Questions on brain imaging, development and learning

Question 1

Discuss the use of animals in medical research from two ethical standpoints.

Accepted Answer

B1 a utilitarian standpoint judges the research by its consequences: it is right if it produces the greatest overall good or the least overall harm B1 a utilitarian may support it when the suffering of a limited number of animals is outweighed by the benefits, such as treatments for large numbers of patients or new understanding of disease B1 a utilitarian may oppose it when the benefits are small or uncertain, when the results may not apply to humans, or when the same results could be obtained without animals B1 a rights-based standpoint holds that some actions are wrong whatever their consequences, because they violate a right or break a duty B1 if animals have a right not to be harmed or used as a means to an end, the research is wrong however great the benefits B1 alternatively, if rights belong only to beings that can reason and accept duties, animals have none and regulated research is permissible, though there is still a duty to avoid cruelty, as in the three Rs

Question 2

When the siphon of the sea slug Aplysia is touched, it withdraws its gill, but after many repeated touches it withdraws the gill only weakly. Explain how this habituation happens at the synapse between the sensory neurone and the motor neurone.

Accepted Answer

B1 the sensory neurone still sends impulses to its presynaptic terminal with each touch B1 with repeated stimulation the calcium ion channels in the presynaptic membrane become less responsive, so fewer open B1 fewer calcium ions enter the presynaptic terminal, so fewer vesicles fuse with the membrane and less neurotransmitter is released B1 less neurotransmitter binds to receptors on the motor neurone, so its depolarisation is smaller and reaches threshold less often B1 so fewer impulses reach the gill muscles and the withdrawal is weaker

Question 3

Describe how you could investigate habituation to touch in a snail, including the variables you would control.

Accepted Answer

B1 place the snail on a clean, damp surface and wait until it has emerged fully with its tentacles extended B1 touch it gently between the tentacles with a damp cotton bud and time how long the tentacles take to extend fully again B1 repeat the touch at a fixed interval after each re-emergence, ten times or more, recording the time each time B1 control the place touched, the pressure of the touch and the person applying it, and keep temperature, light and vibration constant B1 repeat with several snails to check the trend is repeatable, and handle them gently, keep them moist and return them unharmed

Question 4

Explain how functional magnetic resonance imaging (fMRI) is used to find which regions of the brain are active while a person reads aloud.

Accepted Answer

B1 neurones in an active region respire faster and use more oxygen B1 blood flow to the active region increases, bringing more oxygenated blood B1 oxyhaemoglobin and deoxyhaemoglobin behave differently in the magnetic field, so the scanner detects the rise in oxygenated blood B1 scans are taken every few seconds during the reading and compared with scans at rest, and the regions where oxygenated blood has increased are the ones the task uses

Question 5

Compare computed tomography (CT) with magnetic resonance imaging (MRI) as ways of imaging the structure of the brain.

Accepted Answer

B1 CT detects the absorption of X-rays by tissues, whereas MRI detects radio signals from hydrogen nuclei in a strong magnetic field B1 both produce cross-sectional images of brain structure rather than of activity B1 MRI shows soft tissue in finer detail, such as grey against white matter, whereas CT gives poorer soft-tissue resolution B1 CT exposes the patient to ionising radiation but is quicker, whereas MRI uses no ionising radiation but is slower and unsuitable for some people with metal implants

Question 6

Use the idea of a critical period to explain the difference between the results for animal B and animal C in the table. The figures are constructed to show the pattern of Hubel and Wiesel's results.

Accepted Answer

B1 kitten B's eye was closed during the critical period, when the connections from the eyes to the visual cortex are still being sorted by use B1 synapses from the closed right eye carried few impulses, so they were weakened and lost B1 synapses from the open left eye were active, so they were strengthened and took over the cortical neurones, which is why 90% respond to the left eye only B1 in the adult cat C the critical period was over, so the connections made while both eyes were in use were kept even though one eye received no light

Question 7

Within hours of birth, babies turn their eyes and head further to follow a face-like pattern than to follow the same features scrambled. Evaluate the use of this evidence to conclude that the preference for faces is determined by nature.

Accepted Answer

B1 new-born babies have had very little experience of the world, so a preference present at birth is likely to be innate, which supports the conclusion B1 learning begins in the womb, so some experience has already occurred before birth B1 the preference is inferred from eye and head movements, because babies cannot report what they perceive, so the interpretation may be wrong B1 a judgement: the evidence supports a large genetic contribution to a basic preference, but cannot show that later face recognition owes nothing to experience

Question 8

Explain how studies of people with damaged brain areas are used to investigate brain function and development, and give two limitations of these studies.

Accepted Answer

B1 when damage to one region causes a particular ability to be lost, that region is linked to the ability, as Broca linked speech production to an area of the left frontal lobe B1 recovery of an ability after damage, which is often greater in children than in adults, shows other regions can take over, so development is shaped by experience as well as genes B1 one limitation: the damage is rarely confined to one region, so the lost ability may depend on another region that was also damaged B1 another limitation: abilities before the damage are seldom known exactly, and every case differs, so samples are small

Question 9

Describe how a positron emission tomography (PET) scan using labelled glucose shows which regions of the brain are most active.

Accepted Answer

B1 glucose labelled with a positron-emitting isotope is injected into the blood B1 the most active brain cells take up the most glucose, so the tracer collects in the most active regions B1 the radiation produced when the positrons meet electrons is detected by detectors round the head, and a computer locates where it came from to build the image

Question 10

The table shows the percentage of neurones in the visual cortex that responded to light shone into each eye, in an experiment of the kind Hubel and Wiesel carried out. The figures are constructed to show the pattern of their results. Describe what the data show about the effect of closing the right eye.

Accepted Answer

B1 closing the right eye in the kitten (B) reduced the neurones responding to the right eye from 20% to 2%, and those responding to both eyes from 60% to 8% B1 in the kitten the neurones responding only to the open left eye rose from 20% to 90% B1 closing the right eye in the adult cat (C) had almost no effect: the percentages are close to those of the normal kitten (A)

Question 11

Explain why kittens and monkeys were used as animal models in research into the development of human vision.

Accepted Answer

B1 they have forward-facing eyes and binocular vision, with a visual cortex organised much like the human one, so results are likely to apply to humans B1 the experiments required procedures that would not be allowed on humans, such as depriving an eye of light and recording from single neurones in the cortex with electrodes B1 they develop faster than humans, so the critical period can be studied in months, and their rearing conditions can be controlled exactly

Question 12

A snail has habituated to gentle touches on its head, but when it is touched much more firmly it withdraws its tentacles fully. Explain what this observation shows about habituation.

Accepted Answer

B1 the muscles that withdraw the tentacles are not fatigued, because they can still produce a full response B1 the receptors and sensory neurones still work, so the reduced response is not sensory adaptation B1 the change is specific to the repeated, harmless stimulus and lies in the nervous system, at the synapses of the withdrawal pathway

Question 13

People brought up in surroundings with few straight edges and right-angled corners were found to be less easily taken in by the Müller-Lyer illusion. Suggest why this finding does not prove that the way we judge depth from lines is learned.

Accepted Answer

B1 the two populations differ genetically as well as in their surroundings, so the difference could be partly genetic B1 the test was devised in one culture and may be understood or answered differently in another B1 many other environmental factors also differ between the two groups, such as schooling and experience of pictures, so the cause cannot be pinned to the surroundings alone

Question 14

Of CT, MRI, fMRI and PET, state which two expose the patient to ionising radiation, and the source of the radiation in each.

Accepted Answer

B1 CT, which uses X-rays passed through the head B1 PET, which uses a radioactive tracer injected into the blood

Question 15

A student touched a snail between its tentacles ten times and recorded the time for its tentacles to re-emerge after each touch. The results are constructed. Calculate the percentage decrease in the re-emergence time between the first touch and the tenth.

Accepted Answer

M1 (48 − 4) ÷ 48 × 100 A1 a decrease of 91.7% (accept 92%)

Animal	Responding to the left eye only / %	Responding to both eyes / %	Responding to the right eye only / %
A: kitten, both eyes open	20	60	20
B: kitten, right eye closed for its first three months	90	8	2
C: adult cat, right eye closed for three months	22	58	20

Animal	Responding to the left eye only / %	Responding to both eyes / %	Responding to the right eye only / %
A: kitten, both eyes open	20	60	20
B: kitten, right eye closed for its first three months	90	8	2
C: adult cat, right eye closed for three months	22	58	20