The Positional Acuity of the Human Visual System Year 2 Practical Class Dr. Paul McGraw & Mr. Craig Stockdale

Human Positional Acuity To provide guidance on the conventions used in the reporting of studies carried out in a major field of Cognitive Neuroscience (Vision) A brief overview of the practical class presentations: 1.Each group (max of 4 students) will have 10 mins to present their research plus 2 mins for questions. 2.Aim for Introduction (2 mins), Method (2 mins), Results (3 mins) and Discussion (3 mins). 3.Use PowerPoint for your presentation 4.ALL members of the group should try to speak (e.g. present different sections) 5.Each group should try to ask one “good” question to at least one other group!

Guidance for Writing the Lab Report ABSTRACT & INTRODUCTION These should be written in exactly the same format as a conventional Psychology practical report (see 1st year notes). In the Introduction you must discuss relevant literature (show evidence of wide reading) and provide a clear rationale for your study. METHODS Divide into 3 separate subsections with the following headings: Observers — (similar to the Subjects section of a Psychology report) Apparatus & Stimuli — (describe equipment, viewing conditions and stimuli) Procedure — (describe task, dependent variable, conditions used in the experiment, number runs of trials, how threshold values were calculated)

Guidance for Writing the Lab Report OBSERVERS - Number of observers. Give their initials of your group memebers (e.g. TL, PVM and NJP) - Normal or corrected-to-normal acuity (spectacles worn if needed)? - Did they practice the task before formal data collection commenced? APPARATUS & STIMULI - Stimulus generation, presentation and data collection computer controlled - Viewing binocular at a distance of 0.57 m (1 pixel subtended degrees) -Therefore to convert pixel measures into units of visual angle (minutes of arc) all pixel values must be multiplied by a conversion factor of Central fixation - Describe Vernier alignment stimuli (e.g. alignment stimuli in Appendix 1): “Visual stimuli were presented in a three patch Vernier alignment paradigm. The stimulus elements consisted of Gaussian patches with a standard deviation (size) of 10.4 minutes of arc…..”

Guidance for Writing the Lab Report PROCEDURE - Describe how trials (stimulus presentations) were initiated by the observers - Central fixation prior to and after each trial - Centre patch offset chosen to be either left or right with equal probability - Describe task (forced-choice positional judgement) and how responses were collected - Trials self-paced (next stimulus presented only after a response made) - Magnitude of positional offset varied (determined by the stimulus range in method of constant stimuli) Psychophysical procedure used: Method of Constant Stimuli for more information see: Psychophysics: The Fundamentals. Ed. 3. Gescheider, G. A. 1997, Lawrence Erlbaum Assoc. Perception. Sekuler, R. & Blake, R. 2002, McGraw-Hill

Guidance for Writing the Lab Report PROCEDURE - How many blocks of trials were completed by the observer for each condition? -Was the order of testing randomised? - Positional acuity (dependent variable) taken as the standard deviation of a cumulative Gaussian fit to the psychometric data. - Point of Subjective Alignment (PSA) taken as the mean of a cumulative Gaussian fit to the psychometric data.

Guidance for Writing the Lab Report RESULTS Present the results of “positional acuity vs separation” and “point of subjective alignment vs separation” on separate figures Plot each observer’s results on a different graph but try and fit them onto a single page (if possible) On each graph you should include: - Meaningful labels for each axis indicating the measurement units (e.g. arcmin) - On each observers graphs put their initials in one of the corners - Make sure the two axis have the same scales (cover the same absolute range of values) for all observers - Include a figure legend (or title) describing briefly what the graphs show - For your presentation group data plot mean positional acuity thresholds as a function of separation and indicate the standard error of these measures using error bars. Repeat this for measures of perceived positional alignment (PSA).

Guidance for Writing the Lab Report RESULTS How does performance on the task (dependent variable) depend on the independent variable? Do the graphs exhibit consistent and characteristic features across observers? Describe these general features of the data both qualitatively and quantitatively (where appropriate) “…as the separation between the patches is increased positional thresholds show a systematic increase…” Note any (major) individual differences (e.g. in absolute threshold sensitivity, shape, variability) between the observers results as well as the overall similarities Do the same for the data describing PSA as a function of separation.

Guidance for Writing the Lab Report DISCUSSION This should be written in exactly the same way as a conventional Psychology practical report Do the results support your predictions? What do they tell us about the human visual system? Are the results explained by any known sensory laws? Are they consistent with previous research? It is essential to discuss relevant literature (show evidence of wide reading). This can include psychophysics, physiology, brain-imaging, etc. Search keywords associated with your study e.g. separation, position, eccentricity, Vernier (wok.mimas.ac.uk - do cited reference search as well). APPENDIX If you wish you could include a schematic diagram of the experimental stimuli used in the study. You can also present raw data and an example of the data fitted with a psychometric function (either from excel or WhizGraph)