Lecture 1 – Introduction to Psychophysics & Neuroimaging

Ilan Dinstein

Psychophysics

“The scientific study of the relation between stimulus and sensation”

• Behavioral studies of perceptual abilities.• How sensitive are we to different stimuli?

• What affects our ability to detect a stimulus?• Perceptual learning – improving sensitivity with practice.

Where I is the “reference” and ∆I is the additional amount of stimulus needed to detect a difference. K is surprisingly

consistent across multiple amplitudes and sensory domains.

Just noticeable difference

What is the smallest difference in the stimulus that we can detect. Weber’s law:

Contrast sensitivity

Spatial frequency

Con

trast

Contrast sensitivity & threshold

Visual search

Behavioral measures

Accuracy & Reaction time

Can the subject detect something at above-chance levels?How quickly does the subject perform the task?

Things to worry aboutIs the subject cooperative? Motivation? Arousal level? Stress?

What is creating the difference? Slow motor responses?

Magnetic resonance imaging (MRI)

Anatomy - Separating tissues

1T 2T

Anatomy – Cortical thickness

Anatomy – Cortical folding

Brain function

Neurovascular coupling

Vasculature

Changes in oxygenated blood

Heeger et. al. 2002

זמן

fMRI experiment

Experimental results

In fMRI we always compare measures over time

Observing facial expression

Mirror system areas in autism

Dapretto, Nat. Neurosci. 2006

Control > Autism

Electroencephalogram

Control > ASD

Signal = Potential Difference

Source of EEG signals

1. Muscle contractions anywhere in the head.2. Heart beat (ECG).3. Electromagnetic noise – AC, Cell phones, etc…4. Synchronized cortical neural activity over large areas (>1cm) –

Sources and Sinks.

What causes potential changes between the electrodes?

Source of EEG signals

1. The larger the synchronicity, the stronger the signal (potential difference).

2. Topography of the brain – Sulci and Gyri.3. Changing conductance in the brain – CSF, dura, skull (strong

resistor).4. Inverse problem – almost infinite combinations of dipoles can create

the same potentials on the scalp.

Example of Event Related Potential experiment

Differentiating responses to illusory contours…

Show many trials of each stimulus and average across presentations.

See how and where the brain response differs.

You need many trials!

Spatial selectivity of response

Control > ASD

ERPs

EEG Frequencies

EEG coherence

Control > ASD

Similarity of activity across electrodes.