Feedforward processing

Question 1

The fast feedforward sweep

Answer 1

• speeds through visual system within 80ms by meens of feedforward connections
• Provides neurons with their receptive field tuning properties
• Provides fast detection of hardwired features and constellations (rapid extraction of relevant info)
• Provides set of grouping operations
• Enables intelligent sensorimotor reflexes; intelligent reflex arch, enables execution of well trained automatic, visually driven motor programs (baseball player)

Question 2

Feedforward convergence

Answer 2

During feedforward sweep, cells get increasingly larger RFs and progressively more complex tuning properties

There are multiple sweeps, as well as horizontal and feedback connections

Question 3

Horizontal connections

Answer 3

Aside from intelligent feedforward reflexes, horizontal connections create interconnection between cells with distant receptive fields. Mailny with nearby cells, but also more selectively distant ones, that are related to the tuning preference of the cell.

Seen as boutons when injected with tracers

Because the connections interconnect cells with the same orientation preference, they group elements with the same orientation together. Making horizontal connections the neural way of explaining gestalt law of grouping by similarity.

Also, V1 horizontal connections spread along a line, not just orientation, but also line in visual space

Question 4

Local Gestalt Grouping

Answer 4

V1 cells respond more strongly when similar orientation surround the receptive field. BUT only if there is activation in the receptive field itself as well.

Gestalt laws explained by horizontal connections: Proximity, similarity(orientation) and good continuation

Question 5

MT area Gestalt

Answer 5

Most neurons in MT are direction selective. Horizontal connections connect cells with similar direction preference. In line with law of common fate

Question 6

Feedback signals

Answer 6

Contextual modulation of responses; The first 100ms, the cell looks at what happens in their RF (isolated information). After 100 ms, the V1 activity is mudalted by the global visual context, meaning the surrounding context is integrated (integrated information)

When there’s a lesion in V1, contextual modulation is distorted.

Question 7

Figure-ground segregation

Answer 7

Gestalt law of figure-ground is reflected in the modulation. The whole surface of the figure shows elevated activity. Meaning that if you plot just the modulation figure, you’ll see an elevated surface in which the boarders are peaked.

The horizontal connections give the boundary, the feedback connections do the filling in

Question 8

Neurotransmitters

Answer 8

Feedforward processing uses different neurotransmitters than feedback (recurrent) processing

Injecting NMDA receptor blocker (APV)
- selectively reduces recureent processing (feedback processing)
- Feedforward Sweep is still intact (not so much affected)

Injecting AMPA receptor blocker (CNQX)
- Feedforward sweep signals are reduced
- Feedback processing (RP) is not