1. A.F. Lamme and Pieter R. Roelfsema
The distinct modes of vision offered by feedforward and recurrent processing Victor
A.F. Lamme and Pieter R. Roelfsema

2. Dichotomies in the Visual System?
What are three dichotomies that Lamme identifies in the visual system?

3. Dichotomies in the Visual System?
What are three dichotomies that Lamme identifies in the visual system?
Dorsal vs. Ventral stream

4. Dichotomies in the Visual System?
What are three dichotomies that Lamme identifies in the visual system?
Dorsal vs. Ventral stream
Pre-attentive vs. Attentive

5. Dichotomies in the Visual System?
What are three dichotomies that Lamme identifies in the visual system?
Dorsal vs. Ventral stream
Pre-attentive vs. Attentive
Conscious vs. Unconscious

6. The Feed-Forward Sweep
What is the feed-forward sweep?

7. The Feed-Forward Sweep
The feed-forward sweep is the initial response of each visual area “in turn” as information is passed to it from a “lower” area
a single spike per synapse
no time for lateral connections
no time for feedback connections

8. The Feed-Forward Sweep
The feed-forward sweep is the initial response of each visual area “in turn” as information is passed to it from a “lower” area
Consider the latencies of the first responses in various areas

9. The Feed-Forward Sweep
Thus the “hierarchy” of visual areas differs depending on temporal or anatomical features
Three aspects of the visual system account for this fact:
Some neurons in an area don’t receive direct connections from the next “lower” area
multiple feed-forward sweeps progressing at different rates (I.e. magno and parvo pathways) in parallel
signals arrive at cortex via routes other than the Geniculo-striate pathway (LGN to V1)

10. The Feed-Forward Sweep
The feed-forward sweep gives rise to the “classical” receptive field properties
tuning properties exhibited in very first spikes
think of cortical neurons as “detectors” only during feed-forward sweep

11. After the Forward Sweep
By 150 ms, virtually every visual brain area has responded to the onset of a visual stimulus
But visual cortex neurons continue to fire for hundreds of milliseconds!

12. After the Forward Sweep
By 150 ms, virtually every visual brain area has responded to the onset of a visual stimulus
But visual cortex neurons continue to fire for hundreds of milliseconds!
What are they doing?

13. After the Forward Sweep
By 150 ms, virtually every visual brain area has responded to the onset of a visual stimulus
But visual cortex neurons continue to fire for hundreds of milliseconds!
What are they doing?
with sufficient time (a few tens of ms) neurons begin to reflect aspects of cognition other than “detection”

14. Extra-RF Influences
One thing they seem to be doing is helping each other figure out what aspects of the entire scene each RF contains
That is, the responses of visual neurons begin to change to reflect global rather than local features of the scene
recurrent signals sent via feedback projections are thought to mediate these later properties

15. Extra-RF Influences consider texture-defined boundaries
classical RF tuning properties do not allow neuron to know if RF contains figure or background
At progressively later latencies, the neuron response differently depending on whether it is encoding boundaries, surfaces, the background, etc.

16. Recurrent Signals in Object Perception
Can a neuron represent whether or not its receptive field is on part of an attended object?
What if attention is initially directed to a different part of the object?

17. Recurrent Signals in Object Perception
Can a neuron represent whether or not its receptive field is on part of an attended object?
What if attention is initially directed to a different part of the object?
Yes, but not during the feed-forward sweep

18. Recurrent Signals in Object Perception
curve tracing
monkey indicates whether a particular segment is on a particular curve
requires attention to scan the curve and “select” all segments that belong together
that is: make a representation of the entire curve
takes time

19. Recurrent Signals in Object Perception
curve tracing
neuron begins to respond differently at about 200 ms
enhanced firing rate if neuron is on the attended curve

20. Feedback Signals and the binding problem
What is the binding problem?

21. Feedback Signals and the binding problem
What is the binding problem?
curve tracing and the binding problem:
if all neurons with RFs over the attended curve spike faster/at a specific frequency/in synchrony, this might be the binding signal

22. Feedback Signals and the binding problem
What is the binding problem?
curve tracing and the binding problem:
if all neurons with RFs over the attended curve spike faster/at a specific frequency/in synchrony, this might be the binding signal
But attention is supposed to solve the binding problem, right?

23. Feedback Signals and the binding problem
So what’s the connection between Attention and Recurrent Signals?

24. Feedback Signals and Attention
One theory is that attention (attentive processing) entails the establishing of recurrent “loops”
This explains why attentive processing takes time - feed-forward sweep is insufficient

25. Feedback Signals and Attention
Instruction cues (for exaple in the Posner Cue-Target paradigm) may cause feedback signal prior to stimulus onset (thus prior to feed-forward sweep)
think of this as pre-setting the system for the upcoming stimulus

26. Feedback Signals and Attention
We’ll consider the role of feedback signals in attention in more detail as we discuss the neuroscience of attention