An essential step in
understanding visual processing is to characterize the neuronal receptive
fields (RFs) at each stage of the visual pathway. However, RF characterization
beyond simple cells in the primary visual cortex (V1) remains a major
challenge. Recent application of spike-triggered covariance (STC) analysis has
greatly facilitated characterization of complex cell RFs in anesthetized animals.
Here we apply STC to RF characterization in awake monkey V1. We found up to
nine subunits for each cell, including one or two dominant excitatory subunits
as described by the standard model, along with additional excitatory and
suppressive subunits with weaker contributions. Compared with the dominant subunits,
the nondominant excitatory subunits prefer similar orientations and spatial
frequencies but have larger spatial envelopes. They contribute to response
invariance to small changes in stimulus orientation, position, and spatial
frequency. In contrast, the suppressive subunits are tuned to orientations 45°–90°
different from the excitatory subunits, which may underlie crossorientation suppression.
Together, the excitatory and suppressive subunits form a compact description of
RFs in awake monkey V1, allowing prediction of the responses to arbitrary
visual stimuli.
