Psychophysics and Scanner Donald A. Robin, Ph.D. Professor and Chief of Human Performance Division RIC

What is Psychophysics? The quantification of human performance, first applied to quantification of perception and now more broadly includes motor system and perhaps aspects of cognitive processing Assumes that humans are essentially measurement systems that can be systematically analyzed (RT, judgments, skill acquisition

Hemholtz

Motion Lab

Parametric Manipulation Importantly, a key feature of psychophysical experiments is the parametric manipulation of stimulus parameters allowing for fine grained quantification of performance. This makes psychophysical methods ideal for imaging studies. However, integration into the scanner environment is complex.

The Signal to Noise Problem Perceptual experiments require the perceiver to be able to distinguish the SIGNAL from NOISE-not unlike image processing Noise can be external or internal (physiological) Signal Detection Theory (d-prime and beta)

Starry Night (Rizzo & Robin)

Dependent Variables D-prime (accuracy) Beta (response bias) RT (reaction time)

Car as Psychophysical Laboratory

OR as Laboratory

MRI Environment

Issues with MRI Environment As noted by Savoy-the scanner creates a “electromagnetically hostile” experimental environment thus most standard equipment poses potential risks to subjects and critically Can interfere image processing, causing distortions by altering the magnetic field

Other Issues Collecting data in the scanner can also be an issue because of the noise changes how stimuli are presented, the ability to move is limited (beyond movement artifact in the image), grounding issues emerge and others.

Carter et al-2008 Details of Study Protocol Details of Study Protocol Study Design Type—Cross-sectional (case/control), longitudinal, interventional Outcome measure—Brain activation, treatment effect, correlation of brain activation with clinical variable Completeness—Completed/ongoing study; complete/partial reporting of data Approval Cite governing body Funding Cite all sources

Study Protocol 2 Study Subjects Number of subjects/samples Number of subjects contacted/enrolled/studied/completed Reasons for dropout Clinical assessment, behavioral task performance, inadequate imaging data, technical problems Recruitment site e.g., Community, clinic, inpatient unit Demographics Age, gender, race/ethnic group Assessments IQ; handedness; cognitive/social function; educational achievement; psychiatric diagnoses; rating scales (state markers of illness)

Study Protocol 3 Imaging Methods Scanner—Manufacturer, field strength, stability (changes of software, head coils), use of multiple scanners Image acquisition parameters—Sequence, duration, averaging; number of slices, slice thickness (+/- gap), voxel size, field-of-view; field maps magnitude/amplitude), TR, TE, flip angle, number of image volumes Design—Block-design (length), Event-related (efficiency, sensitivity), Sparse

Study Protocol 4 Behavioral Methods Stimulus presentation—Hardware, software, stimulus set (source of stimuli, number of stimuli) Design—SOA, ISI, counterbalancing scheme Behavioral data—Criterion for performance, exclusion criterion, practice effect

Presentation Modes Visual Auditory Tactile Olfactory

Cost –Benefit of Stimulus Presentation Decisions Cost (vision a mirror system is much cheaper than goggles) Spatial/Temporal Resolution/Frequency Response/Color-Intensity Resolution Field of View (Vision) Image Quality Ease of Use Software compatibility

Subjects Responses Voluntary Physiological

Types of Voluntary Responses Simple RT Choice RT Continuous Movement Speech Eye Movements Language and Cognition

Voluntary Response Systems Buttons Cursors Pressure Sensors Microphone Camera

PET Speech Set-up

Physiological Responses Electrical Signals (ECG, EOG, EMG)

Dealing with Movement Artifact Minimize head movement Feedback Immobilize head Foam pads Thermoplastic Masks Plaster head cast Bite bars

Other movements Shoulder Articulator Arm Leg During speech (or other overt responses movement artifact a huge problem)

Solution! Sparse Sampling Scanner turned on after movement ends

Sparse Sampling

Sparse Study-Auditory Feedback and Voice Production

The pitch – shift reflex A voice F0 response is in the opposite direction of a pitch-shift stimulus Stimulus, upward 30 cent pitch shift Stimulus, downward 30 cent pitch shift Black lines --control Red lines --response to stimulus Black lines -- control Blue lines -- response to stimulus

BOLD fMRI Sparse sampling Image acquisition 2 seconds following phonation Reduce movement artifact Eliminate effect of scanner noise

Conditions Identical for fMRI and TMS Sustained phonation for 5 seconds 4 – 6 pitch shifts per vocalization ± 100 cents 200 ms in duration Feedback Response

BOLD fMRI Results

Issues with Sparse Paradigm S/N is poor Therefore requires many more repetitions than blocked and more than event related Subjects are in the scanner a long time

Safety Issues RF heating-wires in the body (e.g., deep brain stimulation unit), burn tissues or heat wires and cause tissue damage Nerve and cardiac effects of rapidly switching gradients Stabilization devices can cause safety issue EEG, EOG are potential issues

Integrating Psychophysics into the MRI lab Commercial systems MR compatible buttons, headphones, microphones Center developed systems Because of complex electromagnetic fields, and numerous wires and electrical sources, getting started can be a real challange

Experimental Platforms Stimulus Generation Stimulus Delivery Performance measure data collection (e.g., RT, verbal response)

Experimental Platforms E-Prime Used by many functional imaging groups Easy to learn Developed in part for fMRI studies Has issues with timing, stimulus control Need at least two computers: One for stimulus presentation and one for data collection

Experimental Platforms MatLab Requires programming skills Highly flexible Multiuses Integrates nicely with many other platforms Compile programs for exceptional timing

Experimental Platforms LabView Can compile Very adaptable Integrates with MatLab Used in many performance laboratories Can control multiple devices Can present and collect data on one computer

Summary Psychophysics is critical to functional imaging (and to some structural questions – voxel based lesion symptom mapping, DTI with performance correlations) Integration in the MRI environment is complex Understanding signal processing and signal detection theory are critical to designing experiments

Summary ANY experimental design depends on the specific questions you ask THUS No single design for all areas or studies Must consider subject group Must understand differences in subject performance levels affect activations