Physical Symbol System Hypothesis A physical symbol system has the necessary and sufficient means for general intelligent action. (Newell and Simon, “Computer Science as Empirical Enquiry...”) What is a Physical Symbol System?

Five conditions 1. A symbol may be used to designate any expression (where ‘designate’ means to affect causally or be affected by); 2. there exist expressions that designate every process of which the machine is capable; 3. any expression can be created or modified in arbitrary ways; 4. expressions can be stored without alteration; 5. memory is unbounded. (One thing these give you is flexible, stored program computing.)

Modal v. Amodal symbols A modal symbol is one tied distinctively to a particular sensory mode or to a motor program (include bound sensory representations together). An amodal symbol is one that is not modal (these are typically characterized as arbitrary)

Perceptual Symbol Systems Human thought consists only of the manipulation or activation of sensory or motor states or stored records of them. On the view of Barsalou et. al, this can include binding units (conjunctive neurons), that cause the appropriate reactivation of the stored records.

Levels of Explanation We should distinguish between the cognitive-functional description of the models and the neural description. Being modal need not be a claim about cortex (that, e.g., anything appearing in sensory or motor cortex is automatically the physical form of a modal representation).

“Tied distinctively to...” What does this mean, then? It’s partly a claim about processing: that the way in which symbols are processed is influenced by the particular modality in which they appear.

Local v. global representation Global form assumption: the same symbol represents a given feature (property) in connection with all relevant concepts. These are typically assumed to be amodal (i.e., not tied to any particular sensory modality). Local form assumption: distinct symbols are used to represent the same property in connection with the various relevant concepts. (These are frequently assumed to be modal.)

Phenomena that might seem hard to for the perceptual SS to explain Categorical inference. Inferences based on the category something belongs to or relation between categories : Sam is a dog. All dogs are animals. All animals can die. Therefore, dogs can die. Therefore, Sam can die. Type-token distinction. We regularly distinguish between individuals (tokens) and the categories to which they belong (types). Abstraction: Forming categories requires ignoring many properties of individual members.

Productivity and Compositionality Human language and thought are generative; an open-ended set of sentences or thoughts can be generated from a relatively small number of rules and unstructured representations. The content of a thought or statement is a function of the content of its parts and combinatorial rules. (Even when it’s not, the meaning is often a function of theoretical factors.)

Conjunction detectors Function as simple markers that reactivate past records of sensory experience. Can code for similarities among past sensory experiences. This coding explains abstraction and can explain how the right combinations of sensory experiences get re-activated.

Reactivation of selected sensory records can account for productivity and compositional semantics. It also accounts for categorical inference and the type-token distinction. The particular sensory record is the token; the type is a collection of aspects (common features) of particular records.

Status of conjunctive neurons Read from p. 88. Are higher-order conjunctive neurons amodal symbols? What determines what they activate and how the results will be read?