Visual response characteristics of neurons in the nucleus isthmi magnocellularis and nucleus isthmi parvocellularis of pigeons.

The visual response characteristics of single cells in the nucleus isthmi (NI) of pigeons were investigated using standard extracellular recording techniques. The results show that both major components of NI, the parvocellular NI (Ipc) and the magnocellular NI (Imc), have a tight retinotopic organization where nasal regions of the visual field are mapped onto the rostral poles of Ipc and Imc, and temporal regions of the visual field are mapped onto the caudal poles. The more ventral regions of these nuclei receive input from more inferior regions of visual space. The receptive fields (RFs) of both Ipc and Imc are large and oval-shaped, and their long axis is oriented vertically in the visual field. Most RFs are distributed on the contralateral visual horizon, and no binocular responses were found in either Ipc or Imc. All of the excitatory RFs of NI cells were surrounded by large inhibitory regions which participated in the dramatic modulation of the driven visual response when a large background pattern was moved across this zone. Although both Ipc and Imc neurons are driven best by small dark spots, some of them also show orientation selectivity to bars which may result from their oval-shaped RF (74% of Imc cells, 20/38, were orientation selective as compared to 10% of the Ipc cells, 3/30). It is suggested that some tectal cells with small RFs, and which originate from a vertically oriented zone may converge onto a single NI neuron to produce the elliptical shaped receptive fields.