The mechanisms that link sensory inputs in spatially separated regions of cortex can be elucidated by analyzing the mechanisms that generate receptive field properties in cortical neurons under conditions that mimic the waking state; a state when learning, memory and the modification of synaptic strength can be most readily demonstrated. Important advances in understanding receptive field mechanisms in sensory cortex have arisen from studying the precise relationship between the mystacial vibrissae or "whiskers" and their neural representation in separate cortical domains or "barrels". The anatomical precision of whisker projections to barrels permits a unique delineation of thalamocortical and intracortical components of cortical cell responses based on latency and security of response to peripheral receptor stimulation. When recorded in awake animals or even under very light anesthesia, cortical neurons show two components to their response to whisker movement. Neurons in layer IV of a whisker's primary projection zone respond with short latency (7-10 msec) and a high response magnitude (two or more action potentials (spikes) per stimulus). This "Center Receptive Field" (CRF) for layer IV cells is generated in large part by sensory fiber inputs from the thalamus. The CRF is restricted to 1.4 whiskers on average and is the only response detectable when cortical responses are depressed by deep anesthesia. In the "waking state" the same neuron often will respond to deflection of 4-6 surrounding whiskers, but only at longer latency (15-40 msec) and with fewer spikes per stimulus. These more labile responses form an excitatory surround receptive field (SRF). Sensory information that is transduced by individual whiskers and that generates the SRF of a cortical neuron achieves this added response complexity through intracortical mechanisms. The control of the mechanisms that determine the dissemination of sensory information within cortex include: (1) regulating the level of GABAergic inhibition; and (2) potentiation or depression of the response level generated by repeated sensory experience. State-dependent "modulatory" inputs to cortex, such as the noradrenergic and cholinergic fiber system, could regulate the degree of horizontal spread of a sensory input, in part through global changes in the level of inhibition and/or regulating the amplitude of cortical responses, thereby determining the level of associative interactions between sensory inputs.