Threshold 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. 

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). 

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. 

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. 

An alternative explanation for these higher thresholds was not that they took greater stimulus intensity (longer flashes) to perceive but that the social unacceptability of the words made the subjects more reluctant to voice them until they were quite certain. Further research showed that this accounts for part of the delay, but there is still a perceptual defense factor. More direct evidence for perceptual defense comes from the fact that physiological reactions associated with emotions, like quick changes in the electrical resistance of the skin, can sometimes be seen when emotionally threatening words are presented that are well below a subject s conscious perception threshold. 

The detection threshold is the lowest stimulus intensity (odour concentration) that the subject can distinguish from an odour-free situation. The subject s response indicates whether the presence of an odour has been perceived or not. Correspondingly, the recognition threshold is the minimum concentration at which an odour can be identified. 

In contrast to its prophylactic effect in cats [191], A -THC in amygdaloid-kindled rats failed to suppress seizures in non-toxic doses, exerted no prophylactic effect on seizure development, and also induced tolerance after repeated administration [ 192]. In hippocampal-kindled cats, A -THC elevated seizure thresholds, but only following weak stimulus intensities [193]. 

Fontana GA, Pantaleo T, Lavorini E, Boddi V, Panucdo P (1997) A noninvasive electromyographic study on threshold and cough intensity in humans. Eur Respir J 10 983-989 Fontana GA, Pantaleo T, Lavorini E, Malucdo NM, Mutolo D, Pistolesi M (1999) Repeatability of cough-related variables during fog challenges at threshold and suprathreshold stimulus intensity in humans. Eur Respir J 13 1447-1450... 

Fiber responses are also characterized with tuning curves, a plot of thresholds for stimulus intensity vs. frequency. Tuning curves for auditory nerve fibers have a minimum intensity (maximum sensitivity) at a characteristic frequency (CF). 

Fig. 19. Small-amplitude spikes recorded intracellularly from granule cells. (A) Trains of SA spikes to be initiated by intracellular injections of depolarizing ramps of current through the recording electrode. Full-blown action potentials are initiated only at higher thresholds which are reached by increasing the slope of the ramps. (B) Antidromic action potentials and associated after-potentials evoked by stimulation in the CAS region near threshold for antidromic invasion (15 V, 50 sec). In the first trace failure of the action potential at the same stimulus intensity causes the after-potential to disappear and reveals the stimulus artifact. The after-potential gives rise to either SA spikes (arrow) or full-blown action potentials. In (C) a SA spike is evoked in the same cell by the intracellular injection of depolarizing current through the recording electrode. (From Assaf et al, 1981.)...

The odor detection-threshold values of organic compounds, water, and mineral oil have been determined by different investigators (Table 2 and 3) and may vary by as much as 1000, depending on the test methods, because human senses are not invariable in their sensitivity. Human senses are subject to adaption, ie, reduced sensitivity after prolonged response to a stimulus, and habituation, ie, reduced attention to monotonous stimulation. The values give approximate magnitudes and are significant when the same techiriques for evaluation are used. Since 1952, the chemistry of odorous materials has been the subject of intense research (43). Many new compounds have been identified in natural products (37—40,42,44—50) and find use in flavors. 

Odors are measured by their intensity. The threshold value of one odor to another, however, can vary greatly. Detection threshold is the minimum physical intensity necessary for detection by a subject where the person is not required to identify the stimulus, but just detect the existence of the stimulus. Accordingly, threshold deterrninations are used to evaluate the effectiveness of different treatments and to estabflsh the level of odor control necessary to make a product acceptable (8). Concentration can also produce different odors for the same matenal. For example, indole (qv) in low concentrations has the smell of jasmine and a low threshold of perception. In high concentrations, it has a strong odor of feces and CX-naphthyl amine as well as a considerably higher threshold of perception. 

Lidocaine (Xylocaine), die representative class I-B drug, raises the threshold of the ventricular myocardium. Threshold is a term applied to any stimulus of the lowest intensity that will give rise to a response in a nerve fiber. A stimulus must be of a specific intensity (strength, amplitude) to pass along a given nerve fiber (Fig. 40-2). 

To further illustrate the threshold phenomenon using plain figures instead of precise electrical values, a certain nerve fiber has a threshold of 10. If a stimulus rated as 9 reaches the fiber, it will not pass along the fiber because its intensity is lower than die fiber s threshold of 10. If another stimulus reaches the fiber and is rated 14, it will pass along the fiber because its intensity is greater than the fiber s threshold of 10. If the threshold of a fiber is raised from 10 to 15, only the stimuli greater dian 15 can pass along the nerve fiber. 

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