Jd. Victor et Kp. Purpura, SPATIAL PHASE AND THE TEMPORAL STRUCTURE OF THE RESPONSE TO GRATINGS IN V1, Journal of neurophysiology, 80(2), 1998, pp. 554-571
We recorded single-unit activity of 25 units in the parafoveal represe
ntation of macaque V1 to transient appearance of sinusoidal gratings.
Gratings were systematically varied in spatial phase and in one or two
of the following: contrast, spatial frequency, and orientation. Indiv
idual responses were compared based on spike counts, and also accordin
g to metrics sensitive to spike timing. For each metric, the extent of
stimulus-dependent clustering of individual responses was assessed vi
a the transmitted information, H. In nearly all data sets, stimulus-de
pendent clustering was maximal for metrics sensitive to the temporal p
attern of spikes, typically with a precision of 25-50 ms. To focus on
the interaction of spatial phase with other stimulus attributes, each
data set was analyzed in two ways. In the ''pooled phase'' approach, t
he phase of the stimulus was ignored in the assessment of clustering,
to yield an index H-pooled. In the ''individual phases: approach, clus
tering was calculated separately for each spatial phase and then avera
ged across spatial phases to yield an index H-indiv. H-pooled expresse
s the extent to which a spike train represents contrast, spatial frequ
ency, or orientation in a manner which is not confounded by spatial ph
ase (phase-independent representation), whereas H-indiv expresses the
extent to which a spike train represents one of these attributes, prov
ided spatial phase is fixed (phase-dependent representation). Here, re
presentation means that a stimulus attribute has a reproducible and sy
stematic influence on individual responses, not a neural mechanism for
decoding this influence. During the initial 100 ms of the response, c
ontrast was represented in a phase-dependent manner by simple cells bu
t primarily in a phase-independent manner by complex cells. As the res
ponse evolved, simple cell responses acquired phase-independent contra
st information, whereas complex cells acquired phase-dependent contras
t information. Simple cells represented orientation and spatial freque
ncy in a primarily phase-dependent manner, but also they contained som
e phase-independent information in their initial response segment. Com
plex cells showed primarily phase-independent representation of orient
ation but primarily phase-dependent representation of spatial frequenc
y. Joint representation of two attributes (contrast and spatial freque
ncy, contrast and orientation, spatial frequency and orientation) was
primarily phase dependent for simple cells, and primarily phase indepe
ndent for complex cells. In simple and complex cells, the variability
in the number of spikes elicited on each response was substantially gr
eater than the expectations of a Poisson process. Although some of thi
s variation could be attributed to the dependence of the response on t
he spatial phase of the grating, variability was still markedly greate
r than Poisson when the contribution of spatial phase to response vari
ance was removed.