Geometry of the representation of the visual field on the superior colliculus of the wallaby (Macropus eugenii). I. Normal projection.

In 13 wallabies (Macropus eugenii, the tammar), microelectrode recordings of the activity of units in the superficial layers of the superior colliculus in response to a flashing light spot were used to make a map of the spatial location of their receptive fields. This article describes the projection of a normal eye to the contralateral colliculus. Ten of the 13 animals had one rotated eye and these projections are analysed in the accompanying paper (James et al., this issue). Units responded briskly to the stimulus at light on and off and had receptive fields about 5 degrees across. The centres of receptive fields from a regular array of recording points on the colliculus were plotted with a perimeter and fitted to a flattened representation of the colliculus according to a spline technique. The visual field of each colliculus extends from 25 degrees ipsilateral to the vertical meridian to 120 degrees temporal contralaterally. The lines of isoazimuth are regularly spaced and parallel and run mediolaterally on the colliculus. The horizon is represented by a line running rostrocaudally and the parallels are more widely spaced near the horizon and become compressed in the superior and inferior fields. The variation of areal magnification factor fits the distribution of density of retinal ganglion cells very well. Anisotropy of the projection means that the increased ganglion cell density of the retinal visual streak is entirely accommodated by magnification in the vertical direction, while the magnification of the azimuthal projection is equal over the whole field. No responses were recorded from the ipsilateral eye even though anatomically there is a direct retinal ipsilateral projection.