(horizontal section)
The image projected onto the retina is inverted due to the optics of the eye. • The
eye, especially the
retina • The
optic nerve • The
optic chiasma • The
optic tract • The
lateral geniculate body • The
optic radiation • The
visual cortex • The
visual association cortex. These are components of the
visual pathway, also called the
optic pathway, that can be divided into
anterior and posterior visual pathways. The anterior visual pathway refers to structures involved in vision before the
lateral geniculate nucleus. The posterior visual pathway refers to structures after this point.
Eye Light entering the eye is
refracted as it passes through the
cornea. It then passes through the
pupil (controlled by the
iris) and is further refracted by the
lens. The cornea and lens act together as a compound lens to project an inverted image onto the retina. ,
Structure of the Mammalian Retina, 1900 Retina The retina consists of many
photoreceptor cells which contain particular
protein molecules called
opsins. In humans, two types of opsins are involved in conscious vision:
rod opsins and
cone opsins. (A third type,
melanopsin in some
retinal ganglion cells (RGC), part of the
body clock mechanism, is probably not involved in conscious vision, as these RGC do not project to the
lateral geniculate nucleus but to the
pretectal olivary nucleus.) An opsin absorbs a
photon (a particle of light) and transmits a signal to the
cell through a
signal transduction pathway, resulting in hyper-polarization of the photoreceptor. Rods and cones differ in function. Rods are found primarily in the periphery of the retina and are used to see at low levels of light. Each human eye contains 120 million rods. Cones are found primarily in the center (or
fovea) of the retina. There are three types of cones that differ in the
wavelengths of light they absorb; they are usually called short or blue, middle or green, and long or red. Cones mediate day vision and can distinguish
color and other features of the visual world at medium and high light levels. Cones are larger and much less numerous than rods (there are 6-7 million of them in each human eye).
Mechanism of generating visual signals The retina adapts to change in light through the use of the rods. In the dark, the
chromophore retinal has a bent shape called cis-retinal (referring to a
cis conformation in one of the double bonds). When light interacts with the retinal, it changes conformation to a straight form called trans-retinal and breaks away from the opsin. This is called bleaching because the purified
rhodopsin changes from violet to colorless in the light. At baseline in the dark, the rhodopsin absorbs no light and releases
glutamate, which inhibits the bipolar cell. This inhibits the release of neurotransmitters from the bipolar cells to the ganglion cell. When there is light present, glutamate secretion ceases, thus no longer inhibiting the bipolar cell from releasing neurotransmitters to the ganglion cell and therefore an image can be detected. The final result of all this processing is five different populations of ganglion cells that send visual (image-forming and non-image-forming) information to the brain: In 2007 Zaidi and co-researchers on both sides of the Atlantic studying patients without rods and cones, discovered that the novel photoreceptive ganglion cell in humans also has a role in conscious and unconscious visual perception. The peak
spectral sensitivity was 481 nm. This shows that there are two pathways for vision in the retina – one based on classic photoreceptors (rods and cones) and the other, newly discovered, based on photo-receptive ganglion cells which act as rudimentary visual brightness detectors.
Photochemistry The functioning of a
camera is often compared with the workings of the eye, mostly since both focus light from external objects in the
field of view onto a light-sensitive medium. In the case of the camera, this medium is film or an electronic sensor; in the case of the eye, it is an array of visual receptors. With this simple geometrical similarity, based on the laws of optics, the eye functions as a
transducer, as does a
CCD camera. In the visual system,
retinal, technically called
retinene1 or "retinaldehyde", is a light-sensitive molecule found in the rods and cones of the
retina. Retinal is the fundamental structure involved in the transduction of
light into visual signals, i.e. nerve impulses in the ocular system of the
central nervous system. In the presence of light, the retinal molecule changes configuration and as a result, a
nerve impulse is generated. as well as other motor responses. A final population of
photosensitive ganglion cells, containing
melanopsin for
photosensitivity, sends information via the
retinohypothalamic tract to the
pretectum (
pupillary reflex), to several structures involved in the control of
circadian rhythms and
sleep such as the
suprachiasmatic nucleus (the biological clock), and to the
ventrolateral preoptic nucleus (a region involved in
sleep regulation). A recently discovered role for photoreceptive ganglion cells is that they mediate conscious and unconscious vision – acting as rudimentary visual brightness detectors as shown in rodless coneless eyes. at the base of the
hypothalamus of the brain. At this point, the information coming from both eyes is combined and then splits according to the
visual field. The corresponding halves of the field of view (right and left) are sent to the left and right
halves of the brain, respectively, to be processed. That is, the right side of
primary visual cortex deals with the left half of the
field of view from both eyes, and similarly for the left brain. Hence selection of visual input information by attention starts at V1 along the visual pathway. Visual information then flows through a cortical hierarchy. These areas include V2, V3, V4 and area V5/MT. (The exact connectivity depends on the species of the animal.) These secondary visual areas (collectively termed the extrastriate visual cortex) process a wide variety of visual primitives. Neurons in V1 and V2 respond selectively to bars of specific orientations, or combinations of bars. These are believed to support edge and corner detection. Similarly, basic information about color and motion is processed here. Heider, et al. (2002) found that neurons involving V1, V2, and V3 can detect stereoscopic
illusory contours; they found that stereoscopic stimuli subtending up to 8° can activate these neurons. .
Visual association cortex As visual information passes forward through the visual hierarchy, the complexity of the neural representations increases. Whereas a V1 neuron may respond selectively to a line segment of a particular orientation in a particular
retinotopic location, neurons in the lateral occipital complex respond selectively to a complete object (e.g., a figure drawing), and neurons in the visual association cortex may respond selectively to human faces, or to a particular object. Along with this increasing complexity of neural representation may come a level of specialization of processing into two distinct pathways: the
dorsal stream and the
ventral stream (the
Two Streams hypothesis, first proposed by Ungerleider and Mishkin in 1982). The dorsal stream, commonly referred to as the "where" stream, is involved in spatial attention (covert and overt), and communicates with regions that control eye movements and hand movements. More recently, this area has been called the "how" stream to emphasize its role in guiding behaviors to spatial locations. The ventral stream, commonly referred to as the "what" stream, is involved in the recognition, identification and categorization of visual stimuli. (red) However, there is still much debate about the degree of specialization within these two pathways, since they are in fact heavily interconnected.
Horace Barlow proposed the
efficient coding hypothesis in 1961 as a theoretical model of
sensory coding in the
brain. Limitations in the applicability of this theory in the primary visual cortex (V1) motivated the
V1 Saliency Hypothesis that V1 creates a bottom-up saliency map to guide attention exogenously. The
default mode network is a network of brain regions that are active when an individual is awake and at rest. The visual system's default mode can be monitored during
resting state fMRI: Fox, et al. (2005) found that "the human brain is intrinsically organized into dynamic, anticorrelated functional networks", in which the visual system switches from resting state to attention. In the
parietal lobe, the
lateral and ventral intraparietal cortex are involved in visual attention and saccadic eye movements. These regions are in the
intraparietal sulcus (marked in red in the adjacent image). ==Development==