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Fig. 2 (A) An example tectal image from a 15-dpf fish. The neuropil (NP) contour of each fish was fitted with an ellipse, with the major axis defining the tectal anterior-posterior (AP) axis. Periventricular layer (PVL), NP, anterior (A), and posterior (P) ends of the tectum are indicated. Inset: larvae were embedded in agarose with one eye facing the projected image for 2-photon calcium imaging. We recorded evoked activity at a depth of 70 μm in response to 20 trials of the stimulus set consisting of spots at positions 45°, 60°, 75°, 90°, 105°, 120°, 135°, 150°, and 165° of the visual field, where 0° was defined as the body axis. (B) Schematic of retinotectal projection showing temporal retinal ganglion cells (green, representing the frontal visual field) projecting to anterior tectum and nasal ganglion cells (blue, representing the rear visual field) projecting to posterior tectum. The dashed black rectangle represents the tectal region shown in (A). (C) Population response of 187 PVL neurons from the fish shown in (A) elicited by two trials of visual stimuli set. Neurons are sorted by their position on the AP axis. (D) Raw receptive fields of 217 and 187 PVL neurons in an example 4 dpf (left) and 15 dpf (right) fish, respectively. Neurons are sorted by their position on the AP axis and display a rough topography. (E) Raw (black) and fitted (blue) receptive field of an example neuron from a 15 dpf fish shown in (C) and (D) (neuron 123). (F) Mean receptive field width did not change over development (t test). (G) More neurons were tuned to the rear visual field at 4 dpf, but this gradually balanced over development. Shading represents SEMs. Comparing the area under the curve grouped by age showed differences as follows: 4 versus 5, n.s.; 4 versus 9, ; 4 versus 15, ; one-way ANOVA with Bonferroni multiple-comparison correction. (H) Neuronal selectivity shifted over development so that at each tectal position, neuronal tuning moved in a frontal direction. Shading represents SEMs. A linear fit to these curves showed similar slopes but different intersection points with the x axis, indicating that maps are shifted (4 versus 15, , for slopes, and for intercepts t test). (I) Topography improved over development (two-sample t test). Variability also decreased over development (4 and 5 versus 9 and 15, , two-sample F-test). (J) At 4 dpf, the population response amplitude was stronger in response to the rear visual field, but this balanced over development to give a more even response amplitude to stimuli across the visual field. (K) At 4 dpf, a larger proportion of neurons were active in response to stimuli in the rear visual field, but again, this balanced over development.