Spectral integration in primary auditory cortex attributable to temporally precise convergence of thalamocortical and intracortical input

Abstract

Primary sensory cortex integrates sensory information from afferent feedforward thalamocortical projection systems and convergent intracortical microcircuits. Both input systems have been demonstrated to provide different aspects of sensory information. Here we have used high-density recordings of laminar current source density (CSD) distributions in primary auditory cortex of Mongolian gerbils in combination with pharmacological silencing of cortical activity and analysis of the residual CSD, to dissociate the feedforward thalamocortical contribution and the intracortical contribution to spectral integration. We found a temporally highly precise integration of both types of inputs when the stimulation frequency was in close spectral neighborhood of the best frequency of the measurement site, in which the overlap between both inputs is maximal. Local intracortical connections provide both directly feedforward excitatory and modulatory input from adjacent cortical sites, which determine how concurrent afferent inputs are integrated. Through separate excitatory horizontal projections, terminating in cortical layers II/III, information about stimulus energy in greater spectral distance is provided even over long cortical distances. These projections effectively broaden spectral tuning width. Based on these data, we suggest a mechanism of spectral integration in primary auditory cortex that is based on temporally precise interactions of afferent thalamocortical inputs and different short- and long-range intracortical networks. The proposed conceptual framework allows integration of different and partly controversial anatomical and physiological models of spectral integration in the literature.

Figure 1.

Temporal development of silencing the intracortical contribution of tone-evoked activity in primary auditory cortex. Shown are the CSD profiles across cortical layers (roman numbers) evoked by acoustic stimulation with the BF represented at the measurement site, after different times following topical application of muscimol and SCH50911 to the auditory cortex. Prominent current sources (So1, So2, So3) and current sinks (S1, S2, S3, iS1) are shown in red and blue, respectively. Inset, Custom-made multichannel shaft electrode used for recording. A, Canonical laminar feedforward CSD profile in A1 (for details, see Results). B, At 10 min after application of the silencing mixture, maximum amplitudes of current sources and sinks were reduced to 20% of the original values (see color bar scaling), and the spatial pattern is indicative of diffusion of the mixture from the top layers to the infragranular layers. C, At 20 min after application, amplitudes were further reduced. D, After 30 min, the tone-evoked CSD pattern has stabilized, and CSD amplitude maxima and minima were <10% of the predrug values. Cortical layers were derived from histological analysis of this particular measurement site (supplemental Note 2, animal 1, available at www.jneurosci.org as supplemental material).

Figure 2.

Dissociation of thalamocortical and intracortical contributions of the laminar CSD profile. A, Laminar processing in A1 differed with stimulation frequency (BF, nearBF, or nonBF stimulation). B, After cortical silencing, remaining sinks were observed exclusively after BF and nearBF stimulation in thalamocortical input layer IV (>30 min) (see Fig. 1). Activations after nonBF stimulation were abolished completely. Note that the onset latency of the nearBF-evoked initial granular sink S1 was faster after drug application. In contrast, cortical silencing had no effect on the onset latency of BF-evoked S1 (see dashed line as reference).

Figure 3.

Quantitative comparison of silencing intracortical activity by GABA_A agonist muscimol alone or by applying muscimol in combination with the GABA_B receptor agonist SCH50911 to block possible side effects of muscimol on presynaptic processing of thalamocortical input (see Results). A, Mean ± SEM peak amplitudes of the granular sink S1 evoked by pure tones of different spectral distance to the best frequency. Note the significant decreases of amplitude values at the BF and nearBF frequencies and the total absence of measurable responses at nonBF frequencies (for quantification of the ratios of decrease, see Table 1) after both silencing methods. B, Mean ± SEM onset latency of granular sink S1 decreased after drug application selectively for nearBF stimulation but was constant for BF stimulation. In all cases, we did not find significant differences between muscimol (n = 10) or muscimol plus SCH50911 (n = 3) induced cortical silencing. *p < 0.001 indicates significant changes after drug application (paired t test).

Figure 4.

Laminar processing of convergent afferent thalamocortical input and intracortical input. A, Mean ± SEM onset latencies are shown for the three main sinks (S1, 400–1100 μm; S2, 0–500 μm; S3, 1200–1600 μm) evoked with BF, nearBF, or nonBF stimulation. B, After cortical silencing, the granular sink S1 was still measurable. Shown are the mean ± SEM onset latencies for this sink, evoked by BF and nearBF frequencies (pooled results from ±1–2 octaves distance from BF) before and after cortical silencing. Note that cortical silencing affected the spatial and temporal structure of the granular sink selectively after stimulation with nearBF stimuli. Cortical layers indicated here were derived from histological analysis of five different measurement sites and represent the mean relations (supplemental Note 2, available at www.jneurosci.org as supplemental material).

Figure 5.

Using the residual CSD for dissociating thalamocortical and intracortical contributions to the laminar activation profile. A, Left, The AVREC waveform reflects the temporal characteristics of the overall current flow at a given measurement site (Givre et al., 1994; Schroeder et al., 1998). Relative residues of the CSD reflect the amount of unbalanced sinks and sources (Harding, 1992). The mutually essentially orthogonal orientations of thalamocortical and intracortical input systems can be expected to contribute differently to the relative residues of the CSD measured with a linear electrode array oriented perpendicular to the cortical surface (see traces). Right, Visualization of the orthogonal thalamocortical and intracortical fiber orientation using SMI32 neurofilament staining of gerbil primary auditory cortex (courtesy of Dr. Eike Budinger, Leibniz Institute for Neurobiology, Magdeburg, Germany) reflecting the canonical pattern of fiber orientations in sensory neocortex (overlaid line schematic following Creutzfeldt, 1983). For additional explanation, see Results. B, AVREC (top) and relative residues of the CSD (bottom) after stimulation with pure tones before drug application. Stimulation with frequencies around the BF of the site (0.5–1.0 kHz) evoked maximal AVREC amplitudes and minimal AVREC latencies (red dots indicate amplitude values >3 SD of the baseline variation). Note that, for these frequencies, the latencies of the AVREC are shorter than the latencies of relative residues, indicative of the dominant thalamocortical drive (dashed line). No such latency differences were found for stimulation frequencies more distant from the BF, indicative of substantial intracortical contributions. C, After pharmacological silencing of all intracortical (namely, horizontal) contributions, tone-evoked relative CSD residues were totally abolished. AVREC amplitudes were measurable after stimulation with frequencies around the BF; AVREC onset latency was not affected.

Figure 6.

Comparison of mean ± SEM onset latencies of AVREC and relative residues. A, Mean ± SEM onset latencies of AVREC were significantly shorter for BF stimulation (paired t test, p = 0.0006), but AVREC and relative residues showed no significant differences for nearBF/nonBF stimulation. Mean ± SEM onset latencies of relative residues showed significant differences for BF versus nearBF stimulation (p = 0.023) but were not significantly different for BF versus nonBF stimulation (p = 0.173). B, Cortical silencing blocked all tone-evoked relative residues. Significant AVREC values vanished also for nonBF stimulation. AVREC mean onset latencies for BF and nearBF stimulation showed no significant difference (p = 0.43) (*p < 0.05, paired t test). n.s., Not significant.

Figure 7.

Averaged ± SEM response bandwidths of the granular sink S1 amplitude, the AVREC, and the relative CSD residues, after stimulation with pure tones (+20 dB with regard to threshold at BF) before and after cortical silencing. Values from the untreated cases (n = 13) were compared with cases applying muscimol (n = 10) and muscimol plus SCH50911 (n = 3). Cortical silencing resulted in a decreased response bandwidth of the granular sink S1, as well as the AVREC. Relative residues were blocked completely after silencing. **p < 0.0001 indicates significant changes after drug application (paired t test). n.s., Not significant.