Auditory Forebrain

1. Auditory Forebrain Auditory Thalamus Auditory Cortex Auditory Projections to Polysensory Cortex Plasticity

2. Parallel Processing Pathways in the Subcortical System • Tonotopic/core: • ICC  MGv  A1 (core) • Non-tonotopic/diffuse: • ICX  MGm/MGd  Belt/parabelt).

3. Auditory Cortex is Comprised of Multiple Fields Bat Cat Macaque Human Human

4. Macaque Topographic Organization • Primary auditory cortical areas (“core” areas) respond best to tones, and are tonotopically organized. • Borders between fields marked by reversal in tonotopic gradient. • “Belt” fields less responsive to tones, respond best to NB noise, yet share tonotopy of adjacent core fields. • Adjacent fields interconnect more extensively than non-adjacent fields. Hackett, Stepniewska, Kaas (1998) JCN 394: 475-495

5. Functional Organization of Isofrequency Slabs in A1: Evidence from Anaesthetized Cat • As in the ICC, receptive field properties are systematically organized in “hypercolumns” within iso-CF slabs: • Spectral selectivity (“integration”; i.e., tuning curve width, Q) • Threshold • Latency • FM sweep direction, sweep rate selectivity • Binaural Interaction

6. Functional Organization of Isofrequency Slabs in A1: Evidence from Anaesthetized Cat • Spectral selectivity or “integration” (tuning width, Q) Q40dB Read et al. (2002) High (rostral) NB = Narrow band BB = Broad band Frequency Low (caudal)

7. Functional Organization of Isofrequency Slabs in Cat A1 • … FM sweep direction preference and rate selectivity Fast Up Slow Down Fast Up Mendelson et al. (1993)

8. Functional Organization of Isofrequency Slabs in Cat A1 • …Binaural Interactions: Represented in elongated bands in cat AC • Bands with similar binaural properties are interconnected across fields ipsilaterally, and contralaterally. EE: Binaural Summation (ITD) EI: Binaural Suppression (IID)

9. Space Map in Auditory Cortex? • Spatial selectivity is relatively poor. • No evidence for systematic map of space. • Alternate theory: Location encoded by populations of neurons with modest spatial selectivity.

10. Functional Organization of Primate A1 • In awake behaving monkeys, evidence for internal organization is not as strong as that in anaesthetized cats (methodological?). • Primate rostral field neurons have longest latencies, and narrowest frequency and intensity tuning • A1 has shortest latencies, moderate tuning. • Caudomedial field has broadest frequency tuning • Lateral field neurons have monotonic R/L functions.

11. Spectral Domain Properties • Classically-defined receptive fields resemble those at thalamic and IC levels. • More “multi-peaked” response areas are found. Recanzone et al. (2000)

12. Spectrotemporal Response Areas • Response areas are the result of interactions between excitation and inhibition. • However, the time course of excitation and inhibition may vary. • STRA’s try to capture the time-dependency of E – I interactions to reveal dynamic spectral filtering. “Classical” Frequency Response Area

13. Spectrotemporal Response Areas • Reverse correlation technique: Find which stimulus feature correlated most strongly with the response. • STRAs can be used to design “optimal stimuli”. DeCharms et al. (1998)

14. Spectrotemporal Response Areas • “Edge detection” (I.e., response to low pass or high pass noise). • “Orientation and direction specificity”: response to FM sweeps with particular modulation direction and speed. • “Optimal” stimuli generate much higher response rates than tonal stimuli.

15. Spectrotemporal Response Areas • “Edge detection” (i.e., response to low pass or high pass noise). • “Orientation and direction specificity”: response to FM sweeps with particular modulation direction and speed. • “Optimal” stimuli generate much higher response rates than tonal stimuli.

16. Spectro-temporal Facilitation (Combination Sensitivity) • Combination sensitivity may underlie rapid discrimination of vowels. Kent & Read (1991)

17. Combination Sensitivity in Primates • 66% of macaque A1 neurons are enhanced by presentation of tone combinations of different frequencies. Brosch et al. (1999)

18. Responses to Complex Sounds and Vocalizations • Many neurons show specificity for sets of complex sounds, e.g., certain vocalizations, even specific combinations of utterances. • But there’s little evidence for specificity to a unique sound (e.g., Grandma’s name cell). Klug et al. (2002)

19. Processing Signals in Noise • In auditory nerve fibers, background noise raises tonal thresholds, shifts rate level functions to the right, compresses dynamic range. • In A1, same stimuli generate increase in threshold, shift to right, but without compression.

20. Outputs of AC • A1 projects subcortically to ICdc, MGB, pontine gray. • AC belt and parabelt regions project to • Superior temporal gyrus and sulcus • Ventral prefrontal (cognition; saccade initiation) • insular (multimodal: limbic, hippocampus) • Caudal AC belt regions project to • LIP, lateral interparietal area (spatial; projects to dorsal premotor cortex.

21. Projections to Prefrontal Cortex • Belt and Parabelt AC project to rostral STS/STG. • …and ventral prefrontal cortex. • Auditory responses prevalent in ventrolateral PFC (areas 12, 45): corresponds to Broca’s area in humans. • Rostro-caudal gradient of projections Romanski et al. (1999)

22. Prefrontal Connections Hackett and Kaas

23. Cortical Plasticity 9 kHz (± 1/3 octave) Before After • Functional organization is maintained by experience. • E.g., Representation of frequency is massively altered by pairing stimulation with cholinergic nucleus basalis of the forebrain. Kilgard and Merzenich (1998) 250 ms tones paired with nucleus basalis stimulation.

24. Control Cortical Plasticity • Experience-dependent plasticity develops over time… • …and dramatically increases with temporal complexity. • Conclusion: auditory cortical organization is strongly influenced by behavioral significance of acoustic stimulus. Train of 15 ms tones paired with nucleus basalis stimulation. Kilgard and Merzenich (1998)