The Paleoneurobiology Group at the Centro Nacional de Investigación sobre la Evolución Humana (CENIEH), led by the researcher Emiliano Bruner, has just published a paper in the journal Perception, on the visual attention patterns associated to the sensory exploration of Paleolithic tools (choppers and handaxes), to discover to what extent attention is influenced by their morphology.

The results of this work, which availed of the collaboration of Timothy Hodgson, professor of Psychology at the University of Lincoln (United Kingdom), suggest that the most functional areas of the tools are considered the most significant and attract more attention, which may be related to identifying the affordances, that is, the characteristics of an object that inform us and allow an action to be taken.

“The worked pebbles or choppers were more closely observed in the center and the top, while for handaxes the gaze was distributed between the base and tip. In general, the knapped surface of both tools drew more attention than the natural or cortical surface”, says María Silva Gago, lead author of this paper.

Factors of attention

There exist different factors that can influence how the attention is directed toward an object or image. On the one hand, the attention can be affected by ascending or "bottom-up" factors, which are the visual features of an image that stand out from the rest because of their sensory properties (such as changes of color, brilliance, texture or orientation). This mechanism is known as visual salience.

Over recent years, computational models have been developed which can quantify to what extent the parts of an image are conspicuous and likely to capture attention. Along these lines, it has been shown that the distribution of the gaze within an image is closely correlated to the salience maps generated by these models.

Nevertheless, the attention can also be drawn toward an image due to descending or "top-down" factors, like the importance of a specific characteristic or an object for a particular function.

In this study, both the salience and visual attention were analyzed for different images of worked pebbles or choppers and handaxes. To determine the visual salience of these images, an algorithm that decomposes the photograph into different layers such as brilliance, color and texture was applied.

“This gives us a visual salience map which is superimposed upon the image of the tool, highlighting those zones which ought to attract most attention. At the same time, we studied visual attention using the eye-tracking methodology with the same images, yielding maps that reveal which zones are the most observed”, explains Silva.

Comparison of these types of maps has shown a clear distinction between the zones really observed and those considered conspicuous. In spite of the tendency to observe the center of an image or object (known as “central bias”), the tools were more closely observed in the upper and knapped zones, whereas the salience maps highlighted the sides and cortical areas. Moreover, the visual behavior was also analyzed for natural pebbles and other types of objects, verifying that the pattern of visualization was exclusive to the stone tools.