Visual Fields and fMRI

Adler’s Physiology of the Eye 11th Ed.Chapter 35 - by Johnson & Wall

http://www.mcgill.ca/mvr/resident/

Visual Fields

• Perimetry and visual field testing- detect functional losses- identify the location of a visual deficit- monitor the status of acute and chronic disease- evaluate efficacy of treatment

• Old technique, but continued improvements- automation- standardization- immediate statistical evaluation- greater efficiency

Visual Fields

Photopic adaptation, in normal eye, and visual system

Hill of Vision

temporalnasal60o 100o

15o

HM

30o

3o

Weber’s LawL/L=C

standard deviationabout 3dB

>300% more for damaged

Visual Fields

• Kinetic Visual Fields- egg shaped isopters for a given target- efficient & flexible method for center and periphery- more variability, more expertise needed, less standards

• Static Visual Fields- most common, good standardization, and prediction- immediate statistics, can monitor reliability, align. & fixation- demanding for patient, high variability for low sensitivity

• Suprathreshold static perimetry- rapid to detect field defects, over entire field- limited quantification, lower sensitivity and specificity,- less validation

Visual Fields

Standard Automated Perimetry (SAP)

~1000 x dimmer than maximum

Glaucoma Hemifield Test

Mean DeviationPattern Standard Deviation

Total Deviation Pattern Deviation (remove generalized sensitivity differences up to 85 percentile)

Visual Fields

• Normal vision, but ‘trigger happy’- abnormally high sensitivity & false positives,hitting the button too often

• Generalized, widespread field loss

• Localized field loss

• Mixture of localized and widespread loss

Visual Fields

• good gaze tracking

• excessive blinking

• droopy eyelid

• fatigue

• alignment problems

Gaze tracking with infrared Purkinje image system

Field Defects

arcuaterespect HM

bitemporalrespect VM central

homonymous

Field Defects

New Perimetric Tests

State of Affairs in 1990’s

• Monkey Visual Cortex Felleman & Van Essen, 1991

• Human Visual Cortex Clarke & Miklossy, 1990; Horton & Hoyt, 1991

• fMRI Kwong et al., 1992; Ogawa et al., 1992

• Cortical Surface Representation Dale & Sereno, 1993

State of Affairs in 1990’s

• Idea of Phase-Encoded Stimuli & fMRI Engel et al., 1994

• Mapping Human Visual Cortex with fMRI Sereno et al., 1995

• Visual Cortex Activation with Stimulation/PET Dobelle et al., 1979; Fox et al., 1986

Magnetic Resonance Imaging

T1 weighted T2 weighted

Brain Images Showing T1 and T2 Contrast

By varying the timing of sending and receivingsignals in the MR scanner, most anatomical MRimages are either T1 weighted or T2 weighted.This is done to create contrast between differenttissues types.

Functional MRI is a newer technique that detectsthe T2 difference between oxy-hemoglobin anddeoxy-hemoglobin

A V A V

The BOLD Effect

oxyhemoglobin

deoxyhemoglobin

Normal Flow High Flow

“Blood Oxygen Level Dependent”

LOCALIZED CHANGES IN BLOOD FLOW AND OXYGENATION

VISUALIZE CHANGES WITH BOLD fMRI

CHANGES IN INFORMATION PROCESSING DEMANDS

LOCALIZED CHANGES IN NEURAL ACTIVITY

fMRI Is an Indirect Measure of Neural Activity

Mapping Visual Cortex

Eccentricity and Polar Angle Stimuli

TIME

TIME

Eccentricity and Polar Angle Analysis

TIME

INTENSITY

Activation Viewed on Brain Slices

Maps of Cardinal Axes on Flattened Cortical Surface

Maps of Cardinal Axes on Flattened Cortical Surface

‘Field-Sign’ is Calculated from Cardinal Axes

Inflated View of Areas

V3A

V3

V2

VPV4v

V1

Inflated View of Areas

View of Areas

monkeyhuman

human

Comparison with Other PrimatesCortical Magnification

Human has highest magnification factor

Current Efforts to Relate Human and Monkey Cortex

Monkey Human

Orban et al., 2004

Use of Standard Landmarks Alone & with FMRI Landmarks

Brodmann Areas

Standard Landmarks Standard & fMRI

Orban et al., 2004

Current Efforts to Relate Human and Monkey Cortex:

Lack of Consensus Exists Even for Monkey

Mapping Ipsilateral Field

18

16

9 10 11 12 13 14 15 16

7 6 5 4 3 2 18

Receptive Fields at 17/18 Border

Callosally Projecting Neurons at 17/18 Border

Bourdet et al., 1996Concept: stabilization of CC connections for neurons with similar receptive fields

Human Ipsilateral Visual Field Representation

Moving stimulus confined to hemifield

Effects of Varying Wedge Size

Cortical Magnification &Other Functional Organization

Retinotopy & Cortical Magnification

V1

* = occipital pole = foveal vision

Tootell et al., 1998b

Ocular Dominance (blind spot)

Tootell et al., 1998a

Contrast Sensitivity

Pelli-Robson Chart

How could V1 be activated selectively?

MT has high contrast sensitivity V1 has lower contrast sensitivity

Tootell et al., 1998b

Well studied in monkeys Distinctive anatomy (myelin) and function direction selectivityPresent in all primates tested.

Tootell et al., 1995

Human Cortical Area MT

Localizing Stimuli

low-contrast motion comparedto low-contrast stationary