Information Encoding and Modality

1. Information Encoding and Modality Veerendra Allada Souvik Ray Neurocomputing Seminar

2. Outline • Information encoding • Types of encoding • Engram • Encoding specificity principle • Modality • Information modality • Spatial organization of neural patterns • Transformation networks

3. Information Processing in Brain • Information encoding – get information into the brain • Information storage – retain the information • Information retrieval – get the information back out later

4. Simplified Model of Human Memory(Multi-store model, Atkinson and Shiffrin, 1968) (Iconic, Echoic) (Semantic) (Acoustic)

5. Memory Models • Individual memories are stored in distinct locations in the brain (Penfield, 1950) • Engram model (Hubbard, 1950) • The engram is the neural network representing fragments of past experiences which have been encoded • Strong evidence that there are distinct elements of memory which involve different parts of the brain (e.g. hippocampus - short-term memory, amygdala – emotional memories, hippocampus, temporal lobes and structures of limbic system – long term memory)

8. Encoding • Active process • Requires selective attention to the material to be encoded • Memories may be affected by the amount or type of attention devoted to the task of encoding the material • Cocktail party phenomenon

9. Types of Encoding • Shallow encoding • Short-term memory – repeating information to memorize it (e.g. phone no.) • Utilizes the brain’s phonological loop (relies on part of brain designed for short-term memory) • Elaborative encoding • Link, associate or connect incoming information with something already in memory • Visualization or guided imagery techniques can also be used to recall memories (Mnemonics)

10. Types of Encoding (based on depth of encoding) • Structural encoding • Emphasis on the physical structural characteristics of the stimulus • Shallow level • Phonemic encoding • Emphasis on the sounds of the words • Intermediate level • Phonemic encoding • Emphasis on the meaning • Deep processing

11. Engrams: How the Brain Stores Memories (Hubbard, 1950) • Recording an event: Strengthening the connections between groups of neurons that participate in encoding the experience • Pattern of connections: Engram • Engrams are spread throughout the brain (equipotentiability) • Engrams remain dormant unless retrieved from memory using cues • Engrams weaken and disappear with time • We tend to remember only important things

12. The Search for the Engram (Unit of Memory) • Biochemical theories • Memory storage occurs in biochemical changes at the synapse • People with Alzheimer’s show a depletion of acetylcholine and glutamate • Neural circuit theories • There may be specific circuits for specific memories

13. Biochemical Theories • Current view (Kandel theory, 1970): • At molecular level, new proteins are manufactured and these proteins stabilize the changes underlying the memory • Every new memory results in a permanent representation in the brain • Routtenberg-Rekart theory (2005): • Memory storage is a dynamic, meta-stable process • Replication of memories across many different brain networks

14. Routtenberg-Rekart theory (contd.) • For example, one’s name is represented in innumerable neural circuits; thus it is extremely difficult to forget • Since no particular neural network lasts a lifetime, it is theoretically possible to forget one’s name (advanced stages of Alzheimer’s disease) • Brain stores long-term memory by rapidly changing the shapes of proteins already present at those synapses activated by learning

15. Neural Circuit Theories • Short-term or immediate memory: • Temporary information is maintained in “reverberating neural circuits” which sustain a nerve impulse by channeling it repetitively through the same network • Long-term memory: • Memories are stored in a distributed fashion- as synaptic strengths (weights) in a neural network • No localization • Graceful degradation • Issue: Difficulty of correlating artificial neural networks with biological ones lies in the way weights are modified in the former and synaptic weights are modified in the latter • Backpropagation model (unrealistic)

16. Encoding Specificity Principle (Tulving) • A specific way a person thinks about, or encodes an event determines what gets into the engram • The recollection of an event depends on the interaction between the properties of the encoded event and the properties of the encoded retrieval information

17. Encoding Specificity Principle: Context-Dependent Memory • Golden and Baddeley (1975) • some scuba divers learned words on land and others underwater • at test, each group was split in two and half were tested underwater and half were tested on land

19. Encoding Specificity Principle: State-Dependent Memory • Same design as scuba experiment but using pharmacological states as context (Eich, 1980)

20. Problems with the Engram Theory • Conditioning as an alternative • A small fish in shallow, stale waters is bumped and hurt by larger fish trying to eat him • The small fish gets an engram from this occurrence (pain and momentary unconsciousness) • If attacked again, the engram is reinforced and the small fish avoids shallow, stale water in the future

21. Conditioning • The small fish may at a later time figure out that shallow, stale waters do not always include danger and may return to those areas to feed • Conditioning can be unlearned whereas engrams remain until audited out.

22. Encoding +ve and -ve Values • Biased Encoding • Neuron firing at intermediate or background rate represents the value zero • Firing at slower rate represents a negative value while faster rate represents a positive value • Two-Cell Encoding • Two different neurons encode a single numeric value • Firing of neither represents zero; firing of only one cell represents positive and firing of another represents negative value. The magnitude is proportional to the firing rate

23. Encoding Vectors and Complex Numbers • Vectors – quantities that represent both magnitude and direction • Velocity, acceleration and surface orientation of visually perceived objects • 6 neurons are logically grouped in to 3 pairs, each pair representing one component of a vector • Complex numbers are represented as a pair of complex numbers and hence either by two cells if a biased encoding is used or by 4 cells if a two cell encoding is used for each real number.

24. Information Modality • Human brain processes numerous modes of information

25. Spatial Organization of Neural Patterns • Sensory information is encoded as 2-D patterns (in all sensory modalities) • In a visual system, the retina encodes a 2-D representation of the ocular image • Two aspects of sensory organs are of importance - Spatial Organization - Temporal Organization

26. Spatial Organisation • Sensory patterns are composed of several independent sub patterns in registration with each other.

27. Transformation Networks • How can alternate representations of a given sensory pattern be formed ? • Are mechanisms for forming alternate representations prewired (genetically specified) or formed with experience ? • Are the pathways that convey the different representations distinct or same ?

28. Contd.. Control Pattern to be stored Storage and Access Control Storage system for control patterns Information output Information Input Controlled information transformation network The essential components of a sensory processing system capable of learning

29. Dimensionality of Encoded Patterns • Dimensionality of a transformed pattern is not necessarily the same as that of the original pattern • When the transformed pattern is smaller, information is lost

30. Temporal Organization • Temporal organization of information likewise derives either from the external environment or from the neural substrate • With in the time domain, the speed at which the sensory signals are transformed from one representation to another varies greatly, suggesting several different storage systems • For example speech sounds must be processed at the phoneme level( milliseconds), at the syllable and word levels( tenths of seconds), at the sentence level( seconds) and at the thematic level( years).

31. Thank You ….