Receptive Fields In Auditory Cortex Research Articles

Stimulus dependent transformations between synaptic and spiking receptive fields in auditory cortex

Auditory cortex neurons nonlinearly integrate synaptic inputs from the thalamus and cortex, and generate spiking outputs for simple and complex sounds. Directly comparing synaptic and spiking activity can determine whether this input-output transformation is stimulus-dependent. We employ in vivo whole-cell recordings in the mouse primary auditory cortex, using pure tones and broadband dynamic moving ripple stimuli, to examine properties of functional integration in tonal (TRFs) and spectrotemporal (STRFs) receptive fields. Spectral tuning in STRFs derived from synaptic, subthreshold and spiking responses proves to be substantially more selective than for TRFs. We describe diverse spectral and temporal modulation preferences and distinct nonlinearities, and their modifications between the input and output stages of neural processing. These results characterize specific processing differences at the level of synaptic convergence, integration and spike generation resulting in stimulus-dependent transformation patterns in the primary auditory cortex.

ON-OFF receptive fields in auditory cortex diverge during development and contribute to directional sweep selectivity

Neurons in the auditory cortex exhibit distinct frequency tuning to the onset and offset of sounds, but the cause and significance of ON and OFF receptive field (RF) organisation are not understood. Here we demonstrate that distinct ON and OFF frequency tuning is largely absent in immature mouse auditory cortex and is thus a consequence of cortical development. Simulations using a novel implementation of a standard Hebbian plasticity model show that the natural alternation of sound onset and offset is sufficient for the formation of non-overlapping adjacent ON and OFF RFs in cortical neurons. Our model predicts that ON/OFF RF arrangement contributes towards direction selectivity to frequency-modulated tone sweeps, which we confirm by neuronal recordings. These data reveal that a simple and universally accepted learning rule can explain the organisation of ON and OFF RFs and direction selectivity in the developing auditory cortex.

Intention and attention: Top-down influences on the representation of task-relevant sounds.

To explore the role of top-down projections from frontal cortex (FC) to auditory cortex (AC), which may play a role in adaptive reshaping of A1 receptive fields to attended acoustic stimuli during behavior, simultaneous recordings were made from single neurons in FC and AC in behaving ferrets, trained on multiple auditory detection and discrimination tasks in positive and negative reinforcement paradigms. Performance required selective attention to different salient spectral frequency and/or temporal cues. Previous observations revealed task-specific transformations in receptive fields in AC [Fritz et al. (2003); (2005); (2007); (2009); Atiani et al. (2009)]. In contrast, FC neurons showed recognition responses, which categorically distinguished between acoustic foreground and background stimuli. FC responses to targets were often independent of the acoustic properties of the target and thus encoded an abstract representation of the class of target stimuli [Fritz et al. (2009)]. Stimulation of FC, paired with tones, lead to receptive field transformations similar to those observed in behavior. These results emphasize the importance of interactions between multiple areas during selective attention, and the tight coupling of target recognition and auditory attention that enhances auditory cortical filters for attended acoustic stimuli, thus creating a functional representation of task-salient sounds during behavior. [This work was supported by grants from NIDCD, NIH.]

Intracortical Pathways Determine Breadth of Subthreshold Frequency Receptive Fields in Primary Auditory Cortex

To examine the basis of frequency receptive fields in auditory cortex (ACx), we have recorded intracellular (whole cell) and extracellular (local field potential, LFP) responses to tones in anesthetized rats. Frequency receptive fields derived from excitatory postsynaptic potentials (EPSPs) and LFPs from the same location resembled each other in terms of characteristic frequency (CF) and breadth of tuning, suggesting that LFPs reflect local synaptic (including subthreshold) activity. Subthreshold EPSP and LFP receptive fields were remarkably broad, often spanning five octaves (the maximum tested) at moderate intensities (40-50 dB above threshold). To identify receptive-field features that are generated intracortically, we microinjected the GABA(A) receptor agonist muscimol (0.2-5.1 mM, 1-5 microl) into ACx. Muscimol dramatically reduced LFP amplitude and reduced receptive-field bandwidth, implicating intracortical contributions to these features but had lesser effects on CF response threshold or onset latency, suggesting minimal loss of thalamocortical input. Reversal of muscimol's inhibition preferentially at the recording site by diffusion from the recording pipette of the GABA(A) receptor antagonist picrotoxin (0.01-100 microM) disinhibited responses to CF stimuli more than responses to spectrally distant, non-CF stimuli. We propose that thalamocortical and intracortical pathways preferentially contribute to responses evoked by CF and non-CF stimuli, respectively, and that intracortical projections linking frequency representations determine the breadth of receptive fields in primary ACx. Broad, subthreshold receptive fields may distinguish ACx from subcortical auditory relay nuclei, promote integrated responses to spectrotemporally complex stimuli, and provide a substrate for plasticity of cortical receptive fields and maps.

Cortical responses in rats to periodic frequency-modulated sounds

Spectral variations over time are a common characteristic of naturally occurring sounds. They constitute a prevalent feature in communication signals in several species. Studies of mammals have shown that auditory cortex neurons respond to single frequency-modulated (FM) sweeps and that most responses are selective for sweep direction and/or rate. Researchers have also used trains of FM sweeps to estimate spectrotemporal receptive fields in auditory cortex. In the present study, microelectrode recordings were used to explore how the auditory cortex responds to trains of FM sweeps in anesthetized rats. Maps of 20–60 penetrations were made for each subject. Sweep frequencies ranged from 1–16 kHz with FM rates ranging from 4–24 octaves/s and repetition rates from 2–24 sweeps/s. Both down-sweeps and up-sweeps were presented. Neuronal responses were analyzed in terms of onset, offset, oscillatory and directionally selective properties. Several types of responses predominated. Most units responded to sound onset. In contrast, responses to signal offsets were rare. Oscillatory responses were typically evoked only for repetition rates less than 12 sweep/s. Directionally selective responses were limited to these rates. These results indicate that oscillatory responses in the auditory cortex are limited to low repetition rate stimuli, and that directional selectivity depends on repetition rate. [Work supported by NSF.]

Cholinergic modulation of frequency receptive fields in auditory cortex: II. Frequency-specific effects of anticholinesterases provide evidence for a modulatory action of endogenous ACh.

Exogenously applied muscarinic agonists--for example, acetylcholine (ACh) and acetyl-beta-methacholine (MCh)--modify frequency receptive fields in auditory cortex of unanesthetized animals in a frequency-specific rather than global manner. The present study sought to relate these findings to endogenous actions of ACh by using the anticholinesterase agents eserine sulphate and soman (0-1,2,2-trimethylpropylmethylphosphonofluoridate) to facilitate the effects of endogenous ACh. Frequency receptive fields (FRF) were determined by presenting sequences of different isointensity tones before, during, and after application of ACh, MCh, eserine, or soman; also the cholinesterase blockers were applied between applications of ACh or MCh. The major effects produced by the inhibitors were similar to those of the agonists. Predominant effects were frequency-specific changes in FRF. Further, eserine and soman, similar to ACh and MCh, produced shifts in the best frequency (BF) of FRF due mainly to coordinated depression of responses to the BF and increased responses to adjacent, non-BF. The results indicate that exogenous and endogenous ACh, acting via muscarinic receptors, can significantly influence the physiological functioning of cortical neurons and consequently their processing of sensory information.

Cholinergic modulation of frequency receptive fields in auditory cortex: I. Frequency‐specific effects of muscarinic agonists

Previously we reported that acetylcholine (ACh) and acetyl-beta-methacholine (MCh) modify responses of neurons in auditory cortex to individual frequencies. The purpose of this study was to determine whether muscarinic agonists produce frequency-specific alterations or general changes in cellular responses. Frequency-specific modifications would be evident in alterations of frequency receptive fields (FRF) that differed across frequencies while general effects would be seen as changes that were more or less the same over frequencies. Responses of single neurons to designated sets of tones were recorded in the auditory cortex of chronically prepared awake cats before, during, and following ejection of ACh or MCh by iontophoresis or micropressure using multibarrel micropipettes. Frequency receptive fields were determined by presenting isointensity tones across a range of frequencies including the cell's best frequency (BF) to tone onset. FRF for "off" and "sustained (through)" responses were also determined quantitatively. The effects of ACh and MCh were predominantly frequency-specific (77%, 39/51 cells); general changes (19%, 10/51) and no effects (4%, 2/51) were less likely. Frequency-specific effects involved both facilitation and reduction of the same response component to different frequencies within the same neuron. For responses to tone onset (but not "through" and "off" responses), agonists were more likely to produce a decrease at the BF while simultaneously increasing responses to other frequencies. Agonists could increase or decrease frequency selectivity. Effects of agonists could be blocked by atropine, suggesting involvement of muscarinic receptors.