All sensory systems encode information about the external world through the activity of neurons in the primary sensory cortex. With the exception of olfaction, sensory neuronal signals are relayed through the thalamus to the primary sensory cortex and higher cognitive centers for processing and, ultimately, for perception. These neural representations of sensory input become increasingly abstract as they progress throughout the brain. Since organisms use the neural representations to make behavioral decisions, it is important to understand the representation and transformation of neural signals in the early stages of the sensory pathway. In this thesis, we investigate mechanisms of encoding that shape neural representations and transformations in the rat vibrissa system. We describe a thalamocortical transformation that is dependent on stimulus history, construct a functional model that predicts complex responses of cortical neurons, and elucidate differences in encoding properties between transient and steady-state cortical responses.

Rats actively explore the outside world using their vibrissae, and contact with features in the sensory environment induces complex temporal vibrissae deflection patterns. We found that the recorded response of cortical neurons to temporal patterns of vibrissa deflection was nonlinearly dependent on the stimulus history due to interactions between excitation and suppression. We constructed a functional model that predicted cortical activity in response to novel deflection patterns, based on the amount of suppression induced throughout a stimulus. By constructing a biophysical model of layer IV cortical circuitry, driven by recorded thalamic inputs, we found that the temporal precision of thalamic spikes was crucial for shaping cortical responses to transient stimuli. However, during adaptation to periodic deflection patterns, the cortical sensitivity to input precision decreased throughout the stimulus train and was lost at steady-state. Similarly, altering the initial state of the thalamocortical system strongly affected transient cortical responses, yet neurons reached the same steady-state adapted response. Although adaptation has significant effects on transient cortical activity, both transient and steady-state cortical adaptation dynamics were well characterized by our functional model. These results show that the interplay between cortical excitation and suppression dynamically shapes thalamic inputs into cortical responses that ultimately represent sensory features of the external world.