Highlights d Two distinct subtypes of excitatory neurons in superficial retrosplenial cortex (RSC) d Most common neuron in layer 2/3 of RSC is excitatory lowrheobase (LR) neuron d LR intrinsic properties enable precise, sustained encoding of information d Layer 2/3 of RSC is dominated by feedforward, not feedback, inhibition

The granular retrosplenial cortex (RSG) is critical for both spatial and non-spatial behaviors, but the underlying neural codes remain poorly understood. Here, we use optogenetic circuit mapping in mice to reveal a double dissociation that allows parallel circuits in superficial RSG to process disparate inputs. The anterior thalamus and dorsal subiculum, sources of spatial information, strongly and selectively recruit small low-rheobase (LR) pyramidal cells in RSG. In contrast, neighboring regular-spiking (RS) cells are preferentially controlled by claustral and anterior cingulate inputs, sources of mostly non-spatial information. Precise sublaminar axonal and dendritic arborization within RSG layer 1, in particular, permits this parallel processing. Observed thalamocortical synaptic dynamics enable computational models of LR neurons to compute the speed of head rotation, despite receiving head direction inputs that do not explicitly encode speed. Thus, parallel input streams identify a distinct principal neuronal subtype ideally positioned to support spatial orientation computations in the RSG.

The granular retrosplenial cortex (RSG) is critical for both spatial navigation and fear conditioning, but the neural codes enabling these seemingly disparate functions remain unknown. Here, using optogenetic circuit mapping, we reveal a double dissociation that allows parallel circuits in superficial RSG to process navigation- versus fear-related inputs. The anterior thalamus, a source of head direction information, strongly recruits small, low rheobase (LR) pyramidal cells in RSG layer 3. Neighboring regular-spiking (RS) cells are instead preferentially controlled by claustral and anterior cingulate inputs, sources of higher-order and fear-related information. Precise sublaminar axonal and dendritic arborization within RSG layer 1 enable this parallel processing. Synaptic dynamics and computational modeling suggest LR neurons are optimally-tuned conjunctive encoders of direction and distance inputs from the thalamus and dorsal subiculum, respectively. RS cells are better positioned to support contextual fear memories. Thus, parallel input streams to computationally-distinct principal neurons help facilitate diverse RSG functions.

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