There’s much more to visual perception than meets the eye. What we see is not merely a matter of patterns of light falling on the retina, but rather is heavily influenced by so-called ‘top-down’ brain mechanisms, which can alter the visual information, and other types of sensory information, that enters the brain before it even reaches our conscious awareness.
A striking example of this is a phenomenon called inattentional blindness, whereby narrowly focusing one’s attention on one visual stimulus makes us oblivious to other stimuli, even though they otherwise may be glaringly obvious, as demonstrated by the infamous ‘Invisible Gorilla’ study. Now researchers say they have discovered another extreme form of blindness, in which people fail to notice an unexpected image, even when shown by itself and staring them in the face.
Marjan Persuh and Robert Melara of the City University of New York designed two experiments to investigate whether people’s prior expectations could block their awareness of meaningful and important visual stimuli.
In the first, they recruited 20 student volunteers and asked them to perform a visual discrimination task. They were shown a series of images, consisting of successive pairs of faces, each of which were presented for half a second on a computer screen, and asked to indicate whether each pair showed faces of people of the same or different sex.
Towards the end of each session, the participants were presented with a simple shape, which flashed onto the screen for one tenth of a second. They were then asked if they had seen anything new and, after replying, were told that a shape had indeed appeared, and asked to select the correct one from a display of four. This shape recognition task was then repeated in one final control trial.
The participants found that the initial visual discrimination task was very demanding, and responded correctly just less than two thirds of the time. More importantly, the researchers found that half of their participants completely failed to notice the first shape shown that was shown to them, and only two that had not noticed it correctly identified the shape afterwards, as would be expected to happen by chance. By contrast, most who noticed the shape correctly identified it afterwards. And almost all of the study participants noticed, and then correctly identified, the second shape they saw.
The second experiment was designed in exactly the same way, but used a simpler discrimination task, in which 20 other participants were required to determine whether or not pairs of shapes they saw were the same or different colours. They were then shown a photograph of President Obama’s face, asked if they had seen something new, and then to indicate what it was they had seen from a display of four images.
As in the first experiment, the participants performed very well on the discrimination task, responding correctly about 85% of the time. And again, a significant proportion of them – nearly two thirds – completely failed to notice Barack Obama’s face, and of those, just one correctly chose the President’s face from the choice task at the end, far less than would be expected by chance. BY contrast, nearly all of the participants noticed Obama’s face in the final control trial, and then correctly identified him afterwards.
Both experiments demonstrate an extreme form of blindness to a meaningful visual stimulus, even in the absence of a second stimulus that would compete for attentional resources. The researchers name this phenomenon the ‘Barack Obama Blindness (BOB) Effect, and explain it in terms of the participants’ prior expectations. In both experiments, they expected to see particular types of stimuli – faces in the first case, and geometric shapes in the second – and so failed to perceive images that violated their expectations, even when they are displayed prominently.
The researchers believe this is the very first study to show that top-down neural mechanisms can profoundly influence visual perception and prevent us from seeing things that are prominent and highly familiar to us. Exactly how this might happen is still not clear; there is, however some evidence that such top-down mechanisms can directly influence activity in the primary visual cortex, which contains neurons that respond to the most basic features of visual scenes, such as contrast and the orientation of edges.
Persuh, M. & Melara, R. D. (2016). Barack Obama Blindness (BOB): Absence of Visual Awareness to a Single Object. Front. Hum. Neurosci. DOI: 10.3389/fnhum.2016.00118 [Full text]