VI main balance refers to the balance between the two main visual pathways in the brain – the ventral stream and the dorsal stream. The ventral stream, also known as the “what” pathway, is involved in object recognition and form representation. The dorsal stream, or “where” pathway, is involved in spatial awareness and guiding actions. Proper function requires a balance between both pathways in order to see and interact with the world effectively.
The Ventral and Dorsal Visual Streams
The dual visual streams were first proposed by Ungerleider and Mishkin in 1982. According to their model, the ventral stream projects from the primary visual cortex to the inferotemporal cortex and is specialized for object vision and recognition. The dorsal stream projects from the primary visual cortex to the posterior parietal cortex and is specialized for spatial vision and guiding actions.
The Ventral Stream
The ventral visual stream allows us to perceive the forms of objects and recognize what they are. Cells in the inferotemporal cortex respond selectively to complex objects such as faces. Damage to the ventral stream can cause visual agnosia, an inability to recognize objects despite otherwise normal vision.
The Dorsal Stream
The dorsal visual stream allows us to locate objects in space and guide our actions towards them. Cells in the posterior parietal cortex are tuned to respond to stimuli at specific locations. Damage to the dorsal stream can cause optic ataxia, an inability to reach accurately despite normal visual perception.
Balancing the Two Visual Streams
Both the ventral and dorsal streams are essential for normal visual function. The ventral stream allows us to know what we are looking at, while the dorsal stream allows us to interact with it properly. To pick up a cup of coffee, for example, we need to both recognize the object through the ventral stream and locate its position in space through the dorsal stream.
Clinical Implications
Imbalances between the two streams can occur after brain injury or neurodegenerative disease:
| Condition | Effects |
|---|---|
| Damage to ventral stream | Visual agnosia – can’t recognize objects |
| Damage to dorsal stream | Optic ataxia – can’t reach accurately |
Rehabilitation aims to restore balance through compensatory strategies. For example, visually scanning items systematically helps dorsal stream problems, while naming items aloud helps ventral issues.
Perceptual Biases
Even in healthy people, biases towards one stream or the other can influence behavior and personality. A ventral stream bias is associated with recognition skills and professions involving fine visual judgments. A dorsal bias is associated with spatial skills and athletic professions.
Neuroimaging of the Dual Streams
Functional magnetic resonance imaging (fMRI) allows researchers to visualize the two visual streams in action. Viewing and recognizing objects strongly activates the ventral temporal regions, while spatial processing and motion perception activates parietal and occipital regions of the dorsal stream.
Object Recognition
When participants view images of faces, houses, animals, or manmade objects, distinct ventral regions show selective activation:
| Object Type | Activated Brain Region |
|---|---|
| Faces | Fusiform face area |
| Places | Parahippocampal place area |
| Body parts | Extrastriate body area |
This category-specificity allows accurate object recognition through the ventral stream.
Spatial Processing
When participants track moving targets or navigate virtual environments, dorsal stream areas are activated:
| Task | Activated Brain Region |
|---|---|
| Parietal eye fields | Lateral intraparietal area |
| Reaching | Superior parieto-occipital cortex |
| Self-motion | Vestibular cortex |
These regions coordinate spatial awareness and body movements through the dorsal stream.
Development of the Dual Streams
The ventral and dorsal visual pathways develop at different rates through childhood and adolescence. The dorsal stream matures earlier, supporting basic spatial abilities in early childhood. In contrast, the ventral stream develops more gradually, supporting increasingly sophisticated object recognition and visual perception skills.
Infancy
Newborns already demonstrate basic functions of the dorsal stream, such as tracking moving targets. However, ventral stream object recognition abilities are very limited. Infants struggle to discriminate similar objects or recognize across changes in viewpoint or illumination. Recognition improves gradually across the first year.
Early Childhood
Between ages 2-4 years, basic object recognition matures, but fine discrimination is still developing. The dorsal stream supports reaching and grasping of objects, but some spatial errors persist. Visual-spatial integration, such as drawing, is very limited at this age.
Middle to Late Childhood
Between ages 5-12 years, object recognition abilities, visual search, and visual memory improve substantially through ventral stream development. Fine motor skills depending on the dorsal stream also refine during this period. However, children may still struggle with complex spatial tasks and perceptual reasoning.
Adolescence
The ventral stream continues developing through adolescence, reflected in more adult-like abilities to recognize faces, objects, and scenes. The dorsal stream also reaches maturity, enabling skilled navigation and visuomotor coordination. However, some executive integration functions may still be developing.
Evolution of the Dual Streams
Comparative studies suggest that a similar dual visual system is present across mammals, reflecting specialized divisions of labor between object vision and spatial vision. However, the degree of specialization increased markedly in primates, supporting their advanced visual capabilities.
The Ventral Stream
In monkeys, inferotemporal cortex shows similar object sensitivities as the human ventral stream. Neurons respond selectively to faces, bodies, and complex shapes. Damage leads to recognition deficits. But monkeys’ object vision likely remains less specialized than humans’.
The Dorsal Stream
Parietal regions in monkeys are also tuned for spatial processing and guiding actions like the human dorsal stream. But monkey parietal cortex may play a more dominant role in high-level cognition. The expansion of frontal cortex in humans may have shifted some functions away from parietal regions.
In Other Mammals
Studies in mice, rats, and other mammals reveal similar dual stream organization. But primates, especially humans, demonstrate the most pronounced specialization and hemispheric lateralization of the two streams. This likely supports our superior object recognition and spatial reasoning abilities.
Interactions Between Dual Streams
While the ventral and dorsal streams are specialized for different functions, their operation is highly interconnected. Information flows bidirectionally between the pathways, shaping how we perceive and interact with our visual world.
Bottom-up Processing
Sensory information flows upwards from early visual areas to both the ventral and dorsal streams. This bottom-up processing allows higher areas to extract object identity or spatial relationships. Rapid bottom-up sweeps are important for orienting us to salient stimuli.
Top-down Feedback
Higher ventral stream areas send feedback to early visual regions, modulating responses based on expectations and context. For example, knowing you are looking at a face enhances activity in early areas tuned for facial features. Similar top-down effects occur from dorsal regions.
Direct Interactions
The ventral and dorsal streams interface directly through cross-talk between areas. For example, object recognition areas send shape and identity information to parietal regions to guide actions. Parietal areas send back spatial information to recognition networks. Breakdowns in communication between streams can impair visually-guided behavior.
Computational Models
Computational models provide theoretical insights into the workings and development of the dual visual systems. Simple feedforward network models can mimic some specialized responses, but feedback interactions between regions are needed to achieve human-like object and spatial vision.
Deep Neural Networks
Deep convolutional neural networks (CNNs) demonstrate that hierarchical feedforward architectures can learn sophisticated invariant object recognition abilities akin to the ventral stream. However, CNNs lack dorsal stream-like spatial functions.
Generative Models
Generative models such as Bayesian networks better capture the top-down and contextual influences on object perception found in the brain’s ventral stream. These models use prior knowledge to guide interpretation of sensory data.
Sensorimotor Networks
Recurrent neural networks that integrate sensory inputs with motor outputs mimic some dorsal stream functionality. As they interact with environments, they learn spatial relationships supporting accurate actions. Developing hybrid models that capture both ventral and dorsal dynamics remains an active area of research.
Outstanding Questions
While our understanding of the dual visual streams has advanced considerably, many open questions remain:
– How is visual information seamlessly integrated between the streams? The neural mechanisms supporting complex visuomotor behaviors are still unclear.
– How do the streams interact with top-down attention and cognitive control processes? Prefrontal cortex likely regulates distribution of processing between streams.
– How do developmental disorders differentially impact the two streams? Dysfunctions in either pathway may contribute to conditions like autism or ADHD.
– Can artificial dual stream systems achieve human-like sophistication? Advances in deep learning and sensorimotor integration make this an exciting prospect.
– How does stream specialization vary across animal species? Comparisons of multiple mammals could reveal how dual vision evolved.
Conclusion
In summary, the dual visual streams are specialized cortical pathways essential for human visual experience and behavior. The ventral stream supports object recognition, while the dorsal stream supports spatial awareness and visuomotor control. An intricate balance between the streams allows us to both perceive the visual world and interact with it. Advances in neuroscience and computational modeling continue to reveal new insights into how these systems develop and function.
