Figure 2.

Basic visual processing in the accessory optic system in birds. (A) and (B), Respectively, show Nissl-stained coronal sections through the pigeon lentiformis mesenceophali (LM) and nucleus of the basal optic root (nBOR). (C) Indicates that most nBOR and LM neurons have large receptive fields in the contralateral visual field and exhibit directional tuning in response to largefield motion (e.g., Burns and Wallman, 1981; Winterson and Brauth, 1985). (D) Shows the response of a nBOR neuron to upward (excitation) and downward (inhibition) motion of a large drifting sine-wave grating (from Crowder and Wylie, 2001). (E) Shows a directional tuning curve of a typical nBOR neuron. Firing rate (spikes/s) is plotted as a function of the direction of motion in polar coordinates, and the gray circle represents the neuron's spontaneous firing rate. The directions are indicated as follows: U, upward; D, downward; F, forward or temporal-to-nasal (T-N), and B, backward or nasal-to-temporal (N-T). (F) Shows a distribution of the direction preferences of LM neurons in pigeons: most prefer forward motion (adapted from Wylie and Crowder, 2000). (G) Shows a distribution of the direction preferences of nBOR neurons in pigeons: most prefer upward, downward, or backward motion (Crowder et al., 2003).