AUDITORY PERCEPTION Pitch Perception - PowerPoint Presentation

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AUDITORY PERCEPTION

Pitch Perception

We

associate pitch (the high or low quality of a sound) with

frequency.

Pitch

can vary due to factors other than frequency, such as the intensity or context of a stimulus.

Most frequencies are systematically encoded by the auditory system through

tonotopic

organization.

Neurons

responding to one frequency are located next to neurons responding to similar frequencies.

As

a result, one cue for assessing the

frequency

of a sound is the location of active neurons.

From

the level of the basilar

membrane

up through the spiral ganglia, the medial

geniculate

nucleus, and the auditory cortex, different frequencies are processed at different locations.

Georg Von B

é

k

é

sy

’s place theory explains the tonotopic organization of the basilar membrane. According to this theory, the peak of the wave traveling along the length of the basilar membrane is correlated with a sound’s frequency.

Loudness Perception

Loudness is perceived in decibels.

Decibels

describe the physical qualities of the sound stimulus, whereas loudness is the human perception of that

stimulus

. Loudness doubles with each 10 dB increase in stimulus intensityIn other words, a stimulus that is 10 dB greater than another is perceived as only twice as loud.Our ability to detect loudness varies with the frequency of a sound. By

allowing participants to adjust the intensity of different tones until they sound equally loud, we can plot functions known as equal loudness contours.

low

frequencies are usually perceived as quieter than high frequencies at the same level of intensity.

At

very high intensities of 80 to 100dB, you can see that all frequencies are perceived as being nearly equally loud.

Auditory neurons can respond to higher sound amplitudes by increasing their rate of response.

However, the range of sound amplitudes we can hear is too broad to be completely encoded in this manner.

Normally, a single neuron can respond to a range of about 40 dB, whereas at some frequencies, humans can perceive a range of 130dB.

Although a single neuron might have a limited range of 40dB, a population of neurons with different ranges can provide the coverage we require.

In addition, although auditory neurons have a preferred frequency to which they respond, they will in fact respond to similar frequencies if amplitude is high enough.

The recruitment of these additional neurons contributes to our perception of loudness.

Localization of Sound

Our primary means of localizing sound in the horizontal plane (in front, behind, and to the side) is a comparison of the arrival times of sounds at each ear.

Because

arrival times from sounds immediately in front or behind you are identical, these sounds are very difficult to localize accurately

.

Distinctions between arrival times of sound at each ear are made by neurons in the superior olive. These neurons are known as binaural neurons because they receive input from both ears. Binaural neurons respond most vigorously when input from both ears reaches them simultaneously.

If

input from the two ears arrives at slightly different times, the cells will respond less vigorously.

Several

unique physiological features allow auditory neurons to produce the rapid responses required for sound localization.

Many

cells in the auditory system respond and reset more rapidly than other types of neurons, due at least in part to large numbers of potassium channels. Synapses are unusually large, providing the ability to release large amounts of neurotransmitter within a given time. Specialized glutamate receptors, found only in the auditory system, are also built for rapid transmission.

Finally, many brainstem auditory neurons have myelin not only on their axons but also on their cell bodies

.

In

addition to arrival times, we also assess the differences in the intensities of sound reaching each ear.

Because the head blocks some sound waves, a sound

“shadow” is cast on the ear farthest away from the source of sound, producing a weaker signal to that ear. This system does not work for lower-frequency sounds, which move around the head without producing a noticeable shadow.

The pinna of the ear is essential for localizing sounds in the vertical plane (above or below).

When different-shaped false pinnas were attached to human participants, sound localization was impaired.

However

, with practice wearing their new

pinnas

, the participants learned to localize sounds correctlySound localization also involves vision. While watching a movie, we perceive sound as originating from the actors’ lips, in spite of the fact that the speakers producing the sound are typically above and to the sides of the screen. This integration of visual and auditory information occurs in the caudal areas of the auditory cortex, which become more active when monkeys watch a video than when they simply listen to sound