Visual processing

Visual processing is a term that is used to refer to the brain's ability to use and interpret visual information from the world around us. The process of converting light energy into a meaningful image is a complex process that is facilitated by numerous brain structures and higher level cognitive processes. On an anatomical level, light energy first enters the eye through the cornea, where the light is bent. After passing through the cornea, light passes through the pupil and then lens of the eye, where it is bent to a greater degree and focused upon the retina. The retina is where a group of light-sensing cells, called photoreceptors are located. There are two types of photoreceptors: rods and cones. Rods are sensitive to dim light and cones are better able to transduce bright light. Photoreceptors connect to bipolar cells, which induce action potentials in retinal ganglion cells. These retinal ganglion cells form a bundle at the optic disc, which is a part of the optic nerve. The two optic nerves from each eye meet at the optic chiasm, where nerve fibers from each nasal retina cross which results in the right half of each eye's visual field being represented in the left hemisphere and the left half of each eye's visual fields being represented in the right hemisphere. The optic tract then diverges into two visual pathways, the geniculostriate pathway and the tectopulvinar pathway, which send visual information to the visual cortex of the occipital lobe for higher level processing (Whishaw and Kolb, 2015). Visual processing is a term that is used to refer to the brain's ability to use and interpret visual information from the world around us. The process of converting light energy into a meaningful image is a complex process that is facilitated by numerous brain structures and higher level cognitive processes. On an anatomical level, light energy first enters the eye through the cornea, where the light is bent. After passing through the cornea, light passes through the pupil and then lens of the eye, where it is bent to a greater degree and focused upon the retina. The retina is where a group of light-sensing cells, called photoreceptors are located. There are two types of photoreceptors: rods and cones. Rods are sensitive to dim light and cones are better able to transduce bright light. Photoreceptors connect to bipolar cells, which induce action potentials in retinal ganglion cells. These retinal ganglion cells form a bundle at the optic disc, which is a part of the optic nerve. The two optic nerves from each eye meet at the optic chiasm, where nerve fibers from each nasal retina cross which results in the right half of each eye's visual field being represented in the left hemisphere and the left half of each eye's visual fields being represented in the right hemisphere. The optic tract then diverges into two visual pathways, the geniculostriate pathway and the tectopulvinar pathway, which send visual information to the visual cortex of the occipital lobe for higher level processing (Whishaw and Kolb, 2015). Top-Down and Bottom-Up Representations The visual system is organized hierarchically, with anatomical areas that have specialized functions in visual processing. Low-level visual processing is concerned with determining different types of contrast among images projected onto the retina whereas high-level visual processing refers to the cognitive processes that integrate information from a variety of sources into the visual information that is represented in one's consciousness. Object processing, including tasks such as object recognition and location, is an example of higher-level visual processing. High-level visual processing depends on both top-down and bottom-up processes. Bottom-up processing refers to the visual system's ability to use the incoming visual information, flowing in a unidirectional path from the retina to higher cortical areas. Top-down processing refers to the use of prior knowledge and context to process visual information and change the information conveyed by neurons, altering the way they are tuned to a stimulus. All areas of the visual pathway except for the retina are able to be influenced by top-down processing. There is a traditional view that visual processing follows a feedforward system where there is a one-way process by which light is sent from the retina to higher cortical areas, however, there is increasing evidence that visual pathways operate bidirectionally, with both feedforward and feedback mechanisms in place that transmit information to and from lower and higher cortical areas (Gilbert & Li, 2013). Various studies have demonstrated this idea that visual processing relies on both feedforward and feedback systems (Jensen et al., 2015; Layher et al., 2014; Lee, 2002). One study that recorded from early visual neurons in macaque monkeys found evidence that early visual neurons are sensitive to features both within their receptive fields and the global context of a scene, suggesting that top-down interaction that is mediated by recurrent feedback connections in the early visual cortex. Two other monkey study used electrophysiology to find different frequencies that are associated with feedforward and feedback processing in monkeys (Orban, 2008; Schenden & Ganis, 2005). Studies with monkeys have also shown that neurons in higher level visual areas are selective to certain stimuli. One study that used single unit recordings in macaque monkeys found that neurons in middle temporal visual area, also known as area MT or V5, were highly selective for both direction and speed (Maunsell & Van Essen, 1983). Disorders of Higher Level Visual Processing