Ecological Validity, External Validity, and Mundane Realism in Hearing Science : Ear and Hearing

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Point of View

Ecological Validity, External Validity, and Mundane Realism in Hearing Science

Beechey, Timothy

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Ear and Hearing 43(5):p 1395-1401, September/October 2022. | DOI: 10.1097/AUD.0000000000001202
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Abstract

Editor’s Note: The following article is categorized as a “Point of View” article. As originally described (Jerger 2000): “Our second type of new publication, the Point of View article, is a publication with a slant or opinion. This type of article should have a fresh point of view, a clear logic to its propositions, and a clarity of presentation. The article must have a well-reasoned point of view, but the view does not have to be balanced. Our long-term goal for the Point of View article is to stimulate the field’s interest in and to enhance the appreciation of the author’s area of expertise.”

INTRODUCTION

Like researchers in related experimental fields such as psychology and vision science, hearing scientists are concerned with the extent to which our experimental findings may generalize beyond the immediate experimental conditions in which data are collected. Recognition of discrepancies between laboratory results and self-reported hearing ability and hearing device benefit and satisfaction (see Cord et al. 2004; Cox et al. 2016; Pronk et al. 2018; Souza et al. 2000; Working Group on Speech Understanding and Aging 1988; Wu et al. 2019, for examples) has led many researchers to develop new experimental methods which incorporate aspects thought to be similar to conditions encountered by people outside the laboratory. The term ecological validity has come into frequent usage in hearing science, and many other fields (see Holleman et al. 2020, for a recent review), to refer to experimental methods aimed at ensuring similarity to the “real-world”, and to characterize results obtained using such methods. A recent special issue of this journal devoted to ecological validity (Keidser & Naylor 2020) exemplifies this approach, with articles describing experimental methods and variables, which should be considered when designing hearing experiments with the aim of maximizing generalizability of results beyond the laboratory (see Keidser et al. 2020 for a review).

Use of the term ecological validity to refer to similarity to the real world in fields such as psychology has received criticism both recently and in previous years (Holleman et al. 2020; Araújo et al. 2007; Hammond 2001; Hammond & Stewart 2001; Hammond, 1998). Much of this criticism within the psychology literature has focused on researchers’ failure to clearly define what is meant by either “ecological validity” or the “real world”. For example, Holleman et al. (2020) argue that labeling methods or findings as either high or low in ecological validity is often based on the subjective judgment and specific research interests of individual researchers, since clearly defined metrics are rarely, if ever, provided.

In the context of existing criticism within psychology, Keisder et al. (2020) provide an admirable level of detail and specificity in their definition of ecological validity and the broader framework they propose around realistic experimental methods. This paper will argue, however, that issues remain in the usage of the term ecological validity within hearing science. This article seeks to consider these issues in the specific context of hearing science, but this should not be taken to imply that the issues discussed are unique to this field.

In attempting to improve the generalizability of experimental findings, it is important to remember that realism is not the same as generalizability, and to note that changes in generalizability resulting from using realistic experimental methods are rarely verified. Confusing these two concepts can lead to a view that realism is the end goal in itself. That generalization must be our goal and that realism merely serves this goal can be seen if one considers whether one would choose an expensive and difficult to implement realistic experimental method over a simpler less realistic method if both methods were shown to provide equal generalizability. However, the use of the term ecological validity within hearing science appears to conflate realism, which is a means to potentially improve generalizability, and validity which is a method of quantifying generalization.

These distinct concepts are referred to in the psychology literature by two separate terms: mundane realism which is the degree to which experimental conditions are similar to everyday conditions, and external validity which is the degree to which findings of a laboratory experiment would correlate with findings that would be obtained were a comparable task to be administered outside the laboratory. The term ecological validity, meanwhile, refers to an entirely different concept which is, nevertheless, highly relevant to hearing science: the reliability of cues received at the sense organs in terms of the correlation between those cues, and objects or events in the environment (see Table 1 for a summary of terms). The confusion of these terms is not simply a matter of disagreement over the precise wording of a definition; it may reduce our ability to benefit from, and contribute to, cumulative science (Araújo, Davids, & Passos, 2007; Hammond 1998). I hope to show that efforts to increase the generalizability of hearing research can be further advanced by a familiarity with the concept of ecological validity, as originally defined by Brunswik (1952,1956a).

TABLE 1. - Summary of terms
Term Description
Mundane realism Qualitative similarity between experimental conditions and everyday conditions experienced by the subject.
Psychological realism The extent to which psychological states or processes elicited by an experimental task are similar to those that occur during comparable tasks outside the laboratory.
External validity The extent to which results obtained in one context can be generalized to another context.
Ecological validity The trustworthiness or ambiguity of a cue available to a perceiver in relation to some object or event in the environment.
Functional validity (achievement) Overall accuracy of perception in terms of the correlation between a percept and an object or event in the environment.
Representative design A framework for the design of experiments which incorporates sampling perceptual targets from the environment.
Ecological perception An organism’s perception of information about objects and events in the physical environment.

This article will first describe the problematic nature of recent usage of the term ecological validity within hearing science in terms of a lack of distinction between realism and generalization. It will be argued that the term “ecological validity” as it is currently used should be replaced by the separate existing terms mundane realism and external validity. This is followed by a summary of the concept of ecological validity as proposed by Brunswik (1952,1956a). The potential usefulness for hearing science of Brunswik’s ecological validity will be described, and it will be argued that we should guard against the loss of this concept through redefinition. The following section then outlines how the theory of Representative Design, within which ecological validity is foundational, may contribute to our efforts to improve the generalizability of hearing experiments and encourage greater specificity regarding what aspects of real-world conditions and behaviors hearing experiments seek to represent. The final section contrasts two different uses of the term ecological validity within hearing science in order to illustrate how the current usage of this term may be problematic and to point to the usefulness of Brunswik’s concept of ecological validity.

EXTERNAL VALIDITY, MUNDANE REALISM, AND PSYCHOLOGICAL REALISM

Measures of validity are traditionally defined in terms of correlations between quantities of interest, such as test results (Campbell 1959). External validity can be defined as the correlation between a test result obtained in a laboratory setting and performance on an equivalent task in some other context outside the laboratory. External validity encapsulates the general concept expressed by a recent definition of ecological validity provided by (Keidser et al. 2020, p. 7S).

“In hearing science, ecological validity refers to the degree to which research findings reflect real-life hearing-related function, activity, or participation.”

If external validity captures the meaning being attributed to ecological validity in recent usage within hearing science, the latter term should not be used for this purpose because, as described below, it was originally defined, and continues to be used by researchers in related fields, to refer to a fundamentally different and useful concept.

To the extent that the definition of ecological validity assumed within hearing science is more specific and therefore in need of a term other than external validity, it is in terms of a concern with the use of realistic experimental methods to achieve generalizability of experimental findings. That is, the use of the term ecological validity in hearing science tends to imply a measure of similarity between real-world and experimental conditions in addition to generalizability of findings. For example, Keidser et al. (2020, p. 7S) note that the development of realistic methods within hearing science has generally been motivated by a concern that:

“… many of the established methodologies used to assess hearing and hearing devices lack sufficient realism to produce adequately meaningful findings about a person’s hearing function, activity, or participation in real life.”

This usage of the term ecological validity conflates what is a likely pre-condition for generalization: realism, naturalness, or similarity to the environment; with a measure of generalization: validity or correlation between two measured quantities. But these are two different aspects of science. To the extent that our methods incorporate aspects of realism, we may assume our findings will be generalizable outside the laboratory. But the method of measuring degree of generalization in the form of correlation is unrelated to the method of facilitating generalization through realistic experimental conditions. The former is a mathematical comparison between results obtained in the laboratory and results that would be obtained from comparable real-life tasks in some everyday settings; the latter is a qualitative comparison between the characteristics of an experimental condition and the characteristics of some specific situations outside the laboratory. In order to avoid the conflation of these concepts, we should use two different terms to represent them. Such terms are already in common use: external validity as described above, and mundane realism, described below.

Mundane realism originated in the field of psychology and is used to describe experimental methods that are thought to reflect real-life or everyday conditions. The concept of mundane realism, coined by Aronson and Carlsmith (1968), is defined by The Encyclopedia of Social Psychology as:

“the degree to which the materials and procedures involved in an experiment are similar to events that occur in the real world. (Kelly 2007, p. 599)”

This term is used across various fields, including psychology (e.g., Blascovich et al. 2002), medicine (e.g., Persky 2011), and hearing science (e.g., Mansour et al. 2019). Referring separately to mundane realism and external validity can help to avoid conflating methods with outcomes, a form of circular reasoning.

While realistic experimental methods may in fact improve generalizability of findings of hearing experiments, we should be cautious about using the realism of our methods as a type of proxy measure of generalization. It is important to recognize that there is no single target realism: there are infinite possible realistic scenarios. In this sense, realism is impossible to generalize. A better goal may be to create experimental conditions, which are representative of conditions and task demands experienced by people in their daily lives. This more targeted goal is captured by the concept of psychological realism: the extent to which psychological states or processes elicited or required by experimental conditions reflect those that occur in everyday life (Wilson et al. 2010). As Goldrick (2011) points out, the concern of psychological realism is functional, rather than algorithmic or neural. The psychological reality of an experiment is the degree to which it provides an accurate reflection of an individual’s functional capacity. In the context of a perceptual field such as hearing science, we may ask whether the perceptual demands imposed by an experimental task match those required for perception outside the laboratory. Where an experiment has high psychological realism, we can expect that performance measured during that experiment should closely resemble performance outside the laboratory.

ECOLOGICAL VALIDITY AND THE LENS MODEL

Ecological theories of perception are concerned, in general, with how organisms are able to perceive objects or events in their environment (see Gibson 1966,2015, for examples). Brunswik (1952,1956a) describes an ecological theory of perception, named Probabilistic Functionalism, based on the premise that perceptual cues received by our sense organs provide probabilistic rather than deterministic information about the environment. Proximal cues received by our senses are uncertain. As a result, the environment is uncertain for the perceiver. For example, if a person walking in a forest hears a sound, the pressure wave that stimulates the person’s auditory system constitutes proximal cues, such as frequency and intensity, which may ultimately evoke a percept of an animal’s movement—an event in the environment at some distance from the perceiver. The person perceives the distant event by means of physical stimulation at the peripheral nervous system. But the sound may have been caused by an animal, the wind, or a falling branch—the relationship between the sound and its cause in the environment is not completely certain—there is only a probabilistic link between cues and source. Were the cues received by the perceiver to be deterministic, they would uniquely identify an object or event with complete certainty. To adapt to and survive in an uncertain environment, organisms must develop mechanisms to successfully perceive the environment despite this uncertainty. The Lens Model (Brunswik 1952,1956a), of which ecological validity is a component (see Figure 1), was developed as an explanation of how organisms achieve the goal of perceiving objects or events in the environment through optimal utilization of multiple uncertain proximal cues.

F1
Fig. 1.:
Ecological validity within the Lens model, adapted from Brunswik (1956b). D indicates a distal object or event in the environment, P represents a proximal cue received at a perceiver’s sense organs, C is a unitary central percept that is formed by utilizing proximal cues, and r is the Pearson correlation between each proximal cue and distal object or event. For clarity, some aspects of the Lens Model including inter-cue correlations and functional validity are not shown.

Organisms are assumed, within the Lens Model, to have knowledge of the trustworthiness of proximal cues, gained either through direct experience (e.g., Chasseigne et al. 1997) or through evolutionary adaptation. The certainty or ambiguity of a proximal cue (Pi) in relation to a distal object or event in the environment (D) can be calculated as the Pearson correlation between that cue and the object or event in the environment, rPiD. This results in a correlation coefficient between 0 and 1. The degree of the correlation between a cue and a distal object or event in the environment is that cue’s ecological validity (Brunswik 1956a, p. 30). For example, Brunswik (1956b) describes the calculation of ecological validities of perceptual depth cues, such as vertical position and brightness, in the form of correlation coefficients between those cues and the objective distances to various objects in the environment.

For any object or event, there are multiple cues, each with their own ecological validity. In what Brunswik (1956b) refers to as the utilization phase of perception, perceivers weight the various cues available to them based on their ecological validities, resulting in a single percept. Where the reliability of particular cues is reduced because of environmental conditions or sensory impairment, a perceiver will alter the extent to which they utilize different cues (Souza et al. 2018). This, then, is a theory of how perceivers efficiently use redundant cues. It is in this sense that Bregman (1994) refers to the heuristic use of multiple auditory cues with “partial validity” (p. 314) to determine the probable origin of a percept in the environment; and Turgeon and Bregman (2001) refer to the ecological validity of auditory grouping cues in terms of the relative importance of multiple interacting and competing cues in real-world situations. The use of redundant cues in accordance with their ecological validities means that accuracy of perception, referred to within the Lens Model as functional validity or achievement, may be higher than the ecological validity of any one proximal cue available to the perceiver (Brunswik 1943).

Ecological validity is closely related to the concept of precision, the inverse of variance, as it is used in Bayesian theories of cognition (e.g., Knill & Pouget 2004). For example, Clark (2013a,2013b) hypothesizes that perceivers prioritize the relative use of top-down and bottom-up cues on the basis of the precision of bottom-up cues received via sensory input. When cues received by the senses are judged to have low precision, or low ecological validity, the perceiver will rely more heavily on top-down predictive processing. Ecological validity and the Lens Model are also the main difference between Brunswik’s approach to ecological perception and that of James Gibson (Gibson 1966,2015). Gibson’s theory of ecological perception holds that perception of events or objects in the environment is deterministic rather than probabilistic because organisms directly perceive higher-order information (see Rosenblum 2004, for an auditory example). However, there have been recent efforts to adapt the Lens model to incorporate Gibson’s approach to ecological perception (Vicente 2003).

From the above description, it can be seen that ecological validity is not a property of an experiment nor of experimental findings. Rather, ecological validity is a quantification of the degree to which a perceiver may rely on a probabilistic cue, and as such enables efficient use of probabilistic information. It is also important to emphasize that ecological validity is not a measure of realism or naturalness. Proximal cues which are available to perceivers in their natural environment can, and often do have low ecological validity (Brunswik 1943). For example, segmental cues such as formant transitions in running speech are highly uncertain or ambiguous cues to consonant identity, as indicated by experiments which have considered the intelligibility of small, excised portions of running speech (e.g., Ernestus et al. 2002; Pickett & Pollack 1963). Part of the challenge of perception faced by an organism is that the environment supplies cues with limited ecological validity (Brunswik 1952). We should not be striving to achieve high ecological validity. Rather, we should aim to design experiments, which involve the presentation of stimuli consisting of cues which are realistically uncertain or ambiguous, given a specific environment. This is the approach suggested by the concept of psychological realism. It should be noted that psychological realism and mundane realism are not mutually exclusive. Psychological realism can be seen as one aspect of the more general approach of mundane realism.

REPRESENTATIVE DESIGN

Ecological validity in its original definition intersects with the goal of increasing the generalizability of experimental results in the concept of representative design (Brunswik 1956a). While the Lens Model is a theory of how perceivers use uncertain cues on the basis of ecological validities, representative design provides one method for incorporating cues with realistic ecological validities into experimental methods.

Experimenters have traditionally been concerned with representative sampling of research subjects from the population as a method of generalizing findings. Representative design adds to this a requirement to sample the objects of perception from subjects’ environments (Brunswik 1943; Hammond 1948). By sampling objects from the environment, the stimuli used in an experiment will be characterized by cues with realistic levels of uncertainty; that is, realistic ecological validities. Sampling perceptual objects in this manner may have the result of increasing mundane realism but, crucially, the aim is not to find and use stimuli characterized by high ecological validities, but rather to represent the probabilistic, uncertain nature of cues available to perceivers in their environment. In this way, it can be ensured that the redundancy of cues will be representative, and that participants’ perceptual processing will be representative in terms of the utilization of multiple cues based on their ecological validities.

Representative design is defined in opposition to what Brunswik (1956b) referred to as systematic design, which seeks to isolate variables of interest from any other sources of potential influence (Hammond 1954). While systematic design is responsible for many valuable insights, representative design and ecological validity provide a useful way of understanding the shortcomings which nevertheless may be attributed to this type of reductionist approach. Comparison between systematic and representative design can inform the general goal of developing experimental methods, which maximize generalizability. In a systematic experimental design, a given cue type which is of interest to an experimenter may be presented in the absence of other cues that would naturally accompany it, and in a manner which is distinctive enough to ensure that the cue is sufficient for subjects to reliably perceive a contrast of interest. But, this is likely to place undue importance on the cue type in question if, in natural contexts, such a cue type has low ecological validity and co-occurs with other cues to the same distal object or event. That is, experimental stimuli may be unrepresentative of signals ordinarily encountered by hearers because they are constructed in a way that creates cues with artificially high ecological validities. Taking again the example of segmental cues, an experiment may be designed to study perception of consonant place of articulation by presenting CV syllables. If CV syllables are produced in isolation, formant transitions are strong cues to consonant place (Sharf & Hemeyer 1972). But the usefulness of formant transitions as cues to consonant place in these relatively artificial stimuli is much higher than in running speech (Warner & Tucker 2011). The successful perception of running speech is based on the utilization of additional cues available in the speech signal since formant transitions have low ecological validity (Tucker & Ernestus, 2016).

Brunswik’s theory of representative design has received criticism, particularly from the perspective that implementing experimental methods where objects of perception are sampled from the environment is time-consuming and expensive (see Dhami et al. 2004, for a review). However, in many ways, the ideas contained within the theory of representative design have long been embraced within hearing science where it is common to measure perception across multiple environmental conditions. This is increasingly the case in terms of the use of virtual environments (e.g., Hohmann et al. 2020). Perhaps the clearest example of the compatibility of existing methods within hearing science with Brunswik’s representative design is the use of ecological momentary assessment (EMA). In EMA hearing research, participants are prompted to provide subjective ratings related to auditory perception at quasi-random intervals in an effort to assess hearing demands or performance in a representative sampling of actual situations encountered in an individual’s daily life. While not typically recognized, ecological validity and representative design are among the fundamental bases of EMA (see Stone & Shiffman 2010, for discussion), which is an increasingly important method in hearing science for generalizing findings beyond the laboratory. Brunswik (1943, p. 264), for example, describes a study of perceptual size constancy, which is reminiscent of EMA:

“Purposely, one subject only was used. The person was interrupted frequently during her normal daily activities and asked to estimate the size of the object she just happened to be looking at.”

As this quote illustrates, the emphasis in representative design is to measure perception for individuals across many scenarios, preferably sampled from everyday life, closely matching the aims of EMA research.

CONTRASTING USES OF ECOLOGICAL VALIDITY IN THE STUDY OF PERCEPTION OF TALKER STATES AND CHARACTERISTICS

One area of hearing science research where the term ecological validity has been used in very different ways is in studies of the perception and processing of speech prosody and voice quality relevant to the recognition of talker state, such as emotion or fatigue, and talker characteristics, such as age, sex, or personality.

Scherer (2003) reviewed studies of emotional speech and analyzed the ecological validity of several cues in relation to true emotional states of talkers including fundamental frequency (F0), formant frequencies and bandwidths, intensity, frequency spectrum and speech rate, stating that “Using Brunswik’s terminology, one can call the degree to which such characteristics actually correlate with underlying speaker state ecological validity [original emphasis]” (p. 229). Similarly, Scherer (1978) investigated the ecological validity of voice quality cues to talker personality by calculating the correlation between phonetitians’ classification of voice quality and peer classifications of 24 talkers’ personalities, resulting in correlation coefficients between nine voice qualities and five personality types. This work provides valuable understanding of which acoustic cues may be most important for listeners in attributing states and characteristics to talkers and could form the basis of a better understanding of how hearing impairment may impede understanding beyond word or sentence intelligibility and how different acoustic cues should be presented in experimental studies in order to represent the perceptual demands likely to be encountered by listeners. By investigating perceivers’ use of auditory cues and the relationship of specific cues to events in the environment (talkers’ emotions or personalities), these studies address psychological realism rather than the more general issue of mundane realism.

The use of ecological validity in Scherer (1978,2003) can be contrasted with the use of the same terminology in studies with similar aims of investigating perception of emotional speech. For example, Robins et al. (2009) report an imaging study of audiovisual perception of emotion cues using videos of professional actors producing sentences with intonation and facial expressions corresponding to anger, fear, and happiness, in addition to a neutral condition. Robins et al. (2009) use the term “ecologically valid” comparatively when describing the stimuli which were presented to participants as more natural than static images with accompanying sound. Further, the authors argue that such dynamic stimuli are more representative of “real-world social interactions” (p. 270) and that this increases the possibility of generalizing experimental findings beyond the laboratory.

When referring to ecological validity, Robins et al. (2009) appear to be pointing to the level of similarity of experimental stimuli to those likely to be encountered outside the laboratory. However, it is very hard to judge with any certainty how the results of this study should be considered in relation to results of studies using less natural stimuli because the authors do not go beyond a broad, qualitative comparison with previous methods. It is undoubtedly true that videos are more realistic stimuli that static photographs. However, the comparison offered by Robins et al. (2009) does not provide any information about how representative the experimental task was of perceptual tasks outside the laboratory, or which particular real-world situations the experimental findings might be generalized to. In particular, the authors do not consider whether the emotional cues contained in the stimuli are similar in strength or type to those actually relied on by perceivers, especially given the small, closed set of emotions and the forced-choice task. As Bänziger and Scherer (2005) have argued, when completing closed set emotion perception tasks, participants may rely on discrimination rather than recognition and are likely to show artificially high performance.

HAS BRUNSWIK’S THEORY EVOLVED?

In considering original definitions found in a theory that first appeared in the 1950s, it is reasonable to question whether such early definitions are still appropriate or whether they have evolved over decades of use by other researchers. In this article, I have attempted to summarize Brunswik’s definition of ecological validity within the Lens Model, but this should not be taken to imply that Brunswik’s definition must be definitive and could not be improved on by other researchers. For example, in attempting to reconcile Brunswik’s approach to ecological perception with that of Gibson (1966,2015), Vicente (2003) modified the Lens Model to account for perception of higher order information which is central to Gibson’s theory of ecological perception.

CONCLUSION

Achieving greater generalizability of experimental findings within hearing science is an important goal. For this reason, it is also important to differentiate a means of achieving this goal—realism—with a method of measuring the extent to which results actually generalize to other contexts—validity. By combining these ideas, it becomes too easy to assume that making experimental methods more realistic is the end goal itself. Such conflation is made more likely by the use of the term ecological validity to refer to both similarity of experimental conditions to realistic conditions outside the laboratory, and generalization of laboratory results to those more natural conditions. Fortunately, there are separate, established terms for these concepts, mundane realism and external validity, to help us avoid this confusion. The use of mundane realism and external validity to refer to these concepts is also highly desirable to avoid redefining an existing concept that has been referred to as ecological validity for several decades. Ecological validity, as originally defined, is a useful concept for hearing science. Where we collectively forget useful ideas, we will eventually need to reinvent them.

The current focus on realism rather than generalizability may stem from the fact that we do not have good tools for easily determining how well experimental findings actually generalize. However, given the lack of tools for measuring generalization, we should be careful about focusing too heavily on realism before we can determine what increased realism buys us in terms of improved generalizability.

ACKNOWLEDGMENTS

The author wishes to thank Adam Weisser, Peggy Nelson and Jörg Buchholz for feedback on an earlier draft of this paper.

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Keywords:

Ecological perception; Ecological psychology; Ecological validity; External validity; Hearing; Mundane realism; Realism; Representative design; Speech perception

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