Perception Chap 3 II

The time required for a complete adaptation depends on which factors?

1. In general, the greater the area of the stimulus, the faster the adaptation. 2. The greater the intensity or force, the slower the adaptation 3. the less sensitive areas of the body adapt the quickest, often within a few seconds

It was originally thought that adaptation occurred due to reduced electrical discharges by mechanoreceptors. This has largely been rejected on which two grounds?

1. We now know that the sustained type of mechanoreceptors SA-I and SA-II continue to produce action potentials as long as the stimulus is applied. A strictly neural explanation is unsatisfactory given that some mechanoreceptors at the site of stimulation are unlikely to become completely adapted. 2. There is now evidence that adaptation occurs largely because of mechanical reasons and that the key parameter is the lack of stimulus movement. We become less and less inclined to notice a stimulus if it is not dynamic enough or if the stimulus fails to remain in motion. Even though the peripheral receptors may be signalling a response to a constant tactile stimulus, the situation is not the same as at the time of its onset or say for a stimulus that changes with time. Both of these situations produce a contrasting condition that may elevate the sensory experience and our appreciation of it.

Point localization error

A sequential test where a person must indicate the point on the body that was previously touched with a probe. The error between the two points is taken as a measure of tactile discrimination.

Esthesiometer

A set of calibrated fibres, each producing a different force when applied to the skin. The early sets used animal hair, whereas modern ones are developed from nylon fibres. Max von Frey discovered that horse hairs tend to apply a single downward force that depends on the thickness and stiffness of the hair. By using a set of calibrated hair fibres, von Frey created the first esthesiometer. The early studies showed that human skin was very sensitive to touch and that small forces were sufficient to elicit sensation. The more recent work on touch thresholds relies on the use of nylon filaments, which were invented shortly after WWII. The Semmes-Weinstein esthesiometer is a set of 20 nylon fibres of different diameters attached to Plexiglas handles. As with von Frey hairs, the nylon fibres can be used to determine the minimal force needed to elicit sensation and determine the absolute threshold of touch over various parts of the body.

Hyperacuity

A term applied to the very low tactile discrimination thresholds that are found with spatially extensive stimuli in comparison to the traditional two-point test. This term has often been applied to spatial discrimination thresholds that are much lower than the traditional two-point limen.

Vibrotactile Stimulation

A vibrating, tactile stimulus. Vibratory stimuli produce different threshold values than static stimuli. The two-point limen is lower if a vibrating probe is used. Magnitude estimation studies have also provided different results.

Study - Sidney Weinstein - Two-Point Threshold

He found that the two-point discrimination profile over the body does not exactly correspond to the pressure sensitivity profile. Although the facial regions are most sensitive to touch, it is the fingers that show the highest spatial discrimination ability. Much of the upper torso is quite sensitive to touch but shows poor spatial resolution. The feet display just the opposite pattern, relatively good discrimination ability but poor sensitivity. It is worth pondering these data and how they relate to our own everyday experiences in tactile perception.

Hyperacuity Further Description

If human subjects were asked to discriminate size differences between two successively applied disks or edges, then the tactile spatial threshold became 4-10 times smaller than the classical two-point threshold. Similar threshold values were also found if subjects were tested on a line-separation task or asked to determine whether two bars were more or less separated than a comparison pair. The low threshold values that were found in these experiments suggested that there may be multiple forms of spatial discrimination ability depending on the type of stimulus used and the way in which the experiments are performed.

Ronald Verrillo - Duplex Theory

In the 1960,s, Ronald Verillo showed that if a more punctuate stimulus is used instead, such as a vibrating pin, then the temporal frequency does not significantly affect threshold values. Only low temporal frequencies in the range of 20-40 Hz, were effective in this case. Beyond this frequency range, the detection threshold for punctate stimuli became too large. This finding led Verrillo to propose the duplex theory in which 2 separate and independent channels mediate the temporal nature of touch - a frequency-independent channel that operated only at low frequencies and a frequency-dependent channel that operated only at high frequencies

Difference Thresholds

It was the early studies by Ernst Weber on tactile perception that led him to formulate his law. Weber was concerned with understanding the relationship between perceived magnitude and stimulus intensity. He saw the need to establish the relationship between difference thresholds an stimulus intensity as a means of achieving this. The study of difference thresholds in the somatosensory system began at the earliest moments in the history of experimental psychology. Weber's law only holds true for a limited range of tactile intensities. At very low or very high intensities, the constant in Weber's law can change dramatically, thereby leading to a failure in the law since Weber's constant no longer remains constant. Another factor that affects difference thresholds is the site on the body where the measurements are made. Skin areas that have low touch thresholds are more sensitive to stimulus change and have LOWER Weber constants. The actual experimental way in which difference thresholds are measured can have a bearing on the outcome. Over the past century, values in the range of 0.02 to 0.30 have been reported by various researchers. Given that man variables can influence the Weber constant, there is no one value commonly used to portray the tactile sense.

Meissner's corpuscules vs. Pacinian corpuscules

Meissner's corpuscules are more sensitive to low-frequency stimulation. Pacinian corpuscules are sensitive to high-frequency stimuli.

Callus

The absolute threshold for pressure detection can also be influenced by age, gender and condition of the skin. A reduction in skin elasticity will reduce sensitivity and raise threshold. A callus is a good example where a marked increase in detection threshold occurs simply because the thickened skin is less elastic and not as easily deformed by mechanical pressure. A more gradual loss of skin elasticity occurs with age, which in turn causes progressive increments in detection threshold. Men and women differ in how the rate of detection threshold can change with age. It is not uncommon to hear that women are more sensitive than wen. Weinstein found in his studies that pressure thresholds were generally lower in women than men in nearly all sites he studies, with the exception of the tongue where they were nearly equal.

Absolute Thresholds

The absolute threshold of touch is very small when considered in terms of the actual mechanical force that needs to be applied. Touch is by no means the most sensitive among our sensory systems. The absolute thresholds of vision and hearing are nearly a billion times lower when considered in terms of stimulus energy. As with all of the senses, the actual threshold value can depend on a number of different factors. One of the most important factors that affects touch threshold is the actual site on the body where it is measured.

Mechanoreceptor Properties related to vibrotactile processing

The psychophysical functions relating detection threshold to temporal frequency fit in very nicely with neural responses at the receptor level. Temporal changes in touch can best be detected by fast-adapting mechanoreceptors. The sensitivities of the two types of FA receptors - Meissner and Pacinian corpuscules - are different.

Point localization test vs. compass test

The stimuli are presented successively in the localization test. The stimuli are presented simultaneously in the compass test. Localization errors are 3-4 times smaller than 2-point discrimination.

The temporal factors affecting touch perception can be broken down into which two forms? What are they based on?

They can be broken down into: 1. the effects of a prolonged, steady stimulation => Adaptation 2. the effects of short, repetitive stimulation => Vibrotactile Stimulation

Compass Test

Use of a standard compass - a two-pronged probe where the spatial separation between the points can be varied - to explore the minimum separation that is detectable as two separate points on the skin.

von Frey hairs

Weinstein undertook a study using von Frey hairs to examine how absolute thresholds varied across the body surface. There is considerable variability in touch sensitivity across the body surface. Facial regions around the mouth and nose are among the most sensitive to pressure i. e. have the lowest thresholds. The extremities, particularly at the lower end of the body, require relatively large forces to produce a touch sensation. This is consistent with out everyday experience. A small puff of air, for example, will much more likely be detected if aimed at our lips than our palms or sores.

When is the vibration frequency more apparent?

When measuring the absolute threshold of detection. The threshold value to be lowest for a certain vibrational frequency is typically in the range of 200-300 Hz. The threshold rises rapidly if lower or higher temporal frequencies are used. This function appears to be true only if the stimuli encompass a fairly large area on the skin.

Adaptation

When the perceived intensity of a sensory stimulus is reduced, often to the point where it is no longer consciously experienced. This happens whenever a constant stimulus is applied for a prolonged period of time. A tactile stimulus becomes progressively less effective in producing a sensation the longer it is maintained. Adaptation becomes complete when the stimulus is no longer detectable.

Mechanoreceptor properties related to two-point discrimination

The gradient of spatial acuity from the base of the palm to the fingertips is striking. This region has been used to explore the neural basis of two-point discrimination function and the factors that underlie differences in the values. The smaller the receptive field's size, the greater the ability to discriminate two different points of stimulation. The field sizes should be much smaller for mechanoreceptors on the fingertip than those in the palm. This relation should be especially apparent for mechanoreceptors linked to the FA-I and SA-I type afferents since these have the most superficial location. Although there are systematic changes in FA-I and SA-I afferents in the manner expected, Vallbo and Johansson found that the change in receptive field size was not as great as the change in two-point discrimination across the hand. An additional factor had to account for the superior tactile resolution in the fingertips. That factor turned out to be the actual density of mechanoreceptors. Both FA-I and SA-I afferents are densely packed in the fingertip and become much sparse outside of this region, a relationship that parallels the psychophysical gradient of two-point discrimination in the hand and the fingers. Differences in packing density of both types of mechanoreceptors play a important role in determining tactile acuity. The receptive field size and density of FA-II and SA-II afferents does not change appreciably across the hand. It would be unlikely that these mechanoreceptors would play an important role in setting the limits of tactile acuity, given that they are located deep within the skin and have fairly large receptive field sizes.

Law of Outward Mobility

The greater the mobility of the body, the greater the spatial resolution. Although this is true for the upper extremities, it does not necessarily hold true for other body parts.

Sensory Magnitudes

The main distinctions arise from whether the stimulus is a single indentation or a vibrating one. Single indentations in hairless skin were shown to produce sensation magnitudes that increased linearly with depth of skin displacement. The power law exponent of the function was close to 1.0. However, the same experiment on hairy skin produced an exponent value of 0.4. Clearly, the response characteristics of the two types of skin are different and yield different sensory magnitudes as stimulus intensity is increased.

The two-point-limen

The minimal separation of two simultaneous indentations that can still be perceived as two separate points. The term limen is synonymous with threshold. The lower the threshold value, the greater the spatial resolution or acuity. It was defined by Weber as the smallest separation of two points applied simultaneously to the skin that can still be discriminated. They evoke the sensation of two separate points. At spatial separations below this value, the sensations merge and the two points feel as a single indentation.