1. The longer one looks, the less one sees --- catching the features
before they merge into objects
Li Zhaoping, Nathalie Guyader,
University College London
Presented at the meeting of Experimental Psychology Society,
Birmingham, UK, April 10, 2006.

2. First, a demo: visual search for orientation singleton
Now, superpose on each bar a vertical or horizontal bar.

4. Difficult --- even though the target still has an uniquely oriented bar.
Is this due to dissimilarities between distractors in background? --- Duncan & Humphreys only partly!

5. In a control condition:
Singleton 20 deg from vertical
Now, superpose on each bar a vertical or horizontal bar.

7. Target not as difficult to spot
Target has a unique shape

8. Comparing the two stimuli:
Add horizontal/vertical bars to each bar
Asimple
Bsimple
A: target difficult to find
B: target easy to find

9. A: target difficult to find B: target easy to find
Outline of the talk:
A: target difficult to find
B: target easy to find
(1) To find the target, bottom up saliency by unique orientation feature, pre-attentively is mostly sufficient.
(2) Target object recognition is unnecessary.
(3) Viewpoint invariant object recognition (involving attention): Target in A, but not B, is identical to distractors, confusing!
(4) Object/attention processes interfere with feature/pre-attentive processes. Thus the title.

10. V1 (primary visual cortex)
Low level image features:
Higher level objects:
view-point independent …
IT (infero-temporal cortex), LOC parietal cortex, (Tanaka, Rolls, Logothetis, Humphreys, Riddoch & Price, Kourtzi, etc)
V1 (primary visual cortex)
Hubel & Wiesel 1968
Representation requires no attention Wolfe & Bennett (1997).
Representation requires attention.
Stankiewicz, Hummel & Cooper (1998)
When gaze is not on target.
When gaze is on target.
Hoffman (1998): attention at gaze positions in free viewing (except very briefly before saccades)
Bottom up, unique feature attract attention.
(Treisman, Julesz, Duncan & Humphreys, Wolfe, Itti &Koch, Li, etc)
Object/shape viewpoint invariant recognition (Treisman, Humphreys et al, Grill-Spector, Kourtzi, Kanwisher, etc.)

11. Low level image features: Higher level objects: view-point independent …
Unique feature to attract attention
Pre-attentively
Sufficient to locate the target. B
In A but not B, target is identical to distractors, confusing!
Attentively
Typically, more is better, …..
Perhaps not here!

12. In a visual search experiment:
Fixation
Stimulus until button response:
Subject’s eye movements were tracked
Time
Task: locate the target with a uniquely oriented bar, ASAP, by pressing left/right button for target in the left or right half of display A B
Bsimple
Asimple
Stimuli:
Hence, can not use feature based top-down attention to locate target initially.
The uniquely oriented bar maybe randomly tilted right or left: or
The task irrelevant bar on target maybe randomly or or

13. Display span 46x32 degrees in visual angle Condition A

14. Gaze arrives at target after a few saccades
Mainly by the bottom up saliency of the unique orientation.
Then ...

15. We call this an arrival-abandon-return (AAR) trial
Gaze dawdled around the target, then abandoned and returned.
We call this an arrival-abandon-return (AAR) trial

16. Condition B

17. Gaze arrival at target

18. Followed immediately by button response
Measurements:
RThand Reaction time of button press
RTeye of gaze arrival at target.
Performance: Button correct?
Eye scan paths

19. RTeye A and B had similar RTeye RT data: for hand and gaze:
(3 subjects: red/green/blue)
RTeye
Asimple
Bsimple A B
A and B had similar RTeye

20. RT data: for hand and for gaze: RTeye dark bars
(3 subjects red/green/blue)
RTeye dark bars
RThand – RTeye(light bars)
Asimple
Bsimple
3-4 second delay before button press! A B
Percentage of correct button presses

21. Eye scan paths Target location
An arrive-abandon-return (AAR) trial
A non-AAR trial
An example for A
An example for B
Percentage of AAR trials
Note: visual attention is directed to eye position --- top down interference to task in A A B
RThand – RTeye for non-AAR trials

22. To probe the time course of the interference
Experiment 2: visual search with time limit
Mask stimulus:
Fixation
Search stimulus:
Search stimulus
Time
Seemingly random time interval between them
Task: Subject were instructed to take their time to button press for target location, before or after the mask appearance as they please, guess if necessary.

23. Subjects unaware of links between mask onset times and eye positions
Gaze Contingency
fixation
Search stimulus onset
Gaze arrival at target
Mask onset
time t
Target viewing time without attention T
Target viewing time with attention
Subjects unaware of links between mask onset times and eye positions
Interleaved with control trials of random and earlier times for mask onsets, such that

24. Comparing conditions A and B (interleaved in each session)
Same performance in the beginning! B
Improve marginally for B with time A
Deteriorate significantly for A with time
Before eye arrival on target
Target (attentive) viewing time T (ms)
i.e., eye-to-mask delay

25. The longer one looks, the less one sees
Not because of any strategy to respond according to eye position at mask onset.
Only 71% of button presses agree with eye position at mask onset (averaged over all trials in session).
First a good guess, as good as without mask at all!
Finally, clarified!
Percent correct button presses
For condition A only in blocked sessions
Then, confused!
Before eye arrival on target
Target (attentive) viewing time T (ms)
i.e., eye-to-mask delay

26. Dissection of events in the task:
(1) Before gaze arrival at target,
viewing target without attention
Hoffman (1998) Visual attention is at the eye position in free viewing (except very briefly before saccades).
Condition A
Wolfe & Bennett (1997) --- pre-attentive objects are bundle of shapeless features
seen as +
viewing target as unbound stimulus features,

27. Dissection of events in the task:
(2) Still before gaze arrival at target, without attention,
unique feature (orientation) attract attention and gaze. Treisman, Julesz, Duncan & Humphreys, Wolfe, Koch & Ullman, Itti & Koch, etc.
V1, as neural basis for a bottom up saliency map, via intra-cortical interactions, regardless of their feature preferences Li (1999, 2002)
V1 activities drive saccades, via superior colliculus, to receptive field location of the active cell. Tehovnik, Slocum, & Schiller 2003
Bottom up feature based decision by V1
--- No object/orientation recognition necessary.

28. Dissection of events in the task:
(3) After gaze arrive at target location, viewing the target attentively
Features are bound into object shape by attention Treisman, etc.
Recognized as the same object Stankiewicz, Hummel, Cooper (1998) --- Priming with attention is insensitive to viewpoint
IT, parietal, LOC as neural basis. Tanaka, Rolls, Logothetis, Lawson & Humphreys, Riddoch & Price, Kourtzi, Grill-Spector, Treisman, etc.
Object based decision by the higher level processes interferes with bottom up feature based decision by V1.
Interference did not have to occur, data indicate that it actually does.

29. Dissection of events in the task:
Catching the features before they merge into objects !!
At least 100 ms with attention is needed to achieve viewpoint invariant object representation.
Prediction: lesioning the brain area responsible for viewpoint invariant object recognition (e.g., by TMS or in clinical populations) should improve task performance.
Object based decision by the higher level processes interferes with bottom up feature based decision by V1.

30. Subjects quickly learned to reduce/remove interference (condition A).
Second session
Percent correct button presses
First session
Target (attentive) viewing time T (ms)
i.e., eye-to-mask delay

31. Summary Bottom up inattentive processes Attentive processes Interfere
High level object processes
Interfere
Low level feature processes
This is demonstrated in a visual search task.
Interference can be removed/reduced by limiting processing time.
Implications on interactions between different processing levels.
More info:

32. Comparing the two stimuli:
A: target difficult to find
B: target easy to find
They have the same background distractors
Both targets have a unique feature, an oriented bar,, in the whole image --- should be salient (by bottom up saliency)
In A, the target object is a rotated and/or mirrored version of all distractors. In B, target object is uniquely shaped