Page 46 - Volume 12, Issue 2 - Spring 2012

Fig. 3. Possible pathways to drive and modulate neurons in auditory cortex (AC) including the primary field (AI). These routes may explain changes in acoustic response properties, such as frequency selectivity, following fear conditioning, i.e., association of simple tones or complex frequency-modulated (FM) sounds with foot shock, direct and indirect effects of neuromodulatory substances, modulatory effects of non-auditory stimuli, and feedback modulation from higher brain regions. AC, auditory cortex; ACh, acetylcholine; AI, primary auditory field; NE, norephinephrine; vMGB and mMGB— ventral and medial nuclei of the medial genicu- late body.
noise ratio, which allowed stimulus-relat- ed response periods to be more easily detectable (Ojima et al., 2010).
The different brain states seem to be reflected in the degree of background activity of a restricted population of neu- rons. The discharge pattern evoked dur- ing different brain states may contribute differentially to the processing, and potentially the perception, of stimuli because of the varied signal-to-noise ratio. There are several possible converg- ing pathways that can mediate changes in neuronal responses evoked by relevant sound stimuli (Fig. 3). In particular, neu- romodulators may have long-lasting effects, since they are suitable for changes in background firing, and they contribute to plasticity, cognitive functions, and changes in brain states (e.g., Aston-Jones and Cohen, 2005).
Audition is not only related to sound
identification, but is linked also to emo-
tional effects and, in turn, to behavioral
responses. Little is known about how emo-
tional conditions are reflected in brain
state-dependent perceptual changes. In
the future, continuous and simultaneous
monitoring of brain state, behavior, and
neuronal discharges are required to inter-
pret important basic questions in the audi-
tory field: (1) how can ambient sounds be
largely ignored and how does a particular
sound attract one’s attention; and (2), how
can an acoustically identical sound be per-
ceived as either attractive or aversive,
depending on distinct internal and exter-
nal conditions? Only by carefully evaluat-
ing sound-induced behavioral actions and emotional states will we be able to fully understand cortical sound processing and perceptual correlates.
Rapid plasticity in auditory and prefrontal cortex during active listening
The brain is an extraordinarily adaptive and predictive machine: it adapts to present demands and predicts the future. The brain also undergoes tremendous plasticity in multiple diverse forms, from birth through adulthood. Neuronal plasticity is a fundamental property of neurons and neuronal circuits, because it facilitates adaptation to new environments, dynamically adjusts cortical sensory fil- ters to improve processing of salient stimuli to optimize task performance, enables prediction of reward, and provides the basis for learning from experience. Depending upon the time scales and mechanisms involved in induction and per- sistence of receptive field (RF) plasticity, these changes may be described as ephemeral stimulus-driven adaptive plastic- ity, rapid attention-driven plasticity, or consolidated learn-
ing-induced plasticity. Though there are likely to be com- mon molecular and synaptic mechanisms underlying all rapid RF transformations, there may also be some striking differences.
Rapid plasticity is generated so that the brain can adapt to the current environmental context and accentuate responses to the most salient cues in the present moment, to optimize processing of the most important stimuli. One of the major functions of rapid RF plasticity is contrast enhancement of the attended auditory object against the acoustic background. There are many ways of achieving this, not only by simple response enhancement at the task-rele- vant or conditioning stimulus tone frequency, but also by more complex changes in RF shape, gain, and neuronal activ- ity and connectivity, depending upon stimulus features, task, and behavioral context. Hence, it is important to emphasize that in the adult brain, there are likely to be multiple forms of plasticity, each with its own characteristic context of sensory experience, or behavioral challenge, and its unique set of cel- lular mechanisms and sites of plasticity. One of the major
Auditory Cortical Function 45