Synaptic mechanisms of direction selectivity in primary auditory cortex

Abstract

Frequency modulation (FM) is a prominent feature in animal vocalization and human speech. Although many neurons in the auditory cortex are known to be selective for FM direction, the synaptic mechanisms underlying this selectivity are not well understood. Previous studies of both visual and auditory neurons have suggested two general mechanisms for direction selectivity: (1) differential delays of excitatory inputs across the spatial/spectral receptive field and (2) spatial/spectral offset between excitatory and inhibitory inputs. In this study, we have examined the contributions of both mechanisms to FM direction selectivity in rat primary auditory cortex. The excitatory and inhibitory synaptic inputs to each cortical neuron were measured by in vivo whole-cell recording. The spectrotemporal receptive field of each type of inputs was mapped with random tone pips and compared with direction selectivity of the neuron measured with FM stimuli. We found that both the differential delay of the excitatory input and the spectral offset between excitation and inhibition are positively correlated with direction selectivity of the neuron. Thus, both synaptic mechanisms are likely to contribute to FM direction selectivity in the auditory cortex. Finally, direction selectivity measured from the spiking output is significantly stronger than that based on the subthreshold membrane potentials, indicating that the selectivity is further sharpened by the spike generation mechanism.

Figure 1.

Schematic illustration of two potential circuit mechanisms for direction selectivity. A–C, Differential latency mechanism. Tone pips at frequencies f1 and f2 evoke responses with long and short latencies, respectively (B). When FM sound sweeps from f1 to f2 (preferred direction), the responses to f1 and f2 arrive simultaneously, giving rise to a large summed response (C, solid line). For null direction, the responses to f1 and f2 arrive at different times, resulting in a summed response with low amplitude (C, dotted line). D–F, Asymmetric inhibition mechanism. Tone f1 evokes an excitatory response, and tone f2 evokes a delayed inhibitory response (E). For preferred direction, FM sound sweeps across the excitatory region first and then evokes a delayed inhibition, so the inhibitory response fails to suppress the excitatory response (F, solid line). For null direction, inhibitory response is evoked first, which effectively suppresses the subsequent excitatory response (F, dotted line).

Figure 2.

Whole-cell recording from direction-selective neurons in A1. A–C, An example neuron that preferred upward FM stimulus. D–F, Another neuron that preferred downward FM stimulus. A, D, Superimposed traces depict membrane potential changes evoked by 60 repeats of upward and downward FM stimuli (at 70 octaves/s). Each trace represents response in one trial. Calibration: 10 mV, 20 ms. B, E, PSTHs of spikes extracted from the raw traces. C, F, Subthreshold membrane potentials extracted from the raw traces, averaged across all trials. The DIs of these two cells are 1 and −0.62 (based on spike rate) or 0.54 and −0.18 (based on amplitude of subthreshold responses), respectively.

Figure 3.

Separation of excitatory and inhibitory conductances under voltage clamp. A, Synaptic currents evoked by a tone pip (duration indicated by horizontal gray bar), recorded at three holding potentials. Each trace was averaged from 100 trials. Calibration: 30 pA, 30 ms. B, Synaptic current amplitudes at 30 ms after stimulus onset (indicated by vertical dashed line in A) plotted against the clamping membrane potentials. Each point represents data from one trial. Solid line, linear regression; bigger gray circles, averaged value across trials. The slope and x-intercept of the linear regression represent the total conductance (1.83 nS) and synaptic reversal potential (−56.6 mV), respectively. C, Separated excitatory (black) and inhibitory (gray) conductances in response to the tone pip. Calibration: 0.3 nS, 30 ms.

Figure 4.

Excitatory STRF exhibits differential latency at different sound frequencies. A, STRF of excitatory conductance of an example neuron. Circle indicates the time of peak response at each frequency. The stimulus onset time is defined as 0. Note the systematic shift of peak amplitude time in the excitatory STRF. B, Fourier transform of the STRF at 10–70 ms from stimulus onset. FF, Fourier frequency of the sound frequency; TF, temporal frequency. The contour plot (gray lines) shows higher amplitude at first and third quadrants, which corresponds to the latency shift in A. The predicted DI of excitatory inputs (based on the Fourier transform) was 0.39, and the DI measured with FM stimuli was 0.24. C, D, Predicted and actual excitatory conductances evoked by upward and downward FM sweeps, respectively. E, DI of excitatory input predicted from the Fourier transform of STRF versus DI of the neuron measured with FM sweeps. Each circle represents one cell (n = 38). Thick dashed line indicates linear fit (cc = 0.59, p = 0.0001).

Figure 5.

Inhibitory STRF exhibits less extended differential latency at different sound frequencies. A–D, Same cell and same analysis as that shown in Figure 4, except that the inhibitory STRF is examined. The predicted DI of inhibitory inputs (based on the Fourier transform) was −0.05, and the DI measured with FM stimuli was 0.24. FF, Fourier frequency of the sound frequency; TF, temporal frequency. E, DI of inhibitory inputs predicted from the fast Fourier transform (FFT) of STRFs versus DI of the neuron measured with FM sweeps. Each circle represents one cell (n = 37). Thick dashed line indicates linear fit (cc = 0.39, p = 0.02).

Figure 6.

Spectral offset between excitatory and inhibitory STRFs. A, STRFs of excitatory and inhibitory inputs of an example neuron. B, Integrated excitatory (black) or inhibitory (gray) conductances over 10–70 ms after stimulus onset (between dashed lines in A), as a function of the sound frequency. Arrowheads indicate the center of mass of each function. The distance between inhibitory and excitatory tuning curves (E–I distance) is measured by the spectral distance between the two arrowheads, a value of 0.40 octave for this cell. C, D, Predicted (left) and actual (right) excitatory (G_e) and inhibitory (G_i) conductances evoked by upward (C) and downward (D) FM sweeps, respectively. Circles denote the onset time of excitatory or inhibitory conductance. For upward sweep, G_e precedes G_i in the prediction and actual measurement by 12 and 24 ms, respectively. For downward FM sweep, the values were 2 and −7 ms, respectively. E, E–I distance versus DI of the neuron measured by FM sweeps. Each circle represents one cell (n = 29). Thick dashed line represents linear fit of the data (cc = 0.43, p = 0.02).

Figure 7.

Sharpening of direction selectivity by spike generation. DI measured from spike rate is plotted against that from subthreshold membrane potentials. Data are from 11 neurons, each measured at multiple sound intensities. Each circle represents a test at a particular intensity (n = 31). Solid line indicates linear fit (slope of 1.72, y-intercept = −0.0002, cc = 0.59, p = 0.0004).