1.3 Higher visual processing areas

Higher visual processing areas are crucial for interpreting complex visual information. These regions, including the ventral and dorsal streams, work together to recognize objects, perceive motion, and process spatial information. Understanding these areas helps explain how we make sense of the visual world around us.

The brain's ability to process visual information goes beyond simple perception. Higher visual areas enable us to recognize faces, interpret color and depth, and even influence our aesthetic experiences. These complex processes shape our understanding of the visual environment and our interactions with it.

Ventral and dorsal streams

• The ventral stream, also known as the "what" pathway, is involved in object recognition and form perception, processing visual information related to the identity and features of objects
• The dorsal stream, also known as the "where" or "how" pathway, is involved in spatial processing and visually guided actions, processing information about the location and motion of objects
• These two streams originate from the primary visual cortex (V1) and diverge into separate pathways, with the ventral stream projecting to the temporal lobe and the dorsal stream projecting to the parietal lobe

Form and object recognition

Inferior temporal cortex

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• The inferior temporal cortex (IT) is a key region in the ventral stream involved in object recognition and categorization
• IT neurons respond selectively to complex visual stimuli, such as faces, objects, and scenes
• Damage to the IT can lead to visual agnosia, a condition characterized by the inability to recognize objects despite intact visual perception
• The IT is organized in a hierarchical manner, with increasing complexity and specificity of visual representations as information flows from posterior to anterior regions

Face-selective regions

• Within the IT, there are specialized regions that respond selectively to faces, such as the fusiform face area (FFA) and the occipital face area (OFA)
• These face-selective regions are involved in the perception and recognition of faces, processing information about facial features, identity, and expression
• Damage to these regions can result in prosopagnosia, a specific impairment in face recognition
• Studies using functional magnetic resonance imaging (fMRI) have shown increased activation in the FFA and OFA when participants view faces compared to other objects

Motion perception

Middle temporal area (MT/V5)

• The middle temporal area, also known as MT or V5, is a key region in the dorsal stream involved in the perception of motion
• MT neurons are highly sensitive to the direction and speed of moving stimuli
• Damage to MT can lead to motion blindness, a condition characterized by the inability to perceive motion despite intact visual acuity
• MT receives input from the primary visual cortex (V1) and projects to other areas in the dorsal stream, such as the medial superior temporal area (MST)

Medial superior temporal area (MST)

• The medial superior temporal area (MST) is another region in the dorsal stream involved in motion perception
• MST neurons respond to complex motion patterns, such as optic flow and self-motion
• MST is involved in the integration of visual motion signals with vestibular and proprioceptive information, contributing to the perception of self-motion and navigation
• Damage to MST can impair the ability to perceive heading direction and navigate through the environment

Visuospatial processing

Posterior parietal cortex

• The posterior parietal cortex (PPC) is a key region in the dorsal stream involved in visuospatial processing and attention
• The PPC integrates visual, somatosensory, and proprioceptive information to create a unified representation of space
• Damage to the PPC can lead to spatial neglect, a condition characterized by the inability to attend to and respond to stimuli in the contralesional side of space
• The PPC is involved in the planning and execution of visually guided actions, such as reaching and grasping

Spatial attention networks

• The PPC is part of a network of brain regions involved in spatial attention, including the frontal eye fields (FEF) and the superior colliculus
• These regions work together to allocate attentional resources to relevant locations in space and to guide eye movements
• The dorsal attention network, which includes the PPC and FEF, is involved in the voluntary control of attention, such as when searching for a specific target
• The ventral attention network, which includes the temporoparietal junction (TPJ) and the ventral frontal cortex, is involved in the automatic orienting of attention to salient or unexpected stimuli

Color perception

V4 and color processing

• Area V4 is a region in the ventral stream that is involved in color processing and perception
• V4 neurons are sensitive to color and respond selectively to different wavelengths of light
• Damage to V4 can lead to achromatopsia, a condition characterized by the inability to perceive color despite intact visual acuity
• V4 receives input from the primary visual cortex (V1) and projects to higher-order regions in the temporal lobe, such as the inferior temporal cortex (IT)

Color constancy mechanisms

• Color constancy refers to the ability to perceive the color of an object as relatively stable despite changes in illumination
• The visual system achieves color constancy through a combination of retinal and cortical mechanisms
• Retinal mechanisms, such as chromatic adaptation and color opponency, help to maintain stable color perception under different lighting conditions
• Cortical mechanisms, such as the integration of contextual information and the use of prior knowledge, also contribute to color constancy
• The interaction between V4 and higher-order regions in the temporal lobe is thought to play a key role in color constancy

Depth perception

Binocular disparity cues

• Binocular disparity refers to the slight difference in the images projected onto the left and right retinas due to the horizontal separation of the eyes
• The visual system uses binocular disparity cues to extract information about the relative depth of objects in the environment
• Neurons in the primary visual cortex (V1) and other visual areas, such as V2 and V3, are sensitive to binocular disparity and respond selectively to different disparity values
• The integration of binocular disparity information across multiple visual areas allows for the perception of stereoscopic depth

Monocular depth cues

• Monocular depth cues are sources of depth information that can be extracted from a single retinal image, without the need for binocular vision
• Examples of monocular depth cues include occlusion, relative size, linear perspective, and texture gradient
• The visual system uses a combination of monocular and binocular depth cues to create a unified perception of depth and three-dimensional space
• The integration of depth cues occurs in higher-order visual areas, such as the inferior temporal cortex (IT) and the posterior parietal cortex (PPC)

Visual memory

Parahippocampal place area

• The parahippocampal place area (PPA) is a region in the ventral stream that responds selectively to scenes and landmarks
• The PPA is involved in the encoding and recognition of spatial layouts and navigational cues
• Damage to the PPA can lead to topographical disorientation, a condition characterized by the inability to navigate familiar environments
• The PPA is thought to play a key role in the formation of cognitive maps and the representation of spatial context

Perirhinal cortex

• The perirhinal cortex (PRC) is a region in the medial temporal lobe that is involved in object recognition memory
• The PRC is critical for the encoding and retrieval of object-related information, such as the identity and features of objects
• Damage to the PRC can impair the ability to recognize objects and faces, particularly when the discrimination requires fine-grained visual analysis
• The PRC is thought to work in conjunction with the hippocampus and other medial temporal lobe structures to support long-term declarative memory

Artistic processing

Aesthetic experiences

• Aesthetic experiences, such as the appreciation of beauty and the emotional response to art, involve a distributed network of brain regions
• The ventral visual pathway, particularly the inferior temporal cortex (IT), is involved in the perceptual analysis of visual art, processing information about form, color, and composition
• The prefrontal cortex (PFC) and the anterior cingulate cortex (ACC) are involved in the cognitive and emotional evaluation of art, integrating perceptual information with personal experiences and cultural knowledge
• The reward system, including the orbitofrontal cortex (OFC) and the ventral striatum, is activated during aesthetic experiences, suggesting a link between art appreciation and pleasure

Creativity and visual imagery

• Creativity and visual imagery involve the generation and manipulation of mental images in the absence of direct sensory input
• The default mode network (DMN), which includes the medial prefrontal cortex (mPFC), the posterior cingulate cortex (PCC), and the inferior parietal lobule (IPL), is involved in self-generated thought and mental simulation
• The frontoparietal control network (FPCN), which includes the dorsolateral prefrontal cortex (DLPFC) and the anterior inferior parietal lobule (aIPL), is involved in the top-down control and manipulation of mental images
• The interaction between the DMN and the FPCN is thought to support creative cognition and the generation of novel ideas

Synesthesia

Types of synesthesia

• Synesthesia is a neurological condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway
• There are many different types of synesthesia, such as grapheme-color synesthesia (associating letters or numbers with specific colors), chromesthesia (associating sounds with colors), and lexical-gustatory synesthesia (associating words with tastes)
• The most common form of synesthesia is grapheme-color synesthesia, which affects approximately 1-2% of the population
• Synesthesia is thought to arise from increased connectivity or cross-activation between different sensory or cognitive regions in the brain

Neural basis of synesthesia

• The neural basis of synesthesia is not yet fully understood, but several theories have been proposed
• The cross-activation theory suggests that synesthesia arises from direct connections between sensory or cognitive regions that are normally separate, such as the visual word form area (VWFA) and the color processing area V4 in grapheme-color synesthesia
• The disinhibited feedback theory proposes that synesthesia results from a reduction in the normal inhibitory feedback from higher-order regions to lower-order sensory areas, leading to the activation of additional sensory experiences
• Neuroimaging studies have shown increased activation and connectivity in the sensory or cognitive regions associated with the specific type of synesthesia, supporting the idea of cross-modal interactions in the brain

Disorders of higher visual processing

Visual agnosia

• Visual agnosia is a neurological condition characterized by the inability to recognize objects, faces, or scenes despite intact visual perception
• There are different types of visual agnosia, such as apperceptive agnosia (difficulty in perceiving the form and structure of objects) and associative agnosia (difficulty in attributing meaning to objects despite intact perception)
• Visual agnosia can result from damage to the ventral visual pathway, particularly the inferior temporal cortex (IT) and the fusiform gyrus
• Patients with visual agnosia may be able to copy or match objects but fail to recognize them, suggesting a dissociation between perception and recognition

Balint's syndrome

• Balint's syndrome is a rare neurological condition characterized by a triad of symptoms: simultanagnosia (inability to perceive multiple objects simultaneously), oculomotor apraxia (difficulty in initiating voluntary eye movements), and optic ataxia (difficulty in reaching for objects under visual guidance)
• Balint's syndrome typically results from bilateral damage to the posterior parietal cortex (PPC), a key region in the dorsal visual pathway involved in visuospatial processing and attention
• Patients with Balint's syndrome may experience a restricted "spotlight" of attention, being able to perceive only one object at a time and having difficulty in shifting their attention to other objects in the visual field
• The oculomotor and reaching deficits in Balint's syndrome highlight the role of the PPC in the coordination of eye movements and visually guided actions

Top-down influences

Attention and visual processing

• Attention is a top-down cognitive process that allows us to selectively focus on relevant information while ignoring irrelevant distractors
• Attention can modulate visual processing at multiple stages, from early sensory areas to higher-order regions in the ventral and dorsal pathways
• The frontoparietal attention network, which includes the frontal eye fields (FEF) and the posterior parietal cortex (PPC), is involved in the top-down control of attention and the allocation of attentional resources
• Attention can enhance the neural responses to attended stimuli, increase the signal-to-noise ratio, and improve the efficiency of visual processing

Expectations and prior knowledge

• Expectations and prior knowledge can influence visual processing in a top-down manner, shaping our perception and interpretation of visual information
• The predictive coding framework suggests that the brain constantly generates predictions about incoming sensory input based on prior experience and updates these predictions based on the mismatch between expected and actual input
• Top-down expectations can modulate activity in sensory areas, such as the primary visual cortex (V1), leading to a more efficient processing of expected stimuli and a suppression of responses to unexpected or irrelevant stimuli
• The integration of bottom-up sensory information and top-down expectations is thought to occur in higher-order regions, such as the prefrontal cortex (PFC) and the posterior parietal cortex (PPC), which send feedback signals to sensory areas to guide perceptual processing.