PHYSIOLOGICAL OPTICS VISP325 MODULE LEADER: PROF A LOGVINENKO

Transcription

1 DIVISION OF VISION SCIENCES SESSION: 2007/2008 DIET: 1ST PHYSIOLOGICAL OPTICS VISP325 LEVEL: 3 B.Sc./B.Sc. (HONS) OPTOMETRY MODULE LEADER: PROF A LOGVINENKO MAY 2008 DURATION: 3 HOURS CANDIDATES SHOULD ATTEMPT FIVE QUESTIONS IN TOTAL AND AT LEAST ONE QUESTION FROM EACH SECTION PLEASE READ THE QUESTIONS CAREFULLY Students for whom English is not their first language are permitted to use a Standard English/Foreign Language dictionary, e.g. French/English/English/French. Please Page 1 of 6

2 ensure that the dictionary does not contain any notes or other materials and note that electronic dictionaries are not permissible MATERIALS TO Lined Examination Script Books BE SUPPLIED/ALLOWED: Unlined Examination Script Books Other Materials, e.g. Graph paper, statistical tables (please specify) Page 2 of 6

3 SECTION A 1. We discussed, in class, simple cells responses to orientation and their relation to the tilt after effect. This question concerns an analagous case of grating sizes (or spatial frequencies). Consider a set of vertical gratings (each grating consisting of white bars and (neutral) gray bars): (this is a picture of a vertical grating) The set consists of: a 1.66 cycle per degree grating (each white and each gray bar having a width of 0.3 ) a 2.5 cycles per degree grating (each white and each gray bar having a width of 0.20 ) a 5 cycles per degree grating (each white and each gray bar having a width of 0.10 ) a 6.25 cycles per degree grating (each white and each gray bar having a width of 0.08 ) a 15 cycles per degree grating (each white and each gray bar having a width of ) One grating is presented at a time. Consider a simple cell which has a vertically oriented receptive field with an on center of width (i.e width in the horizontal direction) of that is centered on the center of a white bar of the gratings. a) Present, in detail (set up, stimuli, results, etc), a single unit extra cellular electrophysiological recording experiment to determine the receptive field of this cell. Indicate where in the visual system you would expect to find this cell. (6) b) Using diagrams of the stimuli on the receptive field and time lines showing spiking, illustrate the cell's response to each of the five gratings flashed on the cell's receptive field. (6) c) From this, draw the "tuning curve" of the cell's response as a function of grating bar size. (3) (If you find doing parts b and c for grating stimuli too difficult, you will obtain substantial partial credit by just considering a set of six single white Page 3 of 6

4 bar stimuli (one white bar being presented at a time) with the six widths listed above.) d) Design a demonstration of a spatial frequency after effect, using gratings. Illustrate what you would use as adapting and test gratings and what you expect subjects to see. (5) 2. a) Suppose that there are two patients who suffered brain damage producing fairly large damage to their cortex (i.e. macro-lesions, rather than microlesions). Name what areas or regions (one structure per deficit is all that is required) could be damaged by such a macro-lesion to produce: i) Achromatopsia (colour blindness) without any other deficit; (2) ii) Motion blindness without any other deficit; (2) iii) Inability to deal with spatial relations and perform visually guided action; (2) iv) Inability to recognise objects. (2) b) Name which substructure (one per deficit is all that is required, but you need to specifically name the substructure) that could be damaged by a micro - lesion to produce: i) Localised achromatopsia (colour blindness) without motion blindness; (1) ii) Localised motion blindness without colour blindness. (1) c) Draw a circuit diagram for the circuit to compute stereo, clearly indicating the sample stimuli, and as they would appear on the two retinas. Show how the cells are connected and what their receptive fields are. (10) SECTION B 3. a) Define spatial visual acuity. (2) b) Give three types of spatial acuity and a brief example of how each is measured. (6) c) Using an example, explain why hyperacuity worsens less with defocus than Snellen acuity. Include diagrams in your answer where possible. (12) 4. This question concerns the spatial contrast sensitivity function (SCSF) a) Sketch the SCSF that you would obtain from a normal human observer in high ambient light levels. Briefly explain your plot. (5) Page 4 of 6

5 b) Using diagrams where possible explain in detail why it is thought that the SCSF represents a measure of V1 Simple Cell rather than Ganglion Cell performance. (Hint: contrast adaptation experiments.) (15) SECTION C 5. a) What is the basic tenet of the Gestalt theory of visual perception? (2) b) Who were its three main proponents? (3) c) How is Julesz s Texton theory related to Gestalt psychology and to Marr s primal sketch? (15) 6. What is agnosia? Describe the types of visual agnosia that can occur and the deficits that they incur. (20) SECTION D 7. a) Describe and discuss Hurvich & Jameson s model of colour vision. (10) b) Briefly describe the hue cancellation technique and its relevance to the model. (5) c) What is the physiological evidence supporting this theory? (5) Page 5 of 6

6 8. a) What is meant by the term pseudoisochromatic plates (PIC) used in colour vision tests? (4) b) List the four test designs used in PIC colour vision tests. (4) c) Which of these designs are the most successful in detecting colour vision anomaly? (2) d) Show how the choice of colours used in these designs is influenced by the location of the iso-chromatic confusion lines drawn in the chromaticity chart. (10) END OF PAPER Page 6 of 6