Page 47 - Volume 12, Issue 2 - Spring 2012

challenges for the field is to come up with a synthesis that explains the origins and mechanisms, roles, and interactions of the different kinds of plasticity.
We have studied the dynamic, RF transformations in the auditory cortex that accompany top-down attentional modu- lation of auditory processing. The dynamic changes reveal that the brain is able to make nimble, adaptive changes from one moment to the next as acoustic context and task demands change (Fritz et al., 2005a,b; 2010). These transfor- mations occur at the level of synapses, single neuron recep- tive fields, and also at the level of brain networks. They are related to issues such as rapid, “automatic” RF adaptive plas- ticity that is not driven by attention, possible common mech- anisms of RF plasticity contributions of the broader atten- tional network to task-driven plasticity, and insights from human neuroimaging studies that may help to put the results of animal studies in perspective.
The essence of our approach is to record from single neurons in primary auditory cortex (A1) and frontal cortex, while the animal performs a variety of different auditory tasks (Fig. 4). Our goal is to quantify the nature and time- course of state-dependent, task-dependent adaptive plastici- ty in the auditory cortex on a cellular and network level. Our approach to quantification of RF changes is illustrated (Fig. 5) with examples of rapid RF changes during tone detect and two-tone discrimination tasks.
Overall, a distinct pattern of change was found in spec- tro-temporal RFs (STRFs), i.e., there was selective enhance- ment not only at a target tone frequency, but also by an equal- ly selective depression at the reference tone frequency. When single-tone detection and frequency discrimination tasks were performed sequentially, neurons responded differen- tially to identical tones, reflecting distinct predictive values of
stimuli in the two behavioral contexts. Our findings show that A1 neuronal responses can swiftly change to reflect both sensory content and the changing behavioral meaning of incoming acoustic stimuli (Fritz et al., 2005a).
Rapid auditory task-related plasticity is an ongoing process that occurs as the animal switches between different tasks and dynamically adapts auditory cortical STRFs in response to changing acoustic demands. Rapid plasticity modifies STRF shapes in a manner consistent with enhanc- ing the behavioral performance of the animal. The specific form of the STRF change is dictated by the salient acoustic cues of the signals in the behavioral task, and is modulated by general influences reflecting the animal’s state of arousal, attention, motor preparation and reward expectation.
Top-down signals from frontal cortex are important in cognitive control of sensory processing. We compared activ- ity in ferret frontal cortex and A1 during auditory and visual tasks requiring discrimination between classes of reference and target stimuli. Frontal cortex responses were behavioral- ly gated, selectively encoded the timing and invariant behav- ioral meaning of target stimuli, could be rapid in onset, and sometimes persisted for hours following behavior. These results indicated that attention led to rapid, selective, persist- ent, task-related changes in spectrotemporal receptive fields. This suggests that A1 and frontal cortex dynamically estab- lish a functional connection during auditory behavior that shapes the flow of sensory information and maintains a per- sistent trace of recent task-relevant stimulus features (Fritz et al., 2010).
Acoustic motion processing in auditory cortex
Within extrastriate visual cortex of humans, monkeys and cats, individual cortical areas are specialized for spatial or
Fig. 4. Design of experimental stimulus presentations. Upper left: On a given trial during a behavioral session, a random number of temporal orthogonal ripple combina- tions (TORCs), i.e., 1–6 noisy reference signals, were followed by a ‘target’ tone. The panels at right illustrate spectrograms of three such TORCs and of the following tonal target. Responses to each TORC are collected in post-stimulus time histograms (PSTHs) at middle right that are cross-correlated with the TORC spectrograms to estimate the spatio temporal reference field (STRF) above. Although the animal behaves in anticipation of the target, all spike measurements to derive the STRF are made during the presentation of the reference TORCs. Lower right: Similar design for a two-tone discrimination task in which the ferret is presented with a random number of TORC tone combinations (1–6 reference signals in which the reference tone is fixed in frequency) followed by a target TORC-tone combination (in which the tone component changes to a different frequency than that of the reference tone). The panels at left represent a schematic of various possible experimental paradigms, including (A) tone detection, (B) two-tone discrimination, (C) gap detection, and (D) click rate discrimination. All follow the same basic design. The reference signals are (or include) TORCs used to measure the STRF. The target varies from one experiment to another. (Reprinted with permission from Fritz et al. 2005b).
46 Acoustics Today, April 2012