Top-down signals from frontal cortex (FC) are conjectured to play a critical role in cognitive control of sensory processing. To explore this interaction, we compared activity in ferret FC and primary auditory cortex (A1) during auditory and visual tasks requiring discrimination between classes of reference and target stimuli. FC responses were behaviorally-gated, selectively encoded the timing and invariant behavioral meaning of target stimuli, could be rapid in onset, and sometimes persisted for hours following behavior. This mirrors earlier findings in A1that attention triggered rapid, selective, persistent, task-related changes in spectrotemporal receptive fields. Simultaneously recorded local field potentials (LFPs) revealed behaviorally-gated changes in inter-areal coherence, selectively modulated between FC and focal regions of A1 responsive to target sounds. These results suggest that A1 and FC dynamically establish a functional connection during auditory behavior that shapes the flow of sensory information and maintains a persistent trace of recent task-relevant stimulus features.

A variety of attention-related effects have been demonstrated in primary auditory cortex (A1). However, an understanding of the functional role of higher auditory cortical areas in guiding attention to acoustic stimuli has been elusive. We recorded from neurons in two tonotopic cortical belt areas in the dorsal posterior ectosylvian gyrus (dPEG) of ferrets trained on a simple auditory discrimination task. Neurons in dPEG showed similar basic auditory tuning properties to A1, but during behavior we observed marked differences between these areas. In the belt areas, changes in neuronal firing rate and response dynamics greatly enhanced responses to target stimuli relative to distractors, allowing for greater attentional selection during active listening. Consistent with existing anatomical evidence, the pattern of sensory tuning and behavioral modulation in auditory belt cortex links the spectro-temporal representation of the whole acoustic scene in A1 to a more abstracted representation of task-relevant stimuli observed in frontal cortex.

A new stereotaxic brain atlas of the Mongolian gerbil (Meriones unguiculatus), an important animal model in neurosciences, is presented. It combines high-quality histological material for identification of brain structures with reliable stereotaxic coordinates. The atlas consists of high-resolution images of frontal sections alternately stained for cell bodies (Nissl) and myelinated fibers (Gallyas) of 62 rostro-caudal levels at intervals of 350 μm. Brain structures were named according to the Paxinos nomenclature for rodents. The accuracy of the stereotaxic coordinate system was improved substantially by comparing and matching the series of histological sections to in vivo brain images of the gerbil obtained by magnetic resonance imaging (MRI). The skull outlines corresponding to the MR images were acquired using X-ray computerized tomography (CT) and were used to establish the relationship between coordinates of brain structures and skull. Landmarks such as lambda, bregma, ear canals and occipital crest can be used to line up skull and brain in standard atlas coordinates. An easily reproducible protocol allows sectioning of experimental brains in the standard frontal plane of the atlas.

In bats conventional stereotaxic methods do not yield sufficient positional accuracy to allow reliable recordings and tracer injections in subnuclei of the auditory system. In a newly developed stereotaxic system experimentally measured patterns of skull profile lines are used to define the animal's brain position with an accuracy of _+ 100 ~m. By combining the neurophysiological stereotaxic procedure with a standardization of the neuroanatomical processing of the brains, the location of recordings, stimulations or injections can be readily transformed into brain atlas coordinates. This facilitates the compilation and comparison of data within and among animals. The system is not restricted to use in bats and can be readily adapted to other experimental animals.

Responses of auditory neurons to complex stimuli were recorded in the dorsal belt region of the auditory cortex of two taxonomically unrelated bat species, Rhinolophus rouxi and Pteronotus parnellii parnellii, both showing Doppler shift compensation behaviour. As in P.p.parnellii (Suga et al., J. Neurophysiol., 49, 1573 - 1626, 1983), cortical neurons of R.rouxi show facilitated responses to pairs of pure tones or frequency modulations. Best frequencies for the two components lie near the first and second harmonic of the echolocation call but are in most cases not harmonically related. Neurons facilitated by pairs of pure tones show little dependence on the delay between the stimuli, whereas pairs of frequency modulations evoke best facilitated responses at distinct best delays between 1 and 10 ms. Facilitated neurons are found in distinct portions of the dorsal cortical belt region, with a segregation of facilitated neurons responding to pure tones and to frequency modulations. Non-facilitated neurons are found throughout the field. Neurons are topographically aligned with increasing best delays along a rostrocaudal axis. The best delays between 2 and 4 ms are largely overrepresented numerically, and occupy approximately 56% of the cortical area containing facilitated neurons. A functional interpretation of the large overrepresentation of best delays approximately 3 ms is proposed. Facilitated neurons are located almost entirely within layer V of the dorsal field.