Ge. Baker et al., SPATIAL-FREQUENCY TUNING AND GENICULOCORTICAL PROJECTIONS IN THE VISUAL-CORTEX (AREA-17 AND AREA-18) OF THE PIGMENTED FERRET, European journal of neuroscience, 10(8), 1998, pp. 2657-2668
We have examined the spatial-frequency selectivity of neurons in areas
17 and 18 of the adult pigmented ferret, by measuring how the amplitu
de of response depends on the spatial-frequency of moving sinusoidal g
ratings of optimal orientation and fixed contrast. Neurons in area 17
of the ferret respond optimally to low spatial frequencies [average 0.
25 cycles per degree (c/deg)], much lower than the optima for cat area
17. The tuning curves are of the same form as those found in cat and
monkey: unimodal with bandwidths in the range 0.8-3.5 octaves. Neurons
in area 18 of the ferret respond optimally to even lower spatial freq
uencies (average 0.087 c/ deg) than area 17 neurons, and the distribut
ions of optimal spatial frequency for areas 17 and 18 hardly overlap.
In both cortical areas, the bandwidth of the tuning curves is inversel
y correlated with optimal spatial frequency. This marked difference in
tuning between the two cortical areas is probably attributable to dif
ferential geniculocortical projections. Small injections of fluorescen
t latex microspheres or horseradish peroxidase (HRP) were made into ar
ea 17 or area 18 in order to investigate the populations of geniculate
neurons projecting to the two cortical areas. After injections into a
rea 17, labelled neurons are found predominantly in the geniculate A l
ayers, with a few neurons labelled in the C layers. Conversely, after
an area 18 injection, similar numbers of labelled neurons are found in
the C layers as in the A layers. Soma-size analysis of the neurons in
the A-layers suggests the existence of two populations of relay neuro
ns, which project differentially to areas 17 and 18. The different gen
iculate inputs and the different spatial-frequency tuning in areas 17
and 18 may imply that the two cortical areas process visual informatio
n more in parallel than in series.