Abstract

What we consciously see in our everyday life is not an exact copy of the information that our eyes receive from the external world. Our brain actively elaborates and transforms the light that impinges onto the two dimensional surface of our retinas to create complex three dimensional scenes full of colorful objects of different shapes and sizes, motion and depth. Our visual perception is not a passive reception of information: our brain actively decodes and separates the retinal information into relevant and significant objects and compares this information with previous memories. One remarkable example is the ability of the visual system to decode the information that arrives from the eyes into recognizable visual objects and scenes. We are able to recognize objects even under conditions of low illumination or low contrast, when these objects are partially occluded, presented among other objects or when they are defined by textures, for example. In addition to this process of recognition, the visual system generates the conscious sensations of those objects and scenes. Even though we need our eyes to see the world these are not enough by themselves to generate visual perception. The light impinging on our retinas needs further elaboration in higher areas of the brain to generate perception. Several situations in which vision is separated from perception can demonstrate this. For example, we can imagine some object with our eyes closed in our “mind’s eye” or we can generate images in our dreams that are completely independent of external stimulation. Experimentally, it is possible to manipulate visual perception while keeping visual stimulation to the eyes constant. Examples of this are the changes in perception that occur with multi-stable phenomena (i.e, as with the Necker cube) or when subjects are presented with dissimilar images to each eye, a condition called binocular rivalry. Without a functioning visual brain we are not able to properly see. Lesions to the visual cortex produce a wide variety of visual deficits ranging from blindness to achromatopsia (the impossibility of perceiving color), akinetopsia (the impossibility of perceiving motion) and/or visual agnosias (the difficulty in recognizing objects through vision). Also, extensive neuroimaging experiments have shown that visual information causes the activation of wide areas of the brain and the role of many of these areas has been studied in the last two decades. However, currently the neuroscientific study of perception can only be addressed with limited resources. We cannot measure the activity of the 10 billion neurons that constitute our brain. Neither can scientists manipulate the human brain by disrupting, modifying or altering the activity of neuronal circuits. The current imaging methods for studying the brain can only provide incomplete information at different spatial levels of analysis and with different temporal resolutions. Thus, the conclusions that neuroscientists we can extract from these data are only partial attempts to reach to a better understanding of the functioning of the brain. Despite these limitations, few neuroscientists would disagree today with the fact that visual perception has a basis on distributed neuronal circuits in the brain. In the same line of thought, it is agreed that there must be circuits of neurons that code for the conscious perception of those objects. Consciousness has always been considered as one of the major mysteries of life. The study of the origins of sensations and “feelings” from the operations of our brain is for many scientists one of the final challenges for the biological sciences (Koch, 2003). The mystery of consciousness is considered to be at the same level of the mystery of the creation of the universe and the mystery of the origin of life out of inanimate matter. The topic of this thesis is the study of visual consciousness. Considering its complexity, we do not intend to provide a final answer to the explanation of consciousness. Instead, this thesis will focus on the study of vision and it will describe some properties of conscious vision as opposed to unconscious vision. We will explore the fate of unseen vision: the information that reaches the retina but that does not generate any conscious sensation. We will analyze the processing and limits of unseen visual stimuli and we will compare them with conscious processing of the same objects. In doing this we expect to shed light into some of the properties of conscious and unconscious visual perception and on the role that visual awareness might have had in evolution.