Neurobiological Analysis of Backward Masking in Visual Perception, Slides of Ophthalmology

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Background

Contrast

Energy

Chromaticity

Criterion

STA

Form

SOA

Spatial Layout

Dichoptic

Monoptic

Contour

ISI

Background

Contrast

Energy

Chromaticity

Criterion

STA

Form

SOA

Spatial Layout

Dichoptic

Monoptic

Contour

ISI

Neurobiological correlates

of

backward masking

at

cortical levels

Does masking lead to the absence of neural activity?Does masking affect only certain parts of the neural response?Does masking affect the integration of the neural response?…

Single-unit recording from V

(average of 30 on-off-response complex neurons of alert rhesus monkey)

target only 

onset transient and after-discharge standing wave of invisibility 

no onset transient and after-discharge mask only 

excitatory response … partly in receptive field 

obscures sustained response target only 

onset transient and after-discharge forward masking 

no onset transient and after-discharge backward masking 

no after-discharge

Multi-unit recording from layer 4B of an anesthetized squirrel monkey:

The mask was moved further away so thatit is outside of the target’s receptive field.Onset response and after-dischargeUnder forward masking conditions, thetarget’s transient onset response wasinhibited. Under backward masking conditions, thetarget’s transient after-discharge wasinhibited.

Forward masking inhibits the transient on-response and the after-dischargeto the target.

Backward masking inhibits the transient after-discharge to the target.

Sustained portions of responses can be inhibited by the mask but they areless important in determining the visibility of targets.

Max. masking effect 100msafter the target was turnedoff (100ms STA).

They did not measure behavioral effects of masking in the monkeys, but the time ofminmum visibility in a similar paradigm fits nicely with the electrophysiological results:

Masking Interrupts Figure-Ground Signals in V

Lamme VAF, Zipser K, Spekreijse H

THEORIES OF BACKWARD MASKING:

The

later

mask

catches

up

with

the

target

before

it^

reaches

conscious levels as proposed by feedforward models of backwardmasking (e.g. Breitmeyer).Masking

interferes

with

recurrent

feedback

processing

only

when information from higher levels is coherent with information atlower

levels,

the

stimuli

is

sufficiently

processed

to

allow

for

conscious recognition. Masking occurs when by the time the high-level signals reach the lower levels, info at these lower-levels is notabout the first stimulus but about the mask (e.g. diLollo).

Mask

Target

Target

TASK:Saccade to the figure in the first display.2x2 DESIGN MATRIX:- receptive field on figure or background- receptive field on 45° or 135° orientation(non-/preferred)Instead of the pattern mask, also a light mask wasused (blank screen of approximately equal meanluinance as the textured figure-ground scene).NOTE:Not the ISI, but stimulus duration (=SOA) was variedfrom 14-110ms.RECORDING:Multi-unit activity in V1 of awake monkeys.

Experimental Design

To what extent is

figure-ground modulation

influenced by texture pattern backward masking?

Figure-ground modulation:Difference

between

neural

activity

evoked by texture elements that belongto a figure (solid line) and activity evokedby the same elements of the background(thin line). 

No figure-ground modulation for short target durations/SOAs. 

Mean figure-ground modulation from 100ms to 140ms (dotted interval only)was analysed

To what extent is

orientation selectivity

influenced by texture pattern backward masking?

A+B

vs.

C+D

Orientation selectivity arises at much shorter latenciesthan figure-ground modulation.Orientation selectivity is hardly affected by backwardmasking: At SOAs where the animals do not perceivethe stimuli (line), orientation selectivity (bars) is almostequally strong as at SOAs where performance is nearperfect.

To what extent is

orientation selectivity

influenced by texture pattern backward masking?

Macknik

shows

that

metacontrast

masking affects the neuronal after-discharge. Lamme reveals that later regimes ofactivity

are

targeted

by

backward

pattern masking.

SUMMARY: V

Brightness Discrimination

SOA 0ms extensive training … interleaved with …

Metacontrast trials

Simultaneous

:^

SOA 0ms vs.

Delayed: SOA @ max.masking effect

Conditions are physically identical

but

perceptually different.

Single cell recordings in monkey V

(divided into early and late components)