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Abstract:
During visually guided behavior relevant information must be extracted from the objects in the environment. Which information about particular stimuli is actually used can provide insights into strategies underlying behavior. Here we quantify which parts of simple geometric shapes humans and monkey observers used during a visual discrimination task. The stimulus set consisted of three rotated versions of a square with a single protrusion (left, up or right). Each shape was associated with a unique behavioral response (button press or eye movement). We determined relevant regions in each stimulus by presenting the shapes behind occluding surfaces with randomly placed windows, while the observers continued to perform the discrimination task. In this paradigm, the diagnostic value of a shape region is reflected in the way in which its occlusion affects the performance of the observer. While occlusion of an unimportant shape feature will leave discrimination performance unaffected, occlusion of the diagnostic features will render the observer unable to identify the shape correctly. For the human observers, each pattern had one diagnostic region consisting of the protrusion. Similar results were obtained in a monkey subject for two of the three shapes. Interestingly, no region was identifiable for the third shape, even though the monkey could identify the shape with about the same accuracy as the other two shapes. We suggest that the lack of a diagnostic region for the third shape reflects the strategy of the monkey, who tended to choose the third shape whenever neither of the protrusions corresponding to the first two shapes was visible. We conclude that although there was close agreement for the majority of stimuli, we also found strategy differences between human and monkey observers.
