Physics II Lab

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[edit] Spring 2007: Psychophysics Experiment

Main Questions-

What physical conditions lead to sensations of hot/cold?
How are these conditions translated by the organism?

Notes from journal club 3/21-

Receptors vary in distribution across the body
Overall(?) cold outnumber hot 30:1
Sensation of pain (and hot/cold?) scales with affected area
Therefore, must compare like with like in terms of body part and exposed area

Want to get our hands on a graph of neural stimulation as function of temperature for hot/cold

Want to get a scale drawing of the top layers of skin showing the A-beta, A-delta, and C fibers.

All three kinds of fibers sensitive to hot, cold, and pain
Subjective sensations can be "neutral", "cold", "warm", "pain"
For laptop scenario, neutral is 30-36 degrees C, warm is 36-40, uncomf. warm is 40-42, pain is >= 42.

Notes from journal club 3/28-

Jackie: Test subjects werer told to expect sensation of cold. People's sensation depended on whether the person held the object themselves. When people held the object, the verbally reported sensation of cold was diminished. [Implications for protocol] Holding it dropped the reported sensation from 4 to 3. (This was not a likert scale with 3 as neutral.) "Corollary discharge theory" is supposed to explain differences between self-generated sensation and other sensation. Subjects were blindfolded. Hypothesis is that neuroprocessing has a role to explain this.

Amin: Questions about environmental temperature. Journal of anesthesiology; thermal factors in pain relief. Respondents rate sensation from "not warm" to "extreme warm" on a 100-point scale presented visually. (Also "not cold" to "extreme cold", and "unpleasantness".) Three environments: neutral (25C), cool (15C), warm (35C). Skin and core body temp. monitored throughout. Instead of touching objects, 1 cm^2 electrodes were used to deliver heat energy (as well as mechanical stimulation) to the skin. [What are the "cold" electrodes doing? How does that work?] Electrodes deliver for 5-second pulses as people go from room to room. Animal studies showed that being in a cool environment leads to endorphins and improved mood. This study found that in a cold environment, a hot stimulus was felt less intensely. A warm environment had no effect. In a cool room, the heat/pain threshhold was 48.5C, in neutral it was 47.5C, hot room was 47C. Ten subjects with one trial per room each. [Is this convincing? Is it large in any case?] In the cold room, the average skin temperature was 3.5C less than in the neutral and warm rooms. [Makes effect seem more important.] Possible explanations: The neural pathway that transmits pain sensations is A-delta. Local temperature is low -> constricts blood flow Restricting blood supply blocks the A-delta pathway and allows pain to hog the bandwidth. Low ambient temperatures. Bottom line: in a cold room, people underrate the warmth of a warm stimulus [goes against the hypothesis], and also underrate the coldness of a cold stimulus [goes with the hypothesis]. In the neutral room, the skin temperature was about 33.4C on the forearm.

Scott: Anatomy & physiology of the skin. Overview. Action potential propagates along the dendrite at a speed determined by the spacing between notes of Lanvier and the myelination. Cold sensation travels faster because of these properties in the C fibers. Chemicals can induce indentical subjective sensations of hot & cold as environmental factors, as well as other bodily responses beside subjective sensations (e.g. sweating; but can you get shivering from a chemical?). A breeze "feels cold" if you're wet, when that same breeze might not "feel cold" when you're dry.

From a text: hot and cold sensors lie at different depths. warmth are deeper.

With temp change of < 0.5C/min, the person will remain unaware of changes so long as it remains in neutral zone.

Bob: what contributions different cues make to discrimination. In first experiment, two index fingers placed in boxes and asked to determine which material feels colder. Not told which is correct. Copper, bronze, stainless steel, plastic, foam, epoxy. In 2nd experiment, three fingers from one hand in holes and tell which hole has a different substance from the other two. During the process, temperature of the skin nearby was monitored. Contact coefficient: sqrt(conductivity*density*specific heat). Hands are reset to 33C in between trials by a chiller. Design notes: thermal cues arise from changes in skin temperature that occur when an object contacts the skin. there are warm and cold thermal receptors in skin. ability to perceive thermal changes depends on many factors, including the amplitude and rate of temp. change, site of stimulation.

What is the timescale for the diffusion of temperature in the skin environment? Is thermal equilibrium the right approximation for this process?