Visual Neuroscience - Face Processing

Face Processing

A recent study using Event-Related Potentials (ERPs) linked a increased neural activity in the occipito-temporal region of the brain to the visual categorization of facial expressions. Results focus on a negative peak in the ERP that occurs 170 milliseconds after the stimulus onset. This action potential, called the N170 was measured using electrodes in the occipito-temporal region, an area already known to be changed by face stimuli. Studying by using the EEG, and ERP methods allow for an extremely high temporal resolution of 4 milliseconds which makes these kinds of experiments extremely well suited for accurately estimating and comparing the time it takes the brain to perform a certain function. Scientists used classification image techniques, to determine what parts of complex visual stimuli (such as a face) will be relied on when patients are asked to assign them to a category, or emotion. They computed the important features when the stimulus face exhibited one of five different emotions. Stimulus faces exhibiting fear had the distinguishing feature of widening eyes, and stimuli exhibiting happiness exhibited a change in the mouth to make a smile. Regardless of the expression of the stimuli's face, the region near the eyes had a impact on the EEG before the regions near the mouth. This revealed a sequential, and predetermined order to the perception and processing of faces, with the eye being the first, and the mouth, and nose being processed after. This process of downward integration only occurred when the inferior facial features were crucial to the categorization of the stimuli. This is best explained by comparing what happens when participants were shown a face exhibiting fear, versus happiness. The N170 peaked slightly earlier for the fear stimuli at about 175 milliseconds, meaning that it took a participants less time to recognize the facial expression. This is expected because only the eyes need to be processed to recognize the emotion. However when processing a happy expression, where the mouth is crucial to categorization, downward integration must take place, and thus the N170 peak occurred later at around 185 milliseconds. Eventually visual neuroscience aims to completely explain how the visual system processes all changes in faces as well as objects. This will give a complete view to how the world is constantly visually perceived, and may provide insight into a link between perception and consciousness.