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British Medical Bulletin 63:95-105 (2002)
© 2002 The British Council

Microelectrode and neuroimaging studies of central auditory function

Alan R Palmer and A Quentin Summerfield

MRC Institute of Hearing Research, University of Nottingham, Nottingham NG7 2RD, UK


   Abstract
Top
 Abstract
Physiological and imaging...
 Frequency, tonotopicity, and...
 Intensity and loudness
 Spatial position and movement
 Complex sounds and speech
 References
 
Imaging studies in humans are revealing parallels with the functional organisation of the auditory brain discovered in microelectrode studies in animals: the rate of amplitude modulation generating the strongest response declines systematically from the lower brain stem to the cortex; an increase in sound level induces a higher level and a greater extent of activity; spectra are represented tonotopically in multiple cortical areas. There are also differences: evidence of organisation reflecting the sound level of the stimulus is absent in animals, but has been found in humans. Additionally, imaging has revealed functional specialisations which have not (yet) been located in animals: areas that respond more strongly to sounds with stronger pitches and to sounds that move in space. Microelectrode studies suggest that vocalisations are represented by spatially distributed populations of neurones in secondary auditory areas. In humans, likewise, activation progressively more specific to speech is found as the search moves from primary to secondary to accessory areas.


   Physiological and imaging methodologies to investigate central auditory function
Top
 Abstract
 Physiological and imaging...
Frequency, tonotopicity, and...
 Intensity and loudness
 Spatial position and movement
 Complex sounds and speech
 References
 
Microelectrodes capable of recording from a single neurone or a small cluster of neurones have been the primary tools for investigating brain function for the last half century. For many purposes, they still provide unique information, although the information can be enhanced and extended by coupling microelectrode recordings with other techniques. For example, recorded cells can be filled with dyes to reconstruct their structure and connections. Neurotransmitters and their agonists can be injected locally to determine the nature of a cell's synaptic connections. Single ion channels can be studied to reveal biophysical mechanisms at the cell membrane. These combinations of techniques have allowed progressively finer resolution of the spatial and temporal properties of single neurone responses.

In the last two decades, a new set of weapons has been added to the research arsenal that has the potential for powerful complementarity with microelectrode techniques. These are imaging methodologies that allow the responses of large ensembles of neurones to be measured in awake human subjects. The oldest of these techniques, electroencephalography (EEG), measures electrical signals at the surface of the head that reflect activation of remote populations of neurones. By making up to 128 simultaneous spatially separated measurements, it is possible to calculate the location of one or more dipole sources within the brain that could have given rise to the voltages at the different electrodes. In this way, the location of the neural activity is inferred. A variant of the method, magnetoencephalography (MEG), detects not the voltage, but the magnetic field generated by the current flows that result from neural activity. MEG also requires multiple sensors to allow the location of a dipole source to be inferred. Both EEG and MEG achieve very high temporal resolution (of the order of milliseconds, but poor spatial accuracy due to the dispersion of the signals within the head before they reach the scalp.

Two other widely used imaging methods do not depend on the neural activity directly, but on the metabolic demands that neural activity imposes. To meet the requirement for oxygen by active neural tissue, the blood supply to the active regions in the brain is increased. The increase in blood flow is measured directly in positron emission tomography (PET) by monitoring the concentration within the brain of radioactive oxygen injected into the subject just before the start of the experiment. Functional magnetic resonance imaging (fMRI) also relies on the metabolic response to neural activity by detecting changes in the ratio of oxy- to deoxyhaemoglobin in blood. PET and fMRI provide good spatial resolution (down to millimetres), but their temporal resolution is generally poor, because they rely on cardiovascular changes which take place over several seconds. Recent developments in fMRI such as single-event paradigms are bringing this limit nearer to fractions of a second.

In the sections that follow we relate data from electrophysiological studies in animals are related to data from imaging studies in humans.


   Frequency, tonotopicity, and pitch
Top
 Abstract
 Physiological and imaging...
 Frequency, tonotopicity, and...
Intensity and loudness
 Spatial position and movement
 Complex sounds and speech
 References
 
Microelectrode studies have revealed that the most striking organising principle of all auditory nuclei, in all animal species, is tonotopicity. From the point at which the auditory nerve enters the brain in the cochlear nucleus up through the diverse nuclei of the auditory brain stem, mid-brain and thalamus to the multiple fields in the auditory cortex, each nucleus has a tonotopic organisation, replicating the logarithmic increase in frequency that occurs with distance from the apex along the cochlear epithelium1. The restricted place representation of any one frequency in the cochlea expands to become a slab of cells in the cortex, all responsive to the same frequency range, and so providing the substrate for parallel processing of various aspects of sound. The mere fact of tonotopicity, however, does not imply that the tuning curves of central neurones are simple ‘V’ shapes like those of auditory nerve fibre responses. Instead, convergence of excitatory and inhibitory inputs results in increasingly complicated response areas at higher levels of the system (e.g. Phillips et al2). Nonetheless, at the level of the primary auditory cortex, the topographical organisation of response types is systematic, with neurones at the centre of the region having the lowest thresholds, shortest latencies, and the narrowest and most symmetrical frequency response areas3.

Electrophysiology in humans has provided a direct demonstration of tonotopicity along Heschl's gyrus – the site of the primary auditory cortex. Using electrodes implanted to detect the foci of seizures in epileptic patients4, a lateral progression in frequency sensitivity from high frequency (3360 Hz) to lower frequency (1480 Hz) was found. A similar progression has been found using MEG: the source of the response in Heschl's gyrus again moves more laterally as frequency decreases5. This experiment also located an orthogonal tonotopic gradient in the secondary auditory cortex in planum temporale running from high to low frequency in a posterior direction.

Similar results have been obtained by using high resolution MRI to image the response to narrow-band noise stimuli6. By using high (above 2490 Hz) and low (below 600 Hz) bands, it was possible to identify regions on the supratemporal plane that were more responsive to one or other of the sounds. Eight regions were found (four that were activated more strongly by the low-frequency sound and four by the high-frequency sound) widely dispersed across the auditory cortex including Heschl's gyrus and planum temporale, suggesting the existence of multiple functionally distinct regions each with its own tonotopic axis. At least some of these regions coincided with anatomical divisions of the cortex, including the primary area on Heschl's gyrus.

Potentially, primary auditory cortex could be organised spatially with respect to pitch rather than frequency. Pitch is the subjective attribute of sounds such that they may be ordered on a musical scale: it is related to the repetition rate of the waveform of a sound. Early studies in monkey favoured frequency7, although recent studies in the gerbil have indicated that pitch could be an additional organising principle8. This result awaits replication in other species. Whatever the outcome, a distinct cortical region responsive to the strength of pitch in sounds has been located in humans. It is possible to create stimuli whose spectral structure is not resolvable by the human auditory system and whose pitch is determined by their temporal structure alone. PET has been used with such stimuli to reveal bilateral areas of auditory cortex where the response magnitude increased parametrically with the temporal regularity, and hence the pitch strength, of the stimulus9. On the right, the region appeared to be in primary auditory cortex, while on the left the activation was more lateral extending onto the surface of the superior temporal gyrus. When the stimuli were presented in sequences forming melodies, additional regions more anterior (anterior temporal lobes) and posterior (superior temporal gyrus) to the primary auditory cortex showed activation dependent on the pitch strength. Temporal integration on a scale of ten's of milliseconds is required to detect pitch, and on a scale of seconds to detect melody. Given the limited temporal resolution of cortical neurones, it is likely that the primary areas were responding to evidence passed up from sub-cortical neurones which have the temporal resolution required to extract pitch, while the anterior and posterior areas themselves may be responsible for extracting the melody. Indeed, the nuclei of the brain stem show parametric increases in MRI activation as the temporal regularity of the stimulus increases10.


   Intensity and loudness
Top
 Abstract
 Physiological and imaging...
 Frequency, tonotopicity, and...
 Intensity and loudness
Spatial position and movement
 Complex sounds and speech
 References
 
The perception of loudness, spatial position, and distance depend on sound level. How loud a sound appears is largely a reflection of its intensity, but also of its spectral content. In general, the higher the level of a sound or the greater the extent of its components in frequency, the louder it appears and the greater the magnitude and spread of the neural activation that it produces. In the auditory nerve, increases in the intensity of a single tone first cause an increase in the discharge rate of fibres tuned to that frequency and second a spread of activity to fibres tuned to adjacent frequencies. Eventually, at moderate levels of 70–80 dB SPL, virtually the whole nerve may be active to some extent11. Progressing centrally, the typically monotonic sigmoid function for discharge rate with sound level seen in auditory nerve fibres becomes less common, as inhibition shapes the range of sound levels to which neurones respond. By the level of the cortex, sharply non-monotonic rate-level functions can be found with each neurone responding only over very narrow ranges of sound levels12, but in most mammals there is little evidence that the neurones are organised spatially by best or most effective sound level (referred to as an ampliotopic organisation).

In humans, in contrast, imaging studies have found some evidence of ampliotopy. MEG has revealed that the depth of the dipole source in response to 1000 Hz tones decreases monotonically with level from 30–80 dB HL (dB above hearing threshold at 1000 Hz)13. PET has also shown evidence of ampliotopic organisation: in the right temporal lobe, the location of the maximal response varied with the intensity, but not with the frequency of the stimuli14. More commonly, however, both with speech stimuli and with tones, an increase in the magnitude of activation15 or in the volume of activated cortex16,17 or both14 have been found to accompany increases in sound level. These increases, even with simple tonal stimulation, are not restricted to the primary auditory areas and do not appear to saturate at high sound levels. For example, in medial geniculate and auditory cortex, activation by 4 kHz tones continued to increase between the high sound levels of 80 and 90 dB HL14. Increases in cortical activation at high sound levels have also been shown18 with different growth functions for high and low frequency tones that were reminiscent of the growth functions for single neurones shown electrophysiologically19.


   Spatial position and movement
Top
 Abstract
 Physiological and imaging...
 Frequency, tonotopicity, and...
 Intensity and loudness
 Spatial position and movement
Complex sounds and speech
 References
 
The spatial position of a sound source is computed from three types of cue: monaural spectral cues generated by the pinna and concha, and binaural differences in the timing of low-frequency sounds and the level of sounds, at the two ears, that result from longer transmission paths and head shadowing respectively20. Moving sound sources generate dynamic spatial cues which, when quite slow (2–6 Hz), lead human listeners to perceive a moving source. Lesion studies show that the auditory pathway up to the cortex is necessary for the perception of sound source position and motion21,22, although the initial processing takes place in the brain stem.

The dorsal division of the cochlear nucleus has been implicated in the processing of pinna spectral cues, which are particularly important in determining sound source elevation. Somatosensory input to the dorsal cochlear nucleus may allow the position of head, neck, and pinna to be taken into account in evaluating the spectral cues23. To account for sensitivity to interaural timing, a network of coincidence detectors linked by delay lines was proposed originally by Jeffress24. Subsequent physiological studies25 have shown that precisely timed action potentials from the auditory nerve are conducted via large high-fidelity synapses through the ventral cochlear nucleus to the binaural comparators in the superior olive. In the medial superior, olive cells receiving input from both ears act as coincidence detectors, firing when the inputs arrive simultaneously. Since the transmission paths to the two ears are not equal, coincidences occur only when the internal transmission delay exactly compensates for the interaural delay due to the sound source position: in the simplest form, the cells may be thought to be tuned for a particular direction in the horizontal plane. A pathway to the lateral superior olive subserves processing of interaural level differences. In this case, the path from the contralateral cochlear nucleus contains an extra synapse onto an inhibitory interneurone in the medial nucleus of the trapezoid body. The net result is that cells in the lateral superior olive are fed by inhibition from one ear and excitation from the other and are exquisitely sensitive to the exact balance of their inputs and hence to the interaural level difference. Microelectrode studies have shown sensitivity to interaural time and level differences throughout the auditory pathway up to the cortex, but sensitivity to the direction of auditory motion only appears beyond the auditory mid-brain25.

The inferior parietal lobes, particularly on the right side, display greater MRI activation during localisation tasks than during other auditory discrimination tasks26. Selective activation of the parietal cortex by moving auditory sources has also been shown using PET and MRI27. In these studies, brain activations were compared under conditions in which the dynamic interaural time and level cues were complementary, resulting in perceived motion, or opposing resulting in little motion perception. A region of right parietal cortex was active only when the subject perceived motion not when the same dynamic cues were presented in opposition, nearly cancelling the motion percept. In contrast, primary auditory cortex displayed bilateral activation to both versions of the stimulus, and was not differentially activated when the subject perceived motion.

A movement sensitive area on the right supratemporal plane located on the planum temporale lateral to Heschl's gyrus has been identified with fMRI28. A complex broad-band stimulus was amplitude modulated either in phase at the two ears (stationary condition) or 90° out of phase at the ears (moving condition) relying on interaural level differences alone to generate the sound movement. Both moving and stationary signals activated different fields of the auditory cortex bilaterally compared to silence, but only the area in right planum temporale showed stronger activation to the moving stimulus.


   Complex sounds and speech
Top
 Abstract
 Physiological and imaging...
 Frequency, tonotopicity, and...
 Intensity and loudness
 Spatial position and movement
 Complex sounds and speech
References
 
Natural sounds, including speech, contain time-varying changes in spectral content and amplitude. Relatively simple temporally varying sounds have often been used to investigate the neural representation of complex sounds as a means of probing the basis for the analysis of speech sounds.

Responses to amplitude and frequency modulation have been studied at every level of the auditory pathway. Auditory nerve fibres discharge in synchrony with the modulation envelope up to frequencies determined by their tuning and phase locking capabilities, forming a low-pass transfer function29. From the cochlear nucleus upwards, neurones tend to be tuned to particular modulation rates, with progressively lower rates being favoured at higher processing stages such that at the cortex preferred rates are only a few tens of Hz. Remarkable parallels are found using MRI with humans30,31 as shown in Figure 1: optimal rates of 32–256 Hz in the cochlear nucleus and lower brain stem decrease to 2–4 Hz in secondary auditory cortex.



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  		Fig 1. Sensitivity to different rates of amplitude modulation at different nuclei along the auditory pathway. The animal data are taken from a variety of different studies using pure tone carriers and represent the range of best modulation frequencies. The human data are from an fMRI study30 using broad-band noise and represent the modulation frequencies that best activated those regions.

 
At the mid-brain and cortex, maps of best modulation frequency that might serve pitch perception have been reported8,32,33. The existence and significance of these maps is controversial, however34. Similarly, only one imaging study, using MEG35, has provided evidence of a map of modulation sensitivity in human cortex, and the results are equivocal. Other imaging studies show a postero-lateral region of the superior temporal gyrus to be activated selectively by frequency and amplitude modulated sounds36–38. Within the auditory region responsive to modulated signals, there appears to be a clustering of responsiveness to different modulation frequencies, but no systematic topographic organisation30.

The number of neurophysiological studies of responses in auditory cortex to species-specific vocalisations peaked in the 1970s. Interest has revived recently with the development of techniques for studying awake behaving primates. The decline occurred because early expectations of populations of cortical feature-detector neurones were not sustained. Clearly, in primary auditory cortex, large numbers of cells are not specialised for vocalisations. Various alternatives have been proposed involving representations based on the collective response of populations of neurones whose individual responses reflect variations in frequency, tuning width, tuning asymmetry, etc (for example39). Neurones selective for vocalisations do exist, but are more common in secondary cortical areas40–43. Recently, evidence has been presented from neurophysiological and imaging studies that such stimuli can activate separate pathways originating in primary auditory cortex that determine ‘what’ sound has been produced and ‘where’ its source is located43–46.

In imaging studies, speech generates wide-spread activation in auditory cortex including primary and non-primary areas in both hemispheres. Here we focus on speech-specific activation within and adjacent to the human auditory cortex.

Temporal analysis, which is a particular requirement in the encoding of acoustic cues for speech perception, is enhanced in areas of the left auditory cortex. Differential responses to voiced and voiceless stop consonants in primary and secondary auditory areas on the left, but not right, have been recorded from electrodes implanted along the supratemporal plane of epilepsy patients47. The auditory cortex on both sides responded strongly to the syllables, but responses were time-locked to the different components of the syllable only on the left. The asymmetries in the temporal processing in these areas were also found for simplified acoustical models of the stimuli, that were not perceived as speech.

MRI and PET, possibly because of their relatively poor temporal resolution, often fail to show hemispheric asymmetries between speech and non-speech sounds, but have provided evidence of an anterior/ventral network involved in processing speech. Activation has been measured to a range of stimuli that included sequences of words and tones, white noise, reversed speech, and pseudowords48. No difference was found in the strength of activation in primary or secondary cortex, in either hemisphere, between sequences of words and sequences of pure tones of different frequencies. Activation by the words spread ventrally towards the superior temporal sulcus, to a greater extent on the left side. Primary auditory cortex responded equally to all of the stimuli. In contrast, surrounding non-primary auditory areas, responded more strongly to tone sequences than to white noise, suggesting a preference for sound sequences of changing frequency. The lateral surface of the superior temporal gyrus responded to words more than tones suggesting the emergence of some degree of speech specificity. Ventrolateral areas, primarily in left superior temporal sulcus, were unresponsive to noise, weakly responsive to the tones, but strongly responsive to the words, pseudowords and reversed speech. It remains to be determined whether this result originates in differences in acoustical, phonetic, or lexical processing. Other findings also suggest that, as analysis moves from primary cortex across the superior temporal gyrus to the superior temporal sulcus, processing becomes more specific for speech49. Activation differences were measured with a variety of speech and speech-like sounds configured to have equivalent spectral and temporal complexity, but to vary in phonetic information and intelligibility. While the lateral surface of the superior temporal gyrus and posterior superior temporal sulcus were activated by all stimuli that contained perceptible phonetic features, regardless of intelligibility, the anterior part of the superior temporal sulcus was specifically activated only by intelligible stimuli.

A possible synthesis of these results is shown in Figure 2: speech processing involves distinct subsystems in the left temporal lobe that form a dorsal-ventral-anterior pathway. Low-level auditory cues are processed in primary and surrounding secondary auditory areas, while prelexical processing of phonetic cues and their sequencing may take place on the lateral surface of superior temporal gyrus. Activation by intelligible speech occurs in the anterior superior temporal sulcus which projects widely to other brain areas including the prefrontal cortex and medial temporal lobe.



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  		Fig 2. Activation of the human brain by various speech and speech-like sounds as determined by fMRI in a variety of studies. The primary auditory cortex in humans is on the transverse gyrus located on the supratemporal plane in the Sylvian fissure (from Palmer and Hall50, with permission).

 

   Footnotes
 
Correspondence to: Prof. Alan R Palmer, MRC Institute of Hearing Research, University Park, Nottingham NG7 2RD, UK


   References
Top
 Abstract
 Physiological and imaging...
 Frequency, tonotopicity, and...
 Intensity and loudness
 Spatial position and movement
 Complex sounds and speech
 References
 

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