  We investigate theoretically the effect of spatial attention on the
contrast-response function (CRF) and orientation-tuning curves in early visual
areas.We look at a model of a hypercolumn developed recently (Persi et al.,
2008), that accounts for both the contrast response and tuning properties in
the primary visual cortex, and extend it to two visual areas.
  The effect of spatial attention is studied in a model of two inter-connected
visual areas, under two hypothesis that do not necessarily contradict. The
first hypothesis is that attention alters inter-areal feedback synaptic
strength, as has been proposed by many previous studies. A second new
hypothesis is that attention effectively alters single neuron input-output
properties. We show that with both mechanisms it is possible to achieve
attentional effects similarly to those observed in experiments, namely
contrast-gain and response-gain effects, while keeping the orientation-tuning
curves width approximately contrast-invariant and attention-invariant.
Nevertheless, some differences occur and are discussed. We propose a simple
test on existing data based on the second hypothesis.
