Chapter 3 - Study Questions Flashcards
(14 cards)
- Describe the experiment that demonstrated the effect of lateral inhibition in the Limulus.
The Limulus eye is made up of hundreds of tiny structures called ommatidia, and each ommatidium has a small lens on the eye’s surface that is located directly over a single receptor. Each lens and receptor is roughly the diameter of a pencil point, so it is possible to illuminate and record from a single receptor without illuminating its neighbouring receptors.
When Harline and coworkers recorded from the nerve fiber of receptor A, the found that illumination of that receptor caused a large response. But when they added illumination to the Three nearby receptors at B, the response of receptor A decreased. They also found that further increasing the illumination of B decreased A’s response even more. Thus, illumination of the neighboring receptors at B inhibited the firing caused by stimulation of receptor A. This decrease in the firing of receptor A is caused by lateral inhibition that is transmitted from B to A across the Limulus’s eye by the fibers of the lateral plexus.
- How can lateral inhibition explain the “spots” that are perceived at the intersections of the Hermann grid?
Because the response of 60 associated with receptor A (at the intersection) is smaller than the response of 76 associated with receptor D ( in the corridor between the black squares), the intersection should appear darker than the corridor. This is exactly what happens – we perceive gray images at the intersections. Although the initial responses of bipolar A and D are the same, their final responses are different, because D receives less lateral inhibition that A. Lateral inhibition therefore explains the dark images at the intersection.
- What are Mach bands, and how can lateral inhibition explain our perception of them? Be sure to understand the calculations used in conjunction with the circuit in Figure 3.11.
Mach bands are illusory light and dark bands near a light-dark border.
The circuit on page 57 explain the Mach band effect based on lateral inhibition. The circuit works like the one for the Hermann grid, with each bipolar cell sending inhibition to its neighbours. If we know the initial output of each receptor and the amount of lateral inhibition, we can calculate the final output of each bipolar cell.
Calculation based on one tenth of the cell’s response so Z x 0.1 = A units of inhibition.
- What is simultaneous contrast? How has it been explained by lateral inhibition? What are some problems with this explanation?
Simultaneous contrast occurs when our perception of the brightness or colour of one area is affected by the presence of an adjacent or surrounding area.
In lateral inhibition, some neurons are stimulated to a greater degree than others. A highly stimulated neuron (principal neuron) releases excitatory neurotransmitters to neurons along a particular path. At the same time, the highly stimulated principal neuron activates interneurons in the brain that inhibit excitation of laterally positioned cells. Interneurons are nerve cells that facilitate communication between the central nervous system and motor or sensory neurons. This activity creates greater contrast among various stimuli and results in greater focus on a vivid stimulus. Lateral inhibition occurs in sensory systems of the body including olfactory, visual, tactile, and auditory systems.
Simultaneous contrast is also the result of lateral inhibition. In simultaneous contrast, the brightness of a background affects the perception of brightness of a stimulus. The same stimulus appears lighter against a dark background and darker against a lighter background.
It is difficult for lateral inhibition to explain the following perception: If we start at the edge of one of the center squares and move toward the middle of the square, the lightness appears to be the same, all across the square. However, because lateral inhibition would affect the square more strongly near the edge, we would expect that the square would look lighter near the border and darker in the center. The fact that this does not occur suggests that lateral inhibition cannot be the whole story behind simultaneous contrast.
that lateral inhibition cannot be the whole story behind simultaneous contrast.
- How does White’s illusion demonstrate that there are some perceptual “lightness” effects that lateral inhibition cannot explain? What principle has been used to explain White’s illusion? What does this mean about the location of the mechanism that determines lightness perception?
Because one area receives more lateral inhibition than another area, an explanation based on lateral inhibition would prefict that area B should appear darker. But in some circumstances the opposite happens, such as the case of White’s illusion. Which means that this illusion can’t be explained by lateral inhibition.
The principle of belonginess has been used to explain White’s illusion. This mean that an area’s appearance is influenced by the part of the surroundings to which the area appears to belong. According to this idea, a perception would be affected by the white background because it appears to be resting on the white background, that is behind the black bars. Similarly, our perception of rectabgle B would be affected by the dark bards, because it appears to be resting on the. Thus, the principle of belonginess proposes that the light area makes area A appear darker and the dark bars make area B appear lighter.
It means you can’t explain what is happening in the retina because there is still much more processing to be done before perception occurs. This processing happens later in the visual system, in the visual receiving area of the cortex and beyond.
1) What is the receptive field? What did Hartline’s research indicate about receptive fields?
Receptive field is the area that, upon receiving appropriate stimulus, causes the neuron to fire.
A fiber’s receptive field covers a much greater area than a single rod or cone receptor.
A fiber’s receptive field covers a much greater area than a single rod or cone receptor. The fact that a fiber’s receptive field covers hundreds or even thousands of receptors means that the fiber is receiving con- verging signals from all of these receptors. Finally, Hartline noted that the receptive fields of many different fibers overlap (Figure 3.20b). This means that shining light on a particular point on the retina activates many ganglion cell fibers.
*** cat discovery changed definition of the receptive field: the retinal region over which a cell in the visual system can be influenced (excited or inhibited) by light
2) What are the characteristics of the receptive fields of a cat’s optic nerve and LGN neurons? What new properties were associated with the discovery of these receptive fields? How did these properties require that the definition of receptive field be changed?
The cat receptive fields, it turns out, are arranged in a center-surround organization, in which the area in the “center” of the receptive field responds differently to light than the area in the “surround” of the receptive field (Barlow et al., 1957; Hubel & Wiesel, 1965; Kuffler, 1953).
For the receptive field in Figure 3.21a, presenting a spot of light to the center increases firing, so it is called the excitatory area of the receptive field. In contrast, stimulation of the sur- round causes a decrease in firing, so it is called the inhibi- tory area of the receptive field. This receptive field is called an excitatory-center, inhibitory-surround receptive field. The receptive field in Figure 3.21b, which responds with inhibi- tion when the center is stimulated and excitation when the surround is stimulated, is an inhibitory-center, excitatory- surround receptive field.
The discovery that receptive fields can have oppositely responding areas made it necessary to modify Hartline’s definition of receptive field to the retinal region over which a cell in the visual system can be influenced (excited or inhibited) by light.
3) What function has been suggested for the LGN?
The lateral geniculate nucleus (LGN) receives 90 percent of the optic nerve fibers that leave the eye (the other 10 percent travel to the superior colliculus) and it is a complex structure containing millions of neurons. One proposal of LGN function is based on the observa- tion that the signal sent from the LGN to the cortex is smaller than the input the LGN receives from the retina (Figure 3.26). This decrease in the signal has led to the suggestion that one of the purposes of the LGN is to regulate neural informa- tion as it flows from the retina to the cortex.
Another important characteristic of the LGN is that it receives more signals from the cortex than from the retina (Sherman & Koch, 1986; Wilson et al., 1984). This “backwards” flow of information, called feedback, could also be involved in regulation of infor- mation flow, the idea being that the information the LGN receives back from the brain may play a role in determining which information is sent up to the brain
4) Describe the characteristics of simple, complex, and end-stopped cells in the cortex. Why have these cells been called feature detectors?
Hubel and Wiesel found cells in the striate cortex with receptive fields that, like center- surround receptive fields of neurons in the retina and LGN, have excitatory and inhibitory areas. However, these areas are arranged side by side rather than in the center-surround con- figuration (Figure 3.27a). Cells with these side-by-side recep- tive fields are called simple cortical cells. cell with this receptive field would respond best to vertical bars.
Hubel and Wiesel (1965) discovered that many cortical neurons respond best to moving barlike stimuli with specific orientations. Complex cells, like simple cells, respond best to bars of a particular orientation. However, unlike simple cells, which respond to small spots of light or to stationary stimuli, most complex cells respond only when a correctly oriented bar of light moves across the entire receptive field. Edge of slide Figure 3.28 When Hubel and Wiesel dropped a slide into their slide projector, the image of the edge of the slide moving down unexpectedly triggered activity in a cortical neuron. © Cengage Learning Further, many complex cells respond best to a particular direction of movement (Figure 3.29a). Because these neurons don’t respond to stationary flashes of light, their receptive fields are indicated not by pluses and minuses but by outlining the area that, when stimulated, elicits a response in the neuron.
end-stopped cells, fire to moving lines of a specific length or to moving corners or angles.
Because simple, complex, and end-stopped cells fire in response to specific features of the stimulus, such as orientation or direction of movement, they are sometimes called feature detectors.
5) How has the psychophysical procedure of selective adaptation been used to demonstrate a link between feature detectors and the perception of orientation? Be sure you understand the rationale behind selective adaptation experiment and also how we can draw conclusions about physiology from the results of this psychophysical procedure.
The idea behind selective adaptation is that this firing causes neurons to eventually become fatigued or adapt. This adaptation causes two physiological effects: (1) the neuron’s firing rate decreases, and (2) the neuron fires less when that stimulus is immediately presented again. According to this idea, presenting a vertical line causes neurons that respond to vertical lines to respond, but as these presentations continue, these neurons eventually begin to fire less to vertical lines. Adaptation is selective because only the neurons that were responding to verticals or near-verticals adapt, and neurons that were not firing do not adapt.
6) How has the procedure of selective rearing been used to demonstrate a link between feature detectors and perception of orientation? Be sure you understand the concept of neural plasticity.
Further evidence that feature detectors are involved in perception is provided by selective rearing experiments. The idea behind selective rearing is that if an animal is reared in an environment that contains only certain types of stimuli, then neurons that respond to these stimuli will become more prevalent. This follows from a phenomenon called neural plasticity or experience-dependent plasticity—the idea that the response properties of neurons can be shaped by perceptual experience. According to this idea, rearing an animal in an environment that contains only vertical lines should result in the animal’s visual system having neurons that respond predominantly to verticals.
7) Describe Gross’s experiments on neurons in the inferotemporal cortex of the monkey. Why do you think his results were initially ignored?
He based this decision on research that showed that remov- ing parts of the IT cortex in monkeys affected the monkeys’ ability to recognize objects, as well as on research on a human condition called prosopagnosia, in which people with tem- poral lobe damage were unable to recognize faces. Finding neurons that responded to real-life objects like hands and faces was a revolutionary result. Apparently, neural processing that occurred beyond the initial receiving areas studied by Hubel and Wiesel had created these neurons. But sometimes revolutionary results aren’t accepted imme- diately, and Gross’s results were largely ignored when they were published in 1969 and 1972
8) What is the sensory code? Describe specificity, distributed, and sparse coding. Which type of coding is most likely to operate in sensory systems?
sensory coding: how the firing of neurons represents various characteristics of the environ- ment.
The idea that the firing of single neurons is the key to understanding sensory coding is called specificity coding.
Specificity coding proposes that a particular object is rep- resented by the firing of a neuron that responds only to that object and to no other objects.
Distributed coding is the representation of a particular object by the pattern of firing of a large number of neurons.
Sparse coding occurs when a particular object is represented by a pattern of firing of only a small group of neurons
features or objects are represented by the pattern of firing of groups of neurons. Sometimes the groups are small (sparse coding), sometimes large ( distributed coding).
9) What is the mind-body problem? What is the difference between “easy” problem of consciousness and the “hard” problem of consciousness?
mind–body problem: How do physical processes such as nerve impulses or sodium and potassium molecules flowing across membranes (the body part of the problem) become transformed into the richness of perceptual experience (the mind part of the problem)?
Research on sensory cod- ing, which focuses on the relationship between stimuli in the environment and how neurons fire, is often referred to as research on the neural correlate of consciousness (NCC), where consciousness can roughly be defined as our experi- ences.
Researchers often call finding the NCC the easy problem of consciousness because it has been pos- sible to discover many connections between neural firing and experience
But if NCC is the “easy” problem, what is the “hard” problem? We encounter the hard problem when we approach Bernita’s question at a deeper level by asking not how physi- ological responses correlate with experience, but how physi- ological responses cause experience. To put it another way, how do physiological responses become transformed into expe- rience? We can appreciate why this is called the hard problem of consciousness by stating it in terms of the flow of sodium and potassium ions we described in Chapter 2 (see page 38): How are sodium and potassium flows across a membrane or the nerve impulses that result from this flow turned into experiencing a person’s face or the color red
