Tactile psychophysics.

Tactile psychophysics.

the study of how the skin perceives and processes mechanical stimuli such as pressure, vibration, and texture.

Von Frey hairs

Apply constant-pressure stimuli. These calibrated hairs bend when a given force is reached as they are pressed into the skin

Solenoid and bell ringer coils

durative pressure pulses because they will indent the skin for the duration of the applied voltage.

Shakers and piezoceramic reed benders

present vibratory stimuli to the skin over the frequency range of .1 Hz to 200 Hz or more.

Shakers vs benders

shakers: able to deliver a wider range of intensities than the benders because they are some­ what independent of the impedance of the skin, and are less susceptible to damping.

Electrostatic forces

produce contactless mechan­ical stimulation of the skin

Air puffs

provide a noncontacting mechanical stimulus

Arrays (e.g., Optacon)

presenting two-dimensional patterns of stimuli to the skin

Arrays limitation

mechanical design prevents the experimenter from having precise control of stimulus amplitude and waveform.

Electrocutaenous stimulus

Produce sensations indistinguishable from mechanical taps

Chemical Stimulation

Substances or solutions are used to stimulate the skin, typically for experiments that involve chemical irritants or topical applications.

Absolute threshold

The minimal energy that can be felt

Lowest vibration stimulis amplitude

0.2 Microns

Pressure threshold on face

5mg

Pressure threshold on hand

250 Hz

Females are slightly more sensitive at all body sites by ______

0.4 to 0.6 log units

Frequency threshold

Threshold sensitivity can vary significantly with frequency, especially within certain ranges.

However, when the stimulus is a train of pulses, the threshold (and suprathreshold loudness) becomes less dependent on the pulse rate, assuming all other conditions remain constant

Temperature threshold

inhibiting or exciting individual receptors that are sensitive to both touch and thermal stimuli such as the Pacinian

Stimulus conditions and threshold

Larger contactor areas lower the threshold due to spatial summation.

Individual factors effecting threshold

Age, hormone levels, and even gender differences affect tactile thresholds. For example, females generally have slightly lower thresholds than males by about 0.4 to 0.6 log units.

Neural Encoding

At the level of first-order afferents, stimulus intensity appears to be encoded by the rate of neural response(number of action potentials in a given period).

Psychophysical "Loudness":

The perceived intensity of a tactile stimulus follows a power-law relationship. Increases in stimulus intensity result in proportional increases in perceived loudness.

Loudness Function:

The exponent of the loudness function can vary depending on body site, stimulus conditions, and other factors. For example, moving the stimulus from the fingertip to the forearm can increase the exponent from 0.90 to 1.30

The _____ the stimulus, the _____ the apparent intensity.

The larger the stimulus, the greater the apparent intensity.

How growth of loudness is affected by frequency

the growth of loudness is unaffect­ ed by changes in other factors such as frequency.

Two point limen

separation at which two points of contact on the skin are perceived as distinct

Temporal summation

said to occur when either threshold decreases or the loudness of the stimulus increases with duration, at least up to about 300-500 ms.

Temporal factors

can have an effect on the perceived intensity of tactile stimuli, but in two different ways depending on the duration of the stimulus.

Adaptation

the increase in threshold or the reduction in the apparent intensity of a stimulus with prolonged stimulation

Adaptations mechanisms

The mechanisms involved in adaptation are not readily apparent.

masking

Occurs when one tactile stimulus is reduced or masked by another stimulus, either preceding or following it within a short time frame.

error of localization

both a test of spatial acuity and spatial memory. Either the same or another point is touched, and the observer is asked to report “same” or “different.”

Which stimuli is strongest for spatial acuity

Mechanical

The limen, or threshold for successivenes

Is about 5 ms

Ear threshold

0.01 ms

the eye threshold

25 ms

Highest sensitivity located at the 

Fingertips