Introduction to Psychophysics

 

Although you may have never heard of it before psychophysics is a well-established field of study more than 150 years old which makes psychophysics as one of the oldest areas of psychological science that is still a highly active area of contemporary research. So, psychophysics is a quantitative discipline, mapping psychology upon physics. This is a field of science relating perceptual responses of an organism such as a human or animal to visual, auditory, tactile, or chemical stimuli, categorized as physics. Sometimes it refers to the collection of methodology to study sensation and perception in general ranging from designs of experiments to analysis of data and formulating mathematical models.

Many of the classical techniques and theories of psychophysics were formulated by these two founding figures in the field. Ernst Weber who was a German physiologist who championed the first psychophysical law named after him. Inspired by Weber, another contemporary German physicist Gustav Fechner used the term psychophysics to describe the research relating physical stimuli to human sensation and perception. He also developed the classical methods to measure the relationship between the two. According to Fechnerian psychophysical world, there are three components that are related to each other first there are external stimuli being perceived by the observer which is categorized as physics. So here the external stimulus is a ball reflecting the red color from the visible spectrum of light then the reflected light hits the eyes then the light energy is converted to a neuronal signal internally, mediating the psychophysical relationship between the physics and psyche, which is the physiology component. And the physiological response invokes the sensation of the stimuli and perceptual impression is made upon them by the psyche component.

From this relationship, Fechner thought that there are two ways to study the relationship between these three components, namely inner and outer psychophysics. The outer psychophysics deals with the relationship between a stimulus and perception which is the psychophysics as we know of today. On the other hand, the inner psychophysics deals with the relationship between the neural events and perception invoked by the physics. So, Fechner himself realized that in his day he could only do what he called outer psychophysics because of the limited technology at his time and Fechner anticipated a later science of inner psychophysics with the technological advances. As he predicted, technology has been advanced so fast that people can now practice inner psychophysics. Today we can at least indirectly find the shared brand region or activity when they are going through the same perceptual experience. For example, we can use the state of our technology such as an fMRI functional magnetic resonance imaging to correlate our perception to some brain activity without having to open up our skull and stab electrodes into our head. Here the logic is to measure the brain activity when the patient is shown a color patch to see what relationship there is to the perception of a specific color. So, if we can find the common brain region or activity with respect to a specific color across people then we can be more confident in that relationship between the brain region and the perceptual experience. In fact, we can now directly measure the neural activity to literally open up the skull and stab electrodes into the brain like in this picture. So, this kind of recording has been only allowed in the field of physiology and neuroscience using animals. However, this kind of technique is now also available to humans only for medical purposes. 

So, combining both inner and outer psychophysics as Fechner envisioned is a highly interdisciplinary field of study however in this part of the visual neuroscience, we will talk about only the outer psychophysics which is the psychophysics in more classical and narrow sense. Historically it has been a long tradition of physiology to identify the internal structures by breaking the system open and dissect it into smaller pieces. However, people soon realized that it might not be the best way to understand the system that way. In fact, it was also recognized that behaviors displayed at the systems level may not be apparent at the level of individual components that make up the system. Moreover, it may be too complicated to describe the behavior displayed at the systems level by considering the individual components in isolation.

 In contrast psychophysicists study the overall performance of a human visual system by examining the input-output relationship. In a sense, a psychophysical approach to visual perception is similar to a branch of science called systems science. In this approach people are more interested in what a system does than what it's made of. Engineers now realize that for such a complex system as visual systems, simple summation of individual parts is not adequate to explain the function of the visual system as a whole. So, the major goal of the systems analysis is to come up with a mathematical model of a system to predict an output when it is given a specific input. So, from this line of thinking, psychophysical approaches may treat the visual system as a black box. In a typical psychophysical experiment, a precisely controlled input is presented to the system and the output is measured from the subject's response. So, from this input and output relationship the functional characteristics of the system can be determined without much consideration of what's actually inside the visual system to process the input. With an appropriate and careful appraisal of the input and output the performance of human visual system can be understood with relatively straightforward mathematics and functionally plausible components.