Fig 2. Avian song system and branched chain network with adapting auditory feedback.
a: Diagram of the avian song system. HVC is a sensory-motor integration area that receives auditory input from high-level auditory nuclei such as NIf (nucleus interfacealus), and sends temporally precise motor controls signals to nucleus RA (robust nucleus of the arcopallium), which projects to the vocal brainstem areas. There is a pre-motor latency of 30–50 ms (ΔT Motor) between activity in HVC and subsequent vocalization. Additionally, there is a latency of 15–20 ms (ΔT Auditory) for auditory activity to reach HVC. This makes for a total auditory-motor latency between pre-motor activity and resulting auditory feedback of 45–70 ms. b: Example of a branch point in a probabilistic sequence (left). Syllable ‘a’ can transition to either syllable ‘b’ or ‘c’. Such probabilistic sequences can be produced by a branched chain network (right). Here, each syllable is produced by a syllable-chain, in which groups of HVCRA neurons (grey dots in red ovals, grouped in grey rectangles for a given syllable) are connected unidirectionally in a feed-forward chain (black lines with triangles are excitatory connections). The end of chain-a connects to the beginning of chain-b and chain-c. Spike activity propagates through chain-a and drives downstream neurons in RA to produce syllable a. At the end of chain-a, the activity continues to chain-b or chain-c via the branched connections. Only one syllable chain can be active at a time, as enforced by winner-take-all mechanisms mediated through local feedback inhibition from the HVCI neurons (red lines are inhibitory connections). c: The branched chain network with adapting auditory feedback for generating repeating sequences of syllable ‘b’. The end of chain-b reconnects to its beginning and to chain-c. Auditory feedback from syllable ‘b’ is applied to chain-b, and biases the repeat probability when the activity propagates to the branching point. The feedback is weakened as syllable ‘b’ repeats due to use-dependent synaptic depression which models stimulus-specific adaptation of the auditory signal.