arbib: cs564 - brain theory and artificial intelligence, usc, fall 2001. lecture 7. object...

Arbib: CS564 - Brain Theory and Artificial Intelligence, USC, Fall 2001. Lecture 7. Object Recognition

CS564 – Lecture 7. Object Recognitionand Scene Analysis

Reading Assignments:TMB2: Sections 2.2, and 5.2

“Handout”: Extracts from HBTNN 2e Drafts: Shimon Edelman and Nathan Intrator: Visual Processing of Object StructureGuy Wallis and Heinrich Bülthoff: Object recognition, neurophysiologySimon Thorpe and Michèle Fabre-Thorpe: Fast Visual Processing

(My thanks to Laurent Itti and Bosco Tjan for permission to use the slides they prepared for lectures on this topic.)


Bottom-Up Segmentation or Top-Down Control?


Object Recognition

What is Object Recognition? Segmentation/Figure-Ground Separation: prerequisite or consequence?

Labeling an object [The focus of most studies] Extracting a parametric description as well

Object Recognition versus Scene Analysis An object may be part of a scene or Itself be recognized as a “scene”

What is Object Recognition for? As a context for recognizing something else (locating a house by the tree in the garden)

As a target for action (climb that tree)


"What" versus "How” in Human

Visual

Cortex

Parietal

Cortex

Inferotemporal

Cortex

How (dorsal)

What (ventral)

reach programming

grasp programming

AT: Goodale and MilnerLesion here: Inability to

verbalize or pantomime size or orientation

DF: Jeannerod et al.Lesion here: Inability to Preshape(except for objects with size “in the semantics”

Monkey Data:Mishkin and Ungerleider on“What” versus “Where”


Clinical Studies

Studies with patients with some visual deficits strongly argue that tight interaction between where and what/how visual streams are necessary for scene interpretation.

Visual agnosia: can see objects, copy drawings of them, etc., but cannot recognize or name them!

Dorsal agnosia: cannot recognize objectsif more than two are presented simulta-neously: problem with localization

Ventral agnosia: cannot identify objects.


These studies suggest…

We bind features of objects into objects (feature binding)

We bind objects in space into some arrangement (space binding)

We perceive the scene.

Feature binding = what/how streamSpace binding = where stream

Double role of spatial relationships: To relate different portions of an object or scene as a guide to recognition

Augmented by other “how” parameters, to guide our behavior with respect to the observed scene.


Inferotemporal Pathways

Later stages of IT (AIT/CIT) connect to the frontal lobe, whereas earlier ones (CIT/PIT) connect to the parietal lobe. This functional distinction may well be important in forming a complete picture of inter-lobe interaction.


Shape perception and scene analysis

- Shape-selective neurons in cortex- Coding: one neuron per objector population codes?

- Biologically-inspired algorithmsfor shape perception

- The "gist" of a scene: how can we getit in 100ms or less?

- Visual memory: how much do we rememberof what we have seen?

- The world as an outside memory and our eyes as a lookup tool


Face Cells in Monkey


Object recognition

- The basic issues- Translation and rotation invariance- Neural models that do it- 3D viewpoint invariance (data and models)- Classical computer vision approaches: template matching and matched filters; wavelet transforms; correlation; etc.

- Examples: face recognition.- More examples of biologically-inspired object recognition systemswhich work remarkably well


Extended Scene Perception

Attention-based analysis: Scan scene with attention, accumulate evidence from detailed local analysis at each attended location.

Main issues:- what is the internal representation?- how detailed is memory?- do we really have a detailed internal representation at all!!?

Gist: Can very quickly (120ms) classify entire scenes or do simple recognition tasks; can only shift attention twice in that much time!


Thorpe: Recognizing Whether a Scene Contains an Animal

Claim: This is so quick that only feedforward processing can be involved


Eye Movements: Beyond Feedforward Processing

1) Examine scene freely

2) estimate material circumstances of family

3) give ages of the people

4) surmise what family hasbeen doing before arrivalof “unexpected visitor”

5) remember clothes worn bythe people

6) remember position of peopleand objects

7) estimate how long the “unexpected

visitor” has been away from family


The World as an Outside Memory

Kevin O’Regan, early 90s:

why build a detailed internal representation of the world?

too complex…not enough memory…

… and useless?

The world is the memory. Attention and the eyes are a look-up tool!


The “Attention Hypothesis”

Rensink, 2000

No “integrative buffer”

Early processing extracts information up to “proto-object” complexity in massively parallel manner

Attention is necessary to bind the different proto-objects into complete objects, as well as to bind object and location

Once attention leaves an object, the binding “dissolves.” Not a problem, it can be formed again whenever needed, by shifting attention back to the object.

Only a rather sketchy “virtual representation” is kept in memory, and attention/eye movements are used to gather details as needed


Challenges of Object Recognition

The binding problem: binding different features (color, orientation, etc) to yield a unitary percept. (see next slide)

Bottom-up vs. top-down processing: how much is assumed top-down vs. extractedfrom the image?

Perception vs. recognition vs. categorization: seeing an object vs. seeing is as something. Matching views of known objects to memory vs. matching a novel object to object categories in memory.

Viewpoint invariance: a major issue is to recognize objects irrespective of the viewpoint from which we see them.


Four stages of representation (Marr, 1982)

1) pixel-based (light intensity)2) primal sketch (discontinuities in intensity)3) 2 ½ D sketch (oriented surfaces, relative depth between surfaces)

4) 3D model (shapes, spatial relationships, volumes)

TMB2 view: This may work in ideal cases, but in general “cooperative computation” of multiple visual cues and perceptual schemas will be required.

problem: computationally intractable!


VISIONS

A computer vision system from 1987 developed by Allen Hanson and Edward Riseman on the basis ofthe HEARSAY system for speech understanding (TMB2 Sec. 4.2)

and Arbib’s Schema Theory (TMB2 Sec. 2.2 and Chap. 5)

This is schema-based and can be “mapped” onto hypotheses about cooperative computation in the brain.

Key idea: Bringing context and scene knowledge into play so that recognition of objects proceeds via islands of reliability to yield a consensus interpretation of the scene.

See TMB2 Sec. 5.2 for the figures.


Biederman: Recognition by Components

Biederman et al. (1991 – )

“geons”: units of3D geometric structure


JIM 3 (Hummel)


Collection of Fragments (Edelman and Intrator)


Collection of Fragments 2


Viewpoint Invariance

Major problem for recognition.

Biederman & Gerhardstein, 1994:

We can recognize two views of an unfamiliar object as being the same object.

Thus, viewpoint invariance cannot only rely on matching views to memory.


Models of Object Recognition

See Hummel, 1995, The Handbook of Brain Theory & Neural Networks

Direct Template Matching:Processing hierarchy yields activation of view-tuned units.A collection of view-tuned units is associated with one object.View tuned units are built from V4-like units,

using sets of weights which differ for each object.

e.g., Poggio & Edelman, 1990; Riesenhuber & Poggio, 1999


Computational Model of Object Recognition

(Riesenhuber and Poggio, 1999)


the model neurons aretuned for sizeand 3D orientationof object



Hierarchical Template Matching:Image passed through layers of units with progressively more complex features at progressively less specific locations.Hierarchical in that features at one stage are built from features at

earlier stages.

e.g., Fukushima & Miyake (1982)’s Neocognitron:

Several processing layers, comprising simple (S) and complex (C) cells.

S-cells in one layer respond to conjunc-tions of C-cells in previous layer.

C-cells in one layer are excited by small neighborhoods of S-cells.



Transform & Match:First take care of rotation, translation, scale, etc. invariances.

Then recognize based on standardized pixel representation of objects.

e.g., Olshausen et al, 1993, dynamic routing model

Template match: e.g., withan associative memory based ona Hopfield network.


Recognition by Components

Structural approach to object recognition:

Biederman, 1987:

Complex objects are composed so simpler pieces

We can recognize a novel/unfamiliar object by parsing it in terms of its component pieces, then comparing the assemblage of pieces to those of known objects.


Recognition by components (Biederman, 1987)

GEONS: geometric elements of which all objects are composed (cylinders, cones, etc). On the order of 30 different shapes.

Skips 2 ½ D sketch: Geons are directly recognized from edges, based on their nonaccidental properties (i.e., 3D features that are usually preserved by the projective imaging process).


Basic Properties of GEONs

They are sufficiently different from each other to be easily discriminated

They are view-invariant (look identical from most viewpoints)

They are robust to noise (can be identified even with parts of image missing)


Support for RBC: We can recognize partially occludedobjects easily if the occlusions do not obscure the setof geons which constitute the object.


Potential difficulties

Edelman, 1997

A. Structural description notenough, also need metric info

B. Difficult to extract geonsfrom real images

C. Ambiguity in the structu-ral description: most oftenwe have several candidates

D. For some objects,deriving a structural repre-sentation can be difficult


Geon Neurons in IT?

These are preferredstimuli for some IT neurons.


Fusiform Face Area in Humans


representation

• Image specific• Supports fine

discrimination• Noise tolerant

• Image invariant• Supports

generalization• Noise sensitive

visual processing

Standard View on Visual Processing

Tjan, 1999


Early visual processing

Face

Place

Common objects ?(e.g. Kanwisher et al; Ishai et al)

primary visual processing

(Tjan, 1999) Multiple memory/decision sites


primary visual processing

memory memory memory...“R1” “Ri” “Rn”

IndependentDecisions

1 i nDelays

Homunculus’Response the first arriving response

SensoryMemory

Tjan’s “Recognition by Anarchy”


A toy visual system

Task: Identify letters from arbitrarypositions & orientations

“e”


normalizeposition

normalizeorientation

Image

down-sampling

memory


memory

normalizeposition

normalizeorientation

Image

down-sampling

memory memorySite 1 Site 2 Site 3


Test stimuli:1) familiar (studied) views,

2) new positions, 3) new position & orientations

1800 {30%} 1500 {25%} 800 {20%} 450 {15%} 210 {10%}

Signal-to-Noise Ratio {RMS Contrast}

Study stimuli:5 orientations 20 positions at high SNR


raw image

norm. pos.

norm. ori.

Site 3

Site 2

Site 1

Processing speed for each recognition module dependson recognition difficulty by that module.


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Familiar views

Black curve: full model in which recognition is based on the fastest of the responses from the three stages.


Experimental techniques in visual neuroscience

- Recording from neurons: electrophysiology- Multi-unit recording using electrode arrays- Stimulating while recording- Anesthetized vs. awake animals- Single-neuron recording in awake humans- Probing the limits of vision: visual psychophysics- Functional neuroimaging: Techniques- Experimental design issues- Optical imaging- Transcranial magnetic stimulation

arbib: cs564 - brain theory and artificial intelligence, usc, fall 2001. lecture 7. object...

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