Introduction.
In Spanish both the Simple Present (canta) and the Present Progressive (está cantando) can convey a Progressive meaning (‘is singing’). While some consider that these markers are in free variation [1,2], others recognize that they are not free alternants, but still fail to provide specific contextual conditions for their use [3,4].
To better understand the markers’ distribution, I study their diachronic trajectories within a grammaticalization path in which a Present Progressive marker gets recruited to express a Progressive meaning whence it encroaches upon the more general Imperfective domain, restricting the contexts of use of older markers, such as the Simple Present [5,6]. Diachronic approaches to this distributional puzzle observe that the Spanish Present Progressive arose from a locative construction, and slowly developed into an autonomous device to signal an aspectual distinction with the Simple Present [7]. But why would locative information matter? I argue that the availability of each marker to convey this meaning ultimately responds to a key feature of the (extra)linguistic context: the use of the Simple Present marker to convey a Progressive meaning is crucially restricted to contexts in which speaker and hearer share perceptual access to the event at issue. By contrast, the Present Progressive marker gradually loses the need for contextual support.

Methods.
A diachronic corpus study analyzed the role of Shared Perceptual Access in linguistic marker choice in Peninsular Spanish of 14th, 17th, and 21st centuries. Present Progressive and Simple Present tokens where these markers express a Progressive meaning (n=1279) were coded for Time Period, and for the presence/absence of Shared Perceptual Access to the relevant event.

Results.
Linear-mixed effect analyses show a significant effect of Time Period (p<.01), due to increased Present Progressive use over time. A main effect of Shared Perceptual Access (p<.001) indicates that the presence of this contextual feature favors the use of the Simple Present. There is crucially an interaction effect between Time Period and Shared Perceptual Access (p<.001). While in the 14th century, Shared Perceptual Access is not a significant predictor in the choice of marker, by the 17th century this contextual feature favors Simple Present use, an effect that increases in size by the 21st century.

Discussion.
We confirm a steady frequency increase of Present Progressive use since its inception. This growth is driven by the development of an aspectual opposition with the Simple Present, in its specialization for the Progressive meaning. In the 14th century, the need for shared perceptual access as a means to disambiguate the meaning at play was similar for both markers. By the 17th century, the Present Progressive became an autonomous device to express the Progressive meaning, while the Simple Present marker still needed contextual support to be felicitous in this use. This difference in need for contextual information increases in the 21st century, where we observe that the Simple Present marker is only available to convey the Progressive meaning when the context guarantees shared perceptual access between speaker and hearer.