Cortical receptive fields represent the signal preferences of sensory neurons. Receptive fields are thought to provide a representation of sensory experience from which the cerebral cortex may make interpretations. While it is essential to determine a neuron's receptive field, it remains unclear which features of the acoustic environment are specifically represented by neurons in the primary auditory cortex (AI). We characterized cat AI spectrotemporal receptive fields (STRFs) by finding both the spike-triggered average (STA) and stimulus dimensions that maximized the mutual information between response and stimulus. We derived a nonlinearity relating spiking to stimulus projection onto two maximally informative dimensions (MIDs). The STA was highly correlated with the first MID. Generally, the nonlinearity for the first MID was asymmetric and often monotonic in shape, while the second MID nonlinearity was symmetric and nonmonotonic. The joint nonlinearity for both MIDs revealed that most first and second MIDs were synergistic and thus should be considered conjointly. The difference between the nonlinearities suggests different possible roles for the MIDs in auditory processing.

Sensory cortical anatomy has identified a canonical microcircuit underlying computations between and within layers. This feedforward circuit processes information serially from granular to supragranular and to infragranular layers. How this substrate correlates with an auditory cortical processing hierarchy is unclear. We recorded simultaneously from all layers in cat primary auditory cortex (AI) and estimated spectrotemporal receptive fields (STRFs) and associated nonlinearities. Spike-triggered averaged STRFs revealed that temporal precision, spectrotemporal separability, and feature selectivity varied with layer according to a hierarchical processing model. STRFs from maximally informative dimension (MID) analysis confirmed hierarchical processing. Of two cooperative MIDs identified for each neuron, the first comprised the majority of stimulus information in granular layers. Second MID contributions and nonlinear cooperativity increased in supragranular and infragranular layers. The AI microcircuit provides a valid template for three independent hierarchical computation principles. Increases in processing complexity, STRF cooperativity, and nonlinearity correlate with the synaptic distance from granular layers.auditory cortex ͉ cortical laminae ͉ information ͉ spectrotemporal receptive field S ensory cortical processing is achieved through a basic anatomical and functional microcircuit that appears to be repeated across modalities with only minor modifications. It represents a hierarchy of connection patterns, with information proceeding to elements of the circuit in a largely sequential manner. In the main excitatory feed-forward pathway, thalamus sends projections to granular cortical layers. Information proceeds, largely serially, to supragranular and infragranular layers, where it is distributed to cortical and subcortical targets (1, 2). The basic structure of this microcircuit is present in auditory cortex (3), although despite our anatomical knowledge, little is known about whether and how processing principles differ between layers (4).In primary auditory cortex (AI), the organization of each layer is complex, and much like a nucleus, has specific sources, targets, and local projection patterns, in addition to intralaminar and interlaminar connections (5). Further, the ascending and descending outputs originating from each layer likely serve different purposes, because each targets different regions of the auditory system (6). Because of this complexity, general rules that capture how stimulus representations change between layers remain unclear. To date, only a few parameters have been identified that remain relatively constant across cortical depth, including preferred frequency (7) and aurality (8-11). However, even for those parameters, deviations from uniformity have been observed (12, 13).The complexity of auditory cortical anatomy leads to two main schemes. The first is that no general processing rules exist, because it has been speculated that microcircuits are too diverse to generate universa...