By anatomical techniques it has been shown that folia VIc-IXc of the pigeon cerebellum receive inputs from the following groups of neurons: the medial and lateral pontine nuclei, the superficial synencephalic nucleus, the medial spiriform nucleus, the inferior olive, and the deep cerebellar nuclei. From all but the last of these, the projection is mainly crossed, though the uncrossed component from the lateral pontine nucleus is not insubstantial. The input from the superficial synencephalic nucleus provides a direct pathway from the retina to the cerebellum (folia VIc, VII, VIII and IXc). Less direct visual pathways reach the cerebellum via the following routes: (i) the superficial synencephalic nucleus projects ipsilaterally to the lateral pontine nucleus and sparsely to the inferior olive; (ii) the tectum projects ipsilaterally to the lateral and medial pontine nuclei, though the latter connection is sparse. In electrophysiological experiments, the importance of the tecto-pontine component of the projection has been demonstrated by cooling the tectum while recording visual responses from the cerebellum. The visual receptive fields of pontine cells have been analysed. They vary in extent from 10 degrees to the whole monocular field. They respond best to moving targets, preferring speeds of 20 to 60 degrees/second, and are usually direction-selective.

The optic tectum of the pigeon has been mapped physiologically, including much of the inferior surface. There are two separate regions of high magnification factor, the projection areas of the fovea and the middle of the red area in the retina, each of which has a correspondingly high density of neurones in the ganglion cell layer.Many avian retinae differ from those of mammals in that they contain two regions of increased ganglion cell density instead of one. In hawks there are two foveae: one central looking laterally, and the other in the temporal retina looking forwards [Rochon-Duvigneaud, 1943]. In pigeons there is only one true fovea for each eye, but there is a second area in the superior temporal retina, known as the 'red area', whose ganglion cell density almost equals that of the fovea [Whitteridge, 1965;Galifret, 1968;Binggeli and Paule, 1969], and whose synaptic organization is particularly complex [Yazulla, 1974]. The red area receives the image of the visual field in the region of the beak, for all stationary positions of the head, since the static component of ocular counterrotation following head movements is less than 10% [Benjamins and Huizinga, 1927].These facts suggest that the red area has a specialized function, and Nye's [1973] behavioural experiments suggest that it is superior to even the fovea for analyzing image structure, motion, colour and pattern. It is therefore of interest to locate the red area's projection site on the tectum and to determine its magnification factor. Previous electrophysiological maps [e.g. Hamdi and Whitteridge, 1954;Bilge, 1971] are inadequate for the lower-tectal surface where the red area projects, and the map of McGill, Powell and Cowan [1966], based on fibre degeneration following retinal lesions, is unavoidably coarse. We have therefore mapped the tectum in some detail, including much of the lower surface, and have compared the magnification factor for the red area with that for other retinal regions. METHODS Retinal histologyThe eyes were fixed in Kolmer's fluid [Walls, 1938], stored in terpineol for several weeks, embedded in paraffin wax, cut in horizontal or vertical planes, and stained with cresyl violet. Of four eyes taken, three provided acceptable sections 10 ,um thick. On the assumption that 351.. ICA

1. Units were recorded in the primary and secondary visual areas (V1 and V2) of the sheep. They were stimulated binocularly, using an adjustable prism to vary the disparity. 2. Cells in V1 responded optimally to stimuli with very small or zero disparities, but cells in V2 frequently preferred disparities of several degrees crossed or uncrossed. Many cells in V2 were particularly selective to disparity, often giving no response to a monocular stimulus. 3. Cells preferring the same disparity occur in discrete columns, about 400 muM wide. Changes between columns result from a step displacement of the receptive field of one eye. The disparities encoded in successive columns seem to follow a regular sequence: crossed, zero, uncrossed, zero, etc. 4. In both V1 and V2, cells are clustered, perhaps in columns, according to their orientation preference and ocular dominance. In V2, the constant disparity columns appear to be independent of the orientation clusters.

1. A stereotaxic method for the sheep brain is described. 2. At its widest part the primary visual area (Visual I) of each hemisphere extends approximately 20 mm anteroposteriorly and, when unfolded, approximately 35 mm from side to side. It occupies both walls of the lateral sulcus, and extends medially to the medial wall of the hemisphere and to the depth of the ectolateral sulcus laterally. 3. The most lateral part of the primary visual area includes 10‐15 degrees of the ipsilateral field; the contralateral field is represented to 135 degrees from the mid line. 4. Visual II also includes a strip of ipsilateral representation on its medial edge and extends to the supra‐sylvian sulcus on the lateral surface of the brain. The furthest lateral representation recorded was 130 degrees lateral. 5. Most of both visual areas is concerned with the area centralis and the visual streak. The remainder of the retina has very little cortical representation. 6. Most cells in Visual I are simple with orientational and sometimes directional sensitivity. Some complex and hypercomplex cells have been seen in Visual I, and these predominate in Visual II. Receptive field sizes from 0‐25 to 10 degree were found. Within 15 degrees of the vertical meridian, binocular cells are common in both Visual I and II.

SUMMARY1. Visual responses were sought in the cerebella of decerebrate pigeons using extracellular micro-electrodes, and were found in folia VIc-IXb, especially folia VII and VIII. The responses were mainly, but not exclusively, from the ipsilateral eye. Four binocular units were recorded.2. In the anterior and posterior walls of folium VII the organization was clearly, though rather crudely, retinotopic. The temporal field was represented laterally, on the ipsilateral side, and the nasal field medially; the superior field was represented superficially, and the inferior field towards the base of the folium. In the lateral wall of folium VII there was a small anomalous region innervated by the contralateral eye.3. The visual input arrived via the mossy fibre system. 4. Units exhibited a strong preference for moving targets, 20-60'/sec being the range of optimal speeds. About three quarters of the units responded most strongly to a particular direction of target motion. The preferred direction was most frequently upwards or backwards.5. Units in the granule layer were to some extent clustered according to their direction-preference, which tended to change gradually as the electrode advanced along the granule layer.6. Units were classified as Gr-units (granule cells or mossy fibre rosettes) or P-units (Purkinje cells) or were left unclassified. Receptive fields of Gr-units were usually 5-30°across; they were much larger for P-units in folium VIII, but not obviously so for those in folium VII. Gr-units were more frequently direction-sensitive than were P-units.