However confident we feel about the way we perceive the visual world around us, there is not a one-to-one relation between visual stimulation and visual perception. Our eyes register reflections of the visual environment and our brain has the difficult task of constructing 'reality' from this information. In doing so, the brain has to rely on internal information, like inferences and assumptions, in addition to external information, being the visual stimulation.
External visual information is processed in specialized sensory brain regions located in the occipital lobe of the brain. Our investigations tested the general belief that internal information is processed in cognitive brain regions located in the parietal and frontal lobe. To this end we presented participants with ambiguous images that elicit two - mutually exclusive - perceptual interpretations. Because perception differs while the stimulus remains the same, ambiguous images enable the separate investigation of neural activations associated with external and internal influences on perception.
We focused our studies on two perceptual phenomena that reflect a contribution of internal information to the perception of ambiguous images. The first phenomenon is the occurrence of spontaneous changes in the perception of a constant ambiguous stimulus, back and forth between two possible interpretations of the stimulus. Since there are no concomitant changes in the visual stimulation, these changes reflect an internal process in the mind of the observer. Using intracranial recordings of neural activity, we found that sensory brain regions that are located in the occipital lobe activate when spontaneous perceptual changes occur. This finding shows that these brain regions, which are specialized in the processing of external information, also process internal perceptual information.
The second phenomenon we studied is perceptual memory. The way we perceive an ambiguous image is partly determined by the way we have perceived it previously. We found that perceptual memory modulates neural processing soon after the onset of the stimulus, preceding the completion of the perceptual interpretation. This was observed using electro-encephalography, a technique that measures neural activity at a high temporal resolution. In an additional study we used functional magnetic resonance imaging, a technique with a high spatial rather than temporal resolution. Perceptual memory was associated with activations in sensory brain regions located in the occipital lobe. Moreover, the involved sensory brain regions differed between stimuli and, per stimulus, were specialized in the processing of that stimulus specifically. Perceptual memory is thus processed in brain regions that are also dedicated to the processing of stimulus-specific external information.
In summary, contrary to general belief we found that internal information that contributes to our perception of ambiguous stimuli is associated with activations of sensory rather than cognitive brain regions. Given the known role of these sensory regions in the processing of external information, our findings indicate that internal and external contributions to perception are not processed in separate brain regions, but instead are inextricably linked in terms of neural processing. It thus seems that objective perceptual processing based solely on external information does not occur in the brain.