Chapter 4 – Processing Beyond the Striate Cortex
G8 Ch 4 Outline
Following the Signals from Retina to Cortex
The Visual System
Processing in the Lateral Geniculate Nucleus
Receptive Fields of LGN Neurons
Information Flow in the Lateral Geniculate Nucleus
Organization by Left and Right Eyes
Organization as a Spatial Map
Receptive Fields of Neurons in the Striate Cortex
Do Feature Detectors Play a Role in Perception?
Selective Adaptation and Feature Detectors
Grating Stimuli and the Contrast Threshold
Selective Rearing and Feature Detectors
Maps and Columns in the Striate Cortex
Maps in the Striate Cortex
Columns in the Striate Cortex
Location Columns
Orientation Columns
Ocular Dominance Columns
Hypercolumns
How is an Object Represented in the Striate Cortex
Streams: pathways for What, Where, and How
Streams for Information About What and Where
Streams for Information about What and How
The Behavior of Patient D.F.
The Behavior of People Without Brain Damage
Modularity: Structures for Faces, Places, and Bodies
Face Neurons in the Monkey’s IT Cortex
Areas for Faces, Places, and Bodies in the Human Brain
Something to Consider: How Do Neurons Become Specialized
How Neurons Can Be Shaped by Experience
View of the outside surface of the left hemisphere
I’ve illustrated the left hemisphere to make this figure correspond to G8’s figure on p. 88.
V1 is the striate cortex – the occipital lobe.
V2, V3, and V4 cortical areas receiving input from V1. V3 is buried in a fold.
IT is the inferior temporal cortex
P is the parietal lobe.
MT is the middle temporal cortex, shown dashed because its not visible from the surface of the cortex.
STP is the superior temporal polysensory area.
Play VL 4.11 “What and Where Streams” here. (It’s pretty simplistic.)
From G8 p 88 - the
Another artist’s conception of the right hemisphere of the brain.
(From Popular Science, March, 2013)
Lobes have been characterized for clarity. You should remember the previous view. This is shown to give you some exposure to the variety of ways the brain areas may be represented.
The figure above is part of an article on the control of seizures through implantation of electrodes. The device shown in the figure is the artist’s conception of a device to stimulate neurons in the temporal lobe.
The Two streams.
The division of visual information that began in the retina with the division of ganglion cells into M and P cells continues in the higher level processing centers.
The streams have been labeled the “What” stream and the “Where” stream.
Parietal or “Where / How” stream
The “Where / How” stream begins with the M ganglion cells, continues through layers 1 and 2 of the LGN and to area V1. The higher order area involved in this stream is the parietal lobe.
The “Where” stream is involved in the primary processing of location and movement (where) and probably action (how).
Current thinking - This stream directs and guides our activities, such as orienting and reaching.
Temporal or “What” stream.
The “What” stream begins with the P ganglion cells, continues through layers 3-6 of the LGN and to area V1. The higher order area involved in this stream is the temporal lobe.
The “What” stream is involved in the primary processing of form and color.
Current thinking –This stream is responsible for our visual awareness of object and events, for “seeing”
Modules G8 p. 91
Module: A region of the cortex which processes a specific complex set of aspectsof sensory information.
The existence of the two streams suggests that such specialization exists at a gross level – we have a “where/how” module and a “what” module.
But does it exist on a more specific level? Are there specialties within the streams – streams within streams?
Suggestions of specialized modules.
1) Module for processing of movement in the medial temporal lobe. Very likely a retinotopic map of neurons “watching” for movement in their receptive fields. There is much evidence for the existence of such a module.
2) Module for representing generic shapes in the inferotemporal lobe.
Neurons which response to specific stimulus shapes. Adjacent neurons respond to different shapes in the same part of the visual field. A retinotopic map of shape recognition neurons? Again, there is evidence for such a module.
3) May be a separate area in the inferotemporal lobe containing neurons which respond to face-like stimuli. The fusiform face area has been identified in humans. It’s under the temporal lobe.
4) Ramachandran has suggested that there may be as many as 30 different processing modules. Each one contains neurons which respond to specific generic stimulus types in their receptive field. Probably retinotopic maps in each.
(23:38 minutes) Listen to the first case study – about 6 minutes.
The binding problem
So, if the perception of a complex unique object or scene is not the response of a single neuron, what is it?
If complex objects result in the responses of neurons in many different modules, each processing a different aspect of the complex object – one its location, one its movement, one the colors of various parts of it, one the shapes of various parts of it – if it is analyzed into a whole jumble of features, how is it that it is still perceived as a unitary thing, a single thing, an object?
How can the diverse responses of the neurons in different modules be synthesized into a unitary perception.
The synchrony hypothesis – Unitary experience is associated with synchronous activity in multiple modules.
Beyond the striate cortex - 110/26/2018