Page 26 - 2016Spring

Evolution of Mammalian Sound Localization
Sound-Localization Acuity
is Related to Vision
Accumulating data forced us to reject many possible expla- nations for the variation in sound-localization acuity, such as whether an animal was large or small, a predator or prey, diurnal or nocturnal, or had large or small binocular visual fields. We then discovered that sound-localization acuity was closely correlated with the size of an animal’s field of best vision. Specifically, the narrower an animal’s field of best vi- sion, the better its sound-localization acuity.
This observation led to the proposal that the primary func- tion of sound localization is to direct the eyes to the source of a sound (Heffner and Heffner, 1992). Thus, animals with narrow fields of best vision, such as humans and cats, require good sound-localization acuity to direct their best vision to scrutinize sound sources, whereas animals such as gerbils and cattle that have broad fields, known as visual streaks, do not require such acuity because most objects on the horizon are already within their field of best vision.
For comparative purposes, the width of an animal’s field of best vision is estimated from microscopic analysis of its ret- ina in which the density of retinal ganglion cells is mapped (or, in the case of primates that possess a fovea, the density of the receptors). The region of best acuity is then defined as the width of the horizontal visual field, in degrees, that subtends the portion of the retina containing ganglion cell densities greater than or equal to 75% of the maximum den- sity. Examples of retinal maps are shown in Figure 4.
Figure 4. Retinal ganglion cell isodensity contours for the cat, Nor- way rat, and cattle. The region of best vision is defined as the 75% isodensity contour. Note the small area of best vision for the cat com- pared with the broad visual streak for cattle. The X indicates the lo- cation of the blind spot. Modified from Heffner and Heffner (1992).
The relationship between the width of the field of best vi- sion and sound-localization acuity, shown in Figure 5, il- lustrates that mammals with narrow fields of best vision are more accurate localizers than mammals with broader fields (r = 0.89). Note that the definition of the field of best vi-
sion does not depend on absolute acuity, just the best acuity available. Indeed, correlational analysis indicates that, of the two factors, only the width of the field of best vision, and not absolute visual acuity, is related to sound-localization acuity (Heffner and Heffner, 1992). In short, it appears that the pri- mary function of sound localization in mammals is to direct the field of an animal’s best vision to the source of a sound for further analysis.
Figure 5. Relationship between the width of the field of best vision (75% isodensity contour) and sound-localization thresholds for 22 species of mammals. Species with narrow fields of best vision have smaller thresholds than those with larger fields of best vision. Modi- fied from Heffner et al. (2001).
Some Mammals Do Not Use
All Three Locus Cues
As previously noted, mammals have three sound-localiza- tion cues available to them: binaural time difference, bin- aural intensity difference, and monaural pinna cues. With careful testing, it is possible to demonstrate the ability of an animal to use each of these cues. Although initially surpris- ing, results reveal that some species do not use all three cues.
Binaural Time and Intensity Difference Cues
The ability to use the binaural locus cues can be demon- strated by determining the ability of an animal to perform a left-right locus discrimination for pure tones presented from loudspeakers located in front of the animal at a fixed angle of separation, typically ±30°. This is because low-frequency pure tones must be localized using the binaural time-differ- ence cue, whereas high frequencies must be localized using the binaural intensity-difference cue.
24 | Acoustics Today | Spring 2016