Previous studies of cat visual cortex have shown
that the spatiotemporal (S-T) structure of simple cell
receptive fields correlates with direction selectivity.
However, great heterogeneity exists in the relationship
and this has implications for models. Here we report a
laminar basis for some of the heterogeneity. S-T structure
and direction selectivity were measured in 101 cells using
stationary counterphasing and drifting gratings, respectively.
Two procedures were used to assess S-T structure and its
relation to direction selectivity. In the first, the S-T
orientations of receptive fields were quantified by fitting
response temporal phase versus stimulus spatial phase data.
In the second procedure, conventional linear predictions
of direction selectivity were computed from the amplitudes
and phases of responses to stationary gratings. Extracellular
recording locations were reconstructed histologically.
Among direction-selective cells, S-T orientation was greatest
in layer 4B and it correlated well (r = 0.76)
with direction selectivity. In layer 6, S-T orientation
was uniformly low, overlapping little with layer 4B, and
it was not correlated with directional tuning. Layer 4A
was intermediate in S-T orientation and its relation (r
= 0.46) to direction selectivity. The same laminar patterns
were observed using conventional linear predictions. The
patterns do not reflect laminar differences in direction
selectivity since the layers were equivalent in directional
tuning. We also evaluated a model of linear spatiotemporal
summation followed by a static nonlinear amplification
(exponent model) to account for direction selectivity.
The values of the exponents were estimated from differences
between linearly predicted and actual amplitude modulations
to counterphasing gratings. Comparing these exponents with
another exponent—that required to obtain perfect
matches between linearly predicted and measured directional
tuning—indicates that an exponent model largely accounts
for direction selectivity in most cells in layer 4, particularly
layer 4B, but not in layer 6. Dynamic nonlinearities seem
essential for cells in layer 6. We suggest that these laminar
differences may partly reflect the differential involvement
of geniculocortical and intracortical mechanisms.
