HCC Research Methods Psychophysical Methods: Psychophysical Methods | Lecture notes Psychology

CAP5108:)HCC)Research)Methods)Psychophysical)Methods)Lecture)Mar)22)

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Lecture'Mar'22:'Psychophysical'Methods'

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Psychophysics)is)a)term)used)that)describes)the)investigation)of)the)relationships)between)a)

physical)stimuli)and)human)perception.)The)term)psychophysics,)in)relation)to)a)research)study,)is)

characterized)by)psycho,)a)term)that)refers)to)a)mental)or)subjective)experience)attempting)to)be)

measured,)in)this)case)in)a)person,)and)its)physical*or)objective)measurement.)The)tools)used)to)

study)psychophysics)are)grouped)into)a)set)of)psychophysical)methods.))All)of)these)tools)attempt)

to)measure)thresholds)and)perception)in)people.)Those)methods)will)be)of)topic.)

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In)order)to)fully)describe)the)meaning)of)psychophysics)let)us)define)the)properties)of)physical)and)

psycho)further.)A)physical)property)is)one)that)can)be)directly)measured)and)can)be)objectively)

stated.)An)objective)measurement)could)be)thought)of)as)a)measurement)or)property)that)is)

unchanging)and)is)independent)of)the)method)used)to)make)the)measurement.)Some)examples)of)

objective)measurements)of)physical)properties)include)the)length)of)an)object,)the)mass)of)an)

object,)the)illuminance)of)a)light)source)or)even)the)time)a)task)takes.)For)example,)time)has)a)

universal)measurement)in)seconds,)minutes,)or)hours.)One)researcher’s)measurement)by)his)clock)

or)stopwatch)will)measure)the)same)variable)time)as)the)others’.)Other)units)of)measurement)like)

meters,)kilograms)and)lux)also)have)objective)values.)

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Psycho)properties)are)those)that)are)subjective)to)the)experience)of)the)user.)Some)examples)of)

psycho)measurements)in)humans)may)be)those)such)as)their)perception)of)the)size)of)an)object,)

their)perceived)heaviness'of)an)object)or)even)their)perceived)distance)to)an)object.)For)example)

how)does)one)compare)my)perception)of)“that)tree)being)far*away”)and)your)perception)of)the)

distance?)As)one)can)see,)it)is)much)more)difficult)to)measure)the)percept)of)a)physical)

characteristic)in)humans.)The)percept)is)the)object)of)the)perception)in)the)human)of)a)physical)

property.)An)example)relating)the)two)described)properites)is)the)physical)property)length)and)its)

percept)“size”)or)the)physical)property)distance)and)“closeness”.)

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Psychophysical)methods)are)used)to)study)the)relationships)between)those)physical)quantities)

and)their)respective)percepts.)For)example)if)a)researcher)wanted)to)study)the)perceptible)level)of)

vibration)in)the)fingers)of)person)in)researching)a)new)phone’s)motor)power)requirement,)he)or)

she)could)study)the)just-noticeable-difference)(JND),)or)when)users)initially)start)to)sense)the)

vibration)of)the)motor/phone.)The)researcher)could)then)could)find)the)correlation)between)the)

physical)power)level)the)motor)needed)to)be)vibrating)at)in)order)for)the)person)to)perceive)the)

phone)is)in)fact)vibrating.)

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Historical'Perspective'

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In)order)for)us)to)understand)the)psychophysical)methods)employed)in)research)let)us)first)look)at)

the)history)of)psychophysics,)dating)back)to)the)1800s.))The)foundation)of)this)area)of)study)into)

human)perception)has)been)attributed)to)originating)with)the)works)of)Ernst)Heinrich)Weber,)who)

stated)what)is)now)known)as)Weber’s*Law,*and)the)later)investigation)by)Theodor)Fechner)

describing)what)is)now)known)as)the)Weber-Fechner*Law.)Fechner)was)also)known)for)the)Fechner*

color*effect)and)his)study)of)where)colors)are)seen)and)viewed)in)a)moving)pattern)of)black)and)

white.))

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Lecture Mar 22: Psychophysical Methods

Psychophysics is a term used that describes the investigation of the relationships between a physical stimuli and human perception. The term psychophysics, in relation to a research study, is characterized by psycho , a term that refers to a mental or subjective experience attempting to be measured, in this case in a person, and its physical or objective measurement. The tools used to study psychophysics are grouped into a set of psychophysical methods. All of these tools attempt to measure thresholds and perception in people. Those methods will be of topic. In order to fully describe the meaning of psychophysics let us define the properties of physical and psycho further. A physical property is one that can be directly measured and can be objectively stated. An objective measurement could be thought of as a measurement or property that is unchanging and is independent of the method used to make the measurement. Some examples of objective measurements of physical properties include the length of an object, the mass of an object, the illuminance of a light source or even the time a task takes. For example, time has a universal measurement in seconds, minutes, or hours. One researcher’s measurement by his clock or stopwatch will measure the same variable time as the others’. Other units of measurement like meters, kilograms and lux also have objective values. Psycho properties are those that are subjective to the experience of the user. Some examples of psycho measurements in humans may be those such as their perception of the size of an object, their perceived heaviness of an object or even their perceived distance to an object. For example how does one compare my perception of “that tree being far away” and your perception of the distance? As one can see, it is much more difficult to measure the percept of a physical characteristic in humans. The percept is the object of the perception in the human of a physical property. An example relating the two described properites is the physical property length and its percept “size” or the physical property distance and “closeness”. Psychophysical methods are used to study the relationships between those physical quantities and their respective percepts. For example if a researcher wanted to study the perceptible level of vibration in the fingers of person in researching a new phone’s motor power requirement, he or she could study the just-noticeable-difference (JND), or when users initially start to sense the vibration of the motor/phone. The researcher could then could find the correlation between the physical power level the motor needed to be vibrating at in order for the person to perceive the phone is in fact vibrating. Historical Perspective In order for us to understand the psychophysical methods employed in research let us first look at the history of psychophysics, dating back to the 1800s. The foundation of this area of study into human perception has been attributed to originating with the works of Ernst Heinrich Weber, who stated what is now known as Weber’s Law, and the later investigation by Theodor Fechner describing what is now known as the Weber-Fechner Law. Fechner was also known for the Fechner color effect and his study of where colors are seen and viewed in a moving pattern of black and white.

Weber was most interested in this idea of JND and studied the physical property weight (or mass) and its human percept “heaviness”. He devised a study where participants were asked to report a “noticeable” difference between a weight and its heavier counterpart. An initial weight of say 1 kg was given to the participant and subsequently added to in small increments of say grams until the participant perceived a difference in weight. Science says that an addition of even just 0. grams is enough to make something weigh more but the participants reported that objects didn’t become “heavier” until weights met what became known as Weber’s Law (for his study of weights) : k = ΔI/I Where ΔI is defined as the change in weight, I is defined as the initial weight given to the participant and k is defined as Weber’s coefficient. This means that if k=2, then if 1 kg was given to a participant then a participant would conclude that 3 kg is “just noticeably heavier” than 1 kg. Gustav Fechner then extended Weber’s Law in order to construct a psychophysical scale to describe the relationship between the physical magnitude of a stimulus and its perceived intensity change. The Weber-Fechner Law is described by the differential equation: Δp = k * ΔI/I Where Δp can be described as the ‘perception difference’ or one unit of perceived change. K is the coefficient estimated by experimental data for a stimulus from Weber’s Law. It is important to note that k is not universal for all types of study (eg, heaviness, loudness, etc). The value ΔI is the change in intensity of the stimulus and I is the level of the initial stimulus. The level of perception, p, can be solved for using simple integration techniques. We can then find that the relationship of p due the stimulus, I and Io: p = k * ln(I/I 0 ) In going back to our weight example with Io=1 kg and k=2, we find that if I=Io=1 kg, then p=0. That means that if you are handed a 1 kg weight and then the same 1 kg weight you will notice no difference at a p=0. In order to notice a change in the weight equal to p=1, you will need to be handed a weight of ~1.6 kg. By using Weber’s Law to determine k in your area of research, whether it is in sound, weight or many other areas, you are able to determine a scale for perception from the Weber-Fechner Law.

Classic Psychophysical Methods

We now work on describing the three classical psychophysical models for testing a subject’s stimulus detection thresholds. The three classical models are Method of Constant Stimuli, Method of Limits and Method of Adjustment. The three will be presented below for different cases. These methods have been developed to research thresholds in perception levels in people.

Method of Limits Another psychophysical method is the Method of Limits. An example we will use in investigating this limits method will be that of sound. As an investigator we may like to know at what level of sound do people start to hear (or that level of sound in that we start to near no more). The method of limits uses a descending or ascending or mixed testing procedure to determine this threshold. In the descending procedure an audible sound will be given to the participant and slowly decreased in sound level (dB) while asking the participant Yes/No if the sound is audible. The experiment can then be repeated from a sound level that is not audible and gradually increased. The descending and ascending trials can be average to find the threshold. A mixed “staircase” procedure can be used like that shown in Figure 2: Figure 2 - Mixed method for the Method of Limits [2] Here the procedure may start ascending or descending but change direction in sound level adjustment when the user starts to identify a presence or lack of sound. The resulting Yes/No will cause an oscillation like of that as you see in the figure until a threshold is accepted to have been found. This staircase method has been employed to attempt to alleviate errors related to ascending and descending methods [3]. Some of the potential problems with the ascending or descending methods is that participants may become accustomed to hearing a stimulus and incorrectly keep identifying the presence of the stimulus after it has gone away. This is referenced as the error of habituation. A second possible error source is that the participant may “jump the gun” and think the signal is about to become undetectable and act on this false assumption and not actually reach the true threshold. This is reference to as the error of anticipation [3]. Method of Adjustment The last and final method is the Method of Adjustment. In this method a user is in control of the level of the stimulus presented to them. In the example of a color matching experiment, a user might be presented a square with a blue color with RGB of [0,0,255]. Users may be given a dial(s) that allow them to control color parameters until they have created a matching color swatch. The users will be asked to repeat the trial over and over in ascending and descending manners from

different initial starting RGB values in order to control for bias by randomness. A grand mean RGB value can be calculated from all the trials. Many or all of these methods may be valid for particular psychophysical research and have been developed over time with the intent to measure human perception and threshold levels. Regardless of the choice of method, the thresholds that are measured are affected by many factors including variations in human nervous systems, the intensity of the signal being presented and individual participant differences in criterion and standards for what he or she has determined to be a change in a signal [4]. Drew’s Psychophysical Example I have presented many examples in explaining the meaning of physical and psycho properties as well as in the explanations of Weber’s law and the three classical methods. Here I present a mock study one may propose utilizing one of the psychophysical methods to display my understanding. Let us say we want to study the threshold of “warmth”. The physical characteristic of warmth can be determined by a true measurement such as degrees in F/C. Let us find the threshold for when users determine a cup of liquid starts to feel ‘hot’. The percept is “warmth”. We will use the Method of Constant Stimuli for one important reason: safety. We will present participants with varying levels of hot cups that may feel harmful if touched for too long. Participants in the Constant Stimuli study will be able to quickly pull their hand away if a cup feels too warm. We will suppose that the threshold for hot and cold will occur at around the average temperature of human skin. We estimate that to be around 85F. We will present 20 different temperatures to the person ranging from 85- 10 5F for we assume ‘hot’ will occur above skin temperature based on scientific principles of heat conduction. We will present each temperature randomly throughout the study a total of 10 times for a total of 200 trials. Users will report Yes/No to the cup being ‘hot’. In our analysis we will use a similar graph and the 50% threshold to determine the threshold for a human to perceive a cup as being ‘hot’. We will be able to find a threshold such as “the threshold for a liquid to be perceived as ‘hot’ is around X degrees F.” Applications of psychophysical methods Optical Illusions Some more examples of how these psychophysical methods can be employed is in the research of some interesting optical illusions. Optical or visual illusions are those where the perceived image differs from objective reality. The three main types of illusions can be categorized as literal, physiological and cognitive. One example is determining the threshold for perceptions of humans in being able to determine if two lines are longer (between the two areas) when presented images such as:

[4] Classical Psychophysical Methods. http://ucalgary.ca/pip369/mod1/measuring/classical. Date Visited: 3/29/16. [5] Optical Illusions. APA. http://opl.apa.org/contributions/EC/Nill.htm. Date Visited: 3/25/16.

HCC Research Methods Psychophysical Methods: Psychophysical Methods, Lecture notes of Psychology

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Lecture Mar 22: Psychophysical Methods

Classic Psychophysical Methods