Use a pen on the test

The distinct modes of vision offered by feedforward and

recurrent processing

Victor A.F. Lamme and Pieter R. Roelfsema

Dichotomies in the Visual System?

• What are three dichotomies that Lamme identifies in the visual system?

Dichotomies in the Visual System?

• What are three dichotomies that Lamme identifies in the visual system?

• Dorsal vs. Ventral stream

Dichotomies in the Visual System?

• What are three dichotomies that Lamme identifies in the visual system?

• Dorsal vs. Ventral stream

• Pre-attentive vs. Attentive

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

The Feed-Forward Sweep

• What is the feed-forward sweep?

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

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

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)

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

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!

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?

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”

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

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 responds differently depending on whether it is encoding boundaries, surfaces, the background, etc.

Extra-RF Influences

• How do these data contradict the notion of a “classical” receptive field?

Extra-RF Influences

• How do these data contradict the notion of a “classical” receptive field?

• Remember that for a classical receptive field (i.e. feature detector):

– If the neuron’s preferred stimulus is present in the receptive field, the neuron should fire a stereotypical burst of APs

– If the neuron is firing a burst of APs, its preferred stimulus must be present in the receptive field

Extra-RF Influences

• How do these data contradict the notion of a “classical” receptive field?

• Remember that for a classical receptive field (i.e. feature detector):

– If the neuron’s preferred stimulus is present in the receptive field, the neuron should fire a stereotypical burst of APs

– If the neuron is firing a burst of APs, its preferred stimulus must be present in the receptive field

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?

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

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

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

Feedback Signals and the binding problem

• What is the binding problem?

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

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?

Feedback Signals and the binding problem

• So what’s the connection between Attention and Recurrent Signals?

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

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

Feedback Signals and Attention

• We’ll consider the role of feedback signals in attention in more detail as we discuss the neuroscience of attention