The retinotopic organization of striate and several extrastriate areas
of ferret cortex has been established. Here we describe the representation
of the visual field on the Suprasylvian visual area (Ssy). This cortical
region runs mediolaterally along the posterior bank of the suprasylvian
sulcus, and is distinct from adjoining areas in anatomical architecture.
The Ssy lies immediately rostral to visual area 21, medial to lateral
temporal areas, and lateral to posterior parietal areas. In
electrophysiological experiments we made extracellular recordings in adult
ferrets. We find that single and multiunit receptive fields range in size
from 2 deg × 4 deg to 21 deg × 52 deg. The total visual field
representation in Ssy spans over 70 deg in azimuth in the contralateral
hemifield (with a small incursion into the ipsilateral hemifield), and
from +36 deg to −30 deg in elevation. There are often two
representations of the horizontal meridian. Furthermore, the location of
the transition from upper to lower fields varies among animals. General
features of topography are confirmed in anatomical experiments in which we
made tracer injections into different locations in Ssy, and determined the
location of retrograde label in area 17. Both isoelevation and isoazimuth
lines can span substantial rostrocaudal and mediolateral distances in
cortex, sometimes forming closed contours. This topography results in
cortical magnifications averaging 0.07 mm/deg in elevation and 0.06
mm/deg in azimuth; however, some contours can run in such a way that
it is possible to move a large distance on cortex without moving in the
visual field. Because of these irregularities, Ssy contains a coarse
representation of the contralateral visual field.