lecture 9 - phd vision

Question 1

What are the three levels of analysis in Marrian analysis of pRF models?

Answer 1

1. Computational: Defines the goals or principles of the process.
2. Algorithmic: Describes rules and transformations used to achieve the process.
3. Implementation: Explores the biological basis of the process, such as mechanisms and hardware.

Question 2

What is the size ratio ϕ in cortical size-response curves?

Answer 2

• σ2/σ1
• It determines the decrease of responses to stimuli beyond the optimal size.

Question 3

What might the size ratio ϕ reflect in cortical models?

Answer 3

• The balance between feedforward/feedback projections.
• Modulation by receptors related to feedback (e.g., NMDA) and inhibition (e.g., GABA).

Question 4

How do neurotransmitter receptors function in divisive normalization models?

Answer 4

Neurotransmitter receptors act as algorithmic modulators, bridging molecular mechanisms (receptor densities) with computational principles.

Question 5

Which neurotransmitter receptors correlate with suppression?

Answer 5

• GABA and 5-HT1A (serotonin subtype) reduce baseline excitatory activity or increase inhibitory activity.
• They affect the activation constant (b), leading to stronger suppression of baseline activity.

Question 6

Which neurotransmitter receptors correlate with compression?

Answer 6

• 5-HT1B and 5-HT2A (serotonin subtypes) influence input sensitivity.
• They affect the normalization constant (d), leading to nonlinear response compression.

Question 7

What is psilocybin, and what role does it play in DN models?

Answer 7

Psilocybin is a serotonin receptor agonist (5-HT2A/1A) that affects neural and visual processing, particularly influencing divisive normalization (DN).

Question 8

What was the experimental design for the psilocybin study?

Answer 8

1. Double-blind crossover study with placebo, 5 mg, and 10 mg psilocybin doses.
2. Measurements included fMRI, psychophysics (e.g., perception tasks), and questionnaires.

Question 9

What key measurements were tracked during the psilocybin study?

Answer 9

1. Neural activation.
2. Subjective effects.
3. Pupil size.
4. Behavioral changes over time.

Question 10

How did psilocybin affect perception in the Ebbinghaus illusion task?

Answer 10

Psilocybin amplified perceptual bias by overweighting context (surround), which increased noise in judgments.

Question 11

How did psilocybin alter DN model constants?

Answer 11

Psilocybin decreased the DN model activation constant
b (suppression) throughout the visual system.

Question 12

What conclusions were drawn from the psilocybin study?

Answer 12

1. Psilocybin systematically alters perception and cortical responses.
2. It does not add noise but alters the fundamental computation (DN suppression).
3. Effects are observed deep in the cortical hierarchy.
4. Provides causal evidence of receptors as algorithmic modulators of DN.