WEEK 3: DISORDERS OF PERCEIVING THE VISUAL WORLD

W3480Y WEEK 3: DISORDERS OF PERCEIVING THE VISUAL WORLD (BASIC VISION)

READINGS (REQUIRED & OPTIONAL)

REQUIRED READINGS on BLINDSIGHT and APPERCEPTIVE AGNOSIA

1. Milner, A. D., & Goodale, M. A. (1995). Cortical Blindness. In The Visual Brain in Action (pp. 67-86). Oxford University Press: Oxford.
2. Fendrich, R., Wessinger, C. M., & Gazzaniga, M. S. (1992). Residual vision in a scotoma: Implications for blindsight. Science, 258, 1489-1491.
3. Kentridge, R. W., Heywood, C. A. & Weiskrantz, L. (1997). Residual vision in multiple retinal locations within a scotoma: Implications for blindsight. Journal of Cognitive Neuroscience, 9(2), 191-202.
4. Vecera, S.P., & Gilds, K.S. (1998). What processing is impaired in apperceptive agnosia? Evidence from normal subjects. Journal of Cognitive Neuroscience, 10(5), 568-580.

OPTIONAL BACKGROUND READINGS

To give you the 'big' picture you might want to skim through:

Riddoch, J., & Humphreys, G. W. (2001). Object Recognition. In B. Rapp (Ed.), The Handbook of Cognitive Neuropsychology: What Deficits Reveal About the Human Mind, Philadelphia: Psychology Press. (Much this chapter is review from Chapter 4 in Gazzaniga, et al: Cognitive Neuroscience.)

If you are less familiar with the anatomy and physiology of the visual system also look over:

Milner, A. D., & Goodale, M. A. (1995). Visual processing in the primate visual cortex, The Visual Brain in Action (pp. 25-43). Oxford University Press: Oxford.

PART 1: QUESTIONS FOR CRITICAL THINKING (Due FEB 1st):

1. A roommate tell you about JC, a friend who suffered a suffered a blow to the back of the head that resulted in cortical blindness in the entire left visual field and part of the right. Your roommate (having recently returned from a camping trip with this friend) remarks about how surprisingly good JC is at grabbing fireflies that should have been located in the "blind" field especially since JC purports to be "totally guessing" about where the fireflies are located and is honestly surprised when he successfully catches them 9/10 times. You are curious whether JC has "blindsight", and if so, what the underlying cognitive and neural basis might be. Based on the assigned articles, consider the following neuroanatomical hypotheses for the residual vision in blindsight. How you might test which one best accounts for JC's blindsight? What type of evidence would give strong support for Hypothesis A vs. Hypothesis B? Imagine that you can do whatever behavioral and non-invasive neuroimaging tests that you'd like.

Hypothesis A: Blindsight represents processing within the retinotectal pathway (retina to superior colliculus to pulvinar to dorsal stream)
Hypothesis B: Blindsight represents islands of intact V1

2. One approach to resolving divergent theories about a particular disorder (i.e., blindsight, apperceptive agnosia) is to try to systematically manipulate variables that would distinguish these theories in normal individuals and see which best simulate results from patients. Describe how this approach has been used to understand what cognitive processes are affected in apperceptive agnosia - including the results of this study. How could this approach be used to study blindsight? Might the residual vision in blindsight be just like near-threshold vision in normals (do the experiment below)? If blindsight acted like near-threshold vision in normals, would that provide stronger support for one of the two hypotheses above? If so, which one and why?

BLINDSIGHT EXPERIMENT (PART OF QUESTION #2)

First go to this website: "SEEING WHAT YOU DON'T SEE?"

http://serendip.brynmawr.edu/bb/blindsight.html

At this website, youíll find a test of "blindsight" for "normal" individuals like yourself. Read the preface and the instructions. Then test yourself using the parameters below. If you do this question, PLEASE record your data and turn it in to my department mailbox (or email to me) before 4 PM on Wednesday JAN 31st so that I can summarize the results of the experiment for the class discussion.

Set Parameters:

KEEP CONSTANT: speed=5, radius=6

VARY: contrast from ó3 to +3

Do 20 TRIALS at each of these contrasts

Make sure you keep your eyes on the fixation point in the center.

Do all the trials with the same level of ambient light: Standard overhead fluorescent if possible

Record Data:

RECORD how bright it was in the room: Very Bright Average Very Dark

RECORD your % correct for Yes, Maybe and No for each level of contrast in the table below:

Send to me by Jan 31st!	Contrast (vary from ó3 to +3)
	-3	-2	-1	0	+1	+2	+3
YES
MAYBE
NO

ADDITIONAL READINGS IF YOU WANT TO LEARN MORE ABOUT THE TOPIC:

1. Meeres, S. L., & Graves, R. E. (1990). Localization of unseen visual stimuli by humans with normal vision. Neuropsychologia, 28(12), 1231-1237.
2. Azzopardi, P. & Cowey, A. (1997). Is blindsight like normal, near-threshold vision? Proceedings of the National Academy of Sciences USA, 94, 14190-14194.
3. Marcel, A. J. (1998). Blindsight and shape perception: deficit of visual consciousness or of visual function? Brain, 121, 1565-1588.
4. Wessinger, C. M., Fendrich, R., & Gazzaniga, M. S. (1997). Islands of residual vision in hemianopic patients. Journal of Cognitive Neuroscience, 9, 203-221
5. Graves, R. E., & Jones, B. S. (1992). Conscious visual perceptual awareness vs. non-conscious visual spatial localization examined with normal subjects using possible analogues of blindsight and neglect. Cognitive Neuropsychology, 9(6), 487-508.
6. Kolb, F. C., & Braun, J. (1995). Blindsight in normal observers. Nature, 377, 336-338.

SOME ADDITIONAL TERMS (GLOSSARY FOR READINGS)

Bonferroni correction: method of statistically correcting for increased possibility of spurious significance when making lots and lots of pairwise comparisons. Without this type of correction, it becomes increasing likely that one of the many comparisons that are made will be statistically significant simply by chance (i.e., Type I error increases). The down side is that you may "overcorrect" (i.e., comparisons that represent real differences will not be significant after this correction). Therefore, many researchers show what comparisons are significant both with and without correction.

Egocentric space: spatial position in the environment with the viewerís body as a reference point (i.e., the position of the paper relative to your eye or hand). Can be contrasted with allocentric space, which is the relative position of two objects (points) in the external world (i.e., the position of the paper relative to the table).

GSR (Galvanic Skin Response): Electrophysiological change in skin conductance that occurs as the result of slight increases in sweat when an individual becomes cognitively and/or emotionally aroused. It is measured with an electrode on the skin surface.

Homonymous hemianopia: blindness in one entire visual field; occurs following complete unilateral damage to the lateral geniculate nucleus (LGN) of the thalamus, optic radiations (white matter connecting the LGN and V1), or primary visual cortex (V1).

Infarction: damage due to stroke

Macula(r): center of the visual field, often spared following lesions to the visual cortex (V1), but not the LGN or optic radiations. Reasons for selective sparing are unknown but may be due to the fact that the cortex for this region receives blood from both middle and posterior cerebral arteries, making it more resilient to strokes.

Scotoma: region where the patient has no conscious vision.