Visual discrimination strategies of monkey and human observers

During visually guided behavior, relevant information must be extracted from the environment. Which particular information about a stimulus is utilized to perform a task can provide insights into the strategies underlying behavior. Here, we identified the discrimination strategies of monkey (Macaca mulatta) and human observers for different sets of stimuli. One set contained geometrical shapes, the other stimulus set consisted of photographs of natural scenes. Each member of a set was associated with a unique behavioral response. The observers were first taught to execute the correct response upon presentation of each of the stimuli. Relevant stimulus regions were then determined by presenting stimuli behind occluding surfaces with randomly placed windows. The observers continued to perform the discrimination task on these occluded stimuli. 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 stimulus feature will leave discrimination performance unaffected, occlusion of the relevant features will render the observer unable to identify the stimulus correctly. Overall, discrimination strategies were very similar between monkey and human observers. However, the monkeys tended to use smaller regions of the images than the human observers. In addition, we analyzed the eye movement patterns evoked when viewing the same images. The results for the monkey observers demonstrated that relevant image regions were more likely to be fixated than other image regions. Similarly, in human observers, the number of fixations made on a given region correlated with the information content subjectively assigned to that region. In summary, we determined directly how human and monkey observers use stimulus information to perform a visual task. We are using this technique to study the neural representation of objects in macaque temporal cortex