Hearing impairments hidden in normal listeners.

E arly and accurate diagnosis of sensory deficits is a public health concern with major scientific and financial implications. Several fundamental requirements conspire to make this goal a difficult one to achieve; among them is the necessity for welldesigned quick tests that can pinpoint the nature of the deficits. Another is the need for a deep understanding of the functional organization of the sensory system and the cues mediating perception. To appreciate how these requirements shape our current diagnostic paradigms, consider the striking difference between the procedures routinely used to screen children and young adults for visual and auditory impairments. In vision, even the simplest eye examinations test the ability to identify the shapes of letters or the direction of arrows, and hence the acuity of the visual system in encoding the meaningful structure of the visual scene. By contrast, hearing tests are often limited to measuring audiometric thresholds that reveal the just-audible tone intensity as a function of frequency. The relevance of these hearing tests is analogous in vision to asking subjects to indicate the presence or absence of a spot of light falling on different retinal locations. It is therefore understandable why the utility of basic auditory screening procedures is limited to indications of “hard of hearing,” failing in most cases to discover a heterogeneity of hearing disabilities in a population of listeners or to provide an indication of hearing impairments that can benefit from early intervention. Auditory scientists are of course well aware of these limitations and have devised a variety of alternative tests to probe deeper into auditory function. These efforts, however, are hampered by the lack of an agreed-upon set of basic auditory physiological and perceptual cues whose measurements reliably reflect hearing ability and fidelity. Thus, despite enormous progress, we remain largely uncertain about the neurobiological mechanisms underlying perception of basic attributes of sound, such as pitch (1, 2), localization (3, 4), and timbre (5, 6). One important source of this difficulty stems from a fundamental duality of spectral and temporal cues in early auditory processing that has bedeviled auditory research for more than a century (7). The essence of this controversy is illustrated in Fig. 1. It stems from the duality of cochlear representations (Fig. 1A) and the way these cues become intermingled in the midbrain as they contribute jointly in mediating various auditory perceptual attributes (Fig. 1B). The study by Ruggles et al. in PNAS (8) elegantly demonstrates how much can be gained from a careful consideration of auditory principles in diagnosing hearing impairments, both at a scientific level and as a public health goal. Having selected a group of “normal” subjects on the basis of widely used audiometric threshold criteria, the authors design a series of tests with suprathreshold sounds targeted to detect evidence of “temporal dysfunction,” specifically the inability to encode properly the temporal cues thought to be critical for performing well on these tests. The first test is a speech segregation task, typical of scenarios commonly referred to as the “cocktail party problem” A