Stimulus intensity

Saturation is the concentration of a stimulus above which no increase in perception can be detected. It is tme that Weber-Stevens laws can predict the relationship between stimulus intensity and sensory response with some precision however, they do not describe the very common situation of stimuli at or near the threshold or point of saturation. 

Space-clamped (HH) equations relate the difference in electrical potential across the cell membrane (V) and gating variables (0 < m, n, h < 1), for ion channels to the stimulus intensity (7J and temperature (T), as follows ... 

Smith, B., Wilson, R. and Davidson, R., Electrodermal activity and extraversion Caffeine, preparatory signal and stimulus intensity effects. Personality and Individual Differences 5, 59-65, 1984. 

The results of our study clearly established that conditioning enhanced the placebo effect by changing people s expectations. As had the Australian researchers, we found that conditioning increased the placebo effect for the subjects that we had kept in the dark about our manipulation. They came to expect less pain, and they subsequently experienced less pain. But conditioning had no effect at all on the subjects who were told that we were lowering the intensity of the pain stimulus. Knowing that we had lowered the stimulus intensity, they did not come to expect less pain where the placebo had been applied, and since they did not expect less pain, they did not experience it when the intensity was turned up again. 

A response is defined as any stimulus-induced alteration in the activity of the organism s motor apparatus which can (but need not always) result in an alteration of the movement or orientation of the organism. Where nature or occurrence of a response depends upon an increase or decrease in stimulus intensity, an increase may be indicated by the use of the term step-up and a decrease by the term step-down . Thus, a step-up photophobic response is a phobic response which occurs upon an increase in light intensity. The responses are classified as follows ... 

Accumulation. Accumulation in or disperal from a region of higher stimulus intensity, though often confused with a true behavioral response, is the most often observed macroscopic result of many of the phenomena discussed above. 

The behavior of the animal in response to flow is important, not just the flows themselves. An animal searching for the source of an odor moves in the direction of increasing stimulus intensity and stays within the boundaries of the plume. If an animal needs to sample odor in turbulence frequently, it may have to reduce speed, to untenable levels in the case of moths or birds, or to an energetically... 

Sackeim, H.A., Prudic, J., Devanand, D.P., Nobler, M.S., Lisanby, S.H., Peyser, S., Fitzsimons, L., Moody, B.J., and Clark, J. (2000) A prospective, randomized, double-blind comparison of bilateral and right unilateral electroconvulsive therapy at different stimulus intensities. Arch Gen Psychiatry 57 425—434. 

Krystal and colleagues (1993, 1995) have similarly demonstrated differences in the EEG according to electrode placement and stimulus intensity. 

Neuroimaging techniques assessing cerebral blood flow (CBF] and cerebral metabolic rate provide powerful windows onto the effects of ECT. Nobler et al. [1994] assessed cortical CBE using the planar xenon-133 inhalation technique in 54 patients. The patients were studied just before and 50 minutes after the sixth ECT treatment. At this acute time point, unilateral ECT led to postictal reductions of CBF in the stimulated hemisphere, whereas bilateral ECT led to symmetric anterior frontal CBE reductions. Regardless of electrode placement and stimulus intensity, patients who went on to respond to a course of ECT manifested anterior frontal CBE reductions in this acute postictal period, whereas nonresponders failed to show CBF reductions. Such frontal CBF reductions may reflect functional neural inhibition and may index anticonvulsant properties of ECT. A predictive discriminant function analysis revealed that the CBF changes were sufficiently robust to correctly classify both responders (68% accuracy] and nonresponders (85% accuracy]. More powerful measures of CBF and/or cerebral metabolic rate, as can be obtained with positron-emission tomography, may provide even more sensitive markers of optimal ECT administration. 

McEwen BS, Angulo J, Cameron H, et al Paradoxical effects of adrenal steroids on the brain protection vs. degeneration. Biol Psychiatry 31 177-199, 1992 McCarvey K, Zis AP, Brown EE, et al ECS-induced dopamine release effects of electrode placement, anticonvulsant treatment and stimulus intensity. Biol Psychiatry 34 152-157, 1993... 

Nobler MS, Sackeim HA Augmentation strategies in electroconvulsive therapy a synthesis. Convulsive Therapy 9 331-351, 1993 Nobler MS, Sackeim HA Electroconvulsive therapy clinical and biological aspects, in Prediction of Treatment Response in Mood Disorders. Edited by Goodnick P. Washington, DC, American Psychiatric Press, 1996, pp 177-198 Nobler MS, Sackeim HA, Solomou M, et al EEG manifestations during ECT effects of electrode placement and stimulus intensity. Biol Psychiatry 34 321-330, 1993 Nobler MS, Sackeim HA, Prohovnik 1, et al Regional cerebral blood flow in mood disorders. III treatment and clinical response. Arch Gen Psychiatry 51 884-897, 1994... 

Use of caffeine has also been recommended to lower the threshold in patients who do not experience an adequate seizure (104,105 and 106). One report, however, found that caffeine appeared to produce neuronal damage in rats receiving ECS (107). Because adenosine may have neuroprotective effects, one postulated mechanism is the ability of methyixanthines (e.g., caffeine, theophylline) to block adenosine receptors. On a positive note, studies have not found a difference in cognitive disruption between patients receiving ECT with or without caffeine (108). Although the implications of the animal data for humans are not clear, and because shorter seizures may be effective in some patients, a conservative approach would be to augment with caffeine only when seizure duration is less than 20 seconds and response is inadequate ( 38). Alternatively, it may be appropriate to switch to BILAT electrode placement or from methohexital to etomidate when UND electrode stimulation produces inadequate seizure duration (even at maximal stimulus intensity) and response is insufficient ( 97, 98). 

Alterations in stimulus intensity, duration, waveform, or path of current... 

Stimulus intensity is determined in relationship to each individual s motor threshold (MT), with most studies stimulating at 80% to 120% of this threshold. A review by George et al. (206) found that studies using stimulation pulses of lower intensities (e.g., 80% of MT) demonstrated only modest antidepressant effects, whereas higher intensities (e.g., 110% of MT) produced greater efficacy. Intensities greater than 120% of MT are generally avoided because of the possibility of an increased seizure risk. 

Sackeim HA, Pmdic J, Devanand DP, et al. Effects of stimulus intensity and electrode placement on efficacy and cognitive effects of electroconvulsive therapy. N Engl J Med 1993 328 839-846. 

Sackeim HA, Prudic J, Devanand DP, et al. A prospective, randomized, double-blind comparison of bilateral and right unilateral electroconvulsive therapy at different stimulus intensities. Arch Gen Psychiatry 2000 57 425-434. 

When an excitatory pathway is stimulated, a small depolarization or excitatory postsynaptic potential (EPSP) is recorded. This potential is due to the excitatory transmitter acting on an ionotropic receptor, causing an increase in cation permeability. Changing the stimulus intensity to the pathway, and therefore the number of presynaptic fibers activated, results in a graded change in the size of the depolarization. When a sufficient number of excitatory fibers are activated, the excitatory postsynaptic potential depolarizes the postsynaptic cell to threshold, and an all-or-none action potential is generated. 

The least experience of pain that a person can recognise. It is the level at which 50% of stimuli are recognised as painful it must be understood that pain is the experience of the patient, which is difficult to measure, whereas the stimulus intensity can be measured by the psychophysicist as an external event. 

Lee B., Butcher G. Q., Hoyt K. R., Impey S., and Obrietan K. (2005a). Activity-dependent neuroprotection and cAMP response element-binding protein (CREB) kinase coupling, stimulus intensity, and temporal regulation of CREB phosphorylation at serine 133. J. Neurosci. 25 1137-1148. 

Figure 24.3 Temporal analysis of responses measured simultaneously in five different antennal lobe neurons in the moth Manduca sexta. The matrices show patterns of neural synchrony evoked by either of two pheromone components or a binary mixture at two concentrations. The number of synchronous events was averaged over 20 trials and calculated for 500 ms from stimulus onset. The gray scale ranges from 0 to 3.8 coincident spikes per stimulus. The horizontal displays below the matrices show the averaged spiking rate in each single neuron (gray scale ranges from 0 to 5.5 spikes per stimulus period). Neural synchrony was influenced not only by the odor quality but also by both stimulus intensity and blend interactions (redrawn from Christensen eta ., 2000).

In a third study, single unit recordings from the moth Heliothis virescens placed in a wind tunnel corroborated the previously described results by showing how well single neurons follow the fine-scale temporal characteristics of a natural odor plume (Vickers et al., 2001). Also in this study it was clear that the occurrence of a stimulus over time heavily influences the temporal structure of the response to a given stimulus. Both stimulus intensity and dynamics of the odor plume had an effect on the time course of the PN spike pattern. Furthermore, no wave-like periodicity in PN spiking could be observed and PN spike frequency did only rarely match the frequency range of local field potential oscillations that has been reported from moths (M. sexta Heinbockel et al., 1998). 

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