  Neurons in primary auditory cortex (AI) in the ferret (Mustela putorius) that
are well described by their spectro-temporal response field (STRF), are found
also to have a distinctive property that we call temporal symmetry. For
temporally symmetric neurons, every temporal cross-section of the STRF (impulse
response) is given by the same function of time, except for a scaling and a
"Hilbert rotation". This property held in 85% of neurons (123 out of 145)
recorded from awake animals, and in 96% of neurons (70 out of 73) recorded from
anesthetized animals. This property of temporal symmetry is highly constraining
for possible models of functional neural connectivity within and into AI. We
find that the simplest models of functional thalamic input, from the ventral
Medial Geniculate Body (MGB), into the entry layers of AI are ruled out because
they are incompatible with the constraints of the observed temporal symmetry.
This is also the case for the simplest models of functional intracortical
connectivity. Plausible models that do generate temporal symmetry, both from
thalamic and intracortical inputs, are presented. In particular, we propose
that two specific characteristics of the thalamo-cortical interface may be
responsible. The first is a temporal mismatch between the fast dynamics of the
thalamus and the slow responses of the cortex. The second is that all thalamic
inputs into a cortical module (or a cluster of cells) must be restricted to one
point of entry (or one cell in the cluster). This latter property implies a
lack of correlated horizontal interactions across cortical modules during the
STRF measurements. The implications of these insights in the auditory system,
and comparisons with similar properties in the visual system, are explored.
