Abstract
The middle temporal (MT) and dorsomedial (DM) areas are densely myelinated subdivisions of extrastriate cortex which receive direct projections from V1. MT is characterised by strong direction selectivity and DM neurons are narrowly tuned for orientation. It has been established that visual cortical receptive fields in V1 and MT show more extensive spatial summation at lower contrasts, and that surround inhibition is more prominent at higher contrasts. We examined the responses of 129 MT neurons and 79 DM neurons in marmoset monkeys anaesthetised with sufentanil (6mg.kg-1.hr-1) and N2O (70% in O2). Stimuli were drifting sinewave gratings of optimal orientation, presented at optimal direction of motion within rectangular windows at 66% contrast. Lengths and widths of gratings (the latter being the dimension parallel to the direction of motion) were varied independently. Stimuli of near optimal dimensions were then presented at a range of contrasts. In MT, cells that showed peak responses upon presentation of wide gratings were less sensitive to low contrasts (r=0.29, p<0.005), while cells showing significant side-inhibition were more sensitive to low contrasts (p<0.0001). Similarly, in DM, cells that preferred narrow gratings were more sensitive to low contrasts (r=0.33, p=0.003), and those that preferred wide gratings (i.e. showing little or no side-inhibition) were less sensitive to low contrasts (p<0.001). The contrast sensitivity of neurons in both areas was not related to the preferred grating length, or strength of end-inhibition. In summary, in both areas the relationship between summation field size and contrast is more obviously expressed along the width dimension of receptive fields. The common pattern of response in these extrastriate areas, which have otherwise very different response properties, suggest a common biological implementation in the form of a generalized cortical circuit, or computations performed at a common source of inputs.
Meeting abstract presented at VSS 2012