Cell type diversity and (some) functional connectivity of the fish optic tectum. (a) Cells described from classical Golgi studies in the adult goldfish tectum . Fourteen types of neuron were identified on the basis of cell body position and morphology. Modified from . (b) A sampling of neuron morphologies observed in the larval zebrafish tectum using 'genetic Golgi' methods. These include: radial glia (RG), periventricular projection neurons (PVPNs), periventricular interneurons (PVINs) and superficial interneurons (SINs). Retinorecipient laminae in the tectum are indicated by shading. Note the diverse dendrite morphologies of both projection neurons and interneurons in the tectum. In particular, PVINs have been observed containing arbors that are non-stratified (nsPVINs), mono-stratified (msPVINs) or bi-stratified (bsPVINs). (c) Hypothetical neural circuit responsible for size tuning of PVNs in the optic tectum . Retinal afferents targeting the superficial layers of the SO and SFGS form excitatory synapses onto PVINs containing superficial dendrites and an axonal arbor in a deeper layer. These PVINs may mediate the vertical flow of excitation in response to small visual stimuli by activating PVPNs with dendrites located in deeper neuropil layers. In contrast, large visual stimuli additionally activate SIN cells, which inhibit the PVIN-mediated vertical flow of information to PVPNs.