Chapter 7 Visual Summary

Light is bent, or refracted, by the transparent outer layer of the eye, the cornea, focusing an image on the retina in the back of the eye. We vary the thickness of the lens to fine-tune the image. The refracted light forms an image on the retina that is upside down and reversed. Review Figure 7.1 and Figure 7.2, Animation 7.2, Activity 7.1

The retina contains two different types of photoreceptors to detect light forming the focused image. Rods are very sensitive, working even in very low light, and they respond to light of any wavelength. Rods drive the scotopic system, which can work in dim light. Each of the three different types of cones responds better to some wavelengths of light than others, allowing us to detect colors. The cones provide information for the photopic system, which needs more light to function. Photoreceptors adapt to function across a wide range of light intensities. Review Figure 7.3, Figure 7.4, Figure 7.5, Figure 7.6, Table 7.1

The retina consists of layers of neurons, with the photoreceptors in the very back stimulating bipolar cells, which stimulate ganglion cells. The ganglion cells of the retina project their axons to the brain via the optic nerve. Amacrine cells and horizontal cells communicate across the retina, using processes such as lateral inhibition to analyze brightness. Review Figure 7.3, Figure 7.14 and Figure 7.15

The center of our visual field lands on the fovea, the portion of the retina with the greatest density of photoreceptors, an absence of overlying cell layers, and more direct synaptic connections to ganglion cells, providing us with our greatest visual acuity (sharpness of vision). Cones are concentrated in the fovea, and rods are concentrated in the peripheral retina, so our peripheral vision is best for seeing dim objects, but it provides no color information. Review Figure 7.7, Figure 7.8, Figure 7.9

The left visual field falls on the nasal retina of the left eye and the temporal retina of the right eye. Only nasal retinal ganglion cells of each eye send their axons across the midline, forming the optic chiasm, so the left visual field projects to the right hemisphere and the right visual field projects to the left hemisphere. Review Figure 7.11 and Figure 7.12, Animation 7.3

Most ganglion cells of the retina synapse on neurons in the lateral geniculate nucleus (LGN) of the thalamus. The LGN neurons send axons to synapse on neurons in layer IV of the primary visual cortex (V1) in the occipital cortex. V1 sends information to many different cortical areas, called extrastriate cortex (nearly one-third of the human cortex), to further analyze visual information. Review Figure 7.11 and Figure 7.20

The receptive fields of bipolar cells and ganglion cells consist of a circular center and a surround that have opposing effects: either on-center/off-surround or off-center/on-surround. Review Figure 7.13 and Figure 7.14, Animation 7.4

Receptive fields of cells at successively higher levels in the visual cortex change in two main ways: (1) they become larger (occupy larger parts of the visual field), and (2) they require increasingly specific stimuli to evoke responses. For example, they respond best to a bar of light at a particular angle, or to bars that move in a particular direction. Review Figure 7.17, Figure 7.18, Figure 7.19, Figure 7.20, Figure 7.21, Figure 7.22, Animation 7.5

Rods detect light using a pigment called rhodopsin. Our detection of hue (color) depends on the three different cone photopigments (opsins). Each cone responds to wide range of wavelengths, not just a single color. Our perception of hue results from the relative activity of each type of cone. One way of assessing this relative activity is by retinal connections that yield spectrally opponent neurons. Review Figure 7.23, Figure 7.24, Figure 7.25, Figure 7.26, Figure 7.27, Figure 7.28, Figure 7.29

Visual cortical areas are organized into two main streams: a ventral what stream that serves in the recognition of faces and objects, and a dorsal where stream that serves in location and visuomotor skills. Review Figure 7.31 and Figure 7.32

    Previous 1 of 10 Next