found to exhibit corresponding types of azimuthal sensitivity in a previous study. 3. There was a strong correlation between a neuron's binaural interaction characteristics and the form of its IID sensitivity function. Thus, neurons excited by monaural stimulation of only one ear but with either inhibitory, facilitatory, or mixed facilitatory-inhibitory effects of stimulation of the other ear had predominantly contra-max IID sensitivity (if contralateral monaural stimulation was excitatory) or ipsi-max sensitivity (if ipsilateral monaural stimulation was excitatory). Neurons driven weakly or not at all by monaural stimulation but facilitated binaurally almost all exhibited near-zero-max IID sensitivity. The exception to this tight association between binaural input and IID sensitivity was provided by neurons excited by monaural stimulation of either ear (EE neurons). Although EE neurons have frequently been considered to be insensitive to IIDs, our data were in agreement with two recent reports indicating that they can exhibit various forms of IID sensitivity: only 23 of 75 EE neurons were classified as insensitive and the remainder exhibited diverse types of sensitivity. 4. IID sensitivity was examined at two or more intensities (3-5 in most cases) for 84 neurons. The form of the IID sensitivity function (defined in terms of both shape and position along the IID axis) was invariant with changes in ABI for only a small proportion of IID-sensitive neurons (approximately 15% if a strict criterion of invariance was employed), and for many of these neurons the spike counts associated with a given IID varied with ABI, particularly at near-threshold levels. When the patterns of variation in the form of IID sensitivity produced by changes in ABI were classified in a manner equivalent to that used previously to classify the effects of intensity on azimuthal sensitivity, there was a close correspondence between the effects of intensity on corresponding types of azimuthal and IID sensitivity