Bruce Masterton

The Evolution of Human Hearing

Five descriptive parameters of hearing—high‐frequency and low‐frequency sensitivity, lowest threshold. best frequency, and area of the audible field—are compared statistically, first, among mammals in general, and then, among seven animals selected to approximate a phylogenetic sequence of mans ancestors. Three potentially explanatory parameters body size, maximum binaural time disparity, and recency of common ancestry with man—are also explicitly included in the analysis. The results show that: high‐frequency hearing (above 32 kHz) is a characteristic unique to mammals, and, among members of this class, one which is commonplace and primitive. Being highly correlated with functionally close‐set ears, it is probably the result of selective pressure for accurate sound localization. Low‐frequency hearing improved markedly in mankinds line of descent, but the kind and degree of improvement are not unique among mammalian lineages. High sensitivity developed in the earliest stages of mans lineage and has remained relatively unchanged since the simian level. The frequency of the lowest threshold has declined in Mans lineage—the greatest drop probably occurring during the Eocene. The total area of the audible field increased until the Eocene and has decreased since then.

Behavioral Measurements of Absolute and Frequency‐Difference Thresholds in Guinea Pig

Absolute and frequency‐difference thresholds were determined by the conditioned‐suppression technique. The results show that the average frequency range of audibility at +50 dB sound‐pressure level extends from 86 Hz to 46.5 kHz, with a best frequency near 8 kHz. Individual differences in sensitivity are related to body weight and, probably, age. The average frequency‐difference limen is 3.5% from 125 Hz to 42 kHz. Compared to other mammals, the auditory capacities of guinea pig are within one standard deviation of the mammalian mean on each of six dimensions: high‐frequency and low‐frequency cutoff, lowest intensity, best frequency, area of the audible field, and frequency discrimination.

Variation in Form of the Pyramidal Tract and Its Relationship to Digital Dexterity; pp. 175–187

A morphometric analysis of the pyramidal tracts relation to digital dexterity was performed on data from 69 mammals. The results show that the variation in digital dexterity among mammals corresponds most closely to the variation in place of termination of pyramidal tract fibers within the spinal cord, corresponds less closely to the variation in the size of the tract itself and its constituent fibers, and does not correspond reliably with any other feature yet reported. Since the termination of pyramidal tract fibers on or very near spinal motor neurons is a prerequisite even for the peculiar kind of dexterity seen in some non-primates (e.g., raccoon, kinkajou), this one feature alone seems to be a critical factor.

Features of the auditory development of the short-tailed Brazilian opossum, Monodelphis domestica: evoked responses, neonatal vocalizations and synapses in the inferior colliculus

The onset of hearing in anesthetized South American opossums (Monodelphis domestica) was determined by the measurement of evoked potentials to click stimuli from the vertex of the skull immediately over the inferior colliculus. Evoked potentials were first recorded at postnatal day 24 at a threshold of 83 dB SPL; thresholds declined over subsequent weeks to below 58 dB at 40 days. Isolation calls emitted by the pups had stereotypic spectra with peaks at near 13 kHz and an octave higher. Such calls declined in frequency by day 32 and were not emitted at day 40. The peak frequency of the calls matched very closely the best frequency of hearing of adult Monodelphis. The number of synapses in the inferior colliculus increased at day 26; when plotted in relation to the number of cells, synaptic density increased steeply from day 27 after the animal had begun to hear. This suggests that environmental sound has a potent effect on the development of synapses in the auditory system.

An automated sound-localization chamber

A completely automated free-field sound-localization apparatus is described along with a behavioral technique that provides several advantages over previously available methods. With the apparatus, the ability to localize the source of a sound has been tested in a wide variety of mammals including cats, rats, squirrel monkeys, hedgehogs, and tree shews.

Origins of Anthropoid Intelligence; pp. 337–353

The development of the extrastriate visual system relative to the striate system was estimated indirectly by measuring the volumes of the lateral posterior-pulvinar complex and lateral geniculate nucl

Hearing in Two Species of Wild Rodents

As part of a systematic study of the auditory capacities of mammals, behavioral audiograms were determined for two species of wild rodents: the cotton rat (Sigmodon hispidus) and the house mouse (Mus musculus). The method of conditioned suppression was used to assess thresholds for frequencies ranging from 0.5 to 91 kHz. Comparisons of the two audiograms with each other and with an updated sample of those of other mammals yield support for two previously discovered (inverse) relationships between high‐ and low‐frequency hearing and between high‐frequency hearing and interaural distance (maximum Δt). However, a partial correlation in which high‐frequency hearing is held constant reveals that low‐frequency hearing is not related to maximum Δt. The updated sample of mammalian audiograms now provides firmer ground for concluding what does and what does not constitute a bizarre auditory adaptation. For example, bats are neither unique nor bizarre in their ability to hear extremely high frequencies nor in their...

The habituation of auditory reflexes in the decorticate opossum (Didelphis virginiana)

Abstract A lick-suppression technique was used to measure auditory reflexive responses of normal and decorticate opossums. Cases with decortications that included complete disruption of the projections from anterior dorsal and anterior ventral nuclei to cingulate cortex showed a marked deficit in the rate of reflex habituation while cases with decortications that preserved part of that system showed little if any deficit in habituation. However, decortication with or without complete cingulotomy did not result in a loss of sensitivity to sound nor to a systematic change in the nature or duration of the reflexive response to the first sound. Therefore, the habituation deficit is probably not a secondary effect of a perceptual deficit, a motor deficit, nor a change in a more general predisposition. The similarity of these results with results obtained in widely divergent animals suggest that the contribution of the anterior thalamus-cingulate cortex system to habituation may be very primitive and common to all marsupial and placental mammals.

Hearing in Prosimians

As part of a study of the auditory capacities of mammals, behavioral audiograms were determined for three species of prosimians: African bushbaby (Galago senegalensis), potto (Perodicticus potto), and slow loris (Nycticebus coucang). The method of conditioned suppression was used to assess thresholds for frequencies ranging from 125 Hz to 64 kHz. Comparisons of the three audiograms with those of more primitive mammals on one hand, and anthropoids on the other, reveal that prosimians retain several primitive hearing characteristics while in some other characteristics, such as high‐frequency hearing, they more closely resemble Anthropoids.