Psych 30 Lecture Notes: Cochlear Implants and Speech Perception

Professor David Heeger

What you should know from this lecture

• Auditory cortex
• Aphasia: Wernicke's area, Broca's area
• Spectrogram
• Formant
• Formant transition
• Language learning disability
• Cochlear implants

Auditory cortex: Located laterally near top of temporal lobe.

Auditory cortex critical for speech perception and language comprehension. Aphasia refers to the collective deficits in language comprehension and production that accompany brain damage. Brain damage to primary auditory cortex and/or adjacent Wernicke's area causes a certain kind of aphasia, disorder of language comprehension.

Neurons in primary auditory cortex laid out with respect both to tonotopy and binaural summation. Neurons at one end respond to low frequency and neurons at the other end respond to higher frequency tones. Measuring in the orthogonal direction, neurons first sum the responses to signals at the two ears, and then they respond by differencing the signals at the two ears, alternating back and forth across the auditory cortex.

Not much else known about it. Lesion studies on animals, remove auditory cortex, animals can still:

• Detect sounds (e.g., train dog to lift foreleg in response to a tone).
• Discriminate different frequencies.

• Discriminate between different "tunes", that is, temporal patterns of acoustic stimuli.
• Discriminate tone durations.
• Localize the source of the sound.
• Discriminate which ear had detected a sound.

Spectrogram:

Graph of frequency vs. time. Each row is a frequency band. Intensity is amplitude of that frequency band over time. Spectrogram is computed using Fourier Transform. This is very similar to the processing done by the cochlea.

Formants, parallel horizontal bands in the spectrogram = vowels. Formant transitions = consonants. Difference between the "ba", "da", "ga", and "pa" is only in the formant transitions. Some formant transitions are very brief (10-50 msec), like "ba" and "da".  Others are relatively long like "pa" and "ga".

Language learning impairment: Paula Tallal (at Rutgers University) has spent her career studying language learning disabilities, kids that have difficulty learning to understand and produce language. Tallal has demonstrated that these kids have difficulty with speech because they have deficits in the fast (10's of msec) temporal processing needed to distinguish brief formant transistions (like "ba" and "da").

In these experiments, subjects had to discriminate between two stimuli, either a high tone followed by a low tone or a low tone followed by a high tone.  If the tones are separated in time by more than half a second then both normal and language learning impaired (LLI) subjects have no problem performing the task (100% correct).  But for shorter separations (shorter inter-stimulus intervals), LLI children show a dramatic deficit in performance.  Tallal believes that this is cause of their language disability.  Because they can't hear the differences between rapidly changing sounds, they can't discriminate one formant transition and another, so they have trouble understanding and producing speech.

Tallal and Michael Merzenich (a neurobiologist at UCSF) developed a software systems to help kids with language learning disabilities. The software is kind of like a computer game, but is really an auditory psychophysics experiment in disguise. The idea is to give the kids lots of practice making threshold discriminations between sounds. Over time with practice, their performance gets better, and results in better language comprehension and production. Can find out more about this company and their "FastForword" software at the Scientific Learning Corporation website.

Cochlear implants: The cochlear implants is a wonderful example of how we can take the results basic research, our understanding of how the peripheral auditory system (cochlea, 8th nerve) codes sound signals, and put it to use.

Several electrodes mounted on a carefully designed support that is matched to the shape of the cochlea. The design of this support structure is critical because it places the electrodes very near the nerve cells. The computer decomposes a sound signal into its frequency components via fourier transform and sends the separate frequency components to the corresponding electrodes. In other words, is computes a spectrogram mimicking the frequency decomposition performed by the cochlea. Then the implant transforms the spectrogram into a series of current pulses for each of the stimulating electrodes. This transformation into the current pulses is based on what we know about the coding of information in the auditory nerve. Both the temporal and place codes are important for signalling pitch. Both the nerve firing rates and the number of active neurons are important for signalling loudness. The goal is to accurately replicate the neural code that would naturally be communicated along the 8th nerve. Note that need to maintain proper timing of information to within 10's of msec for formant transitions.

How well does it work? In some patients, cochlear implants restore speech nearly perfectly. But that is not the case for most patients (at least at this time). When it doesn't work so well, it can be a detriment for some kids who can't hear well enough to succeed in the hearing community. Consequently, there is some controversy...

Discussion...