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Sample exam question

This is a sample exam question for level 1 psychology, and an outline answer.

The question

Describe the neurophysiology of the eye

The answer outline and feedback on answers seen

"Neurophysiology" refers to the neurons, not the other "accessory" structures of the eye such as the lens etc. Nearly all students who answered this question included descriptions of other parts of the physiology. While this provides a sensible background, it was not required. Similarly, this question only asked about the eye, so students who described the neural structures that follow the eye in the visual system (the LGN, optic chiasm etc.) earned no marks for that. This question was not aimed at the intra-cellular functions (action potentials, neurotransmitters etc.).

The eye has 4 main types of neural cell: photoreceptors, interneurons, bipolar cells, and ganglion cells. The question hoped for information on this architecture and its main functions. The question was mainly marked using a marking scheme that looked for the following points. Marks were given for mentioning these points; irrelevant points were largely ignored; marks were not usually subtracted for false statements. Given that basis, overall marking was adjusted for greater consistency with other questions and markers.

  1. Rods and cones (the 2 types of photoreceptor)
  2. There are 3 types of cone (with 3 different pigments)
  3. Cones support colour vision, rods do not.
  4. Rods are more sensitive than cones i.e. can detect lower levels of light, support night vision. (It is not true that rods see without light. It is not enough to say that rods are "sensitive" without making it clear they are sensitive to light, and more so than cones.)
  5. Interneurones.
  6. Lateral inhibition: inhibitory connections between adjacent sets of photoreceptors, mediated by interneurones, have the function of sharpening local contrast in the resulting signal. This is a function of the neural architecture.
  7. Bipolar cells.
  8. Ganglion cells.
  9. Convergence: many photoreceptors end up feeding each ganglion cell. (Reduces resolution, but probably raises sensitivity to low light levels. The convergence ratio varies across the retina.)
  10. Sequence of cell types: the main sequence of information flow is photoreceptor to bipolar to ganglion.
  11. Sequence of cell types: interneurones take inputs from photoreceptors and give outputs to bipolar cells. (Many students wrote that they take inputs from bipolar cells, forming a layer between bipolar and ganglion cells.)
  12. Fovea. A small area in the centre of the retina. Receptors are more densely packed (per square mm) there; and cones outnumber rods here, while the opposite is true as you move further away on the retina. (It is not true that the light is only focussed here.)
  13. Centre-surround receptive field organisation of ganglion cells.
  14. A final bonus mark was available for mentioning one or more of the following points, not strictly part of the main neurophysiological structure of the eye: the back to front physical layout (so light hits the ganglion cells first), the blind spot (optic disk), the optic nerve.

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