NEURO VISION

What is so Great about it?

Deceptively simple anatomical appearance. Incredibly complicated structure. Developed through millennia of evolution. Hard-wired to prefer certain objects right at

birth. Invariant to position, scale and rotation of

the object. “Tuned” to quickly recognize objects.

IS HUMANVISION

PERFECT ?

PENROSE’S TRIANGLE

Tricks of the brain

I’d love to see someone try to get to the top

PENROSE’S STAIRS

Why is it so? Our vision system neural network has been tuned

to perform recognition, processing and classification of phenomena that was vital to our survival and progress. In this context, every species has a different vision system.

Hence, we are not very good in dealing with artificially generated images, as these phenomena rarely occurred in nature during our evolution.

However, we are the best for natural images!

How human visual perception works.

How human visual perception works

Perceptions of static scenes are inadequate to describe motion

Gibson’s theory of affordances Vision evolved in organisms embedded in a

dynamically changing environment What is important to an organism is a collection of

processes, not a single unique one These processes are at different levels of abstraction.

E.g. We see waves on a shore, and also the innumerable molecules in it moving

How human visual perception works(contd.)

“Seeing involves multi-level process simulations in partial registration using different ontologies, with rich (but changing) structural relations between levels”

Use of structures of various sorts Agglomeration/grouping: Structures of different sizes at

same level of abstraction Interpretation: Structures at different levels of abstraction-

mapping to a new ontology Fragments recognized in parallel, assembled into larger

wholes-may trigger higher level fragments, or redirect processing at lower levels to resolve ambiguities, etc.

Functions of vision

Segment the image (or scene) and recognize the objects distinguished

Compute distance to contact in every direction Provide feedback and triggers for action Provide a low-level summary of the 2-D and/or 3-

D features of the image, leaving it to the central non-visual processes to draw conclusions

Is something left out?

Visual/spatial reasoning Our ability to use diagrams and visual images to

reason about very abstract mathematical problems, like thinking about the complexity of a search strategy

“Seeing” that 7+5=12 by a rearrangement of dots “Seeing” that angles of a triangle add up to a

straight line Visualize infinitely thin and long lines of

Euclidean geometry Many more examples

Visual/spatial reasoning(contd.) Uses of spatial reasoning: Knowing where to search for an object

thrown over a wall, assembling toy crane from a toy set, uses of spatial concepts(notion of search space) in programming design

Reasoning using a grasp of spatial structures requires at least: the ability to see various structures involved in the proof, the possibilities for variatins(rearrangements) in them, the invariant structures during the rearrangements, etc.

In contrast, a reasoning system like logic is completely discrete and all syntactic composition involves function appllication

Specification of the requirements for visual reasoning is very vague, and would not be easy to mechanize

Visual perception involves much more..

Visual perception involves “affordances” Affordances are the possibilities for, and

constraints on action and change in a situation. Seeing the possibility of things that do not exist, but might exist

Example: A person perceiving a chair can immediately see the possibility of sitting on it, that is, the chair "affords" sitting

Visual perception involves much more..(contd.)

POPEYE(1970’s): The Popeye project investigated how it is possible for humans to see structure in very cluttered scenes, where structure exists at different levels of abstraction-it showed that we recognize fll words before individual alphabets

Consider looking at a smiling or a sad face. Does it involve only perceiving the structure of the pattern? We are able to perceive mental states of happiness or sadness

Visual perception involves much more..(contd.)

What may appear to be only one task, might consist of many different tasks in different contexts, e.g. estimating the length of a plank to fit across a ditch

When a number of images are speedily flashed before the eyes in order, the speed with which people can see at least roughly, what sort of scene is depicted by each image, implies that our visual mechanisms are capable of finding low level features, using them to cue in features of the images at various levels of size and abstraction, arriving at percepts involving known types of objects, within 1 or 2 seconds

High level precisions are made in less than 1/2 a second

Artificial Vision systems-what they aim at

recognize objects or people in static images without acquiring or reasoning about the information using 3-D structure

track moving objects represented in simple shapes(points or blobs) often using 2-D representations

explore an environment building a 2-D map of walls, doors, etc. without a possible human understanding of the maps

control a moving robot, regarded as a moving object obtain some 3-D information about the environment, only

to generate new images

What vision systems cannot do After Freddy, the Edinburgh robot built in 1973, there was a

need to move from 2-D to 3-D. Failure due to limitations of computational power, and difficulty of choosing a representation

Consider a cup on a table. Humans can "see" the orientation required for the grasping object at different grasping locations-visual systems cannot. Alignment of grasped surfaces with grasping ones is important

Affordances in the object being grasped, if it has sharp corners, some part of it is more fragile than others-requires a grasp of counterfactual conditionals involving processes that do not actually exist

Why the limitation? To develop a human-like visual system that will

do what a small child/many animals do-need for an adequate analysis of the requirements for such a system

The requirements might seem much simpler than they actually are, if they are not studied in sufficient depth

The failure to achieve set goals is not a fault of the choice of domains, or the representation-it is a problem of overoptimistic predictions

Where is the complexity? Different levels of perception needed. High level of

precision to lift a hair with a pair of tweezers, much lower precision to see something is not graspable

Perception can involve multi-strand relationships requiring much richer forms of representation that just a logical form

Multiple levels of abstraction, affordances, causation-all is needed

Many more subtleties..

Visual Pathway

Hierarchical Neural Network Architecture

Contents

Brain Mechanism of Vision Hubel‘s and Wiesel's hierarchy model

Cerebral Cortex

Evolution of cerebral cortex is one of the great success in the history of living beings.

Insights of cortical organization: Division into different regions having different

functionalities. e. g. , Visual, auditory, somatic sensory, speech

and motor regions

Visual Pathway

Retina to the Visual Cortex

Hubel’s and Wiesel’s Model

Hierarchical model of cortical cells .

The cortical cells are divide into various types Type IV Simple cells Complex cells

Hubel’s and Wiesel’s ModelType IV Cells have circular symmetry. The receptive field of the cell is divided into

on Center. (Excitatory Center and Inhibitory Surrounding) off Center. (Inhibitory Center and Excitatory Surrounding)

Hubel’s and Wiesel’s Model Simple Cells Respond to an optimally oriented line in a

narrowly defined location. Achieved by requiring the centers of layer

Iv cells that lie along the line.

Hubel’s and Wiesel’s Model Complex cells the main feature of complex cell

o They are less particular about the location, Concerned mainly on orientation.

oAquired is from a number of simple cellso Detects motion(direction specific).

Hubel’s and Wiesel’s Model

Biological Visual Systems as Guides

Modelling attempts to imitate primate vision systems

Extended Hubel-Weisel

Hubel-Weisel hierarchical models have been extended to obtain a fine balance between selectivity and invariance.

Simple and complex cells are interleaved at different levels of the inferotemporal (IT) lobes.

Max-like pooling mechanisms have been suggested at certain levels as opposed to a weighted sum of afferents to boost invariancy in scale, position and rotation.

Feedforward Architecture The S cells (simple cells) in the previous figure passed on information

to the C cells (complex cells) by a bell-tuned weighted sum or a max-like operation.

These cells were further arranged in a higher feature-level hierarchy. Some cells bypass a level in propagating information. This model only considers the feedforward architecture model for the

primary visual cortex, V4 and the posterior IT lobe, and a top-level supervised learning mode (coloured regions). [Serre et al. 2007]

Feedforward Architecture Primates have a very advanced level of attention modulation

(fixation) which is a feedback propagation from the IT lobes to the primary visual cortex and lower levels.

This mechanism allows to shift attention from one part of the image to another.

However, crude object recognition is done in a very small duration after stimulus which indicates use of only the feedforward architecture for rapid categorization.

Such a model was attempted at the McGovern Institute for Brain Research at MIT with some simplifications.

The input consisted of 4 different orientations and several scales, densely covering the gray-value input image of 7ºx 7º

Results The model was evaluated against human responses for input

stimulus of 20ms followed by varying inter-stimulus interval. No single model parameter was adjusted to fit the human

data. All unsupervised parts were fixed and constant throughout all the runs.

The supervised mode was tuned differently in different runs using different test images. Humans were also shown these test images.

An evaluation across all such runs for the identification of animal objects was done for both humans and animals. The results were compared.

Results Various categories of

images in different clutter, scale, position, rotation were given.

Maximum similarity was found for ISI until 80ms.

Conclusions

Biologically inspired computation models have shown very promising results. They are versatile and fast learners. Why not learn from nature’s best?

Advances in neuroscience are picking up, allowing us greater understanding. Also, simulations of hypothetical models will help us validate neuroscience findings.

References Talks by Aaron Sloman, Univ of Birmingham, UK

2005 - 2007http://www.cs.bham.ac.uk/~axs/invited-talks.html

http://www.lifesci.sussex.ac.uk/home/George_Mather/Linked%20Pages/Physiol/Cortex.html

Last accessed - 13 April 2008 Brain Mechanism of Vision,

David H. Hubel and Torsten N. WieselScientific American, September 1979

How We See What See - V. Demidov, Mir Publishers, 1986 A feedforward architecture accounts for rapid categorization

Serre et al., PNAS, 2007 Hierarchical Models of Object Recognition in Cortex

Poggio et al., Nature America, 1999 http://www.thebrain.mcgill.ca

Last accessed - 13 April 2008