Abstract

Neurons in the recipient layers of sensory cortices receive excitatory input from two major sources: the feedforward thalamocortical and recurrent intracortical inputs. To address their respective functional roles, we developed a new method for silencing cortex by competitively activating GABA_A while blocking GABA_B receptors. In the rat primary auditory cortex, in vivo whole-cell recording from the same neuron before and after local cortical silencing revealed that thalamic input occupied the same area of frequency-intensity tonal receptive field as the total excitatory input, but showed a flattened tuning curve. In contrast, excitatory intracortical input was sharply tuned with a tuning curve that closely matched that of suprathreshold responses. This can be attributed to a selective amplification of cortical cells' responses at preferred frequencies by intracortical inputs from similarly tuned neurons. Thus, weakly tuned thalamocortical inputs determine the subthreshold responding range, whereas intracortical inputs largely define the tuning. Such circuits may ensure a faithful conveyance of sensory information.

1. Zilkha Neurogenetic Institute, University of Southern California, 1501 San Pablo Street, Los Angeles, California 90033, USA.
2. Department of Physiology & Biophysics, University of Southern California, 1501 San Pablo Street, Los Angeles, California 90033, USA.
3. Neuroscience Graduate Program, University of Southern California, 1501 San Pablo Street, Los Angeles, California 90033, USA.
4. Department of Ophthalmology, Keck School of Medicine, University of Southern California, 1501 San Pablo Street, Los Angeles, California 90033, USA.
5. Present address: Department of Child & Adolescent Psychiatry, School of Medicine, New York University, 550 First Avenue, New York, New York 10016, USA.
6. These authors contributed equally to this work.

Correspondence to: Li I Zhang^1,2 e-mail: liizhang@usc.edu

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