The “What” and “Where” Pathways

William W. Crissman02/08/13

Study Question Overview1. How have the two cortical pathways historically been

modeled by the like of Ungerleider, Mishkin, Goodale, and Milner?

2. How does the de Haan and Cowey (2011) differ from the previous model?

3. What evidence is there to support a more patchwork-like model of cortical info-processing?

4. What does heading disorientation (HD) a characteristically unique form of topographical disorientation?

5. What area of the brain has been heavily implicated in the integration of information about objects in space based on your own frame of reference and changes you make in body direction?

Historical Perspectives

• Lissauer (19th century)• Electrophysiological studies

with animals and neuroimaging studies with humans (20th century)

• Ungerleider and Mishkin – 1st to suggest organization of

2 major paths from V1• Ventral and dorsal paths

Historical Perspectives con.’t

• The Dual-Pathway paradigm (Goodale and Milner, 1995)

– Reconceptualized framework in terms of WHAT the visual info is USED for, rather the KIND of information is relayed

• Thus, the same info is being used differently, rather than different info going to different directions

– Ventral info terminating in the MTL, hippocampus, and amygdala used for visual recognition, memory, and emotional content

– Dorsal info feeds into motor cortex of frontal lobe and uses visual information for action

The Patchwork Model

• Challenges previous model in terms of…– Whether a pathway model is more parsimonious than

a patchwork model?• Latter fails to account for the an organizational structure

that consists of many interrelations b/t different visual maps

– Whether there is sufficient evidence to support a model that suggests strict hierarchical processing of visual information?

Evolution and Innate Design

• Visual architecture and gross organization of visual maps– Both very well-defined– little variability in humans (total extent of striate

cortex)• HOW did the architecture and organization

develop in terms of evolution and natural selection”?– In “one sweep,” or more independently, haphazard

manner?• Evolution supports a patchwork model– Previous model assumes too elaborate of a

“Darwinistic step”

Neural Wiring• Issue on white matter tracts– Very little systematic evidence, but overall picture

does not support linear path model– If no internal organization, then connections should

be equally strong b/t all possible nodes– If organized into hierarchical pathways,

• Then connections within a path would be stronger than connections between the paths,

• And shorter connections would be stronger than more long-range connections

• V1 and V2 > V1 and V3?– Very little systematic evidence concerning this issue

Two Streams (Or are there?)• Hierarchical model works well to conceptualize ventral

“what” stream– Color, orientation, and brightness enable perceptions of objects,

places, and faces• Not so much with dorsal “where/how” stream

– In space, what qualifies are basic or advanced features? – Position feed into depth, or vice versa?– At present, three paths have been postulated in parietal lobe

• Parieto-prefrontal path – top-down eye movements, spatial WM• Parieto-premotor path – visually-guided actions• Parieto-medial temporal path – visuo-spatial navigation info, spatial LT

memory– Four, if we add path from LGN to area MT – Five, if we add path for processing temporal order (“when”

path)

Two Streams (Or are there?)

Too much Cross-talk

• How can two paths with fundamentally content communicate?

• Why even have two paths given the continuous and meaningful cross-talk?

• How is the strict division in terms of conscious perception continually maintained, given the extensive cross-talk?

• How can we explain certain aspects of dorsal processing that can reach conscious awareness?

Integration of Features

• If hierarchical organization, then basic features processed during early stages are integrated in subsequent stages– Maybe at the local, micro-circuit level, in a single area– “End-stopped cells” in V2 integrate orientation and

position/length info• Is this really integration? Or an implicit assumption?

– Theoretically, face recognition, a higher-order process, would call for important visual features that are processed in more posterior visual areas• Role of various visual parameters in face recognition have

shown modest effects at best

Complexity

• Hierarchical model also assumes that the complexity of information increases progressively and systematically as it is processed further down stream (“cascade specification”)

• Is color processing really more “basic,” in terms of complexity and processing characteristics, than reading or face recognition?

Color Perception and Recognition

• Involves numerous separate stages (maps)• Concerns a stimulus-specific perception

system• Involves categorical perception • Leads to conscious perception• Part of processing occurs subconsciously• Heredity does influence ontological

development

Authors’ Concluding Remarks• Prevailing view makes more elaborate assumptions

about evolution of visual system than evidence currently warrants

• Pathway model are not supported by physiological neural wiring between different visual maps

• It is not clear why there should only be two paths• In new model, there are no functional highways, nor

presupposed hierarchy between different visual abilities

• Allows more flexibility in the role of the visual system and visual information from different visual areas

• Not advocating to “throw the baby out with the bath water,” rather reserve hierarchical processing for the columnar and microcircuit level

• Hasimoto, R., Tanaka, Y., Nakano, I. (2010). Heading disorientation: A new test and possible underlying mechanism. European Neurology, 63, 87 – 93.

• http://www.youtube.com/watch?v=uF7-AxleI_0&feature=endscreen

Topographical Disorientation

• Inability to orient one’s self within large-scale, locomotor environments

• Oftentimes the result of diffuse brain damage via stroke, dementia, Alzheimer’s disease, etc.– Also, implicated in focal brain damage as

suggested here– Four unique varieties

Types of TD

• Egocentric Disorientation– Implicated in damage to posterior parietal lobe– Marked inability to represent the location of

objects with respect to one’s self• Unable to reach out and grab objects, point to locations

of objects, or state the spatial relationship between objects (above, below, left, right, near, far), • Pt. GW would turn the wrong way when greeted by

someone she was not facing towards

Types of TD cont.

• Landmark Agnosia– Implicated in damage to bilateral lingual gyrus – Inability to recognize salient environmental stimuli for

purposes of orientation • Anterograde disorientation– Implicated in damage to parahippocampal gyrus– Inability to orient in novel environments

• Pt.’s often able to recall spatial layout of known places before becoming amnestic

– Findings suggest the MTL is not necessary for forming LT declarative memories, such as spatial maps, learned prior to injury

Types of TD cont.

• Anterograde disorientation– Implicated in damage to parahippocampal gyrus– Inability to orient in new environments– Pt.’s often able to recall spatial layout of known

places before becoming amnestic, but nothing about new places after injury

– Findings suggest the MTL is not necessary for forming LT declarative memories, such as spatial maps, learned prior to injury• More ab the hippocampus and its surrounding areas

Heading Disorientation (HD)

• Inability to represent direction of orientation with respect to external environment

• No visuo-spatial agnosia, nor landmark agnosia, but still unable to derive orienting info based on salient environmental cues

• Unable to draw maps or describe routes b/t familiar places

Review of Cases Reports• Patient 1– 67 yr old man that

became lost while driving his usual route home

• Patient 2– 72 yr old man that

became lost while riding his bicycle

• Patient 3– 67 yr old man that

kept forgetting where his hospital room was

R L

Card-Placing Test (CPT)

• Two-part performance-based test designed to examine individuals’ ability to represent spatial locations of surrounding objects placed on the floor

• DEMO

Card-Placing Test (CPT)

Age ± SD CPT part A(mean ± SD)full scores; #correct/30 trials

CPT part B(mean ± SD)full scores; #correct/30 trials

Controls (n = 9) 65.4 ± 4.8 27.6 ± 3.0 25.0 ± 4.3

Pt. 1 67 28 4

Pt. 2 72 26 12

Pt. 3 67 26 17

Discussion

• Common damage– 3 cases, same

localized damage to right retrospenial area

– Damage to left retrospenial reportedly associated with impaired episodic memory, rather HD

Discussion con.’t• Common symptoms– Inability to derive directional info from environmental

cues– TD in novel AND familiar environments– Mild to moderate visual memory impairment– Absence of prominent visuospatial disorientation

• Preserved tapping span• Absence of hemispatial neglect• Intact constructional abilities• Fairly well-preserved general intellectual functioning

• Underlying mechanism of HD?– Anterograde visual memory impairment?– Poor egocentric frame of reference?– Integration of information on changes in body

direction?

Discussion con.’t• Converging sources of evidence– Rat studies• Excision of retrospenial cortex shows impairment of

spatial navigation– Human research via PET and MRI• Retrospenial area active during large-scale navigation

tasks– Head direction cells• Excited when rats maintain a certain heading or

orientation in the environment• Found in retrospenial cortex and many other structures

that may be implicated in the functional circuit of directional information to the self

Study Questions Revisit1. How have the two cortical pathways historically been

modeled by the like of Ungerleider, Mishkin, Goodale, and Milner?– As hierarchically organized paths – the ventral “what”

path which is involved in object recognition and memory, and the dorsal “where” path which is involved in action

2. How does the de Haan and Cowey (2011) model differ from previous ones?– Their patchwork model that does not assume an internal

organizing structure consisting of 2 independent paths, rather a network of connections within the visual maps.

3. What evidence is there to support a more patchwork-like model of cortical info-processing?– More than 2 streams, too much cross-talk, integration,

shared complexity, etc.

Study Questions Revisit

4. What makes heading disorientation (HD) a characteristically unique form of topographical disorientation?– The inability to navigate both novel and familiar

environments through the derivation of directional info from landmarks and changes in body direction, despite normal visuospatial functioning and the absence of landmark agnosia

5. What area of the brain has been heavily implicated in the integration of information about objects in space based on your own frame of reference and changes you make in body direction? – Right retrospenial cortex