Transcript Figure 4.6
Figure 4.6 Receptive fields in the retina. Visual cells’ receptive fields in the retina are often circular with a center-surround arrangement, so that light striking the center of the field produces the opposite result of light striking the surround. In the receptive field depicted here, light in the center produces increased firing in the visual cell, whereas light in the surround produces decreased firing; the arrangement in other receptive fields may be just the opposite. Figure 4.7 Visual pathways to the brain. (a) Input from the right half of the visual field strikes the left side of each retina and is transmitted to the left hemisphere (shown in red). Input from the left half of the visual field strikes the right side of each retina and is transmitted to the right hemisphere (shown in green). The nerve fibers from each eye meet at the optic chiasm, where fibers from the inside half of each retina cross over to the opposite side of the brain. After reaching the optic chiasm, the major visual pathway projects through the lateral geniculate nucleus in the thalamus and onto the primary visual cortex (shown with solid lines). A second pathway detours through the superior colliculus and then projects through another area of the thalamus and onto slightly different areas of the primary visual cortex (shown with dotted lines). (b) This inset shows a vertical view of how the optic pathways project through the thalamus and onto the visual cortex in the back of the brain [the two pathways mapped out in Early 1960’s: Hubel and Wiesel • Studied activity in primary visual cortex of cats – Identified 3 major types of cells in the visual cortex • Simple • Complex • Hypercomplex • Feature Detectors Figure 4.8 Hubel and Wiesel’s procedure for studying the activity of neurons in the visual cortex. As the cat is shown various stimuli, a microelectrode records the firing of a neuron in the cat’s visual cortex. The figure shows the electrical responses of a simple cell apparently “programmed” to respond to lines oriented vertically. Sensory organs and brain are the most sophisticated signalprocessing devices known to man Joseph Fourier and his most remarkable theorem • Proposed in 1807 that any function could be decomposed into a sum of sinusoidal elements when the functions exhibit discontinuities Figure 1.4. Left. The first 8 components of the Fourier expansion of the square wave shown in Figure 1.2. These 8 components consists of a DC component and 7 sinusoids of decreasing amplitude. Right. The waveform obtained by summing successive components of the Fourier expansion.