Experiment design

Research Methods

Fall 2010

Tamás Bőhm

Elements of experiment design

• Question

• Method

• Stimulus

• Control

• Interpretation

Question

• Functional specialization of the cortex

• Development of functions(ontogenesis & phylogenesis)

• Operation of specific functions

Question

• Functional specialization of the cortex

Question

• Development of functions

Question

• Operation of specific functions

What determines the brightness of a surface?

Method

• Behavioral/psychophysics

• Electrophysiology

• Imaging

• Genetics

• …

Stimulus

Ishihara plates:tests wavelength sensitivity(the tuning of retinal red and green cones)

Stimulus

Brightness of the spots need to be controlled1. Constant

brightness (isoluminance):hard to achieve

2. Randomizing light intensity

Stimulus

• Stereopsis (binocular depth perception): based on retinal disparity

• Can break camouflage

Stimulus

Stimulus

Julesz Béla’s random dot stereograms (RDS)

Stimulus

Stimulus

http://ruccs.rutgers.edu/%7Eikovacs/SandP/rds/rds.html

Stimulus

At which level of visual processing does stereopsis happen?– 1950s: at very high level (after figure-ground

separation and form recognition)• Monocular form cues & contours are essential• Could not find neural substrates

– Julesz: it must be early in processing!• No monocular cues on RDSs• Could find the corresponding binocular depth cells

Question:functional specializationMethod: psychophysics, electrophysiology

Stimulus

• Hubel and Wiesel 1962• Barlow, Blakemore and Pettigrew 1967• Bishop 1969• Gian Poggio 1984: disparity selective neurons in V1, V2, V3, V3a

response of a binocular depth cell

Stimulus

Take home message: use ‘clean’ stimuli

Stimulus

Binocular rivalry

Stimulus

At which level of visual processing does thishappen?– 1980s: at low level

• Competition between the two eyes• Reciprocal inhibition of monocular neurons

Question:functional specializationMethod: psychophysics

Stimuli

• Conventional rivalry inducing pair:eye-of-origin and stimulus coherence

Stimuli

• Patchwork rivalry stimulus (Kovács et al PNAS 1996): stimulus coherence only

Stimuli

Stimulus

At which level of visual processing does this happen?– 1980s: at low level

• Competition between the two eyes• Reciprocal inhibition of monocular neurons

– Kovács et al.: later in processing• Competition also between two coherent pictures• After the input layer of V1

Question:functional specializationMethod: psychophysics

Stimulus

Logothetis

Stimulus

Take home message: get rid of extraneous factors in the stimuli