Threshold of perception

The perception of a current through human skin is dependent on frequency, current density, effective electrode area, and skin site/condition. Current duration also is a factor, in the case of DC determining the quantity of electricity and thereby the electrolytic effects according to Faraday s law (see Section 7.8). 

If a DC source coupled to two skin surface electrodes is suddenly switched on, a transient sensation may be felt in the skin. The same thing happens when the DC current is switched off. This proves that many nerve endings are only sensitive to changes in a stimulus and not to a static stimulus. At the moment a DC is switched on, it is not only a DC, it also contains an AC component. DC must therefore be applied with a slow increase from zero up to the desired level, if the threshold of DC perception is to be examined. 

A sensation will start either under one of the electrodes (anode or cathode), or in the tissue between. The chemical reaction at an electrode is dependent on the electrode material and electrolyte, but also on the eurrent level (see Seetion 7.7). A sensation around threshold current level is slowly developing and may be difficult to discern from other sensations (e.g., the mechanical pressure or the eooling effeet of the electrode). After the sensation is clear and the current is slowly reduced to avoid ae exeitation, the sensation remains for some time. That proves that the eurrent does not trigger nerve ends directly, but that the sensation is of a chemical, electroljrtic nature as deseribed by the law of Faraday. The aftercurrent sensation period is dependent on the perfusion of the organ elieiting the sensation. 

On palmar skin, with a surfaee eleetrode of varying area A, the current Iih or current density Jth at the threshold of pereeption follows the following equation for a sensation within 3 min after current onset (Martinsen et al., 2004) 

The perception was only localized under the monopolar electrode, never in the tissue distal to the electrode. Surprisingly, according to Eq. 10.16, the threshold as a function of electrode area A is more dependent on current than current density. There may be more than one reason for this  

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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. 

Diacetyl, acetoin, and diketones form during fermentation. Diacetyl has a pronounced effect on flavor, with a threshold of perception of 0.1—0.2 ppm at 0.45 ppm it produces a cheesy flavor. U.S. lager beer has a very mild flavor and generally has lower concentrations of diacetyl than ale. Diacetyl probably forms from the decarboxylation of a-ethyl acetolactate to acetoin and consequent oxidation of acetoin to diacetyl. The yeast enzyme diacetyl reductase can kreversibly reduce diacetyl to acetoin. Aldehyde concentrations are usually 10—20 ppm. Thek effects on flavor must be minor, since the perception threshold is about 25 ppm. 

PSvcHomrsicAL (eye-brain) RESPONSE TOINCOMIN6 LIGHT "THRESHOLDS OF PERCEPTION FOR CONTRAST COLOR CHANGE -SENSITIVITY TO SIZE,PATTERN DISTRIBUTION OF COLOR. -SUBJECTIVE JUDGMENT OF PERCEIVED IMAGES... 

Sensitivity selection of panelists as a function of their individual thresholds of perception on 5 pure products. Interindividual differences ranging between 1 and 1000 are accepted, as it really occurs. 

Normally, we would use root mean square (RMS) values for the calculations, but, assuming that the voltages are sinusoidal, we could use the maximum values and still derive the same results. It has been found that a flicker frequency of 8 to 10 Hz with a voltage variation of 0.3 to 0.4% is usually the threshold of perception that leads to annoyance. 

Threshold of perception Sensation of pain Increased pain... 

In Fig. 6a data by Hahn (5) are shovm. Hahn determined the effect of three levels of salt concentration, as a function of time, on the threshold of perception. 

Dependence of threshold of perception S ( ) on an arbitrary time course of stimulation ( ) at two levels... 

Human Exposure. The threshold of perception of ammonia varies from person to person and may also be influenced by atmospheric conditions, values as low as 0.4-2 mg/m3 (0.5-3 ppm) are reported in [1374], but 50 ppm may easily detected by everybody [1375]. Surveys [1376] found concentrations from 9-45 ppm in various... 

The ability of our consciousness to separate into two halves which act upon each other, cannot be exclusively limited to the dream-life The cause of this segregation lies in the simultaneous existence of a waking consciousness, a dream-like subconsciousness and also a much deeper unconsciousness, which are separated from each other through two dividing thresholds (a threshold of consciousness and a threshold of perception) This cause is also given when awake. 

As soon as this Dream-I sinks to the lowest threshold of perception, it enters into a danger zone, where the personal and collective subconsciousness flow together, and out of the latter, dark chaotic figures emerge (encounter with the Guardian of the Threshold). 

Should the Dream-I, the transcendental Subject of the Human Being, sink even lower than the threshold of perception, lower than the Limbus, then another segregation would take place, namely, the total segregation of the Spirit from the Soul The transcendental subject would become a demon, and would sink into the collective subconsciousness, into Hades, in which it would even lose the memory of the dreamworld because it crossed the River Lethe. (Condition of Death and Madness)... 

Thresholds of hearing in healthy human subjects for tones with vibration frequencies from 128 to 11,584 were decreased by 10 mg. of morphine. The decrease in acuity of hearing ranged from 5 to 20 decibels for various tones, the responses to higher frequencies being more affected (Macht and Macht, 15) but Wikler et al. (5) reported that morphine did not alter thresholds of perception for hearing in man. 

Hilsmann (16) found no effect on two-point tactile discrimination, while Kremer (17) recorded a definite increase in the minimal distance for two-point discrimination throughout the surface of the body after 10-15 mg. of morphine was administered subcutaneously. David (11) reported recently that tactile discrimination was decreased in 6 of 10 subjects with 10 mg. (0.14 mg./kg.) and was uniformly decreased in all subjects by 15 mg. (0.22 mg./kg.). Mullin and Luckhardt (18, 19) claimed that tactile sensitivity was not appreciably affected by doses of morphine (35-30 mg.) which reduced sensitivity to pain. Further, according to Wikler et al. (5), the administration of morphine did not alter thresholds of perception for touch, vibration, two-point discrimination, or hearing in man, and hence morphine specifically alters pain thresholds. Wikler felt that this inference was open to question because of the variable effects of analgesics on pain as reported by different investigators (5). 

Dalziel CF. 1954. The threshold of perception currents. AIEE Trans Power App Syst 73,990-996. 

The second level (1 mA) is due to the direct electric excitation of nerve endings, which must be a function of the local current (density). The electric current threshold of perception with firm hand grip contact and contact area several square centimeters, is around 1 mA. Threshold current has a surprisingly small dependence on contact area. The reason for this is mentioned previously in the chapter on DC perception. With a small area contact around... 

The charge transferred around threshold level is of the order of 0.2 C. The threshold of perception as a function of stored energy is about 10 pJ. The formation of an arc in the air between the conductor and the skin is possible when the voltage difference is larger than about 400 V. The arc discharge can be heard as a click and felt as a prick in the skin. 

Very intense-smeUing sulfur compounds with low thresholds of perception often act as key aromatic compounds. They often have very similar structures and contain an important odorophore (a group responsible for the odour character), which is sulfur and oxygen in positions 1 and 3 of the molecule (8-120). This structure occurs in mercaptans, thioacetals, mercapto ethers, mer-capto ketones, alkylthio esters and other compounds listed in the following sections. 

Dalziel, C.F. 1954. Threshold of perception currents. Electrical Engineering 73. 

The threshold of perception of ammonia varies with the individual and may also depend on atmospheric conditions. A lower limit of perception in the range of 0.4 to 2 wtppm is given in [5]. In this report it is also concluded that levels of ammonia at the perception threshold may cause changes in the biopotentials of the brain. 

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